Essay on Evolutionary Theory | Species | Biology

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Essay # 1. Introduction to Evolutionary Theory:

Biology came of age as a science when Charles Darwin published “On the Origin of Species.” But, the idea of evolution wasn’t new to Darwin. Lamarck published a theory of evolution in 1809. Lamarck thought that species arose continually from nonliving sources. These species were initially very primitive, but increased in complexity over time due to some inherent tendency.

This type of evolution is called orthogenesis. Lamarck proposed that an organism’s acclimation to the environment could be passed on to its offspring. For example, he thought proto-giraffes stretched their necks to reach higher twigs. This caused their offspring to be born with longer necks.

This proposed mechanism of evolution is called the inheritance of acquired characteristics. Lamarck also believed species never went extinct, although they may change into newer forms. All three of these ideas are now known to be wrong.

Darwin’s contributions include hypothesizing the pattern of common descent and proposing a mechanism for evolution — natural selection. In Darwin’s theory of natural selection, new variants arise continually within populations.

A small percentage of these variants cause their bearers to produce more offspring than others. These variants thrive and supplant their less productive competitors. The effect of numerous instances of selection would lead to a species being modified over time.

Darwin’s theory did not accord with older theories of genetics. In Darwin’s time, biologists held to the theory of blending inheritance — an offspring was an average of its parents. If an individual had one short parent and one tall parent, it would be of medium height. And, the offspring would pass on genes for medium sized offspring.

If this was the case, new genetic variations would quickly be diluted out of a population. They could not accumulate as the theory of evolution required. We now know that the idea of blending inheritance is wrong.

Darwin didn’t know that the true mode of inheritance was discovered in his lifetime. Gregor Mendel, in his experiments on hybrid peas, showed that genes from a mother and father do not blend. An offspring from a short and a tall parent may be medium sized; but it carries genes for shortness and tallness. The genes remain distinct and can be passed on to subsequent generations. Mendel mailed his paper to Darwin, but Darwin never opened it.

It was a long time until Mendel’s ideas were accepted. One group of biologists, called biometricians, thought Mendel’s laws only held for a few traits. Most traits, they claimed, were governed by blending inheritance. Mendel studied discrete traits. Two of the traits in his famous experiments were smooth versus wrinkled coat on peas.

This trait did not vary continuously. In other words, peas are either wrinkled or smooth — intermediates are not found. Biometricians considered these traits aberrations. They studied continuously varying traits like size and believed most traits showed blending inheritance.

Essay # 2. Incorporating Genetics into Evolutionary Theory:

The discrete genes Mendel discovered would exist at some frequency in natural populations. Biologists wondered how and if these frequencies would change. Many thought that the more common versions of genes would increase in frequency simply because they were already at high frequency.

Hardy and Weinberg independently showed that the frequency of an allele would not change over time simply due to its being rare or common. Their model had several assumptions — that all alleles reproduced at the same rate, that the population size was very large and that alleles did not change in form.

Later. R. A. Fisher showed that Mendel’s laws could explain continuous traits if the expression of these traits were due to the action of many genes. After this, geneticists accepted Mendel’s Laws as the basic rules of genetics. From this basis, Fisher, Sewall Wright and J. B. S. Haldane founded the field of population genetics. Population genetics is a field of biology that attempts to measure and explain the levels of genetic variation in populations.

R. A. Fisher studied the effect of natural selection on large populations. He demonstrated that even very small selective differences amongst alleles could cause appreciable changes in allele frequencies over time. He also showed that the rate of adaptive change in a population is proportional to the amount of genetic variation present.

This is called Fisher’s Fundamental Theorem of Natural Selection. Although it is called the fundamental theorem, it does not hold in all cases. The rate at which natural selection brings about adaptation depends on the details of how selection is working. In some rare cases, natural selection can actually cause a decline in the mean relative fitness of a population.

Sewall Wright was more concerned with drift. He stressed that large populations are often subdivided into many subpopulations. In his theory, genetic drift played a more important role compared to selection. Differentiation between subpopulations, followed by migration among them, could contribute to adaptations amongst populations.

Wright also came up with the idea of the adaptive landscape — an idea that remains influential to this day. Its influence remains even though P. A. P. Moran has shown that, mathematically, adaptive landscapes don’t exist as Wright envisioned them. Wright extended his results of one-locus models to a two-locus case in proposing the adaptive landscape. But, unbeknownst to him, the general conclusions of the one-locus model don’t extend to the two- locus case.

J. B. S. Haldane developed many of the mathematical models of natural and artificial selection. He showed that selection and mutation could oppose each other that deleterious mutations could remain in a population due to recurrent mutation. He also demonstrated that there was a cost to natural selection, placing a limit on the amount of adaptive substitutions a population could undergo in a given time frame.

