Definition of Evolution
Evolution, in the broadest sense, is the view that all reality is in a continual state of change. The process of evolution is primary, and flux (constant change) is universal. This shifting reality is best understood studying the forces that underlie change.
General Concept of Evolution
The face of the earth is gradually changing. New mountain ranges are arising at some sites, while high places are worn away at others. Understanding the forces of erosion enables us to explain the wearing away of older mountain ranges. The exciting theory of continental drift provides insight into the building up of new ranges. In the movement of tectonic plates, explains much of the turbulence at boundary areas of these geological plates or landmasses.
Evolution a Biological Perspective
As applied to biology, evolution maintains that all the diverse forms of life that are now in existence have come into being through a gradual and continual process of modification of ancestral forms. This process of “descent with modification” does not lead to a finished final product. Evolution modifies all living things. And it will continue to produce a change in the future as it has in the present and past. Furthermore, all life shares in the history of evolving, so that protists are as highly evolved as humans. But the specific kinds of modifications are quite different for each ancestral line.
Evolutionary Theories A Historical Perspective
No theory better integrates our understanding of the diversity of living organisms. Their relationships, and degrees of relatedness. Their interaction with a changing environment than evolution does. Though called a theory, evolution is accepted many biologists as a fact. Some controversy continues in the community of biologists as to the precise mechanisms involved in evolutionary change. As well as its tempo, but a virtually complete consensus exists as to its occurrence.
Concept of Evolution Before Darwin
Although organic evolution was not an alien idea among ancient Greek philosophers. It was not a prevalent view.
Plato’s Idea About Evolution
In Plato’s idealism, each species was fixed and existed as a perfect archetype or ideal representation. Individuals in nature were imperfect realizations of this archetype. To Aristotle, every species fit into a single hierarchy of increasing complexity. This “scale of nature” was a gross simplification of the nature of life, but it pervaded natural history until the eighteenth century. However, Lucretius, a Roman philosopher who drew upon Greek thought. He suggested that gradual changes in living forms could occur.
Georges-Louis Leclerc de Buffon Evolutionary Perspective
Georges-Louis Leclerc de Buffon (1707-1788) was the first of the serious naturalists of the modern era to develop a concept of evolving living forms. He tended to denigrate the classification schemes of naturalists like Carolus Linnaeus (1707-1778). However, with their emphasis on fixed forms fitting into permanent natural slots. He recognized that species change and utilized findings in comparative anatomy vestigial organs, etc. It is due to buttress his views of fluidity among species.
Erasmus Darwin Concept of Evolution
Erasmus Darwin (1731-1802) also proposed that species change. Largely modifications in individuals during their lifetime. However, which modifications are passed on to their progeny. This view Charles Darwin’s grandfather anticipated the more extensive mechanism for evolution Lamarck.
Georges Cuvier Believe About Evolution
Georges Cuvier (1769-1832) was a firm believer in the fixity of species. However, his contributions to biological theory and practice were extremely useful to those who did work along evolutionary lines. He virtually founded the science of paleontology, the systematic study of fossils. Became skilled in reconstructing whole organisms from fossilized remnants. He a comprehensive system for the classification of animals and instituted remarkably thorough studies in comparative anatomy.
His studies of fossils demonstrated that many kinds of animals alive at one time are no longer extant. He devised the theory of catastrophism to account for a succession of animal populations. This theory states that a series of catastrophes periodically wiped out most of the forms of life then present. New groups were subsequently formed from the living remnants that were left. This accounts for the variation found in the fossil record according to Cuvier. He did not accept the notion of new species arising after each catastrophe. But claiming that these new forms probably existed in some distant part of the world. Migrated to the places where their fossils were found.
