Showing posts with label evolution. Show all posts
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12:48 AM
Water lilies, a kind of adaptations
Water lilies, a kind of adaptations

The results of natural selection in which succeeding generations of organisms become better able to live in their environments are called adaptations. Many of the features that are most interesting and beautiful in biology are adaptations. Specialized structures, physiological processes, and behaviors are all adaptations when they allow organisms to cope successfully with the special features of their environments.

Adaptations ensure that individuals in populations will reproduce and leave well-adapted offspring, thus ensuring the survival of the species. Adaptations arise through mutations—inheritable changes in an organism’s genetic material.

These rare events are usually harmful, but occasionally they give specific survival advantages to the mutated organism and its offspring. When certain individuals in a population possess advantageous mutations, they are better able to cope with their specific environmental conditions and, as a result, will contribute more offspring to future generations than those individuals that lack the mutation.


Over time, the number of individuals that have the advantageous mutation will increase in the population at the expense of those that do not have it. Individuals with an advantageous mutation are said to have a higher fitness than those without it, because they tend to have comparatively higher survival and reproductive rates. This is natural selection.

Natural Selection

Growing on poisonous environtment
Growing on poisonous environtment

Over very long periods of time, evolution by natural selection results in increasingly better adaptations to environmental circumstances. Natural selection is the primary mechanism of evolutionary change, and it is the force that either favors or selects against mutations.

Although natural selection acts on individuals, a population gradually changes as those with adaptations become better represented in the total population. Most flowering plants, for example, are unable to grow in soil containing high concentrations of certain elements (for example, heavy metals) commonly found in mine tailings.

Therefore, an adaptation that conferred resistance to these elements would open up a whole new habitat where competition with other plants would be minimal. Natural selection would favor the mutations, which confer specific survival advantages to those that carry them and impose limitations on individuals lacking these advantages.

Thus, plants with special adaptations for resistance to the poisonous effects of heavy metals would have a competitive advantage over those that find heavy metals toxic. These attributes would be passed to their more numerous offspring and, in evolutionary time, resistance to heavy metals would increase in the population.

Types of Adaptations

Although natural selection serves as the instrument of change in shaping organisms to very specific environmental features, highly specific adaptations may ultimately be a disadvantage. Adaptations that are specialized may not allow sufficient flexibility (generalization) for survival in changing environmental conditions.

The degree of adaptative specialization is ultimately controlled by the nature of the environment. Environments, such as the tropics, that have predictable, uniform climates and have had long, uninterrupted periods of climatic stability are biologically complex and have high species diversity.

Tropical rain forest
Tropical rain forest, complex competition for resources
and intense predator-prey relationships

Scientists generally believe that this diversity results, in part, from complex competition for resources and from intense predator-prey relationships. Because of these factors, many narrowly specialized adaptations have evolved when environmental stability and predictability prevail.

By contrast, harsh physical environments with unpredictable or erratic climates seem to favor organisms with general adaptations, or adaptations that allow flexibility. Regardless of the environment type, organisms with both general and specific adaptations exist because both types of adaptation enhance survival under different environmental circumstances.

Metabolism is the sum of all chemical reactions taking place in an organism, whereas physiology consists of the processes involved in an organism carrying out its function. Physiological adaptations are changes in the metabolism or physiology of organisms, giving them specific advantages for a given set of environmental circumstances.

harsh physical environments, enhance survival under different environmental circumstances
harsh environments, enhance survival under different environmental circumstances

Because organisms must cope with the rigors of their physical environments, physiological adaptations for temperature regulation, water conservation, varying metabolic rate, and dormancy allow organisms to adjust to the physical environment or respond to changing environmental conditions.

Adaptations and Environment

Desert environments, for example, pose a special set of problems for organisms. Hot, dry environments require physiological mechanisms that enable organisms to conserve water and resist prolonged periods of high temperature.

Evolution has favored a specialized form of photosynthesis in cacti and other succulents inhabiting arid regions. Crassulacean acid metabolism (CAM) photosynthesis allows plants with this physiological adaptation to absorb carbon dioxide at night, when relative humidity is comparatively high and air temperatures relatively low.

✯ A Beavertail cactus in Henderson Canyon - CA
Adaptations and Environment

Taking in carbon dioxide during the day would dehydrate plants, because opening the pores through which gas exchange takes place allows water to escape from the plant. CAM photosynthesis, therefore, allows these plants to exchange the atmospheric gases essential for their metabolism at night, when the danger of dehydration is minimized.

Because organisms must also respond and adapt to an environment filled with other organisms— including potential predators and competitors— adaptations that minimize the negative effects of biological interactions are favored by natural selection. Often the interaction among species is so close that each species strongly influences the others and serves as the selective force causing change.

Under these circumstances, species evolve together in a process called coevolution. The adaptations resulting from coevolution have a common survival value to all the species involved in the interaction. The coevolution of flowers and their pollinators is a classic example of these tight associations and their resulting adaptations.

Speciation

Adaptations can be general or highly specific. General adaptations define broad groups of organisms whose lifestyles are similar. At the species level, however, adaptations are more specific and give narrow definition to those organisms that are more closely related to one another.

Slight variations in a single characteristic, such as bill size in the seed-eating Galapágos finches, are adaptive in that they enhance the survival of several closely related species. An understanding of how adaptations function to make species distinct also furthers the knowledge of how species are related to one another.

Why so many species exist is one of the most intriguing questions of biology. The study of adaptations offers biologists an explanation. Because there are many ways to cope with the environment, and because natural selection has guided the course of evolutionary change for billions of years, the vast variety of species existing on the earth today is simply an extremely complicated variation on the theme of survival.
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11:07 PM
In adaptive radiation, numerous species evolve from a common ancestor introduced into an environment with diverse ecological niches. The progeny evolve genetically into customized variations of themselves, each adapting to survive in a particular niche.

In 1898 Henry F. Osborn identified and developed the evolutionary phenomenon known as adaptive radiation, whereby different forms of a species evolve, quickly in evolutionary terms, from a common ancestor.

According to the principles of natural selection, organisms that are the best adapted (most fit) to compete will live to reproduce and pass their successful traits on to their offspring. The process of adaptive radiation illustrates one way in which natural selection can operate when members of one population of a species are cut off or migrate to a different environment that is isolated from the first.

