Speciation and Modes of Speciation

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.

Parapatric speciation involves the partial separation of geography between species. There is cross-breeding but reduced fitness of the heterozygous organism that prevents interbreeding. This speciation occurs because of differences in ecological niches in nearby populations of organisms.

Sympatric speciation involves a single ancestor in the same geographic location that forms different descendent species. It can happen when insects become dependent on different types of host plants in the same geographical area. Ecological factors also play a role in this type of speciation.

The Wallace effect is also referred to as reinforcement. This happens when natural selection reinforces or increases the reproductive isolation of a species. It is sometimes seen when separated populations come together again. If they haven t developed into completely separate species, the organisms can interbreed to produce hybrids that might not be fertile themselves. If they are infertile or subfertile, the reproductive isolation will increase and species formation will happen. This is what happens in the making of mules from horses and donkeys. If the hybrids are more fit, species separation will not occur.

Ecological selection involves the interaction between the organisms and their environment, particularly when it comes to acquiring resources. Ecological niches play a role in all forms of speciation because they exert different pressures on the organisms living in them.

Can new species be formed artificially? It has been used in many instances of animal husbandry. In some cases, the wild animal that led to the domesticated animal can interbreed with each other. It has been done in the laboratory with fruit flies, which were gradually differentiated into separate species through differences in habitat selection. They can also be isolated by differences in nutrient preferences. Anytime a different ecological pressure is exerted on an organism, the tendency toward speciation occurs.

Speciation is possible if polyploidy happens. If there is failure of meiosis to make diploid gametes, the end result is an offspring that has more genes in it than either parent. It causes rapid speciation, even though it sometimes yields sterile offspring. This occurs particularly in plants but it has probably occurred in animals over the course of evolutionary history. Some of these polyploid offspring are able to reproduce asexually.

Sometimes, hybridization between two distinct species causes a new phenotype to appear. If it is fitter than the parents, this phenotype will be selected for. It will become a new species if reproductive isolation happens as well. Because reproductive isolation is difficult to achieve,