Natural Selection is the process by which evolution occurs. It is in it’s own words the way by which a species betters itself thorough the passing of time and adapts to its environment. The first and most striking example of natural selection is the peppered moth. Some time ago, the light form of the peppered moth was common and the dark form was rare. But ever since industrialization took place, the dark form is common, and the light form rare around the industrial area. Experiments were performed which showed that the lighter form is most around places were the tree trunks weren’t blackened from factory smoke and the darker form tends to be eaten more by birds. IN places were the tree trunks were turned black by pollution, the lighter form survives less and the darker form survives more. The birds that eat the moth act as a selective agent and the color of the moth acts as a camouflage and the better camouflaged moth survives more than the moth that can be seen. The peppered moth is a classic example of natural selection. Another example of natural selection would be drug resistance in bacteria. Because of this reason you have to eat all the drugs that are given to you by you’re doctor. If you don’t, some of the bacteria can survive and adapt to the antibiotics. The antibiotics would act as a selective agent. The people that have the drug resistance bacteria sometimes reproduce therefore spreading it, all they have to do is take their antibiotics to make sure that all the bacteria die off and none are left. Darwin found an example of natural selection on the Galapagos Islands which was that there were the same type of bird with 21 types of beaks to do various jobs and tasks which differed at times. This was because there were no Natural Selection pressures and this was possible. Over 10% of mosquitoes are resistant to insecticide. The University of Western Ontario performed an experiment to see how the mosquitoes would fare under a certain chemical by the name of DDT. They put mosquito larva into DDT. Only about 5% of the larva survived. The populations of normal insects and the insects that survived the DDT were exposed to the DDT for 1 hour. All of the normal population died but most of the resistant population survived. The resistance was inhibited to the children. In the normal population the mortality increases with the concentration of DDT. DDT produced a resistant population. The DDT acts as a selective agent which produces the resistant population. This can be related to humans in antibiotic taking, you need to take all antibiotics so no bacteria survive and are all killed as talked above. You need to switch insecticides because the insects eventually become resistant to them just as the mosquitoes to the DDT.

In Utah, Father Escalante soon enough hat to shoot his horses and eat them because he could find no wild game or deer and didn’t have anything to eat. Deer are browsers and feed on shrubbery and there was none for 2 reasons, (1) Grass had never been disturbed, and (2) fires burned regularly. Then the pioneers came and put range maggots (sheep) on the land. The sheep kill grass by eating it and killed it off. The shrubbery came back in and the fires were also stopped from human intervention. The 2 selective agents were taken away and shrubbery appeared. Suddenly there were deer and game all over because natural selection relating to environment didn’t kill them anymore since they had material to eat. Father Escalante also described Utah Lake as one of the clearest trout filled lakes he had ever seen. Then why is it so green now? The range maggots ate the grass and the soil went down into the lake. This soil killed the trout due to lacks of O₂ and there was algae growth. This is natural selection as it killed of the trout and let algae grow in the lake as it is now. The animals that are capable of surviving the lack of Oxygen in the lake are the ones that stay there.

3. Discuss all the evidence for macroevolution.

The three examples we have just considered are among the many that support Darwin’s proposal that natural selection is the mechanism of evolutionary change. In themselves, however, they do not provide evidence for macroevolution – that is, evolutionary change above the level of the species. The evidence for macroevolution, or the process that Darwin called "descent with modification" falls into five broad categories.

One line of evidence, which had been building for more than a century before Darwin, was the existence of an enormous number of species. When observations had been confined to a limited area of the temperature zone, as they were before the great explorations of the eighteenth and nineteenth centuries, it was plausible that each kind of organism had been created separately. Even then, however, the existence of a score or more of very similar beetles (none of which seemed to serve a higher purpose) was something of an embarrassment.

If no coincidence that Darwin and Wallace had come to doubt the doctrine of special creation while traveling in the tropics, where the widest variety of species is found and where, even today, many species new to science are encountered. Moreover, it became apparent that species are not as distinct as had previously been thought. For example, as Darwin traveled up the western coast of South America, he observed gradual changes in various characteristics of the plants and animals – evidence that organisms became modified, according to the different environments in which they live.

The creationists sought to accommodate doubter of fixed and unchanging species by tinkering with the definition of species. Some species had remained the same, they said; others represented altered forms of the originally created species and so were not really species at all in the creation sense. Some creationists said it was the genera that were specially created and not the species at all, and so on. The species problem, made apparent by the nineteenth century explanations, was the first serious fissure in the monolith of special creation. (And, indeed, the question of why there are so many species is still an unanswered one, although not for the same reasons.)