Such isolation can occur from one patch of plantings to another, from one mountain top or hillside to another, from pond to pond, or from island to island. Faced with different environments, the group will diverge from the original population and in time become different enough to form a new species.


Genetic Changes

In a divergent population, the relative numbers of one form of allele (characteristic) decrease, while the relative numbers of a different allele increase. New environmental pressures will select for favorable alleles that may not have been favored in the old environment.

Over successive generations, therefore, a new gene created by random mutation (change) may replace the original form of the gene if, for example, the trait encoded by that gene allows the divergent group to cope better with environmental factors, such as food sources, predators, or temperature.

The result in the long term is that deoxyribonucleic acid (DNA) changes sufficiently through the growth of divergent populations to allow new generations to become significantly different from the original population. In time, they are unable to reproduce with members of the original species and become a new species.

Galápagos Islands Case Study

Adaptive radiation occurs dramatically when a species migrates from one landmass to another. This may occur between islands or between continents and islands. A classic example of adaptive radiation is the evolution of finches noted by Charles Darwin during his trips to the Galápagos Islands off the west coast of South America.

Several species of plants and animals had migrated to these islands from the South American mainland by means of flight, wind, ocean debris, or other means of transport. Finches from the mainland—perhaps aided by winds—settled on fifteen of the islands in the Galápagos group and began to adapt to the various unoccupied ecological niches on those islands, which differed.

Over several generations, natural selection favored a variety of finch species with beaks adapted for the different types of foods available on the different islands. As a result, several species of different finches evolved, roughly simultaneously, on these islands.

Hawaiian Silversword Alliance

Although plants seem unable to "migrate" as birds and other animals do, adaptive radiation occurs in the plant world as well. In the Hawaiian Islands, for example, twenty-eight species of the Asteraceae family are known together as the Hawaiian silversword alliance. The entire group appears to be traceable to one ancestor, thought to have arrived on the island of Kauai from western North America.

The silverswords—which compose three genera, Argyroxiphium, Dubautia, and Wilkesia— have since evolved into twenty-eight species, and this speciation came about due to major ecological shifts. These plants are therefore prime examples of adaptive radiation.

Within the silversword alliance, different species have adapted to widely varying ecosystems found throughout the islands. Argyroxiphium sandwicense, for example, is endemic to the island of Maui and grows at high elevations from 6,890 to 9,843 feet (2,100 - 3,000 meters) on the dry, alpine slopes of the volcano Haleakala.

This species has succulent leaves covered with silver hairs. It is thought that the hairs lessen the pace of evaporative moisture loss and protect the leaves from the sun. In contrast, species of the genus Dubautia that grow in wet, shady forests have large leaves that lack hairs.

Despite their "customized" physiologies, the silverswords that have evolved in Hawaii are all closely related to one another, so much so that any two can hybridize. Studies of the silverswords have provided what geneticist Michael Purugganan called a "genetic snapshot of plant evolution". Adaptive radiation is one window into how new plant structures arise.
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9:00 AM
Anaerobes
Anaerobes
The first organisms to evolve on the earth are thought to have been heterotrophs and anerobes. Heterotrophs are organisms that cannot produce their own food but must fill their energy requirements by consuming organic molecules produced by other processes or organisms. Anaerobes are organisms that do not require free oxygen gas in order to survive; for some anaerobes, free oxygen may be poisonous.

Heterotrophs include many familiar organisms (such as animals) whose existence is tied to primary producers, those organisms that create energy-storing molecules, such as photosynthesizing plants. Anaerobes also are common, though less apparent. Typically, they are microscopic organisms restricted to living in a few surface environments where oxygen is absent.

It may seem strange, then, that these organisms were perhaps the first organisms to have evolved on the earth. Yet the combination of the heterotrophic lifestyle and the anaerobic life requirement is consistent with what is known about the conditions of the early earth’s surface environment.

The earliest anaerobic heterotrophs laid the biochemical foundations for the evolution of photosynthesis, free oxygen in the atmosphere, and the rise of complex organisms. All those events had the adverse impact of limiting the range of environments available to the anaerobes.


The world’s first organism evolved in what has been called a "prebiotic soup" of energy-rich organic molecules. Heterotrophic organisms would exploit this environment by absorbing the molecules. A continuing supply of energy-rich molecules depended on the absence of free oxygen in the early atmosphere and the functioning of the abiotic synthesis.

Fermentation

The energy-richmolecules of the soupwere converted to energy by a series of biochemical reactions. One of the simplest, and therefore perhaps one of the oldest, types of energy conversion reactions is anaerobic fermentation.

During anaerobic fermentation, an energy-rich molecule, such as the simple sugar glucose, is dismantled to release energy and waste by-products. Several lines of evidence suggest that this form of energy conversion was utilized by the early heterotrophs.

One indicator that fermentation is a very ancient biochemical process is that the reaction used to release energy from the glucose molecule is very common among modern organisms. The ability to utilize the fermentation reaction is evident in the anaerobic reaction of yeast using sugar and releasing ethyl alcohol.

Although it is not the primary energy-releasing reaction for most organisms, fermentation’s widespread availability suggests that it is very old and perhaps inherited from an early, simpler ancestor.

The fermentation reaction is not very efficient. For example, it releases two units of energy for every glucose molecule, whereas oxidation of the same glucose molecule releases more than thirty energy units.

Such an inefficient reaction for energy release could not be tolerated by an advanced organism with many energy demands. Alternatively, single-celled heterotrophs surrounded by, and absorbing, energy-rich molecules such as glucose (which is unlikely to decompose in an anoxic environment) do not expend much energy in gathering their food.

The Earliest Organisms

Thus, two very different types of evidence—that which points to an anoxic early atmosphere and evidence for the ancient ancestry of the glucose fermentation reaction—suggest that the earliest organism was a single-celled heterotroph that absorbed energy-rich molecules from the surrounding anaerobic environment.

Modern analogues for such an organism exist. Single-celled bacteria, called obligate anaerobes, exist in a few anoxic environments today.

It is likely that the modern obligate anaerobes have not changed significantly (especially in their morphology, or shape and size) from their Precambrian ancestors. Given this much information, paleontologists know that their search for early Precambrian fossils, the petrified remains of organisms, is not easy.