Jean Baptiste Lamarck Theory of Evolution (Lamarckism)
Jean Baptiste Lamarck (1744-1829) was perhaps the most significant pre-Darwinian contributor to the concept of evolution. Like Charles Darwin, he began his career (in botany) believing in the fixity of species. Later, he switched to zoology and became convinced that all living forms have evolved in a process of diversification. His great work, Philosophie zooiogique (1809), made landmark contributions to classification. As well as providing an impressive list of evidence for an evolutionary process. Most important, it suggested a mechanism for that process and a method (since reevaluated) for the origin of variations in individuals.
Lamarck believed that during the lifetime of any organism those parts that are used tend to develop or enlarge. While parts not challenged use tend to atrophy. This use-disuse theory is illustrated in humans the large arm muscles formed blacksmiths. Other practitioners of great muscular effort or the withering of limbs of those who do not use them regularly. Lamarck believed that such changes occurring during the lifetime of an individual are then transmitted to the next generation the inheritance of acquired characters. In this fashion, the activities of organisms in one generation tend to direct long-term changes in the future.
Evolution, in Lamarck’s view, is fashioned biological need and reflects a pragmatic program. Leading to success in dealing with the challenges of the environment. An often-cited example of Lamarckianism is the long neck of the giraffe. Supposedly produced countless generations of giraffes reaching for the topmost sprigs of leaves on trees. To more effectively compete with other herbivores who were confined to more accessible foliage.
Darwin And Theory of Natural Selection
Charles Darwin (1809-1882) is most closely associated with evolution for two significant reasons. First, he amassed such a comprehensive and convincing body of evidence demonstrating organic evolution. That it was no longer reasonable for biologists or even open-minded laypeople to dispute the existence of such a process. Second, his explorations of the fauna of South America and Africa. During his five-year (1831-1836) voyage as a naturalist aboard the HMS Beagle provided him with the insights necessary to develop a compelling theory of the mechanism of evolution. That mechanism is known as natural selection. It was first presented at a scientific meeting in 1858. In 1859, his landmark work, The Origin of Species Means of Natural Selection, was published in London. It created a storm of controversy but generated a fervent band of supporters.
Theory of Natural Selection
Definition of Natural Selection
Over time the characteristics which confer adaptiveness, or fitness, come to accumulate in the population. While those characteristics which diminish fitness tend to dwindle or die out. It is this last aspect. However, the greater reproductive success of better-adapted forms that are properly termed natural selection.
Pillars of Theory Natural Selection
The theory of natural selection rests upon three major tenets.
There is a remarkable overproduction of young in each generation. Many more than can possibly be supported the limited resources (food, water, shelter, mates) of the environment.
Heritable variations exist within this too-large population of young.
Competition for survival occurs in which those variants that are better adapted to a particular environment. They are successful and continue to produce offspring with their adaptive characteristics.
Explanation of Theory of Natural Selection
At one time natural selection was described in terms of a struggle for existence in which the fittest survive. Such a formulation Darwin’s followers tended to portray nature in terms of incessant fighting and bloodshed. But it failed to take into account the significance of cooperative mechanisms in survival. The concept of differential reproductive success of variant forms is a more accurate summary. It also stresses that over long periods of time the only criterion for continuing success is reproductive. What fails to reproduce cannot be represented in future generations, no matter how to fit. Thus, fitness can be assessed only in retrospect, after reproductive success is considered.
History of Darwin’s Voyage
The strongest influence in shaping Darwin’s speculations about evolution was the meticulous observations. He carried out during the voyage of the Beagle. Since he was prone to seasickness, he welcomed the opportunity to remain on land. Where he studied both extant forms and the fossils of ancient eras. He was singularly impressed the varieties of birds and reptiles on the Galapagos Islands. As well as, their relationship to similar groups on the Ecuadorian mainland.
He also recognized that the presence of marine fossils high up on the Andes mountains was inconsistent. With a concept of static species in a constant environment. Opportunities to revisit many sites, especially along the South American coast. These sites enabled him to study plant and animal life over extended periods. He noted that despite great fecundity in nature. In addition, little change in numbers of a population from generation to generation occurred. This elimination of nature’s bounty helped to shape his recognition of a selection process in nature.
