SPECIATION AND MODES OF SPECIATION Speciation is defined as the evolutionary processes that take place in order to lead to the development of a new species. Charles Darwin said that natural selection was the major cause of speciation and that sexual selection was the main mechanism for this. Later terms related to this were cladogenesis, which is the splitting of lineages into two more descendent lineages, and anagenesis, which is phyletic evolution within a given lineage of organisms. Darwin himself asked why species exist at all. He wondered why, because there are fine distinctions between related organisms, there aren t an infinite number of transitional forms of an organism. Why aren t species easily defined, he asked. He also wondered why there weren t more transitional species occurring over time. He determined that there was something about natural selection that both generated and maintained the different species. One of the issues related to this is the fact that, if there was out-crossing sexual reproduction, which is interbreeding between species, the cost would be that there would be many different species, each of which would be rare and have few members. It would be too difficult to find a mate. Species with higher numbers would increase in number because they would have more sexual mates, while species with lower numbers would be driven quickly to extinction. In addition to reproductive costs, rarity of an organism is costly because their rare features are generally not an advantage to the organism. This leads to species members avoiding mates that have rare features—a phenomenon called koinophilia. This further leads to the reproductive isolation and uniformity of the species. There are four different modes of speciation that each have taken place in the evolutionary process. These are allopatric, parapatric, peripatric, and sympatric modes of speciation. With allopatric speciation, species are created by things like habitat fragmentation that alter the geography of a species. There are dissimilar selective pressures to the different groups, antigenic drift, and the presence of different mutations. Should they come into contact again, the differences that have occurred make them reproductively isolated from one another. This is what happens with island evolution, where organisms are isolated on a small island. Peripatric speciation, a population gets isolated and forms its own species. This is a subtype of allopatric speciation and is related to the founder effect, in which just a few organisms found” a new population. Genetic drift plays a major role in this phenomenon.
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