The process of fossilization—that is, the preservation of the shape of an organism in rock—is best at preserving the details of hard body parts. Hard skeletons and shells or their impressions are easier to preserve than are soft body parts. In the case of early Precambrian fossils, the most likely organisms (bacteria) not only are small but also contain no hard body parts.

Despite these barriers to preservation and despite the very poorly preserved Precambrian rock record, some early Precambrian fossil remains have been found and described. The fossils are usually found preserved in rock called chert, which probably began as a gelatinous material. Microscopic remains of organisms embedded in this gelatin were delicately preserved when the chert lost some of its water and solidified.

The oldest fossil remains identified have been found in cherts from southern Africa. These cherts, part of what is called the Fig Tree Formation, are more than three billion years old. The fossils consist of the wispy, spherical remains of what may have been a type of alga and the rod-shaped remains of a possible heterotrophic bacterium.
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1:56 AM
Angiosperm
Angiosperm
Angiosperms (flowering plants) appeared about 130 million years ago and today dominate the plant world, with approximately 235,000 species.

In early Devonian-age rocks, approximately 363- 409 million years old, fossils of simple vascular and nonvascular plants can be seen. Ferns, lycopods, horsetails, and early gymnosperms became prominent during the Carboniferous period (approximately 290-363 million years ago).

The gymnosperms were the dominant flora during the Age of Dinosaurs, the Mesozoic era (65-245million years ago). More than 130 million years ago, from the Jurassic period to early in the Cretaceous period, the first flowering plants, or angiosperms (phylum Anthophyta), arose. Over the following 40 million years, angiosperms became the world’s dominant plants.

The angiosperms show high species diversity, and they occupy almost every habitat on earth, from deserts to high mountain peaks and from freshwater ecosystems to marine estuaries. Angiosperms range in size from eucalyptus trees well over 100 meters (328 feet) tall with trunks nearly 20 meters (66 feet) in circumference to duckweed, simple floating plants barely 1 millimeter (0.003 inch) long.


Special Characteristics

Some of the defining characteristics of angiosperms involve their physical appearance or morphology and internal anatomy: the presence of flowers and fruits containing seeds, stamens with two pairs of pollen sacs, a microgametophyte (the male, haploid stage of the life cycle contained in the pollen) with three nuclei, a megagametophyte (the female, haploid stage of the life cycle enclosed in the ovary) with eight nuclei, companion cells, and sieve tubes in the phloem (vascular tissue important in the transport of organic molecules).

Some of these characteristics involve life-cycle features, such as double fertilization, that are distinct from almost all other members of the plant kingdom. (Double fertilization is also known in the genera Ephedra and Gnetum, members of the gymnosperms.)

Because angiosperms possess so many unique features, plant taxonomists have long believed that angiosperms originated from a single common ancestor. Because the first flowers and pollen grains appear in fossils from the early Cretaceous period, up to about 130 million years ago, it is probable that angiosperms actually arose more than 130 million years ago.

As the findings of paleobotanists (botanists who study plants in the fossil record) have been combined with more recent knowledge from evolutionary genetics and biochemistry, a clearer picture of angiosperm evolution has emerged.

Proposed Ancestors

Because gymnosperms (the other large group of seed plants) have long been considered ancestral to the angiosperms, researchers have attempted to develop models for the evolution of the ovule-bearing structures of flowering plants from the similar, naked ovule-bearing structures of gymnosperms.

Angiosperm evolution
Angiosperm evolution

Some lines of evidence indicate that groups of extinct cycad-like gymnosperms known as the Bennettitales and the gnetophytes, a modern division of the gymnosperms which show up in the fossil record about 225 million years ago, are the seed plants most closely related to angiosperms.

All three groups, the Bennettitales, the gnetophytes, and the angiosperms, share, or shared, superficially similar flower-like reproductive structures. The strobili, or cones, of some gnetophytes closely resemble flowers, and the xylem (vascular tissue specialized for transporting water) of some gnetophytes is similar to the xylem found in angiosperms.

Seed Ferns

seed ferns (pteridosperms) fossil
seed ferns (pteridosperms) fossil

Other lines of evidence suggest that a group of plants called the seed ferns, or pteridosperms, might represent the ancestors of the angiosperms. The seed ferns, which predate the angiosperms by many millions of years, had seed-bearing cupules and specialized organs that produced pollen. Many plant taxonomists believe that the seed-bearing cupules in some groups of seed ferns could have evolved into the carpels of flowers.

Earliest Flowers

Most paleobotanists assume that the first flowers were small, simple, and green in color and by modern standards were rather unattractive. Their petals and sepals were probably not clearly differentiated.

In November of 1998, Ge Sun and David Dilcher and their colleagues published their discovery of the oldest angiosperm fossil to date, estimated to be at least 122 million years old and possibly as old as 145million years. Either age qualifies it as the oldest.

The fossils were discovered in China, and the fruits show the characteristic enclosed ovule (a carpel) that is distinctive to angiosperms. It was given the scientific name Archaefructus liaoningensis. Given its great age, this find implies that angiosperms may have arisen as early as the Jurassic period, more than 145 million years ago.

Other early flowers produced pollen with a single aperture, or opening, a trait that the monocot branch of the angiosperms shares with cycads and ginkgos. Plant taxonomists believe that pollen with a single opening is an ancestral feature that some plants have kept as they evolved. The pollen of eudicots,with its three apertures, is thought to be a derived feature (that is, a later evolutionary development).

Recent studies of angiosperm evolution, using data from deoxyribonucleic acid (DNA) sequences, have led to the proposal that an obscure shrub from the South Pacific island of New Caledonia, called Amborella trichopoda, represents what is left of the ancestral sister group (a related organism that branched off before the evolution of another group of organisms) to all the angiosperms.

As a sister group to all the angiosperms, it is considered to be the most primitive (in an evolutionary sense) of the angiosperms and therefore should resemble what the ancestor to the angiosperms was like. It does possess some of the expected primitive traits for a primitive angiosperm, such as small, greenish-yellow flowers and a lack of vessels for conducting water from the ground to the leaves.

Angiosperm Classification

Approximately 97 percent of angiosperm species are classified as either Monocotyledones (monocots), with approximately 65,000 species, or Eudicotyledones (eudicots), with about 165,000 species. The monocots include such familiar plants as the grasses, lilies, irises, orchids, cattails, and palms. The more diverse eudicots include most of the familiar trees and shrubs (other than the conifers) and many of the herbaceous plants.