A second, related line of evidence in support of macroevolution comes from what is known as biogeography – the distribution of plants and animals in the various regions of the world. The naturalist explorers – Darwin and Wallace among them – were puzzled by the fact that places similar in climate and topography are often populated by very different organisms. According to creationist doctrine, each species were created specially for a particular way of life and placed in the locality for which it was suited

-- hence, no polar bears in the tropics, for instance.

Darwin began his voyage with this point of view, but many questions soon arose. Why, for example, did remote oceanic islands often have no terrestrial mammals at all but only peculiar species of bats? Why did England and Europe have rabbits galore, whereas similar areas in South America have only the Patagonian here – taxonomically not a rabbit or hare at all but a rodent – and Australia a marsupial that resembled a hare?

Or, speaking of Australia, why did this island continent lack native placental mammals but contain instead a large array of marsupials, all clearly related to one another and found only rarely elsewhere on the planet? Was each of the 57 separate species of kangaroos created separately and dropped off in Australia? And why only Australia? Or, more plausibly, was there perhaps an ancestral marsupial that gave rise to all these clearly related forms? The fossils of extinct marsupials had been described many years before Darwin visited Australia on the Beagle; in his later writings, he remarks on "this wonderful relationship on the same continent between the living and the dead."

With Darwin’s observations on the Galapagos Islands, the question became still more finely focused. For example, the 1

Species of finches on the Galapagos resembled South America species and also resembled one another. Had each been created separately and distributed among these volcanic islands (which the evidence indicated had been formed much more recently than the mainland?) Similar resemblance’s were also noted among the reptiles and plants of the different islands. In The Voyage Of The Beagle, Darwin wrote:

The distribution of the tenants of this archipelago would not be nearly so wonderful, if, for instance, one island had a mocking-thrush, and a second island some other quite distinct genus; -- if one island had its genus of lizard, and a second island another distinct genus, or none whatever; -- or if the different islands were inhabited, not by representative species of the same genera of plants, but by totally different genera … But it is the circumstance, that several of the islands posses their own species of the tortoise, mocking--thrush, finches, and numerous plants, these species having the same general habits, occupying analogous situations, and obviously filling the same place in the natural economy of this archipelago, that strikes me with wonder.

These observations about geographic distribution did not, of course, disprove the possibility of special creation (which, because of the reliance on a supernatural agency, cannot be disproved). However, these examples and a multitude of other biogeographic examples provide strong evidence that living things are what and where they are because of events that occurred in the course of their previous history.

A third line of evidence in support of macroevolution is provided by the fossil record, which reveals a succession of living forms, with simpler forms generally preceding more complex forms. Geologic studies, as well as the collecting of specimens of plants and animals, were among Darwin’s activities aboard the Beagle. The coasts of South America were of particular interest because they showed evidence of widespread upheaval, with many geologic strata exposed. These strata, like those studied by William Smith in the British Isles, contained successive deposits of marine shells, some of the, found high above sea level. Because of the gradually increasing percentage of modern species in the more recently deposited strata, Darwin was able to estimate their relative ages and to correlate strata from different localities, as Smith had before him.

In the course of his geological studies Darwin came across many fossils of extinct mammals. Among the most interesting to him were those of giant armadillos. The fact that extinct armadillos were buried in the same South American plains where the only surviving species of the strange armored mammals lived provided tangible evidence o change and history.

Nowhere in the geological record – either in his own observations or in the reports of others – did Darwin find exactly what he was seeking: evidence of a gradual transition between one species and another. Thus, as Darwin reiterates in The Origin Of Species, the fossil record provided little evidence for him of how evolution came about. For Darwin’s contemporaries, however, as for modern observers, the fossil record provided the overwhelming evidence that evolution had indeed occurred. During the decades immediately following the first publication of The Origin Of Species, many new fossil finds were made. One of these was of Archaeopteryx, in 1862; Darwin describes this find in his later editions, noting its combination of avian and reptilian features.

The most impressive early success in correlating Darwin’s theory of evolution was fossil evidence stemmed form the discovery of a long series of fossil horses. In 1879, Othniel C. Marsh of Yale University published a description of horse genealogy, leading from the tiny Darwin horse, Eohippus, through a number of stages of successively larger animals. These changes in size were accompanied by concomitant major evolutionary changes in teeth, legs, and feet. Thomas H. Huxley, who characterized himself as "Darwin’s bulldog," contending with Darwin’s critics and arguing the case for evolution before the public, learned heavily on horse genealogy in presenting the evidence for evolution. Describing Marsh’s six stages of horses, spanning some 60 million years, before a meeting of The Zoological society in 1880, Huxley stated.

