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.