端木·宇 2008-6-19 22:11
Origin of Life: The Heterotroph Hypothesis
Life on Earth began about 3.5 billion years ago. At that point in thedevelopment of the Earth, the atmosphere was very different from whatit is today. As opposed to the current atmosphere, which is mostlynitrogen and oxygen, the early Earth atmosphere contained mostlyhydrogen, water, ammonia, and methane.
In experiments, scientists have showedthat the electrical discharges of lightning, radioactivity, andultraviolet light caused the elements in the early Earth atmosphere toform the basic molecules of biological chemistry, such as nucleotides,simple proteins, and ATP. It seems likely, then, that the Earth wascovered in a hot, thin soup of water and organic materials. Over time,the molecules became more complex and began to collaborate to runmetabolic processes. Eventually, the first cells came into being. Thesecells were [b]heterotrophs[/b], which could not produce their own foodand instead fed on the organic material from the primordial soup.(These heterotrophs give this theory its name.)
The anaerobic metabolic processes of theheterotrophs released carbon dioxide into the atmosphere, which allowedfor the evolution of photosynthetic [b]autotrophs[/b], which could use light and CO2to produce their own food. The autotrophs released oxygen into theatmosphere. For most of the original anaerobic heterotrophs, oxygenproved poisonous. The few heterotrophs that survived the change inenvironment generally evolved the capacity to carry out aerobicrespiration. Over the subsequent billions of years, the aerobicautotrophs and heterotrophs became the dominant life-forms on theplanet and evolved into all of the diversity of life now visible onEarth.
[b] Evidence of Evolution[/b]
Humankind has always wondered about itsorigins and the origins of the life around it. Many cultures haveancient creation myths that explain the origin of the Earth and itslife. In Western cultures, ideas about evolution were originally basedon the Bible. The book of Genesis relates how God created all life onEarth about 6,000 years ago in a mass creation event. Proponents ofcreationism support the Genesis account and state that species werecreated exactly as they are currently found in nature. This oldestformal conception of the origin of life still has proponents today.
However, about 200 years ago, scientificevidence began to cast doubt on creationism. This evidence comes in avariety of forms.
[i] Rock and Fossil Formation[/i]
Fossils provide the only direct evidenceof the history of evolution. Fossil formation occurs when sedimentcovers some material or fills an impression. Very gradually, heat andpressure harden the sediment and surrounding minerals replace it,creating fossils. Fossils of prehistoric life can be bones, shells, orteeth that are buried in rock, and they can also be traces of leaves orfootprints left behind by organisms.
Together, fossils can be used to construct a [b]fossil record [/b]thatoffers a timeline of fossils reaching back through history. To puzzletogether the fossil record, scientists have to be able to date thefossils to a certain time period. The strata of rock in which fossilsare found give clues about their relative ages. If two fossils arefound in the same geographic location, but one is found in a layer ofsediment that is beneath the other layer, it is likely that the fossilin the lower layer is from an earlier era. After all, the first layerof sediment had to already be on the ground in order for the secondlayer to begin to build up on top of it. In addition to sedimentlayers, new techniques such as radioactive decay or carbon dating canalso help determine a fossil’s age.
There are, however, limitations to theinformation fossils can supply. First of all, fossilization is animprobable event. Most often, remains and other traces of organisms arecrushed or consumed before they can be fossilized. Additionally,fossils can only form in areas with sedimentary rock, such as oceanfloors. Organisms that live in these environments are therefore morelikely to become fossils. Finally, erosion of exposed surfaces orgeological movements such as earthquakes can destroy already formedfossils. All of these conditions lead to large and numerous gaps in thefossil record.
[i] Comparative Anatomy[/i]
Scientists often try to determine therelatedness of two organisms by comparing external and internalstructures. The study of comparative anatomy is an extension of thelogical reasoning that organisms with similar structures must haveacquired these traits from a common ancestor. For example, the flipperof a whale and a human arm seem to be quite different when looked at onthe outside. But the bone structure of each is surprisingly similar,suggesting that whales and humans have a common ancestor way back inprehistory. Anatomical features in different species that point to acommon ancestor are called [b]homologous structures[/b].
However, comparative anatomists cannotjust assume that every similar structure points to a commonevolutionary origin. A hasty and reckless comparative anatomist mightassume that bats and insects share a common ancestor, since both havewings. But a closer look at the structure of the wings shows that thereis very little in common between them besides their function. In fact,the bat wing is much closer in structure to the arm of a man and thefin of a whale than it is to the wings of an insect. In other words,bats and insects evolved their ability to fly along two very separateevolutionary paths. These sorts of structures, which have superficialsimilarities because of similarity of function but do not result from acommon ancestor, are called [b]analogous structures[/b].