The remaining 3 percent of angiosperms are called the magnoliids, a group of plants considered to have primitive features, among them pollen with a single aperture. Many magnoliids also feature oil cells with ether-containing oils providing the characteristic scents of laurel and pepper, for example. The magnoliids are typically divided into the woody magnoliids and paleoherbs.

Woody magnoliids have large, often showy, bisexual flowers with multiple free parts. Magnolia trees and tulip trees (both in Magnoliaceae, or the magnolia family) are examples of this group. The paleoherbs have small, often unisexual flowers and usually just a few flower parts. Modern paleoherbs include the pepper family (Piperaceae), the birth-wort family (Aristolochiaceae), and the water lily family (Nymphaeaceae).

Recent studies of angiosperm evolution, using data from DNA sequences, have also sharpened the understanding of some of the relationships among monocots, eudicots, and magnoliids. If these groups are displayed as an evolutionary tree (or phylogenetic tree), the magnoliids are polyphyletic (that is, they do not share a single common ancestor).

The magnoliids branch off near the base of the tree on several different branches. The monocots are monophyletic (that is, they share a single common ancestor) and form a separate branch from among the magnoliid branches. The eudicots branch off last and represent the most diverse and evolutionarily complex group.

Geographic Origins

As hotly debated, perhaps, as exactly which group of plants were ancestral to the angiosperms is the question of where the angiosperms first evolved. Some botanists believe that angiosperms first developed in the Northern Hemisphere; others look at the Southern Hemisphere.

At the time angiosperms are proposed to have evolved, the continents were not arranged the way they are now. At that time, all of the world’s major landmasses were grouped into a supercontinent called Pangaea.

The southern part of this continent is referred to as Gondwana land, and the northern part is called Laurasia. Based on what is known about late Cretaceous angiosperms and their habitats, some scientists suggest that the westernmost, semiarid regions of Gondwana land may be the place where angiosperms first evolved.

As Pangaea broke up, the separate continents moved in different positions, resulting in new configurations. India collided with Asia, raising the Himalaya Mountains and the Tibetan Plateau. Antarctica slipped over the South Pole, and Australia became isolated. These plate movements created new climatic regimes, opening up new niches that were rapidly exploited by the angiosperms.

Diversification and Spread

Regardless of their geographic origins, by about ninety million years ago the flowering plants were well on their way to dominating the world’s flora. The early angiosperms were well adapted to drought and cold.

Adaptations that conferred resistance to these conditions included strong leaves, efficient water-conducting cells, and tough, resistant seed coats. Some woody flowering plants evolved the ability to lose their leaves, called the deciduous habit.

This characteristic allows the shutdown of metabolism during adverse environmental conditions, such as during seasonal droughts or winter weather. Because of greater climate instability during the past fifty million years or so compared to earlier times, the above-mentioned traits were important in allowing the flowering plants to adapt to different and often harsher climatic conditions.

Pollination

pollination by insects
pollination by insects

A major innovation that likely led to some of the great diversity seen in angiosperms was pollination by insects or other animals. As plants adapted to the various available pollinators, the pollinators also adapted to the plants, sometimes in very specific ways. Many pollination systems include specialized colors or markings, flower shapes, and flower scents.

This process of evolving "together" is called coevolution. Coevolution has also occurred between plants and their predators. Evolution of chemical compounds to deter herbivory have, in turn, led to adaptations in many animal groups to circumvent the toxicity of the chemical compounds.
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7:44 PM
Animal-plant Interactions
Animal-plant Interactions
The ways in which certain animals and plants interact have evolved in some cases to make them interdependent for nutrition, respiration, reproduction, or other aspects of survival.

Ecology represents the organized body of knowledge that deals with the relationships between living organisms and their nonliving environments. Increasingly, the realm of ecology involves a systematic analysis of plant-animal interactions through the considerations of nutrient flow in food chains and food webs, exchange of such important gases as oxygen and carbon dioxide between plants and animals, and strategies of mutual survival between plant and animal species through the processes of pollination and seed dispersal.

A major example of animal-plant interactions involve the continual processes of photosynthesis and cellular respiration. Green plants are classified as ecological producers, having the unique ability, by photosynthesis, to take carbon dioxide and incorporate it into organic molecules.

Animals are classified as consumers, taking the products of photosynthesis and chemically breaking them down at the cellular level to produce energy for life activities. Carbon dioxide is a waste product of this process.


Mutualism

Mutualism is an ecological interaction in which two different species of organisms beneficially reside together in close association, usually revolving around nutritional needs.

One such example is a small aquatic flatworm that absorbs microscopic green algae into its tissues. The benefit to the animal is one of added food supply. The mutual adaptation is so complete that the flatworm does not actively feed as an adult.

The algae, in turn, receive adequate supplies of nitrogen and carbon dioxide and are literally transported throughout tidal flats in marine habitats as the flatworm migrates, thus exposing the algae to increased sunlight. This type of mutualism, which verges on parasitism, is called symbiosis.

Coevolution

Coevolution is an evolutionary process wherein two organisms interact so closely that they evolve together in response to shared or antagonistic selection pressure. A classic example of coevolution involves the yucca plant and a species of small, white moth (Tegitecula).

The female moth collects pollen grains from the stamen of one flower on the plant and transports these pollen loads to the pistil of another flower, thereby ensuring cross-pollination and fertilization. During this process, the moth will lay her own fertilized eggs in the flowers’ undeveloped seed pods.

The developing moth larvae have a secure residence for growth and a steady food supply. These larvae will rarely consume all the developing seeds; thus, both species (plant and animal) benefit.

Although this example represents a mutually positive relationship between plants and animals, other interactions are more antagonistic. Predator-prey relationships between plants and animals are common. Insects and larger herbivores consume large amounts of plant material. In response to this selection pressure, many plants have evolved secondary metabolites that make their tissues unpalatable, distasteful, or even poisonous. In response, herbivores have evolved ways to neutralize these plant defenses.

Mimicry and Nonsymbiotic Mutualism

Mimicry
Mimicry
Inmimicry, an animal or plant has evolved structures or behavior patterns that allow it tomimic either its surroundings or another organism as a defensive or offensive strategy.