With respect to the interpretations of these facts, two, and only two, appear to be imaginable. The one assumes that these successive forms of equine animals have come into existence independently of one another. The other assumes that they are the result of gradual modification undergone by the successive members of a continuous line of ancestry. As I am not aware that any zoologist maintains the first hypothesis, I do not feel called upon to discuss it. The adoption of the second, however, is equivalent to the acceptance of the doctrine of evolution so far as horses are concerned, and in the absence of evidence to the contrary, I shall suppose that it is accepted.

In the twentieth century, a steady stream of new discovered has enormously increased our knowledge of the fossil record, which now extends back more than 3 billion years. In the case of many groups of organisms – vascular plants and vertebrates, for example fossils have bee found that exhibit a graded series of changes in anatomical characteristics, linking older forms with the modern forms and revealing pathways of divergence from common ancestors. The most dramatic of these fossil finds have shed light on the evolution of homids.

A fourth major line of evidence comes from homologous structures and biochemical p-pathways. For example, the forelimbs of animals as diverse as crocodiles, birds, whales, horses, bats, and humans are all constructed of bones arranged in the same pattern. All vertebrates have four limbs – never six, or eight, or a hundred – and all have gill pouches, at least at some stage of their development. Whales and even some snakes retain the vestiges of pelvic and leg bones, for which they have no use. With a few unusual exceptions, virtually all mammals, ranging from mice to giraffes, have seven cervical vertebrate. If, however, one were starting from scratch, one might choose a somewhat different body plan for a giraffe, for instance, than for a meadow mouse. Yet the evidence confronting Darwin repeatedly suggested obsolete patterns built with and-me-down materials.

The cellular basis of life was demonstrated within a decade of the publication of the first edition of The origin Of Species. Since that time, studies of cell ultra-structure and biochemistry have revealed innumerable new homologies at the submicroscopic and molecular levels of organization. The two-ply nature of cell membranes, the role of ribosome’s in protein synthesis, the internal organization of eukaryotic cilia and flagella, glycolysis, the electron transport chain, the multitude roles played by ATP, and, most important of all, the genetic code, make clear the historical unity of all living organisms.

Adaptation is a word with several meaning in biology: Firs, it can mean a state of being adjusted to the environment; every living organisms is adapted in this sense, just as Abraham Lincoln’s legs were, as he remarked, "just long enough to reach the ground." Second, adaptation can mean a process, which may take place either within the lifetime of an individual organism (physiological adaptation) – such as the production of more red blood cells in response to dwelling at high altitudes—or within a population as a whole over the course of many generations (evolutionary adaptation). Third, adaptation is commonly used to refer to a particular characteristic, that which is adapted, such as an eye or a hand. Adaptation in this sense – and its seeming perfection – has often appeared to provide strong support for the doctrine of special creation. It has been argued that it would be impossible for an organ of such perfection as the eye, for example, to come into existence from nothingness. Of what use would half an eye be?

In the course of his career as a naturalist, Darwin assumed an enormous amount of information about living organisms; assembled in someone else’s brain, much of it would have been charming, unrelated trivia. On the basis of this vast knowledge, Darwin knew that not all adaptations – "contrivances", he called them – are perfect. Adaptations are simply as good as they have to be, "long enough to reach the ground." The imperfection of many adaptations, as revealed on close examination, is a fifth strong line of evidence in support of macroevolution.

For instance, Darwin described the highly ingenious workings of an orchid flower by which the flower "arranges for a bee to fall into a pool of water, held in the highly modified petal that forms the lip, so it cannot fly out of the flower. Instead, the bee must crawl through narrow passageways and so deposit and pick up pollen in the course of its exit. However, Darwin also described orchids and other types of plants with much less elaborate flowers, offering insects only bright colors and a few drops of nectar, and finally plants with simple, inconspicuous flowers and windblown pollen. In short, there are gradations and varieties of adaptations, not merely one set of perfect solution s to a given problem.

From his correspondence with Wallace and Bates, Darwin also knew of remarkable examples of defensive mimicry among insects. He mentions a walking-stick insect that not only is shaped like a stick but also is overgrown with material that exactly resemble native mosses. This is an excellent example of how gradual "improvements" each carefully conserved, could be continuously advantageous to the individual organisms – as, of course could a gradual improvement in the capacity to detect light and focus an image, leading ultimately to the eye.

