Change Gene Frequencies example essay topic

... ark moth variety. The light and dark moth varieties belong to the same species and interbreed freely. If pollution had continued, however, the rural moth population would have become entirely light and the industrial entirely dark. Then each population would be subject to somewhat different selective pressures because the two environments vary. In time, the dark and light populations would differ by groups of genes, with each group advantageous locally. The moth populations might eventually become incapable of interbreeding.

At this point, natural selection would have caused them to diverge from a common ancestor into two species. Populations of various species have easily become isolated in different habitats on various islands and have differentiated into new species. Island chains provide a particularly well-studied context of speciation. Darwin himself was struck by the finches of the Gal' Islands, which radiated into 14 species, each with distinctive form and habits, following an invasion of the islands by a single species from the American mainland.

In the Hawaiian Islands some 500 species of fruit flies have descended from one or two founding species in less than 10 million years; they have had a complex history of island hopping and re hopping, in the process forming isolated populations that were the start of new species. Natural selection is not the only source of genetic change in the evolution of species. Gene frequencies may also change by the chance failure of progeny to reproduce the exact gene proportions of their parents. This is termed genetic drift and is most important in small populations, where genes may be lost from the original gene pool simply by not being represented in successive matings. Also, failure to carry the full range of genes in the parent population occurs when a few individuals migrate and found a new, isolated population, which is thus different from the very beginning; this is called the founder effect. Mutations can, of course, also change gene frequencies, but such changes occur at low rates relative to the changes brought about by the recombination of genes in offspring.

Because all the established genes in a population have been monitored for fitness by selection, newly arisen mutations are unlikely to enhance fitness unless the environment changes so as to favor the new gene activity, as in the gene for dark color in the peppered moth. Novel genes that cause large changes rarely promote fitness and are usually lethal. The genes already established by selection are carefully adjusted to one another so their biochemical effects are coordinated; a new gene with a major effect is comparable to the insertion of a chance word or rearrangement of words into a precise set of instructions. Mutations with small effects provide the basis for the genetic changes that are seen to promote fitness in experimental laboratory environments. Indeed, natural selection by a series of mutations appears to be the chief agent of evolution.

Current Evolutionary Debate Although the fact of evolution is scientifically accepted as underlying modern biology, theories that concern themselves with the processes of evolution continue to be debated and refined. Much of this work involves highly sophisticated mathematical studies, as required by the complex interactions of the various elements of the modern synthesis, from gene mutations to population genetics to large-scale ecological interactions over geological time. Because understanding of the actual evolutionary events that took place over earth's long history depends largely on interpretations of an incomplete fossil record, much latitude remains for differences in such interpretations. One of the issues that is currently being debated among theorists derives from a notable fact observed in the fossil record.

That is, when a new species appears in the record it usually does so abruptly and then apparently remains stable for as long as the record of that species lasts. The fossils do not seem to exhibit the slow and gradual changes that might be expected according to the modern synthesis. For this reason, in part, a number of evolutionists-most notably Stephen Jay Gould of Harvard University and Niles Eldredge of the American Museum of Natural History-have proposed a variant concept of "punctuated equilibrial" for species evolution. According to this concept, species do in fact tend to remain stable for long periods of time and then to change relatively abruptly-or rather, to be replaced suddenly by newer and more successful forms. These sudden changes are the "punctuations" in the state of equilibrium that give this concept its name.

Although these proposed periods of rapid change would be abrupt only in terms of the geological time scale and would actually occur over periods of thousands of years, most evolutionists tend to consider the punctuated-equilibrium concept only another possible mode of evolutionary change that could take place along with the processes described by the modern synthesis, rather than as a supplanting model for evolution theory. The very incompleteness of the fossil record does not permit any such clear choice to be made, because the record of almost any species is highly selective over geological time. In addition, the small changes that would make up gradual evolutionary development according to the modern synthesis are themselves not necessarily of a nature that would be apparent in the fossil history of a species, however complete it might be over a given stretch of time. Fossils primarily show gross morphological changes, whereas changes taking place in genetic makeup could be extensive even though overall body structures do not reveal these shifts in populations of species. Arguments from the known nature of small-scale evolutionary change do not, in fact, necessarily establish long-term evolutionary events, as following either the model proposed by the modern synthesis or the one proposed by punctuated equilibrium.

Evolution may just as well have proceeded along both routes. Steps in Evolution Life originated more than 3.4 billion years ago, when the earth's environment was much different than that of today. Especially important was the lack of significant amounts of free oxygen in the atmosphere. Experiments have shown that rather complicated organic molecules, including amino acids, can arise spontaneously under conditions that are believed to simulate the earth's primitive environment.