Certain types of insects, such as the leaf hopper, walking stick, praying mantis, and katydid (a type of grasshopper), often duplicate plant structures in environments ranging from tropical rain forests to northern coniferous forests. Mimicry of their plant hosts affords these insects protection from their own predators as well as camouflage that enables them to capture their own prey readily.

Certain species of ambush bugs and crab spiders have evolved coloration patterns that allow them to hide within flower heads of such common plants as goldenrod, enabling them to ambush the insects that visit these flowers.

In nonsymbiotic mutualism, plants and animals coevolve morphological structures and behavior patterns bywhich they benefit each other but without living physically together.

This type of mutualism can be demonstrated in the often unusual shapes, patterns, and colorations that more advanced flowering plants have developed to attract various insects, birds, andmammals for pollination and seed dispersal purposes. Accessory structures, called fruits, form around seeds and are usually tasty and brightly marked to attract animals for seed dispersal.

Although the fruits themselves become biological bribes for animals to consume, often the seeds within these fruits are not easily digested and thus pass through the animals’ digestive tracts unharmed, sometimes great distances from the parent plant. Some seeds must pass through the digestive plant of an animal to stimulate germination.

Other types of seed dispersal mechanisms involve the evolution of hooks, barbs, and sticky substances on seeds that enable them to be easily transported by animal fur, feet, feathers, or beaks. Such strategies of dispersal reduce competition be- tween the parent plant and its offspring.

Pollinators

Pollinators
Pollinators
Because structural specialization increases the possibility that a flower’s pollenwill be transferred to a plant of the same species, many plants have evolved a vast array of scents, colors, and nutritional products to attract pollinators. Not only does pollen include the plant’s spermcells; it also represents a food reward.

Another source of animal nutrition is a substance called nectar, a sugar-rich fluid produced in specialized structures called nectaries within the flower or on adjacent stems and leaves.

Assorted waxes and oils are also produced by plants to ensure plant-animal interactions. As species of bees, flies, wasps, butterflies, and hawk-moths are attracted to flower heads for these nutritional rewards, they unwittingly become agents of pollination by transferring pollen from stamens to pistils.

Some flowers have evolved distinctive, unpleasant odors reminiscent of rotting flesh or feces, thereby attracting carrion beetles and flesh flies in search of places to reproduce and deposit their own fertilized eggs.

As these animals copulate, they often become agents of pollination for the plant itself. Some tropical plants, such as orchids, even mimic a female bee, wasp, or beetle, so that the insect’s male counterpart will attempt to mate with them, thereby encouraging precise pollination.

Among birds, hummingbirds are the best examples of plant pollinators. Various types of flowers with bright, red colors, tubular shapes, and strong, sweet odors have evolved in tropical and temperate regions to take advantage of hummingbirds’ long beaks and tongues as an aid to pollination.

Because most mammals, such as small rodents and bats, do not detect colors as well as bees and butterflies do, some flowers instead focus upon the production of strong, fermenting, or fruit-like odors and abundant pollen rich in protein. In certain environments, bats and mice that are primarily nocturnal have replaced day-flying insects and birds as pollinators.
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10:52 PM
The domain Archaea represents a diverse group of prokaryotes originally found in environments once considered to be hostile to life, now known to be widely distributed in nature.

The cycling of plant nutrients, such as carbon, nitrogen, and sulfur, requires the activity of microorganisms that convert these elements to forms readily available to plants. These microorganisms, which are generally found in both soil and water, include both prokaryotic organisms of the domain Bacteria and the domain of prokaryotes called Archaea, which play significant roles in nutrient cycling.

Along with Eukarya, to which protists, fungi, plants, and animals belong, the Archaea formone of the three domains of life. The Archaea are related to both Bacteria and Eukarya and, in some respects, appear to bemore closely related to Eukarya.

Biochemical and genetic studies, including information obtained from whole genome sequencing, suggest that Archaea may be closely related to an ancestor that gave rise to both Bacteria and Eukarya. Thus, Archaea may provide some insight into the processes that resulted in the evolution of higher life-forms, including plants and animals.


A Third Domain

For more than fifty years, biologists categorized living organisms into two groups based on their cellular organization and complexity: prokaryotes (originally all classified in kingdom Monera), the single-celled organisms whose chromosomes are not compartmentalized inside a nucleus (which include the domainBacteria), and eukaryotes, consisting of all other organisms, whose cells contain a nucleus. In the late 1970’s studies on a unique group of microorganisms led investigators to question the accepted classification of prokaryotes.

Originally called Archaebacteria by molecular biologist Carl Woese and his colleagues in 1977, these microorganisms were isolated from environments characterized by extremes in heat, acidity, pressure, or salinity, and many were found to be able to utilize sulfur and molecular hydrogen as part of their growth process.

Like all prokaryotes, Archaea do not have a nucleus. However, in their biochemistry and the structure and composition of their molecular machinery, they are as different from bacteria as they are from eukaryotes.

Woese and his colleagues analyzed and compared specific molecules of ribonucleic acid (RNA) present within the ribosome in all organisms, called ribosomal RNA (rRNA). Their findings suggested that all extant life is composed of three distinct groups of organisms: the eukaryotes, or domain Eukarya, which includes plants and animals, and two different prokaryotes, domains Bacteria and Archaea.

In 1990 Woese and others recommended the replacement of the simple prokaryote/ eukaryote view of life with a new tripartite scheme based on three domains: the Bacteria, Archaea, and Eukarya. Since 1990 the three-domain classification has been the subject of considerable debate, and as a consequence, both old and new terminology are used in scientific and popular literature.

Characteristics

Generally, the size and shape of Archaea are similar to those of Bacteria. They are single-celled microscopic organisms that, in some cases, are motile (capable of self-movement) and may be found in chains or clusters.

Archaea multiply in the same manner as bacteria: via binary fission, budding, or fragmentation. Like Bacteria, archaeal chromosomes are circular, indicating the absence of breaks or discontinuities, and many genes are organized in the same fashion as those found in Bacteria.

On the other hand, the specific chemical composition of Archaea plasma membranes and cell walls is unique to the Archaea and is quite different from the composition of these structures typically found in either Bacteria or Eukarya. In fact, the distinctive ether-linked isoprenoid lipids that compose the external membranes of Archaea are a hallmark of these microorganisms.