Charles Lyell’s Book and Darwin’s Travel Records
A geologist friend of Darwin named Charles Lyell had written a book (The Principles of Geology). Whose major thesis was that the natural forces acting on the earth were the same in the distant past as they are at present. This continuity of geological forces produces a constantly changing environment. As glaciers advance and retreat, mountain ranges rise and fall, and rivers erode the land through which they flow. If conditions were constantly changing over long spans of time. It was reasonable to suppose that different kinds of life might have existed under these variant conditions. Darwin was able to observe both. Including the geological changes and the fossil evidence of the existence of different life forms at different times and in different environments.
Return of Darwin From Voyage and Thomas R. Malthus Easy on Population
Shortly after his return to England, Darwin came across an essay on population written a clergyman, Thomas R. Malthus. The thesis of this powerfully drawn work was that the human population tends to increase at a much greater rate than the food supply necessary to sustain the population does.
Specifically, Malthus contended that the world’s human population tends to increase according to a geometric progression (2, 4, 8, 16, 32, …, 2″). While available resources increase in arithmetic fashion (addition of a constant increment, e.g., 1, 2, 3, 4, 5, …. n). Eventually, the ratio of people to food and other resources would reach unmanageable proportions. This lead to a chaotic struggle for bare subsistence.
Malthus suggested that the existence of pestilence, war, and floods. Moreover, similar disasters serve to maintain the population at levels commensurate with available resources. For Malthus, the situation was interpreted as a justified intervention of divine providence to maintain a balance. For Darwin, however, it sowed the seeds for the concept of overproduction of young leading to a struggle for existence.
Concept of Artificial selection
The possibility of a selection process operating in nature was also suggested the practices of plant and animal breeders. Darwin was familiar with the achievements of animal husbandry. By which, in a few generations, purposeful alterations in the characteristics of domestic animals could be affected. Artificial selection for specific traits is carried out humans. That could not a similar process occur in nature. With the gradual accumulation of traits that enhance survival and reproductive success?
Natural Selection: A Modern Synthesis
The weakest link in Darwin’s theory of natural selection was his ignorance of the mechanism of heredity. Without a sufficient appreciation of the laws of genetics. He could not account for the variations that arose as exceptions to the tendency of “like to beget like,”. Even though these variations were essential to his theory.
In 1901 the basic features of Mendelian genetics were discovered. In the next four decades, a clarification of classic genetics was paralleled the development of population genetics as a separate discipline. Among those who provided significant insights into the nature of gene flux within breeding populations were Sewall Wright, Ernst Mayr, Theodor Dobzhansky, and L. C. Dunn.
The enrichment of Darwin’s theory the perspectives of population genetics as well as the findings of paleontology and biogeography became what Julian Huxley called a modern synthesis of evolution. Essentially this modern synthesis (neo-Darwinism) is a recognition. It is populations that undergo the gradual changes that are affected natural selection acting on individuals.
Sickle-cell anemia originated as a single-gene recessive mutation within an African population. At first, it was relatively rare within that population. However, individuals carrying the sickle-cell allele as heterozygotes are relatively immune to malaria. Because their slightly contorted red blood cells do not accommodate the malarial parasite well.
Consequently, the frequency of that allele increased in the population to its present count. It is the enhancement of reproductive potential that is crucial to the spread of this or any allele. The serious debility of individuals carrying two copies of the mutant allele serves to limit the spread of the allele. So that negative and positive influences on reproductive potential are involved in evolutionary changes.
Punctuated Equilibrium (Punctuated Evolution)
Students of evolution, from Darwin to the present generation, have recognized the comparative rarity of transitional, or linkage, forms in the fossil record. Between one species and another, or among major groups such as reptiles and mammals. If all populations undergo slow changes in their evolution into new forms. One would expect to find a continuous spectrum of fossil representatives all through the transition process.