In convergent evolution organisms evolve similar features independently, often because they live in similar habitats. Similar features that evolved through convergent evolution are called analogous characteristics. An example of this would be two different plant families; the cactus family and the spurge family. Both live in the desert, so they have characteristics such as waxy skin to keep water in and spines to keep animals from eating them.

Sometimes bones or other structures are present in an organism but are reduced in size and either have no use or have less prominent function than they do in other related organisms. Such structures which are considered to be evidence of an organism’s evolutionary past are called vestigial structures. A very good example of this is the human appendix. We no longer have a use for our appendix because we no longer digest a lot of cellulose. But other creatures, such as rabbits who graze, have large appendixes, because they do digest a lot of cellulose. We evolved to eat other things.

If you were to examine the embryos of a fish, a tortoise, a chicken, a rabbit and a human, in their first stage, you would probably not be able to tell the difference. But as the embryos develop into further stages, the differences would become more apparent and they would look like their species. The reason scientists believe that most embryos look alike in the first stage, is that our ancestors that we evolved from all were quite similar. And Mother Nature might say - if you have a good plan for something, why change it.

Since Darwin’s time, massive additional evidence has accumulated supporting the fact of evolution – that all living organisms present on earth today have arisen from earlier forms in the course of earth’s long history. Indeed, all of modern biology is an affirmation of this relatedness of the many species of living things and of their gradual divergence form one another over the course of time. Since the publication of The Origin Of Species, the important question, scientifically speaking, about evolution has not been whether it has taken place. That is no longer an issue among the vast majority of modern biologists. Today, the central and still fascinating questions for biologists concern the mechanisms by which evolution occurs.

A major weakness in the theory of evolution, as formulated by Darwin, was the absence of any valid mechanisms to explain heredity. Although Mendel was at work on his experiments with pea plants at the time Darwin was writing The Origin Of Species, his paper was not delivered until 1865 and did not enter the mainstream of biological thinking until after the turn of the twentieth century. The subsequent development of genetics made it possible to answer three questions that Darwin was never able to resolve: (1) how inherited characteristics are transmitted form one generation to the next; (2) why inherited characteristics are not "blended out" but can disappear and then reappear in later generations (like whiteness in pea flowers); and (3) How the variations arise on which natural selection acts.

The combination of Darwin’s theory of evolution with the principle of Mendelian genetics is known as the neo-Darwinian synthesis, or the synthetic theory of evolution. (Here "synthetic does not mean "artificial" which is the connotation it usually has for us, but has its original meaning of "the putting together of two or more different elements.") During the past 50 years, the synthetic theory has dominated scientific thinking about the process of evolution and has been enormously productive of new ideas and new experiments, as biologists have worked to unravel the details of how evolution takes place. Some aspects of the synthetic theory have recently come under challenge – partly as a result of new understanding of genetic mechanisms arising from rapid development in molecular biology, and partly as a result of reevaluations of the fossil record. The current controversies, which primarily involve the rate of macroevolutionary change and the role played by chance in determining the direction of evolution, do not affect the basic principles of the synthetic theory. They do, however, promise to give us a more complete understanding of evolutionary mechanisms that we have at present.

4. Discuss all the evidence for microevolution.

Darwin believed evolution to be such a slow process that it could never be observed directly. However, modern human civilization has produced such extremely strong selection pressures on some organisms that if one looks at small-scale evolutionary phenomena (known as microevolution), it is possible to observe not only the results but also the actual process of evolution by natural selection.

One example of micro evolution involves hemoglobin proteins in humans. Sickle cell anemia is a hereditary disease that affects hemoglobin molecules in human red blood cells. It arises from a single nucleotide change in the gene that codes for beta hemoglobin, and is one of the best understood of all human genetic disorders. Because sickle red blood cells tend to clog tiny arteries, sickle cell anemia is usually deadly. Heterozygous individuals who have both a defective hemoglobin gene and a normal hemoglobin gene make enough functional hemoglobin to keep their red blood cells healthy. Sickle cell anemia affects roughly two out of one thousand African Americans, but is almost not found in any other ethnic group. In central Africa where the sickle cell is thought to have originated, one in every one hundred people is homozygous for the defective allele, and develops the fatal disorder. Natural selection has not eliminated the defective sickle cell from human populations. The allele persists in humans, because having a single copy of the defective allele provides a definite advantage in certain environments. People who are heterozygous for the sickle cell allele are far more resistant to malaria, which is a common disease in Africa. Though natural selection, Africans have formed the sickle cell allele as a defense system against malaria, which is a very dangerous disease. But at the same time, having sickle cell anemia can be fatal. So there is a good and a bad side to this.