Another unique characteristic of Archaea is the composition of the molecular genetic machinery, which is amosaic of the components found in Bacteria and Eukarya. For example, the ribosomes (which are responsible for protein synthesis) of Archaea resemble the ribosomes of Bacteria in shape and composition and are distinct from the ribosomes of Eukarya.

On the other hand, the enzyme utilized by Archaea in the production of RNA, namely RNA polymerase, is quite different from the enzyme found in Bacteria. In Bacteria, RNA polymerase molecules are composed of four major proteins, while in the Archaea, RNA polymerase molecules consist of more than ten proteins and are surprisingly similar to the enzyme found in Eukarya.

In fact, archaeal RNA polymerase is so similar to the eukaryotic enzyme that combining certain proteins from both archaeal and eukaryotic sources results in a functional enzyme, a manipulation that is not possible with any bacterial RNA polymerases.

Among species of the Archaea, there is a variety of metabolic processes that differ greatly from the better-known metabolic routes of Bacteria and Eukarya. Many of the archaeal pathways used to convert food sources to energy and building blocks for growth involve enzymes having biological activities not found in any other biological systems.

In some cases, the enzymes require the involvement of rare metals, such as tungsten. While a requirement for metals in the activity of many bacterial and eukaryotic enzymes is ubiquitous, the use of tungsten appears to be unique to Archaea.

Diversity

A fascinating feature of Archaea is that they are found in niches that support the growth of few other organisms. These include highly reduced (oxygen-free) environments or very high-temperature environments found near hot springs or undersea hydrothermal vents as well as sites that are sulfur-rich and highly acidic.

Archaea are also found in highly saline marine environments and hypersaline lakes where the salinity is as much as ten times that in seawater. Based on the comparison of ribosomal RNA sequences as well as physiological and metabolic characteristics, the Archaea have been divided into three subdomains: Euryarchaeota, Crenarchaeota, and Korarchaeota.

The Euryarchaeota includes members of the methanogenic (methane-producing) and halophilic (salt-requiring) Archaea as well as many that grow at very high temperature, the thermophilic and extremely thermophilic, or hyperthermophilic, Archaea.

Representatives of hyperthermophilic Archaea are found in the Crenarchaeota, which also includes cold-dwelling Archaea that have been isolated in association with certain marine sponges. The Korarchaeota also includes hyperthermophilic Archaea, although these were not isolated or characterized as of 2001, but whose presence in hot spring and deep-sea samples has been identified by molecular biological techniques.

Methanogenic Archaea

Methane-producing Archaea are found in strictly anaerobic environments. They have no tolerance for oxygen: Trace amounts are inhibitory for growth, and too much is lethal. These Archaea obtain energy for growth by a process called methanogenesis, which results in the conversion of carbon dioxide to methane gas.

Methane production requires several enzymes that use coenzymes unique to methanogenic Archaea. The production of methane is of great importance to carbon cycling inmany anaerobic environments, and microorganisms that produce this gas have been known for centuries.

In 1776 the scientist Alessandro Volta demonstrated that air generated from sediments rich in decaying vegetation, such as those present in bogs, streams, and lakes, could be ignited. It is now known that methanogenic Archaea are responsible for generating this "marsh gas".

Because methanogens require an oxygen-free environment for growth, they are found onlywhere carbon dioxide and hydrogen are available and oxygen has been excluded. Thus, methanogens thrive in stagnant water, natural wetlands, paddy fields, and in the rumen of cattle and other ruminants as well as in the intestinal tracts of animals and the hindguts of cellulose-digesting insects, such as termites.

Methanogens are also found in hot springs and the deep ocean and are major components of the anaerobic process in waste treatment facilities. It has been estimated that production of methane by themethanogenicArchaeamay account for almost 90 percent of the total methane released into the atmosphere each year.

In addition to playing a role in carbon cycling, several methanogenic Archaea are also involved in nitrogen cycling, as they are able to convert molecular nitrogen into organic nitrogen via nitrogen fixation, a process that is shared by only a few prokaryotes.

Thermophilic Archaea

Thermophilic Archaea live in environments ranging in temperature from55 degrees Celsius (131 degrees Fahrenheit) to 80 degreesCelsius (176 degrees Fahrenheit). Hyperthermophilic Archaea grow at temperatures near or greater than the boiling point ofwater and as high as 113 degrees Celsius (235 degrees Fahrenheit).

These Archaea have been isolated from hot sulfur springs, sulfur-laden mud at the base of volcanoes, and near very hot deep-sea hydrothermal vents where super heated water is emitted at very high temperatures under pressure.

Species that can use oxygen, as well as those that have no tolerance for oxygen, are known. Many of the anaerobic representatives obtain energy for growth by the metabolism of elemental sulfur.

In addition, many are found in environments that are extremely acidic, including those that are members of Thermoplasmatales. This group is noted for its ability to growat a pHof 2.0 and below(on a scale where pH 7.0 is neutral), which is equivalent to the acid in car batteries.

Arepresentative is Thermoplasma, which does not possess a cell wall but has a chemically unique structure composed of a lipid-polysaccharide (tetraether lipid with mannose and glucose units) that is distinctly different from the unusual ether-linked lipids found in the membrane components of typical Archaea.

Halophilic Archaea

The salt-dependent halophilic Archaea require extremely high concentrations of salt for survival, and some grow readily in saturated brine, where the salt concentration reaches 32 percent (in seawater it is approximately 3.5 percent) and where very alkaline conditions are not uncommon. Halophilic Archaea are found in salty habitats along ocean borders and inland waters such as the Dead Sea and the Great Salt Lake.

The reddish-purple color observed in salt evaporation ponds is due to production of red- and orange-colored carotenoids and other pigments associated with the massive growth of halophilic Archaea.

Some halophilic Archaea are capable of harvesting light to provide energy for growth by a mechanism that does not involve chlorophyll pigments. Light harvesting by these halophilicArchaea is done by a membrane-bound protein called bacteriorhodopsin that is equivalent to the mammalian eye pigment rhodopsin in both function and structure.

Bacteriorhodopsin contains retinal, a purple carotenoid like molecule used for light trapping. Interestingly, retinal is produced via a pathway that contains many of the same enzymes used for the production of lycopene by tomatoes during ripening.