Theory of Punctuated Equilibrium (Punctuated Evolution)
In 1972, Niles Eldredge of the American Museum of Natural History and Stephen J. Gould of Harvard University proposed the theory of punctuated equilibrium. According to which (among its contributions) explained these gaps in the fossil record. In punctuated equilibria, evolutionary changes occur in irregular jumps. It includes very rapid changes are followed long periods of relative constancy.
Under certain conditions, new species form from old. However, the major modifications are compressed into several thousand years rather than many millions. Although, thousands of years can hardly be considered abrupt. In the time spans associated with evolutionary trends, it represents a tiny proportion of the existence of any particular species. Since fossils are found as samples of any era’s flora or fauna and are unearthed in a random fashion. The chances of obtaining records of each form for a short period of intense change are unlikely.
The theory of punctuated equilibrium has excited considerable controversy. Its opponents contend that the compressed time scales for change do not radically alter. The Darwinian view of gradualism, nor does a demonstrated jump in one lineage prove that similar jumps occur in all lineages. Further, the periods of relative constancy hypothesized Eldredge and Gould may not actually occur.
Since profound changes at the molecular and soft tissue levels may be occurring without detection an examination of the fossil record. It is less easy to dispute the contention of the punctuation lists that the discreteness of present-day forms. It is due to the fact that transitional states last a relatively short time. So that we observe present species after they have undergone the changes that produce the relatively fixed form.
Molecular Biology of Evolutionary Change (Molecular Evolution)
Natural selection acts upon individual phenotypes, determining the reproductive success of specific individuals. However, it is the genotypes of selectively favored forms that tend to persist. Since genotypes are passed on in each generation. The sum of all genotypes constitutes the gene pool of a population. The gene pool is the entity that undergoes evolution and defines, at any particular time, a specific population. The character of DNA and its changes are the ultimate bases for diversity in the living world.
What is molecular evolution?
Molecular biology provides the tools for determining the nature (base sequence) of DNA and the proteins it codes for. Mutations, a major source of variation for the evolutionary mill, are for the most part altered base sequences within a DNA strand. The pace of evolution can be gauged the rate of change of DNA sequences in a specific time frame.
While the relatedness of different species can be measured how closely matched their DNA is. Since proteins are specified DNA. The commonality of protein structure (amino acid sequences) may also be used to determine relationships of descent among groups. It is rather remarkable that the evolutionary perspectives of a man who knew nothing of chromosomes. However, their chemical units have been confirmed and reinforced discoveries in molecular biology.
Darwin’s Idea About Molecular Evolution
Darwin believed that all living things could trace their ancestry back to a single set of progenitors. The existence of one genetic code in all living things is strongly consistent with this concept of descent with modification. The basic relatedness of diverse forms is also seen in the similarity of many of their proteins.
Example of Molecular Evolution
Specific proteins such as the a and pi chains of hemoglobin and cytochrome c have been studied extensively. To provide molecular probes for following evolutionary diversification. The pi chains of hemoglobin in gorillas and humans are virtually identical in amino acid composition. The monkey fi chain has fewer than 10 amino acid differences from the human protein. But the /3 chain of jawless fishes differs in more than 100 amino acids from that of the human. These genetic distances confirm the relative relatedness of these forms suggested other data.
Control of Gene Pools and Micro Evolution
The Hardy-Weinberg equilibrium applies to hypothetical populations in which no net change of gene frequencies or genotypic ratios occur. In nature, however, changes in these characteristics do occur. Such changes in a population’s alleles are called microevolution.
A major cause of microevolution is natural selection. The Darwinian process of differential reproduction that results in increasing the adaptability of a population. Alterations of the gene pool in a random (nonadaptive) manner may also occur through genetic drift. In small populations, loss of alleles or genotypes may occur accidentally through a kind of sampling error. Moreover, in which the specific allele or genotype is “overlooked” reproductively.