Cytochrome C is another example of micro evolution. Cytochrome C is a protein in the electron transport chain. It is used in the making of a certain type of amino acid. When this amino acid code is examined and compared with different species, humans have only about one difference in the code with apes. And only about eight differences in it from yeast. This shows that we still have common evolutionary beginnings with other organisms.

One of the best-studied examples of natural selection in action is that of Biston betularia, the peppered moth. These moths were well known to British naturalists of the nineteenth century, who remarked that they were usually found on lichen-covered trees and rocks. Against this background, the light coloring of the moths made them practically invisible. Until 1845, all reported specimens of Biston betularia had been light colored, but in that year one black moth of this species was captured in the growing industrial center of Manchester.

With the increasing industrialization of England, smoke particles began to pollute the foliage in the vicinity of industrial towns, killing the lichens and leaving the tree trunks bare. In heavily polluted districts, the trunks and even the rocks and ground became black. During this period, more and more black Biston betularia were found. Replacement of light-colored moths by dark ones proceeded briskly. By the 1950s, only a few of the light-colored population could be found, and these were far from industrial centers. Because of the prevailing westerly wind in England, pollutants were carried to the east of the industrial towns, and the moths tended to be of the black variety right up to the east coast of England. The few light-colored populations were concentrated in the west, where lichens still grew. (This tendency or dark-colored forms to replace light-colored forms, known as industrial melanism, has been found among some 70 other moth species in England and some 100 species of moths in the area of Pittsburgh, Pennsylvania, a heavily industrialized city. It also has been observed in many species of butterflies.

Where did the black Biston betularia come from? Eventually, it was demonstrated that the black color was the result of a rare, recurring mutation. The black moths had always been there, in very small numbers. But why had their numbers increased so dramatically? In the late 1950s, H. B. D. Kettlewell, a physician who was also an amateur collector of moths and butterflies, hypothesized that the color of the moths protected them from predator, notably birds. In the face of strong opposition from entomologists – all of whom claimed they had never seen a bird eat a Biston betularia of any color—he set out to test his hypothesis. He marked a sample of moths of each color by carefully putting a spot on the underside of the wings, where it could not be seen by a predator. Then he released known numbers of marked individuals into a bird reserve near Birmingham, an industrialized area where 90% of the local Biston betularia population consisted of black moths. Another sample was released into an unpolluted Dorset countryside, where no black moths ordinarily occurred. He returned at night with light traps to recapture his marked moths. From the area around Birmingham, he recovered 40% of the black moths, but only 19% of the light ones. From the area of Dorset 6% of the black moths and 12.5% of the light moths were retaken.

To clinch the argument, Kettlewell placed moths on tree trunks in both locations, focused hidden movie cameras on them, and was able to record birds actually selecting and eating the moths. As his films revealed, they do this so rapidly that it is not surprising it was not previously observed. When equal numbers of black and light moths were available near Birmingham, the birds seized 43 light-colored moths and only 15 black ones; in Dorset, they took 164 black moths but only 26 light-colored forms. Clearly, if you are a moth, it is advantageous to be black near Birmingham but light in Dorset.

More recently, strong controls have been instituted in Great Britain on the particulate content of smoke, and the heavy soot accumulation has begun to decrease. The light-colored moths are already increasing in proportion to the black forms, but it is not yet known whether a complete reversal either in pollution or in the selection process will come about. There is a moral to this story. Note that the black moths is not absolutely superior to the light one, or vice versa. It is, as Darwin realized, entirely a matter of time and place.

Another example of natural selection in action is the development of insecticide resistance. Chemicals poisonous to insects, such as DDT, were originally hailed as major saviors of human health and property. They have fallen into disfavor not only because of their tendency to accumulate in the environment, but also because of the extraordinary increase in resistant strains of insects. At least 225 species of insects are now resistant to one or more insecticides. One species is even able to remove a chlorine atom from DDT molecule and use the remainder as food. A particularly example of insecticide resistance has been found in the scale insects that attack citrus trees in California. In the early 1900s, a concentration of hydrocyanic gas sufficient to kill nearly 100% of the insects was applied to orange groves at regular intervals with great success. By 1914, orange growers near Corona, California, began to notice that the standard dose of the fumigant was no longer sufficient to destroy one type of scale insect, the red scale. A concentration of the gas that had left fewer than 1in 100 survivors in the nonresistant strain left 22 survivors out of 100 in the resistant strain. By crossing resistant and nonresistant strains, it was possible to show that the difference between the two involved a single gene locus. The mechanism for this resistance is not known, but one group of experiments has shown that the resistant individual can keep its spiracles closed for 30 minutes under unfavorable conditions, whereas the nonresistant insect can do so for only 60 seconds.