In addition to homologous and analogous structures, [b]vestigial structures[/b],which serve no apparent modern function, can help determine how anorganism may have evolved over time. In humans the appendix is useless,but in cows and other mammalian herbivores a similar structure is usedto digest cellulose. The existence of the appendix suggests that humansshare a common evolutionary ancestry with other mammalian herbivores.The fact that the appendix now serves no purpose in humans demonstratesthat humans and mammalian herbivores long ago diverged in theirevolutionary paths.
[i] Comparative Embryology[/i]
Homologous structures not present in adult organisms often [i]do [/i]appearin some form during embryonic development. Species that bear littleresemblance to each other in their adult forms may have strikinglysimilar embryonic stages. In some ways, it is almost as if the embryopasses through many evolutionary stages to produce the mature organism.For example, for a large portion of its development, the human embryopossesses a tail, much like those of our close primate relatives. Thistail is usually reabsorbed before birth, but occasionally children areborn with the ancestral structure intact. Even though they are notgenerally present in the adult organism, tails could be consideredhomologous traits between humans and primates.
In general, the more closely related twospecies are, the more their embryological processes of developmentresemble each other.
[i] Molecular Evolution[/i]
Just as comparative anatomy is used todetermine the anatomical relatedness of species, molecular biology canbe used to determine evolutionary relationships at the molecular level.Two species that are closely related will have fewer genetic or proteindifferences between them than two species that are distantly relatedand split in evolutionary development long in the past.
Certain genes or proteins in organisms change at a constant rate over time. These genes and proteins, called [b]molecular clocks[/b]because they are so constant in their rate of change, are especiallyuseful in comparing the molecular evolution of different species.Scientists can use the rate of change in the gene or protein tocalculate the point at which two species last shared a common ancestor.For example, ribosomal RNA has a very slow rate of change, so it iscommonly used as a molecular clock to determine relationships betweenextremely ancient species. Cytochrome c, a protein that plays animportant role in aerobic respiration, is an example of a proteincommonly used as a molecular clock.
[b] Theories of Evolution[/b]
In the nineteenth century, as increasingevidence suggested that species changed over time, scientists began todevelop theories to explain how these changes arise. During this time,there were two notable theories of evolution. The first, proposed byLamarck, turned out to be incorrect. The second, developed by Darwin,is the basis of all evolutionary theory.
[i] Lamarck: Use and Disuse[/i]
The first notable theory of evolution wasproposed by Jean-Baptiste Lamarck (1744–1829). He described a two-partmechanism by which evolutionary change was gradually introduced intothe species and passed down through generations. His theory is referredto as the theory of transformation or [b]Lamarckism[/b].
The classic example used to explainLamarckism is the elongated neck of the giraffe. According to Lamarck’stheory, a given giraffe could, over a lifetime of straining to reachhigh branches, develop an elongated neck. This vividly illustratesLamarck’s belief that [i]use[/i] could amplify or enhance a trait. Similarly, he believed that [i]disuse[/i]would cause a trait to become reduced. According to Lamarck’s theory,the wings of penguins, for example, were understandably smaller thanthe wings of other birds because penguins did not use their wings tofly.
The second part of Lamarck’s mechanism for evolution involved the [b]inheritance of acquired traits[/b].He believed that if an organism’s traits changed over the course of itslifetime, the organism would pass these traits along to its offspring.
Lamarck’s theory has been proven wrong inboth of its basic premises. First, an organism cannot fundamentallychange its structure through use or disuse. A giraffe’s neck will notbecome longer or shorter by stretching for leaves. Second, moderngenetics shows that it is impossible to pass on acquired traits; thetraits that an organism can pass on are determined by the genotype ofits sex cells, which does not change according to changes in phenotype.
[i] Darwin: Natural Selection[/i]
While sailing aboard the HMS [i]Beagle[/i],the Englishman Charles Darwin had the opportunity to study the wildlifeof the Galápagos Islands. On the islands, he was amazed by the greatdiversity of life. Most particularly, he took interest in the islands’various finches, whose beaks were all highly adapted to theirparticular lifestyles. He hypothesized that there must be some processthat created such diversity and adaptation, and he spent much of histime trying to puzzle out just what the process might be. In 1859, hepublished his theory of natural selection and the evolution itproduced. Darwin explained his theory through four basic points:
[list][*]Each species produces more offspring than can survive.[*]The individual organisms that make up a larger population are born with certain variations.[*]The overabundance of offspring creates a competition forsurvival among individual organisms. The individuals that have the mostfavorable variations will survive and reproduce, while those with lessfavorable variations are less likely to survive and reproduce.[*]Variations are passed down from parent to offspring.[/list]
Natural selection creates change within aspecies through competition, or the struggle for life. Members of aspecies compete with each other and with other species for resources.In this competition, the individuals that are the most [b]fit[/b]—theindividuals that have certain variations that make them better adaptedto their environments—are the most able to survive, reproduce, and passtheir traits on to their offspring. The competition that Darwin’stheory describes is sometimes called [b]the survival of the fittest[/b].
