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A Look Into Natural Selection and its Mechanisms

Charles Darwin is credited with outlining the fundamentals of evolution. He was a smart and eager pupil and protégé, and eventually his interests brought him to the HMS Beagle, a ship employed by England to map the South American coastline. He had the opportunity to study the wildlife on the coasts and neighboring islands, and collected various samples to document. As he reviewed his findings after the five year voyage, he noticed that the species that were documented on the Galapagos Islands were in strange ways similar to the mainland variety; this observation was most prevalent in the bird species (finches). This was strange to him because species in similar climates and terrains elsewhere were different. He determined that they must have migrated from the mainland and therefore become isolated (allopatric speciation). Though they were similar, the birds' beak varieties were different. Darwin eventually concluded that this "design" was helpful for the birds' survival, and therefore it was a trait that was easy to pass on, because the birds' chances of surviving to reproduce increased. Through these conclusions, he helped discover evolution and natural selection.

Evolution is the accumulation of inherited traits within a population over a given time. A species evolves due to pressure from evolution's mechanism: natural selection. Natural selection is when a species with a certain trait either succeeds or fails at reproducing. If the animal succeeds in reproducing, it passes the trait on, which means it is favorable. These favorable traits govern the accumulation of traits in the species' following generations. If the organism has changed significantly from its predecessors, or gone through speciation, it is considered to have undergone evolution. There are many reasons why this can occur.  

The following methods of genetic diversity fall under the category of microevolution, which is change in lower taxons.

Divergence increases genetic variety. There are two developmental ideas when it comes to looking at divergence and how it occurs. They are punctuated equilibrium and gradualism. Punctuated equilibrium suggests that evolution occurs in rapid spurts when there is great evolutionary stress. These spurts are separated by stasis, or when the species is stable. The more commonly embraced idea is gradualism, which suggests evolution slowly branches apart. Fossil records are a main component in the support of punctuated equilibrium, because the record is incomplete (or at least thought to be), and that seemingly illustrates that species did not have transitional states (transitional in the sense that there were forms in between the fossil and the present day species). Gradualists easily argue, however, that if more fossils were added to the records, gradual transitional species would be uncovered, and the punctuated equilibrium theory debunked. Divergence is when a previously existing species migrates or becomes separated from the main gene pool. The previous species continues functioning as it would, whereas the separate group's alleles are now exclusive. The separated species is more susceptible to change, because its gene pool is working off only a few species (this is known as the founder effect). This is also referred to as allopatric speciation, and almost all of the evolution of new species is exclusive to allopatric speciation. Adversely, genetic drift is significantly less likely to affect a small population because chance more readily rids the new allele's appearance. Perhaps a mutation is in about 2% of a population. A larger population of about 10,000 has a higher chance of passing it on than the diverged population of 100. In the case where a mutation gives rise to a whole new species in the same geographic location, and it is successful, it is referred to as sympatric speciation. It is a specific case of disruptive selection.

Working against genetic variety are inbreeding and natural selection. Populations that maintain a relatively small geographic location and do not migrate are susceptible to inbreeding. The dominant traits take over, and lower the genetic variation (the recessive traits). However, natural selection can also play a role here, too. Natural selection eliminates unfit traits, and therefore decreases genetic variation. It affects polygenic traits (traits where several genes affect the phenotype) in one of three ways. Stabilizing selection is when natural selection kills off extreme phenotypes. Directional selection is when one, more uncommon phenotype is only slightly more successful than the average type. There is also disruptive selection, which occurs when both of the extreme phenotypes become more successful than the average type. Most of the time, though, the average traits are fit (which are mostly the dominant ones). However, there are cases where the recessive gene is favored, such as heterozygous sickle cell anemia. In malaria prevalent areas, such as Africa, natural selection promotes the heterozygous sickle cell anemia trait, because though sickle cells are prone to causing clots, they are resistant to malaria, which infects the red blood cells. However, in heterozygous cases, very few of the cells are sickled. Therefore, the carrier is resistant to malaria and does not die due to sickle cell anemia. This instance and others like it help maintain genetic variation.

Often in biology, there are discrepancies in determining whether or not a species has diverged. So first the question that arises: what does the term "species" mean? More often than not, scientists agree they are closely related organisms (same gene pool), and that they breed with each other under natural circumstances. This unifying concept is known as the biological species concept. There are also isolation barriers that help draw the line in determining what separate species are.  

There are a few different types of isolation, which are divided into prezygotic and postzygotic occurrences: Temporal isolation is when species are not time compatible, whether it be when they are sexually active or when they bloom. Habitat isolation occurs when species can't interact because they occupy different geographic locations. Sexual isolation is when animals don't receive, interpret, or understand courtship behavior/ sexual messages. Mechanical isolation is when animals can't physically mate. In the very rare case that these barriers are overcome, a hybrid is produced, which will then have extreme difficulty reproducing (or being born) due to the postzygotic occurrences. Hybrid inviability is the most common, which is when a problem develops in the embryonic stage, and the hybrid dies. If the hybrid is successfully born, then the hybrid will have difficulty mating. It may be sexually isolated from either its mother's species or its father's species. Bu the more common issue is that it is sterile. During meiosis, synapsis cannot occur properly, and the hybrid will have a monosomy or trisomy. In other words, the pairing of its chromosomes will not occur properly. If two hybrids manage to reproduce, in an F2 generation, hybrid breakdown is likely to occur. This is when the F2 hybrid cannot reproduce because it is so unstable.

Macroevolution is when large scale phenotypical changes occur in organisms that warrant their placement in new, higher taxons. Methods of this occurrence are preadaptations, developmental regulatory mistakes, and traumatic events. Preadaptation is when pre-existing structures evolve to fulfill other roles. Feathers are a preadaptation for flight, as they were originally an evolution of scales designed for thermoregulation. With gradual modifications, preadaptations can fulfill their old role and take up the new adaptation. Feathers are now used in flight and thermoregulation. Another method of inducing macroevolution is a mistake in growth; specifically, allometric growth is the cause. Regulatory genes exert control over many other genes, and if there is even a small mistake made by these regulatory genes, massive physical changes can occur. Traumatic events or extinctions open up a severe divergence opportunity known as adaptive radiation. It is when an ancestral species diversifies quickly into many different varieties. This is because, due to the lack of other community members, it can take advantage of resources previously unavailable to ancestors, known as adaptive zones. Extinction is constantly occurring, as it is the ultimate fate of all species, but it is divided into background extinction (the extinctions actively happening) and mass extinctions (extinction to many different species due to a traumatic event).

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An article contributed by Plasmodesmata11 on February 8, 2009 for Biology-Online.org.