A more recent example of natural selection concerns the development of drug resistance in bacteria. Following the period of rapid development of antibiotics after World War II, bacteria soon began to appear that were resistant to these agents, so widely hailed as "miracle drugs". What caused this resistance? Changes in the metabolism of individual bacteria or changes in the bacterial population that is, evolution?

The question, important for medical as well as scientific reasons, was answered by a beautifully simple experiment performed by Joshua and Esther Lederberg. A set of petri dishes were prepared, each containing agar( a jelly-like substance) and were spread in a thin layer on one such dish. Within 24 hours, visible colonies began to appear, clones of the original scattered cells. The experimenters prepared a cylindrical block in the same diameter as the inside of the petri dish, covered one end of it with a piece of velveteen and marked it on the side with an arrow. Then the arrow was lined up with the arrow on the petri dish, and the velveteen surface was pressed against the agar, picking up bacteria. The velveteen surface was then pressed against agar containing penicillin in another petri dish. Gain, the position was marked with an arrow. Each colony was thus placed in the new dish in exactly the same position it had occupied originally. Usually the bacterial colonies failed to survive in the penicillin medium, but eventually the Lederberg's found what they were looking for: bacteria growing on the penicillin plate.

In the next step of the experiment, they located the colony on the original, untreated petri dish that corresponded with the colony that had grown on the penicillin-laced agar. They transferred a sample of cells from this untreated colony to another petri dish containing no penicillin. The cells were allowed to grow, and when samples of these cells were tested in a medium containing penicillin, it was found that they too were penicillin-resistant, even though they had never been exposed to penicillin. Like the black moths and the cyanide-resistant scale insects, they were simply variants produced by chance in the original population and selected by the environment.

It is now known that the bacterial genes for drug resistance are carried on plasmids, small DNA molecules that can be transferred from one cell to another. Consequently, the spread within a bacterial population of mutations conferring resistance is much more rapid than would occur as a result of natural selection alone.

5. Explain in detail, all the steps in speculation. You must start with a stable population and end with a species.(see also chapter 15-2, 15-3)

Speciation is the process in which a new species is formed. This happens through a series of successive stages. Because natural selection favors changes that encourage reproduction success, micro evolution continually molds and shapes a species to improve the fit between a species and its environment.

Here is the different steps through which speciation occurs.

For the purpose of study, the unit of evolution is not the individual but the group of interbreeding individuals. The word population is used in a special sense to describe such a group. The study of single individuals provides few clues as to the possible outcomes of evolution because single individuals cannot evolve in their lifetime. An individual represents a store of genes that participates in evolution only when those genes are passed on to further generations, or populations. The gene is the basic unit in the cell for transmitting hereditary characteristics to offspring. Individuals are units upon which natural selection operates, but the trend of evolution can be traced through time only for groups of interbreeding individuals; populations can be analyzed statistically and their evolution predicted in terms of average numbers.

The Hardy-Weinberg law--which was discovered independently in 1908 by a British mathematician, Godfrey H. Hardy, and a German physician, Wilhelm Weinberg--provides a standard for quantitatively measuring the extent of evolutionary change in a population. The law states that the gene frequencies, or ratios of different genes in a population, will remain constant unless they are changed by outside forces, such as selective (differential) reproduction and mutation. This discovery reestablished natural selection as an evolutionary force because the basis of natural selection is differential reproduction by the most fit organisms. Comparing the actual gene frequencies observed in a population with the frequencies predicted, or calculated, by the Hardy-Weinberg law gives a numerical measure of how far the population deviates from a static, or nonevolving, state, called the Hardy-Weinberg equilibrium. Given a large, randomly breeding population, the Hardy-Weinberg equilibrium will hold true, because it depends on the laws of probability.

In genetics a population is defined in terms of its gene pool, the sum total of all alleles (two or more alternate forms of a gene that determine different characteristics) for all genes in the population. The frequencies of alleles vary from generation to generation, and this change is defined as evolution according to population genetics. Changes are produced in the gene pool through mutations, gene flow, genetic drift, and natural selection.