[b] Natural Selection in Action[/b]
One of the best examples of naturalselection is a true story that took place in England around the turn ofthe century. Near an agricultural town lived a species of moth. Themoth spent much of its time perched on the lichen-covered bark of treesof the area. Most of the moths were of a pepper color, though a fewwere black. When the pepper-color moths were attached to thelichen-covered bark of the trees in the region, it was quite difficultfor predators to see them. The black moths were easy to spot againstthe black-and-white speckled trunks.
The nearby city, however, slowly becameindustrialized. Smokestacks and foundries in the town puffed out sootand smoke into the air. In a fairly short time, the soot settled oneverything, including the trees, and killed much of the lichen. As aresult, the appearance of the trees became nearly black in color.Suddenly the pepper-color moths were obvious against the dark treetrunks, while the black moths that had been easy to spot now blended inagainst the trees. Over the course of years, residents of the townnoticed that the population of the moths changed. Whereas about 90percent of the moths used to be light, after the trees became black,the moth population became increasingly black.
When the trees were lighter in color, naturalselection favored the pepper-color moths because those moths were moredifficult for predators to spot. As a result, the pepper-color mothslived to reproduce and had pepper-color offspring, while far fewer ofthe black moths lived to produce black offspring. When the industry inthe town killed off the lichen and covered the trees in soot, however,the selection pressure switched. Suddenly the black moths were morelikely to survive and have offspring. In each generation, more blackmoths survived and had offspring, while fewer lighter moths survived tohave offspring. Over time, the population as a whole evolved frommostly white in color to mostly black in color.
[b] Types of Natural Selection[/b]
In a normal population without selectionpressure, individual traits, such as height, vary in the population.Most individuals are of an average height, while fewer are extremelyshort or extremely tall. The distribution of height falls into a bellcurve.
[align=center][img]http://www.24en.com/d/file/sat/sat2/biology/2008-01-24/26524a6276f1f6154c5a3926ea7ea716.gif[/img][/align] Natural selection can operate on this population in three basic ways. [b]Stabilizing selection[/b]eliminates extreme individuals. A plant that is too short may not beable to compete with other plants for sunlight. However, extremely tallplants may be more susceptible to wind damage. Combined, these twoselection pressures act to favor plants of medium height.
[align=center][img]http://www.24en.com/d/file/sat/sat2/biology/2008-01-24/1fcb25d397770b9eb621ebd23509ad1f.gif[/img][/align]sw)[({%o3b(Z [b]Directional selection[/b]selects against one extreme. In the familiar example of giraffe necks,there was a selection pressure against short necks, since individualswith short necks could not reach as many leaves on which to feed. As aresult, the distribution of neck length shifted to favor individualswith long necks.
[align=center][img]http://www.24en.com/d/file/sat/sat2/biology/2008-01-24/fb274b1c786b83fa06081e44779f49c5.gif[/img][/align]
[b]Disruptive selection[/b]eliminates intermediate individuals. For example, imagine a plant ofextremely variable height that is pollinated by three differentpollinator insects: one that was attracted to short plants, anotherthat preferred plants of medium height, and a third that visited onlythe tallest plants. If the pollinator that preferred plants of mediumheight disappeared from an area, medium height plants would be selectedagainst, and the population would tend toward both short and tallplants, but not plants of medium height.
[align=center][img]http://www.24en.com/d/file/sat/sat2/biology/2008-01-24/9fdc32ff44f908d18f383ba0af2b7959.gif[/img][/align]
[b] The Genetic Basis for Evolution[/b]
Darwin’s theory of natural selection and evolution rests on two crucial ideas:
[list=1][*]Variations exist in the individuals within a population.[*]Those variations are passed down from one generation to the next.[/list]
But Darwin had no idea how those variationscame to be or how they were passed down from one generation to thenext. Mendel’s experiments and the development of the science ofgenetics provided answers. Genetics explains that the phenotype—thephysical attributes of an organism—is produced by an organism’sgenotype. Through the mechanism of mutations, genetics explains howvariations arose among individuals in the form of different alleles ofgenes. Meiosis, sexual reproduction, and the inheritance of allelesexplain how the variations between organisms are passed down fromparent to offspring.
With the modern understanding of genes andinheritance, it is possible to redefine natural selection and evolutionin genetic terms. The particular alleles that an organism inherits fromits parents determine that organism’s physical attributes and thereforeits fitness for survival. When the forces of natural selection resultin the survival of the fittest, what those forces are really doing isselecting which alleles will be passed on from one generation to thenext.
