Sympatric Speciation

1. In African Lakes (Lake Victoria, Lake Malawi, and Lake Tanganyika), hundreds of cichlid species have evolved from common ancestors. Despite living in the same geographic area, these fish have diversified based on their feeding and mating preferences. Similarly, the same species have also diversified in the Crater Lake of Nicaragua. 
2. Apple Maggot Fly (Rhagoletis pomonella) Originally, these flies laid their eggs on hawthorn trees, but some began to use domesticated apples when they were introduced in North America. The two groups are now evolving separately due to different host preferences, as well as differences in timing for laying eggs and developing.
3. Two species of palm trees, Howea forsteriana and Howea belmoreana, have diversified within the same Lord Howe Island, Australia, due to their adaptation to different soil types – calcareous and volcanic soils. Differences in flowering times also contribute to reproductive isolation, leading to speciation.

Sympatric speciation requires reproductive isolation without any physical barrier. Such divergence can occur due to disruptive selection, a process of natural selection that drives natural selection in two different directions. If the selection pressure is strong enough, it will cause the original population to divide into two groups, each with different requirements for their survival. The hybrids of the two subpopulations have reduced fitness for either of the conditions and thus cannot survive. 

There can be a number of factors, such as aneuploidy, polyploidy, sexual selection, and habitat preferences, that cause disruptive selection, leading to sympatric speciation.

Aneuploidy and Polyploidy

Here, individuals are born with an abnormal number of chromosomes due to an error in cell division. Such an individual cannot mate with a normal member of the parent population. They can only interbreed with those individuals who have mutated and have the same abnormal number of chromosomes.

Modern bread wheat (Triticum aestivum), which we commonly use today, is a hexaploid species (having six sets of chromosomes) that resulted from multiple hybridization and polyploidization events involving different wheat species.

Sexual Selection

Sometimes, a species has mating preferences that lead to changes in some traits of a population. This can lead to disruptive selection casing divergence of the parent population. 

Cichlid Fish in Lake Victoria is an example of sexual selection leading to sympatric speciation.

Habitat Preferences

Habitat preferences often lead to sympatric speciation when different groups within a population develop preferences for distinct habitats within the same geographical area. Over time, this separation based on habitat use can lead to reduced gene flow between groups and eventually result in the formation of new species. 

Again, the speciation of Cichlid Fish in Lake Victoria is such an example.

It is difficult to reason how the incipient species can interbreed in spite of genetic divergence. In sympatric speciation, a new species arises within the parent population that shares the same geographic area, which means individuals can still interbreed. This recombination disrupts the correlation between co-adapted groups of genes, which is essential for speciation. Also, this constant gene flow is generally against the law of speciation, as it should make it harder for incipient groups to diverge by forming reproductive barriers.

Strong disruptive selection is needed for speciation, which means individuals with extreme traits should have a fitness advantage over those with intermediate traits. In the absence of strong selection pressure, the genetic flow will close the gap between the diverging groups and stop speciation.

Additionally, behavioral differences (mating preferences) and ecological isolation are needed for successful reproductive isolation to occur, which is difficult to achieve in sympatric speciation.

Finally, even if ecological isolation is achieved, it has to translate into reproductive barriers for sympatric speciation to occur.