Concentration of such molecules evidently led to the synthesis of active chemical groupings of molecules, such as proteins, and eventually to interactions among chemical compounds. A rudimentary genetic system eventually arose and was elaborated by natural selection into the complicated mechanisms of inheritance known today. The earliest organisms must have fed on nonliving organic compounds, but chemical and solar energy sources were soon tapped. Photosynthesis freed organisms from their dependence on organic compounds and also released oxygen so the atmosphere and oceans gradually became more hospitable to advanced life forms. The earliest organisms of which remains exist were already cells, resembling modern bacteria (see Cell). These simple unicellular forms (prokaryotes) were at first anaerobic (living without oxygen), but they diversified into an array of adaptive types from which (formerly known as blue-green algae) descended, including aerobic photo synthesizers.

Advanced cells (eukaryotes) may have evolved through the amalgamation of a number of distinct simple cell types. A large ingesting cell may have incorporated as symbiont's (see Symbiosis) some small blue-green algal cells that evolved into chloroplasts (cell bodies that photosynthesize) and some tiny aerobic bacteria that evolved into mitochondria (cell bodies that release energy during respiration). Other features of advanced cells, such as their large DNA contents, may also have arisen from prokaryotic symbiont's. Single-celled eukaryotes then developed complex modes of living and advanced types of reproduction that led to the appearance of multicellular plants and animals. The latter are first known from about 700 million years ago, and their appearance implies that at least moderate levels of free atmospheric oxygen and a relatively predictable supply of food plants had been achieved.

Between about 700 and 570 million years ago the basic body plans of modern animals were developed during a remarkable burst of evolutionary diversification. The earliest body fossils consist chiefly of impressions belonging to jellyfish and their allies, a rudimentary group. At about the same time, however, fossil burrows appeared, signaling the evolution of burrowing worms with considerably more advanced body structures. Then, beginning just before 570 million years ago, skeletons developed independently in a number of animal lineages.

One worm like lineage that pursued a swimming mode of life evolved a stiff dorsal cord and eventually an articulated internal skeleton that supported the body to improve swimming efficiency; thus, fish arose from the early invertebrates. In order for complex animal communities to develop, plants must first become established to support herbivore populations, which in turn may support predators and scavengers. Land plants appeared about 400 million years ago, spreading from lowland swamps as expanding greenbelts. Arthropods (some evolving into insects) and other invertebrate groups followed them onto land, and finally land vertebrates (amphibians at first) rose from freshwater fish nearly 360 million years ago.

In general, the subsequent radiations of land vertebrates made them increasingly independent of water and increasingly active. Dinosaurs and mammals shared the terrestrial environment for 135 million years; dinosaurs may well have been more active, and certainly were larger, than their mammalian contemporaries, which were small and possibly nocturnal. The mammals, however, survived a wave of extinction that eliminated dinosaurs about 65 million years ago, and subsequently diversified into many of the habitats and modes of life that formerly had been dinosaurian. Evolutionary Patterns The history of life as inferred from the fossil record displays a wide variety of trends and patterns. Lineages may evolve slowly at one time and rapidly at another time; they may follow one pathway of change for some time only to switch to another pathway; and they may diversify rapidly at one time and then shrink under widespread extinctions.

The key to many of these patterns is the rate and nature of environmental change. Species become adapted to the environmental conditions that exist at a given time, and when change leads to new conditions, they must evolve new adaptations or become extinct (see Endangered Species). When the environment undergoes a particularly rapid or extensive change, waves of extinction occur; these are followed by waves of development of new species. The times of mass extinction are not yet well understood. Although the most famous one is that of the dinosaurs, about 65 million years ago, such events appear in the fossil record as far back as Precambrian time, when life first arose. In fact, five mass extinctions on the scale of that at the end of the age of dinosaurs are known over the past 600 million years.

Some scientists also claim to have demonstrated a definite periodicity to smaller periods of mass extinction, and in particular a 26-million-year cycle of eight extinctions over the past 250 million years. Controversy has arisen over the proposal made by some geologists that mass extinctions are related to periodic catastrophes such as the striking of the earth's surface by a large asteroid or comet. Many paleontologists and evolutionary theorists reject such hypotheses as unjustified; they feel that periods of mass extinctions can be accounted for by less spectacular evolutionary processes and by more earthbound events such as cycles of climatic change and volcanic activity. Whatever proposals may eventually prove true, however, it seems fairly certain that periodic waves of mass extinction do occur.

Species adapted to live in environments that are changeable in the short term have broad tolerances, which may better enable them to survive extensive changes. Human beings are uniquely adapted in that they make and use tools and devices and invent and propagate procedures that give them extended control over their environments. Humans are significantly changing the environment itself, however. The effects are most complex and cannot be predicted, and yet the likelihood is that evolutionary patterns in the future will reflect the influence of the human species...