For a long time, population genetics developed as a theoretical field. But, gathering the data needed to test the theories was nearly impossible. Prior to the advent of molecular biology, estimates of genetic variability could only be inferred from levels of morphological differences in populations.

Lewontin and Hubby were the first to get a good estimate of genetic variation in a population. Using the then new technique of protein electrophoresis, they showed that 30% of the loci in a population of Drosophila pseudoobscura were polymorphic. They also showed that it was likely that they could not detect all the variation that was present.

Upon finding this level of variation, the question became — was this maintained by natural selection, or simply the result of genetic drift? This level of variation was too high to be explained by balancing selection.

Motoo Kimura theorized that most variation found in populations was selectively equivalent (neutral). Multiple alleles at a locus differed in sequence, but their fitnesses were the same. Kimura’s neutral theory described rates of evolution and levels of polymorphism solely in terms of mutation and genetic drift.

The neutral theory did not deny that natural selection acted on natural populations; but it claimed that the majority of natural variation was transient polymorphisms of neutral alleles. Selection did not act frequently or strongly enough to influence rates of evolution or levels of polymorphism.

Initially, a wide variety of observations seemed to be consistent with the neutral theory. Eventually, however, several lines of evidence toppled it. There is less variation in natural populations than the neutral theory predicts. Also, there is too much variance in rates of substitutions in different lineages to be explained by mutation and drift alone.

Finally, selection itself has been shown to have an impact on levels of nucleotide variation. Currently, there is no comprehensive mathematical theory of evolution that accurately predicts rates of evolution and levels of heterozygosity in natural populations.

Essay # 3. Macroevolution Mechanisms in Evolutionary Theory:

The following deals with mechanisms of evolution above the species level:

i. Speciation:

Speciation is the process of a single species becoming two or more species. Many biologists think speciation is key to understanding evolution. Some would argue that certain evolutionary phenomena apply only at speciation and macroevolutionary change cannot occur without speciation.

Other biologists think major evolutionary change can occur without speciation. Changes between lineages are only an extension of the changes within each lineage. In general, paleontologists fall into the former category and geneticists in the latter.

ii. Modes of Speciation:

Biologists recognize two types of speciation- allopatric and sympatric speciation. The two differ in geographical distribution of the populations in question. Allopatric speciation is thought to be the most common form of speciation.

It occurs when a population is split into two (or more) geographically isolated subdivisions that organisms cannot bridge. Eventually, the two populations’ gene pools change independently until they could not interbreed even if they were brought back together. In other words, they have speciated.

Sympatric speciation occurs when two subpopulations become reproductively isolated without first becoming geographically isolated. Insects that live on a single host plant provide a model for sympatric speciation. If a group of insects switched host plants they would not breed with other members of their species still living on their former host plant.

The two subpopulations could diverge and speciate. Agricultural records show that a strain of the apple maggot fly Rhagolettis pomenella began infesting apples in the 1860’s. Formerly it had only infested hawthorn fruit. Feder, Chilcote and Bush have shown that two races of Rhagolettis pomenella have become behaviourally isolated.

Allele frequencies at six loci (aconitase 2, malic enzyme, mannose phosphate isomerase, aspartate amino­transferase, NADH-diaphorase-2, and beta-hydroxy acid dehydrogenase) are diverging.

Significant amounts of linkage disequilibrium have been found at these loci, indicating that they may all be hitchhiking on some allele under selection. Some biologists call sympatric speciation micro-allopatric speciation to emphasize that the subpopulations are still physically separate at an ecological level.

Biologists know little about the genetic mechanisms of speciation. Some think a series of small changes in each subdivision gradually lead to speciation. The founder effect could set the stage for relatively rapid speciation, a genetic revolution in Ernst Mayr’s terms. Alan Templeton hypothesized that a few key genes could change and confer reproductive isolation.

He called this a genetic transilience. Lynn Margulis thinks most speciation events are caused by changes in internal symbionts. Populations of organisms are very complicated. It is likely that there are many ways speciation can occur. Thus, all of the above ideas may be correct, each in different circumstances. Darwin’s book was titled “The Origin of Species” despite the fact that he did not really address this question; over one hundred and fifty years later, how species originate is still largely a mystery.

iii. Speciations:

Speciation has been observed. In the plant genus Tragopogon, two new species have evolved within the past 50-60 years. They are T. mirus and T. miscellus. The new species were formed when one diploid species fertilized a different diploid species and produced a tetraploid offspring. This tetraploid offspring could not fertilize or be fertilized by either of its two parent species types. It is reproductively isolated, the definition of a species.