Window to the Past

The extreme conditions in which Archaea are found suggests that these organisms have adapted to environments thought to exist during early life on earth, three billion to four billion years ago. Thus, the Archaea might be considered as a window into the past, and they may shed light on the processes involved in evolution as well as their relationships with Bacteria and Eukarya.

In order to survive in their unique environments, Archaea possess molecules that withstand heat or cold, acids, salt, and in some cases, pressure—characteristics that are tailor-made for specific applications inmolecular biology and biotechnology.

Uses

A number of important applications have been developed as a consequence of studying the Archaea. These include the identification of heat stable enzymes for analyses used in genetic fingerprinting and cancer detection (certain polymerase chain reaction enzymes), the use of halophilic pigments for holographic applications, optical signal processing and photoelectric devices, and methanogenesis as an alternative fuel source.
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9:27 PM
Biochemical Coevolution in Angiosperms
Biochemical Coevolution in Angiosperms
Flowering plants, or angiosperms, produce many compounds that are not directly related to growth and development. These secondary metabolites arise from primary metabolic pathways and act as antiherbivory mechanisms, allelochemicals, or attractants.

Secondary metabolites are biochemicals produced by plants in response to selection pressures. These pressures may be from herbivory, competition, or the need for pollination.

As plants produce compounds to enhance their survival, predators, competitors, and pollinators react and evolve means of adjusting to the plant’s efforts. Chemically simple secondary metabolites may be widespread throughout angiosperm (flowering plant) families, whereas more complex chemicals are often restricted to a single species.

Secondary metabolites are often under high selection pressures, causing individual compounds to have very limited distributions and making them useful in determining the evolutionary relationships between taxonomic groups. Presence of secondary compounds influences the activities of organisms interacting with the plants and, over long periods of time, influences evolution of those species.

Antiherbivory Mechanisms

Antiherbivory chemicals may have a wide range of effects on herbivores (plant-eating animals). Many compounds merely deter grazing. Crystals produced from calcium oxalate (raphides) may be ejected from the vacuoles of cells along with proteinaceous toxins, causing tissue swelling in the mouth of an offending herbivore.

Many monocotyledonous plant families, such as Liliaceae, Heliconiaceae, Rubiaceae, and Arecaceae, produce this type of antiforaging device. This type of defense is especially notable in young tissues that have not developed the toughness found inmature leaves as a herbivory deterrent. Red oaks produce tannins in response to gypsy moth attacks, reducing further herbivory.

Continued feeding on plants containing tannins would lead to slow starvation of a herbivore, as its digestive system could not absorb proteins. The same tannins are not deterrents to squirrels. Squirrels harvest acorns and bury them for later consumption, providing a food source for the squirrel and a dispersal mechanism for the acorn.

Beavers provide another example of the interaction between plant and animal evolution. Some of the beaver’s preferred foods include species that are unpalatable or toxic to other mammals, such as bracken fern, nettles, thistles, and skunk cabbage. This gives the beaver a largely uncontested food source that may involve a metabolic "cost" to the animal.

Other antiherbivory chemicals result in effects more severe than mere deterrence of feeding. Alkaloids such as caffeine, nicotine, and strychnine are potent anti herbivory mechanisms, causing convulsions, comas, and even death in herbivores. These effects may not occur in all herbivores.

Strychnine, for example, is produced by the fruit of some plants that may be eaten by birds without ill effects, but in mammals the same fruit causes failure of the central nervous system and induces seizures.The plant reduces herbivory by mammals, and the seeds get dispersed by birds that are able to detoxify the strychnine.

Grains and seed crops, such as wheat and peanuts, which are particularly attractive to animal and insect herbivores, often produce cyanogenic glycosides that release hydrogen cyanide as the tissues are digested. This compound inhibits cellular respiration, thus killing the herbivore. In each case members of a plant population are consumed by the herbivore, but future generations are spared by the loss.

Allelochemicals

Allelochemicals
Allelochemicals
Allelochemicals are compounds produced by an organism that interfere with the growth or development of another organism.Many phenolic acids act as allelo chemicals, inhibiting root growth of competing species. Many grains are known to release ferulic acid and caffeic acid into the soil, thus inhibiting the germination of weed species.

Phenolics may also act as antifungal compounds, increasing in concentration with fungal infection, thus protecting the plant from further attack. Phenolic compounds produced in tobacco and tomato leaves reduce the growth of these plants’ natural predator, tobacco hornworm, without affecting the growth or activity of the hornworm’s natural predator.

Allelochemicals produced in response to injury by herbivores may also attract predators of the herbivore.Wastes from many species of caterpillars induce the release of terpenoids from green leaves that attract parasitoid insects.

Production of specific combinations of volatiles on the part of the plant signals the predator, which will then reduce further herbivory. The plants have evolved the signal in response to herbivory, and the predators have evolved the ability to detect the signal indicating the location of their host.

Lectins are widely distributed carbohydrate-binding proteins, most commonly found in the Leguminoseae (legume) family. When found in the seeds, these compounds act as broad-spectrum insecticides, whereas in the roots of legumes they maintain bacterial relationships in nitrogen fixing nodules, providing the plant with a source of nitrogen unavailable to plants not producing nodules.

Attractants

Attractants
Attractants
Terpenoids and aliphatic compounds are often the components of essential oils of plants. The volatile nature of these compounds produces a distinctive odor that attracts pollinators. Composition of the volatile compounds often closely matches the natural pheromones produced by the pollinator, mimicking the chemical scent of a female insect in an attempt to attract male pollinators.

Pheromone mimicry is found primarily in members of the Orchidaceae, which are often dependent on single species of wasp for pollination. Other plants may mimic the odor of food. The smell of rotting flesh, attractive to flies, is produced via ammonia and alkylamines, such as cadaverine and putrescine. Methylesters may attract moth pollinators by mimicking the sweet smell of fruit.

Flavonoids often provide color to fruits and flowers and act as visual cues for pollination. Reds, blues, or yellows in varying patterns stand out against a background of green leaves, helping pollinators locate the flower.

Species may have minor chemical differences in their flavonoids that allow for the determination of identity, hybridization between species, and possible coevolution with pollinators. For example, tropical flowers tend to have a more intense red color from anthocyanins than do temperate flowers.

