Biology X | Heredity and Evolution | SPECIATION

SPECIATION

What we have seen so far is micro-evolution. That means that the changes are small, even though they are significant. Also, they simply change the common characteristics of a particular species. But this does not properly explain how new species come into existence. That can be said to have happened only if this group of beetles we are thinking about, splits into two populations that cannot reproduce with each other. When this happens, they can be called two independent species. So, can we extend the reasoning we have used above to explain such speciation?

Consider what would happen if the bushes the beetles feed on are spread widely over a mountain range. The beetle population becomes very large as a result. But individual beetles feed mostly on a few nearby bushes throughout their lifetime. They do not travel far. So, in this huge population of beetles, there will be sub-populations in neighbourhoods. Since male and female beetles have to meet for reproduction to happen, most reproduction will be within these sub-populations. Of course, an occasional adventurous beetle might go from one site to another. Or a beetle is picked up by a crow from one site and dropped in the other site

without being eaten. In either case, the migrant beetle will reproduce with the local population. This will result in the genes of the migrant beetle entering a new population. This kind of gene flow is bound to happen between populations that are partly, but not completely separated. If, however, between two such sub-populations a large river comes into existence, the two populations will be further isolated. The levels of gene flow between them will decrease even further.

Over generations, genetic drift will accumulate different changes in each sub-population. Also, natural selection may also operate differently in these different geographic locations. Thus, for example, in the territory of one sub-population, crows are eliminated by eagles. But this does not happen for the other sub-population, where crow numbers are very high. As a result, the green variation will not be selected at the first site, while it will be strongly selected at the second.

Together, the processes of genetic drift and natural selection will result in these two isolated sub-populations of beetles becoming more and more different from each other. Eventually, members of these two groups will be incapable of reproducing with each other even if they happen to meet.

There can be a number of ways by which this can happen. If the DNA changes are severe enough, such as a change in the number of chromosomes, eventually the germ cells of the two groups cannot fuse with each other. Or a new variation emerges in which green females will not mate with red males, but only with green males. This allows very strong natural selection for greenness. Now, if such a green female beetle meets a red male from the other group, her behaviour will ensure that there is no reproduction between them. Effectively, new species of beetles are being generated.

QUESTIONS

  1. What factors could lead to the rise of a new species?
  2. Will geographical isolation be a major factor in the speciation of a selfpollinating plant species? Why or why not?
  3. Will geographical isolation be a major factor in the speciation of an organism that reproduces asexually? Why or why not?
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