After two species have been reproductively isolated, they may sometimes come bac
ID: 97524 • Letter: A
Question
After two species have been reproductively isolated, they may sometimes come back into contact and interbreed. Their offspring are called hybrids and the area in which hybrids occur is called the hybrid zone (see example for tree frogs below). Knowing what you do about natural selection, what would you predict would happen if hybrids had tower fitness than either parent species (a process called reinforcement)? Alternatively, what would you predict would happen if hybrids had higher fitness than either parent species?Explanation / Answer
(11) If the hybrids had lower fitness than either parental form, selection would act to increase the reproductive isolation because each form would do better not to mate with the other and form the disadvantageous hybrids. Speciation might then be speeded up by favoring genes which caused individuals to avoid mating with hybrids. Reinforcement is the process by which natural selection increases reproductive isolation. Reinforcement can occur as follows:
-When two populations which have been kept apart, come back into contact, the reproductive isolation between them might be complete or incomplete.
-If it is complete, speciation has occurred.
-If it is incomplete, hybrids would be produced.
Reinforcement is a necessary requirement for both the parapatric and sympatric theories of speciation: it is the process by which a hybrid zone develops into a full species barrier. Reinforcement can be simulated by artificial selection experiments. By continually selecting for assortative mating it has been possible to obtain significant changes in prezygotic isolation mechanisms. However, the theoretical conditions for speciation to take place by reinforcement are difficult and it is controversial whether the process takes place in nature. Consider a species that is geographically distributed in a straight line from east to west across America: it is possible that the forms in the east and west are so different that they could not interbreed. Now imagine taking the line and bending it into a circle, such that the end points (formerly in the east and west) come to overlap in space. If they do not interbreed then the geographic distribution of the species will be in the shape of a ring, and they will be 'ring species': the extreme forms do not interbreed in the region of overlap. A ring species has an almost continuous set of intermediates between two distinct species, and these intermediates happen to be arranged in a ring. At most points in the ring, there is only one species; but there are two where the end-points meet.
(12) Hybrid offspring should outcompete parental populations at least in the areas just outside the original hybrid zone, resulting in its expansion. It should expand into the ranges of both parental populations. So reproductive isolation between the species completely dissolves. In this type of hybridization, we need to go back to our BSC roots. We conclude that the parent species were actually not separate species at all, because there is no restriction to gene flow between them. It is possible to get reproductive isolation between hybrids and parent, though. This occurs especially among plants, which are often polyploidy a trait usually fatal to animals. If ploidy levels of hybrids are different than parents, reproductive isolation can follow. Ploidy can do very strange things, even beyond hybrid speciation. One example of ploidy at work for the common good is the seedless watermelon.