Once you see that natural selection is actuallya selection of the passage of alleles from generation to generation,you can further see that the forces of natural selection can change thefrequency of each particular allele within a population’s [b]gene pool[/b],which is the sum total of all the alleles within a particularpopulation. Using genetics, one can create a new definition ofevolution as the change in the [b]allele frequencies[/b] in the genepool of a population over time. For example, in the population of mothswe discussed earlier, after the trees darkened, the frequency of thealleles for black coloration increased in the gene pool, while thefrequency of alleles for light coloration decreased.
[b] Hardy-Weinberg Equilibrium[/b]
The Hardy-Weinberg principle states that asexually reproducing population will have stable allelic frequenciesand therefore will not undergo evolution, given the following fiveconditions:
[list][*]large population size[*]no immigration or emigration[*]random mating[*]random reproductive success[*]no mutation[/list]
The Hardy-Weinberg principle proves thatvariability and inheritance alone are not enough to cause evolution;natural selection must drive evolution. A population that meets all ofthese conditions is said to be in [b]Hardy-Weinberg equilibrium[/b].Few natural populations ever experience Hardy-Weinberg equilibrium,though, since large populations are rarely found in isolation, allpopulations experience some level of mutation, and natural selectionsimply cannot be avoided.
[b] Development of New Species[/b]
The scientific definition of a [b]species[/b]is a discrete group of organisms that can only breed within its ownconfines. In other words, the members of one species cannot interbreedwith the members of another species. Each species is said to experience[b]reproductive isolation[/b]. If you think about evolution in terms ofgenetics, this definition of species makes a great deal of sense: ifspecies could interbreed, they could share gene flow, and theirevolution would not be separate. But since species cannot interbreed,each species exists on its own individual path.
As populations change, new species evolve. This process is known as [b]speciation[/b].Through speciation, the earliest simple organisms were able to branchout and populate the world with millions of different species.Speciation is also called [b]divergent evolution[/b], since when a newspecies develops, it diverges from a previous form. All homologoustraits are produced by divergent evolution. Whales and humans share adistant common ancestor. Through speciation, that ancestor underwentdivergent evolution and gave rise to new species, which in turn gaverise to new species, which over the course of millions of yearsresulted in whales and humans. The original ancestor had a limbstructure that, over millions of years and successive occurrences ofdivergent evolution, evolved into the fin of the whale and the arm ofthe human.
Speciation occurs when two populationsbecome reproductively isolated. Once reproductive isolation occurs fora new species, it will begin to evolve independently. There are twomain ways in which speciation might occur. [b]Allopatric speciation[/b]occurs when populations of a species become geographically isolated sothat they cannot interbreed. Over time, the populations may becomegenetically different in response to the unique selection pressuresoperating in their different environments. Eventually the geneticdifferences between the two populations will become so extreme that thetwo populations would be unable to interbreed even if the geographicbarrier disappeared.
A second, more common form of speciation is [b]adaptive radiation[/b],which is the creation of several new species from a single parentspecies. Think of a population of a given species, which we’llimaginatively name population 1. The population moves into a newhabitat and establishes itself in a niche, or role, in the habitat (wediscuss niches in more detail in the chapter on Ecology). In so doing,it adapts to its new environment and becomes different from the parentspecies. If a new population of the parent species, population 2, movesinto the area, it too will try to occupy the same niche as population1. Competition between population 1 and population 2 ensues, placingpressure on both groups to adapt to separate niches, furtherdistinguishing them from each other and the parent species. As thishappens many times in a given habitat, several new species may beformed from a single parent species in a relatively short time. Theimmense diversity of finches that Darwin observed on the GalápagosIslands is an excellent example of the products of adaptive radiation.
[b] Convergent Evolution[/b]
When different species inhabit similarenvironments, they face similar selection pressures, or use parts oftheir bodies to perform similar functions. These similarities can causethe species to evolve similar traits, in a process called convergentevolution. From living in the cold, watery, arctic regions, where mostof the food exists underwater, penguins and killer whales have evolvedsome similar characteristics: both are streamlined to help them swimmore quickly underwater, both have layers of fat to keep them warm,both have similar white-and-black coloration that helps them to avoiddetection, and both have developed fins (or flippers) to propel themthrough the water. All of these similar traits are examples ofanalogous traits, which are the product of convergent evolution.
Convergent evolution sounds as if it is theopposite of divergent evolution, but that isn’t actually true.Convergent evolution is only superficial. From the outside, the fin ofa whale may look like the flipper of a penguin, but the bone structureof a whale fin is still more similar to the limbs of other mammals thanit is to the structure of penguin flippers. More importantly,convergent evolution never results in two species gaining the abilityto interbreed; convergent evolution can’t take two species and turnthem into one.
