4. What are the two critical conditions for sympatric speciation?

Conditions for speciation by sympatric speciation are very stringent. Gene flow between

incipient species must be eliminated or severely curtailed, or reproductive isolation is not

possible. Whereas isolation by geographic distance as is seen in allopatric and parapatric

speciation can limit gene flow, arriving at sympatric speciation when individuals are found in the same geographic locale is a challenge. Some kind of assortative mating is required. Stating that gene flow is eleminated is not specific enough to answer this question, because elemination of gene flow occurs in all types of speciation. Specifically, assortative mating is the way in which gene flow is eleminated in sympatric speciation.

A key aspect of the theory of sympatric speciation involves the process of natural selection that favors the evolution of discrimination on the part of the female and also the divergence of male traits that females use for mate discrimination. Dobzhansky suggested that natural selection should favor the more finely tuned discrimination mechanisms of the female, in response to the reduced viability that results from hybridization. Thus, a key component of this theory is some form of disruptive selection that selects against hybrid phenotypes, which then favors evolution of mate discrimination mechanisms.

OR

You could also get marks if you explained multiple niche polymorphism in insects. One class of models maintains that there arise multiple-niche polymorphisms. Heterozygotes are inferior because extreme specialization in detoxifying capability is necessary to successful survive and reproduce on different species of plants.

Assortative mating among homozygotes is a key component of this theory.

In general, two such mutations are required, because it is difficult to imagine getting a single mutation that has a pleiotropic effect on detoxifying ability and host-plant preference. The two mutations should also arise on the same chromosome in close proximity to one another. This causes the behavior for host-plant specificity to be tightly correlated with the detoxifying genes. The linkage between the two loci forms a genetic correlation between the behavioral and physiological traits. An insect that ends up with the wrong allele for detoxifying matched up with the wrong preference is in trouble (recall the problem for stripped and spotted snakes). By having the mutations on the same chromosome, a super gene for the detoxifying ability and host-plant preference is produced. It is possible for the genome to become rearranged after the mutations arise. This would require a third chromosomal rearrangement mutation which makes it less likely.