Example of Micro Evolution
In a pea patch population containing five TT, twelve Tt, and four tt individual pea plants. On the contrary, a rabbit may enter and chance alone eat only the five TT plants. Such a loss of the T allele would tend to alter the frequency of that allele for many generations. It is an example of microevolution. Such accidental elimination of a specific genotype would be highly unlikely in a larger population.
Founder Effect and Evolution
A special case of genetic drift is known as the founder effect. If a new and isolated colony is started a small group atypical of the larger population. The individuals of that colony will resemble the founders rather than the original population. The founder effect has been noted in cases in which high frequencies of genetic disorders exist in small groups tracing their ancestry back to afflicted founders. If an island were inhabited humans all of whom were blue-eyed. Subsequent populations would probably all lack the dominant allele for brown eyes.
Gene Flow and Evolution
Gene flow, in which changes in a population’s alleles are produced immigration into or emigration out of a population. It is another mode producing microevolution. Such changes tend to be neutral. However, in an adaptive sense unless migration favors the movement of either less adapted or better-adapted individuals. A change in the net mutation rate is another way in which random changes in gene frequencies occur in a population.
Since complete randomness in mating is one of the prime requisites for stable gene frequencies. A departure from mating randomness may lead to changes in gene frequencies.
In assortative mating, another source of gene alteration in populations. Individuals tend to show preferences in their selection of a mate. In humans, individuals of similar social classes or ethnic identification tend to mate with one another more than with outsiders. Among nonmigratory mammals, mating occurs more readily with near neighbors than with others. Such mating practices can lead to the eventual formation of new varieties within the larger group.
Only natural selection produces a consistent increase in the fitness of populations. As deleterious alleles decrease in frequency and adaptive alleles increase. Since alleles usually operate in coherent groups. The selective process probably influences gene complexes and may be expressed through a variety of interdependent processes such as coadaptation. Moreover, it is a situation in which several independent components evolve together in the production of a major new structure.
Speciation and Evolution
Definition of Species
A species consists of all individuals who can share a common gene pool. This means that they may interbreed with one another to produce fertile offspring but not with members of another species. The species, as the unit of the various kinds of life that exist in nature. It also defined gross anatomical features that characterize the specific group and differentiate it from other species.
Example of Speciation
The horse and the donkey (ass) belong to separate species. They differ in size and other anatomical features. They do not share a common gene pool. Although they are capable of mating with one another to produce offspring. The cross of a male ass with a female horse yields a mule. A male horse crossed with a female ass produces a hinny. But the gene pools of each do not blend because both mules and hinnies are sterile. It represents a reproductive dead end. Over time then, the horse and the ass maintain separate gene pools, the major criterion of species integrity.
Factors Affections Speciation and Reasons for Evolution
Since species are separate from one another based on their reproductive isolation. However, the key to maintaining species discreteness lies in mechanisms that produce separate breeding populations.
One form of reproductive barrier is found in the geographical boundaries that prevent individuals from two populations from reaching one another. Similar inhabitants of two different islands or populations that live on different sides of a high mountain range. They tend to remain isolated and eventually develop such differences in their gene pools that they are reproductively incompatible.
Where geography does not impose barriers, biological mechanisms may arise to maintain separateness between populations. Even in the same locale, two species may remain isolated because of ecological requirements. Differences in moisture requirements may separate worms into the occupation of different strata underground. Differences in the time of reproductive activity may separate two populations that can otherwise interbreed. Species that are active during the day have little likelihood of interacting sexually with nocturnal species. The incompatibility of the reproductive organs may also serve as an isolating barrier.
Evolution Due to Geographical Barriers
All dogs belong to a single species, but a Great Dane cannot readily mate with a Chihuahua. The common gene pool is maintained because each of these breeds can mate with an intermediate variety. Such as a terrier to produce offspring that share the Great Dane and Chihuahua lineage. However, after long periods of time the varieties of dog existing today may very well evolve into separate species with no gene flow between them.