A mutation is an inheritable change in the character of a gene. Mutations most often occur spontaneously, but they may be induced by some external stimulus, such as irradiation or certain chemicals. The rate of mutation in humans is extremely low; nevertheless, the number of genes in every gamete, or sex cell, is so large that the probability is high for at least one gene to carry a mutation.

Populations can exhibit a wide variety of variations, which are promoted by sexual reproduction and by other means, such as self-incompatibility in plants that discourages self- fertilization and encourages out-breeding.

In sexual reproduction, each individual inherits equal numbers of chromosomes and, hence, genes from both parents; the combination of genes is therefore different from that of either parent. The number of genes is so large that a particular combination is probably never repeated in the history of the species. The other process is recombination, in which each pair of chromosomes inherited from the parents trades segments through physical breakage and exchange.

As a result, the original combination of inherited genes is recombined, thus amplifying the number of possible new hereditary patterns. In such asexually reproducing populations as certain bacteria, algae, and fungi, mutation is the only source of inherited variation.

The resulting genetic variability is subject to natural selection. Individuals with characteristics making them more successful in using the resources of the environment are more likely to survive and reproduce, whereas the others with less favorable characteristics are less likely to reproduce. The hereditary patterns controlling the more favorable characteristics are therefore passed on in greater frequency to the next generation. The resulting change in the genetic makeup of the population in the next generation constitutes evolution. New genes can be introduced into a population through new breeding organisms or gametes from another population, as in plant pollen. Gene flow can work against the processes of natural selection.

The Hardy-Weinberg equilibrium holds true only if the population is large. The qualification is necessary because the equilibrium depends on the laws of probability. These laws—the laws of change – apply equally well to flipping coins, rolling dice, or betting at roulette. In flipping coins, it is possible for heads to show up six times in a row, but on average, heads will show up half the time and tails half the time. The more times the coin is flipped, the more closely the expected frequencies of half (0.50) and half (0.50) are approached.

Consider, for example, and allele, say a, that has a frequency of 1 percent. In a population of 1 million, 20,000 a alleles would be present in the gene pool. (Remember that each diploid individual carries two alleles for any given gene; in the gene pool of this population there are 2 million alleles for this particular gene, of which 1 percent, or 20,000, are allele a). But in a population of 50, only one copy of allele a would be present. If the individual carrying this allele failed to mate or were destroyed by chance before leaving offspring, allele a would be completely lost. Similarly, if 10 of the 49 individuals without allele were lost, the frequency of a would jump from 1 in 100 to 1 in 80.

This phenomenon, a change in the gene pool that takes place as a result of chance, is genetic drift. Population geneticists and other evolutionary biologists generally agree that genetic drift plays a role in determining the evolutionary course of populations. Its relative importance, however, as compared to that of natural selection, is matter of current debate. There are at least two situations in which it has been shown to be important.

A small population that branches off from a larger one may or may not be genetically representative of the larger population from which it was derived. Some rare alleles may be over represented or may be lost completely. As a consequence, even when and if the small population increases in size, it will have a different genetic composition—a different gene pool—from that of the parent group. This phenomenon, a type of genetic drift, is known as founder effect.

An example of the founder effect is found in the Old Order Amish of Lancaster, Pennsylvania. Among these people, there is an unprecedented frequency of a recessive allele that, in the homozygous state, causes a combination of dwarfism and polydactylism(extra fingers). Since the group was founded in the early 1770s, some 61 cases of this rare congenital deformity have been reported, about as many as in all the rest of the world’s population. Approximately 13% of the persons in this group, which numbers some 17,000, are estimated to carry this rare allele. This entire colony, which has kept itself virtually isolated from the rest of the world, is descended from only a few individuals. By chance, one of those must have been a carrier of this allele.

Deme means the genetic mix is similar throughout a species. When mutation, variation or genetic drift is introduced to a deme population, speciation begins.

Sometimes phenotypic variations within the same species follow a geographical distribution and can be correlated with gradual changes in temperature, humidity, or some other environmental condition. Such a graded variation in a trait or a complex of traits is known as a cline.

Many species exhibit north-south clines of various traits. House sparrows, for example, tend to have a smaller body size in the warmer parts of the range of the species, and a larger body size in the cooler parts. House sparrows were first introduced into North America between 1852 and 1860 in the form of small founder populations drawn principally from central England and Germany. From an evolutionary standpoint, the time that has elapsed since their introduction is very brief indeed, yet a dramatic degree size differentiation has occurred. In cooler regions, a larger body size is advantageous for heat conservation; consequently many homeothermic animals show geographic variations in size similar to those of the house sparrow. Conversely, among mammals, ears, tails, and other extremities are relatively longer in the warmer areas of a species’ range; such adaptations allow for heat radiation from the animal. Plant growing in the south often have slightly different requirements for flowering or for ending dormancy than the same kind of plants growing in the north, although they may all belong to the same species.