Essay # 4. Extinction of Species and Evolutionary Theory:

i. Ordinary Extinction:

Extinction is the ultimate fate of all species. The reasons for extinction are numerous. A species can be competitively excluded by a closely related species, the habitat a species lives in can disappear and/or the organisms that the species exploits could come up with an unbeatable defense.

Some species enjoy a long tenure on the planet while others are short- lived. Some biologists believe species are programmed to go extinct in a manner analogous to organisms being destined to die. The majority, however, believe that if the environment stays fairly constant, a well-adapted species could continue to survive indefinitely.

ii. Mass Extinction:

Mass extinctions shape the overall pattern of macroevolution. If you view evolution as a branching tree, it’s best to picture it as one that has been severely pruned a few times in its life. The history of life on this earth includes many episodes of mass extinction in which many groups of organisms were wiped off the face of the planet.

Mass extinctions are followed by periods of radiation where new species evolve to fill the empty niches left behind. It is probable that surviving a mass extinction is largely a function of luck. Thus, contingency plays a large role in patterns of macroevolution.

The largest mass extinction came at the end of the Permian, about 250 million years ago. This coincides with the formation of Pangaea II, when all the world’s continents were brought together by plate tectonics. A worldwide drop in sea level also occurred at this time.

The most well-known extinction occurred at the boundary between the Cretaceous and Tertiary Periods. This called the K/T Boundary and is dated at around 65 million years ago. This extinction eradicated the dinosaurs. The K/T event was probably caused by environmental disruption brought on by a large impact of an asteroid with the earth.

Following this extinction the mammalian radiation occurred. Mammals coexisted for a long time with the dinosaurs but were confined mostly to nocturnal insectivore niches. With the eradication of the dinosaurs, mammals radiated to fill the vacant niches. Currently, human alteration of the ecosphere is causing a global mass extinction.

iii. Punctuated Equilibrium:

The theory of punctuated equilibrium is an inference about the process of macroevolution from the pattern of species documented in the fossil record. In the fossil record, transition from one species to another is usually abrupt in most geographic locales — no transitional forms are found.

In short, it appears that species remain unchanged for long stretches of time and then are quickly replaced by new species. However, if wide ranges are searched, transitional forms that bridge the gap between the two species are sometimes found in small, localized areas. For example, in Jurassic brachiopods of the genus Kutchithyris, K. acutiplicata appears below another species, K. euryptycha.

Both species were common and covered a wide geographical area. They differ enough that some have argued they should be in a different genera. In just one small locality an approximately 1.25m sedimentary layer with these fossils is found. In the narrow (10 cm) layer that separates the two species, both species are found along with transitional forms. In other localities there is a sharp transition.

Eldredge and Gould proposed that most major morphological change occurs (relatively) quickly in small peripheral population at the time of speciation. New forms will then invade the range of their ancestral species. Thus, at most locations that fossils are found, transition from one species to another will be abrupt. This abrupt change will reflect replacement by migration however, not evolution. In order to find the transitional fossils, the area of speciation must be found.

There has been considerable confusion about the theory. Some popular accounts give the impression that abrupt changes in the fossil record are due to blindingly fast evolution; this is not a part of the theory.

Punctuated equilibrium has been presented as a hierarchical theory of evolution. Proponents of punctuated equilibrium see speciation as analogous to mutation and the replacement of one species by another as analogous to natural selection. This is called species selection.

Speciation adds new species to the species pool just as mutation adds new alleles to the gene pool. Species selection favors one species over another just as natural selection can favor one allele over another. Evolutionary trends within a group would be the result of selection among species, not natural selection acting within species.

This is the most controversial part of the theory. Many biologists agree with the pattern of macroevolution these paleontologists posit, but believe species selection is not even theoretically likely to occur.

Critics would argue that species selection is not analogous to natural selection and therefore evolution is not hierarchical. Also, the number of species produced over time is far less than the amount of different alleles that enter gene pools over time. So, the amount of adaptive evolution produced by species selection (if it did occur) would have to be orders of magnitude less than adaptive evolution within populations by natural selection.

Tests of punctuated equilibrium have been equivocal. It has been known for a long time that rates of evolution vary over time that is not controversial. However, phylogenetic studies conflict as to whether there is a clear association between speciation and morphological change.

In addition, there are major polymorphisms within some species. For example, bluegill sunfish have two male morphs. One is a large, long-lived, mate-protecting male; the other is a smaller, shorter- lived male who sneaks matings from females guarded by large males. The existence of within species polymorphisms demonstrates that speciation is not a requirement for major morphological change.