Mating is often aborted at an early stage because courtship rituals are not observed the outside species. A variety of precise behaviors must precede mating, especially for birds and mammals. In addition, failure to observe the rules of the mating game effectively precludes a sharing of genes. In many cases in which sexual union does occur between separate species. The gametes do not fuse, or an embryo fails to develop.
Such gametic incompatibility is an effective barrier to a breakdown of species lines. Even when successful mating does occur between members of separate species. However, the hybrid forms are quite often sterile. The boundaries that preserve the separateness of species thus appear to be quite firm.
If reproductive isolation maintains the segregation of species. Then clearly the road to speciation must involve an event that isolates a segment of a species from other populations of that species. As we have seen, in this state of reproductive separation. However, the isolated group develops mutations and recombinant features that are not shared with the parental species. Given enough separation time, the breakaway population may develop into a new species that can no longer share in a common gene pool.
Example of Evolution of Species Due to Reproductive Isolation
If the isolated group is separated physically from the original larger population, this type of speciation is called allopatric. However, a subpopulation within the parent group may develop characteristics that tend to isolate it from its neighbors. It goes its merry developmental way to yield ever-increasing uniqueness while dwelling in the same neighborhood. Such a phenomenon is labeled as sympatric speciation. The smaller the size of the pioneer population, the more likely are the chances that create a separate species.
A particularly rapid type of speciation occurs when some common ancestor reaches an environment in which a great number of distinct opportunities and challenges exist. Such a process is called adaptive radiation, the production of many species from a common ancestral form. Island chains such as Hawaii offer examples of many unique species. On each island developing from some ancestral migrant. Species developing on one island may colonize a different island and become a distinct species. This process generally involving small pioneer groups. Continues, a rapidly branching speciation process may occur.
Microevolution VS Macroevolution
Macroevolution focuses on the creation of taxonomic groups above the level of species. Although many of the same mechanisms involved in speciation operate in macroevolution. In addition, the time spans required is much greater. Much of our knowledge of the broad trends of macroevolution comes from the fossil record.
However, changes within a group that leads to less drastic modifications of a population. Even the creation of a new species (microevolution) can be studied measurements of gene frequencies in a population.
Pattern of Evolution
Patterns of selection that have been discerned include.
In which extremes at either end of a spectrum are disproportionately selected against so that the population, despite continual variation produced in every generation, tend to cluster about its average (mean).
One extreme is favored over its opposite so that the average value slowly moves toward the favored extreme.
Diversifying (disruptive) selection
Two or more subtypes are favored, and the population tends to evolve into several subgroups or new species.
Phyletic Changes in Macroevolution
Diversifying selection may very well operate in both microevolution and macroevolution. Moreover, the directional selection is similar to the macroevolutionary process known as phyletic change. The basic patterns for the broad changes of macroevolution revealed the fossil record are follow.
Phyletic change (anagenesis)
Gradual change in a single lineage so that eventually Descendants are radically different from their ancestors. Anagenesis may be likened to directional selection over long periods of time.
A macroevolutionary trend in which branching occurs so that one lineage gives rise to two or more lineages. Small populations budding from a lineage may be in a particularly favorable position to produce new groups. Cladogenesis has been emphasized as a major macroevolutionary pattern Ernst Mayr.
A relatively sudden formation of many new groups, which can move into and exploit new environments. The relatively rapid diversification of early mammals during the extinction of the dinosaurs is a good example of such a major outreach. Adaptive radiation incorporates features of both cladogenesis and anagenesis, since new lineages formed during this volatile evolutionary period may each be undergoing progressive transitions.
More than 99.9 percent of all the species that have ever evolved is no longer present. This loss of diversity is an inexorable feature of the evolution of all kingdoms. A changing environment renders yesterday’s fit as today’s unfit, doomed to obliteration.