Geographical- If a volcano erupts and forms a giant peak in the middle of the land isolating the populations. This is an example of geographical isolation. An example of this would be that humans were isolated by oceans. When Columbus crossed the Atlantic Ocean he found the Indians and brought Syphilis to the Old World.

Reproductive Cycles- The mallards and pintails are perfectly capable of breeding together. Then why don’t they? This is because they are isolated due to reproductive cycles although they can produce fertile offspring. Mallard’s breed first and can only breed in September . Pintails start to breed in October and there are no active Mallard’s . Therefore they are separated because of the reproductive cycles.

Behavioral- Humans and Gorilla’s don’t breed because they are isolated behaviorally. Because the behavior of a gorilla and a human are different, you can’t breed them.

Physiological- This is prominent in some types of insects. This is if the reproductive organs are off different shapes and won’t fit together. That makes it hard to breed. That’s just like when a kid has wooden blocks and he has to fit them into a board. He can’t fit a square block into a circle unless he takes a hammer and hammers it in but it doesn’t work that way in nature.

Ecological - There are no hybrids within flickers. This is because the flickers don’t want to cross the great plains so the two subspecies are separated ecologically by the Great Plains and there’s not many things to eat on the Great Plains.

Gametic Isolation- When the sperm isn’t able to fertilize the egg, then the two species are isolated by gametic isolation. The sperm and egg do not match chemically so they don’t produce fertile offspring. This is why if material from an oak-tree falls on to a dandy-lion no dandy-oak for you! No matter how much fun or sex you’re having you get no fertile offspring if gametic isolation is present. Isolation is the main key.

There are different types of mutations within different types of species. For example take the gill people and the mountain people. The mountain people will have different types of mutations. A species that is on an island will also have mutations that differ from the species on land. An example of this is the 21 different species of Finches that Darwin found on Galapagos Island. Mutations also depend upon environment. For each species, the environment determines what type of mutations will the species have. If the species is living in sea level altitude, it will be different then a species living in the Alpine, at 10,000 feet. Mutations can make different species and sub-species very different and separate them.

Natural Selection takes Time, Time, Time, Time! Depending on the type of time you have, there will be more mutations and natural selection will take it’s place eventually since natural selection requires a long period of time. For something to spread within a whole population for example in Humans, it might take 100,000 years for something to spread through the whole population. In bacteria this might be different because bacteria can go through 3 generations within one day so mutation, etc. can go through a population quickly therefore evolution takes place rapidly. Changes can go through a bacterial population within weeks and that is why, for example, bacteria can develop drug resistance so fast. Also insects can develop insecticide resistance but not as fast as bacteria. Also, if you have small populations such as after a mass extinction, then the changes can spread quickly and fast through a small population.

Two subspecies can get together and hybridize. Just like Columbus got it on with the Indians there was contact between two different subspecies. Because the Indians are a sub-species(race) and not a different species he was able to breed with them, there are races that can breed but are different sub-species. Because of coyotes, the Red Wolf was hybridized out of existence and now is on the Endangered Species list and is well gone! Another Example would be the Utah Cutthroat which was also hybridized almost out of existence, some of it still exists though rare. Certain species or sub-species such as the Red Wold can be "hybridized" out of existence.

A hybrid organism is the offspring of parents of different species. Hybrids can occur in animals (such as the mule), but they are far more common in plants. Kentucky bluegrass, for instance, is a promiscuous hybridizer and has crossbred with many related species, producing hundred’s of hybrid races, each well adapted to the ecological conditions of the area in which it grows. Such hybrids, spreading by means of rhizomes, are sometimes able to out-compete both parents. Asexually reproducing races of hybrid plants may be considered species in that they are genetically isolated from their parents and from each other, but they do not conform to the working definition of a species because they do not interbreed.

Hybrids in both plants and animals are often sterile because the chromosomes cannot pair at meiosis (having no homologues), a necessary step for producing viable gametes. If, however, Polyploidy occurs in such a sterile hybrid and the resulting cells divide by further mitosis and cytokinesis so that they eventually produce a new individual asexually, that individual will have twice the number of chromosomes as its parent. As a consequence, it is reproductively isolated from the parental line. However, its chromosomes—now duplicated – can pair, meiosis can occur normally, and fertility is restored. It is a new species, capable of sexual reproduction.