This difference correlates with differences in pollinator preferences, indicating a role by natural selection. Birds, such as hummingbirds, prefer red to yellow, whereas bees are not able to discern reds but are attracted to yellows. Carotenoids, such as xanthophyll and beta-carotene, give fruits and flowers distinctive yellow and orange colors.

Color patterns are also important in attracting pollinators. Butterflies are attracted to red/yellow color patterns. Flavonoid compounds not only impart color but also may modify color patterns by absorbing ultraviolet (UV) light.

Bees are capable of seeing in the UV range, so the presence of flavonoids may alter the bees’ perception of the flower. The patterns may also create cues as to the location of nectaries within the flower, guiding the pollinator to its reward.
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7:34 AM
Coevolution
Coevolution

Coevolution is the interactive evolution of two or more species that results in a mutualistic or antagonistic relationship.

When two or more different species evolve in a way that affects one another’s evolution, coevolution is taking place. This interactive type of evolution is characterized by the fact that the participant life-forms are acting as a strong selective pressure upon one another over a period of time.

The coevolution of plants and animals, whether animals are considered strictly in their plant-eating role or also as pollinators, is abundantly represented in every terrestrial ecosystem throughout the world where flora has established itself.

Moreover, the overall history of some of the multitude of present and past plant and animal relationships is displayed (although fragmentally) in the fossil record found in the earth’s crust.

Beginnings


The most common coevolutionary relationships between plants and animals surround plants as a food source. Microscopic, unicellular plants were the earth’s first autotrophs (organisms that can produce their own organic energy through photosynthesis, that is, from basic chemical ingredients derived from the environment).

In conjunction with the appearance of autotrophs, microscopic, unicellular heterotrophs (organisms, such as animals, that must derive food from other sources, such as autotrophs) evolved to exploit the autotrophs.

Sometime during the later part of the Mesozoic era, angiosperms, the flowering plants, evolved and replaced most of the previously dominant land plants, such as the gymnosperms and the ferns. New species of herbivores evolved to exploit these new food sources.

At some point, probably during the Cretaceous period of the late Mesozoic era, animals became unintentional aids in the angiosperm pollination process. As this coevolution proceeded, the first animal pollinators became more and more indispensable as partners to the plants.

Eventually, highly coevolved plants and animals developed relationships of extreme interdependence, exemplified by the honeybees and their coevolved flowers. This angiosperm-insect relationship is thought to have arisen in the Mesozoic era by way of beetle predation, possibly on early, magnolia-like angiosperms.

The fossil record gives some support to this theory. Whatever the exact route along which plant-animal pollination partnerships coevolved, the end result was a number of plant and animal species that gained mutual benefit from the new type of relationship.

Coevolutionary Relationships

Coevolutionary Relationships
Coevolutionary Relationships
Coevolved relationships include an immense number of relationships between plants and animals, and even between plants and other plants.

Among these coevolved situations can be found commensalisms, in which different species have coevolved to live intimately with one another without injury to any participant, and symbioses, in which species have coevolved to literally “live together.”

Such intertwined relationships can take the form of mutualism, in which neither partner is harmed and indeed one or both benefit—as in the relationships between fungi and algae in lichens, fungi and roots in mycorrhizae, and ants and acacia trees in a symbiotic mutualism in which the ants protect the acacias from herbivores. In parasitism, one partner benefits at the expense of the other; a classic example is the relationship between the mistletoe parasite and the oak tree.

Another coevolutionary relationship, predation, is restricted primarily to animal-animal relationships (vertebrate carnivores eating other animals, most obviously), although some plants, such as Venus’s flytrap, mimic predation in having evolved means of trapping and ingesting insects as a source of food. Some highly evolved fungi, such as the oyster mushroom, have evolved anesthetizing compounds and other means of trapping protozoa, nematodes, and other small animals.

One of the most obvious and complex coevolutionary relationships are the mutualisms that have evolved between plants bearing fleshy fruits and vertebrate animals, which serve to disperse the seeds in these fruits.

Over time, plants that produce these fruits have benefited from natural selection because their seeds have enjoyed a high degree of survival and germination: Animals eat the fruits, whose seeds are passed through their digestive system (or regurgitated to feed offspring) unharmed; at times the seeds are even encouraged toward germination as digestion helps break down the seed coat. Furthermore, dispersal through the animals’ mobility allows the seeds to enjoy more widely distributed propagation.

The coevolutionary processworks on the animals as well: Birds and animals that eat the fruits enjoy a higher degree of survival, and so natural selection favors both fleshy-fruit-producing plants and fleshy-fruit-eating animals. Similar selection has favored the coevolution of flowers with colors and smells that attract pollinators such as bees.

Eventually some plant-animal mutualisms became so intertwined that one or both participants reached a point at which they could not exist without the aid of the other. These obligatory mutualisms ultimately involve other types of animal partners besides insects. Vertebrate partners such as birds, reptiles, and mammals became involved in mutualisms with plants.

In the southwestern United States, for example, bats and the agave and saguaro cactus have a special coevolutionary relationship: The bats, nectar drinkers and pollen eaters, have evolved specialized feeding structures such as erectile tongues similar to those found among moths and other insects with similar lifestyles.

In turn, angiosperms coevolutionarily involved with bats have developed such specializations as bat-attractive scents, flower structures that match the bats’ feeding habits and minimize the chance of injuring the animals, and petal openings timed to the nocturnal activity of bats.

Defense Mechanisms

Coevolution is manifested in defense mechanisms as well as attractants: Botanical structures and chemicals (secondary metabolites) have evolved to discourage or to prevent the attention of plant eaters.

Defense Mechanisms
Defense Mechanisms

These include the development of spines, barbs, thorns, bristles, and hooks on plant leaves, stems, and trunk surfaces. Cacti, hollies, and rose bushes illustrate this form of plant strategy. Some plants produce chemical compounds that are bitter to the taste or poisonous.

Plants that contain organic tannins, such as trees and shrubs, can partially inactivate animals’ digestive juices and create cumulative toxic effects that have been correlated with cancer. Grasses with a high silica content act to wear down the teeth of plant eaters.

Animals have counter adapted to these defensive innovations by evolving a higher degree of resistance to plant toxins or by developing more efficient and tougher teeth with features such as harder enamel surfaces or the capacity of grinding with batteries of teeth.
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