Saturday, March 27, 2010
Gene Pools
Jerry Coyne points out that members of the same species have a common gene pool. Individuals of the same gene pool are able to “mate with each other and produce offspring that contain combinations of their genes” (172). Physical and genetic characteristics of DNA vary greatly across the diverse kingdoms of life; for example, our (humans’) linear DNA does not even have the same shape as the circular DNA found in bacteria. When the DNA of two individuals differ too greatly to combine and create fertile offspring, the two individuals can be deemed as members of different species. What are some other examples of physical or genetic differences in DNA among different species, and why might the differences prevent individuals from creating fertile offspring?
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A physical difference in DNA among different species could the number or pairing of chromosomes, that the DNA is contained in, for the species. For example, humans have 23 diploid pairs of chromosomes while chickens have 39 diploid pairs of chromosomes. One set of the pair of chromosomes comes from the mother, and one set comes from the father. We know that even a small mutation in the human chromosome count can cause serious defects in humans; when there are serious difference between the two sets of chromosomes from the two parents, the result is an offspring that is just not viable. During meiosis, the chromosomes will not be able to pair correctly, and unequal cell division will result. The number of chromosomes is just another "barrier ... that [prevents] interbreeding" (172).
ReplyDeleteAlso, Mayr defined species as "reproductively isolated" such that different species have "differences in appearance, behavior, or physiology - that prevents them from successfully interbreeding" (172).
I agree with Frank's idea of different number of chromosomes will inhibit the likelihood of mating; however, the idea of allopolyploidy, an organism created from two different species, must also be taken note. Theoretically, allopolyploidy is not "natural" because it is not possible without some genetic engineering and other factors, but it must be taken note. Example of a species that should not exist because it is a product of two different species is the mule or different types of dogs. A barrier to reproduction between two species, creating a viable offspring is the issue of DNA transfer. Without genetic engineering, there is no way for a human gamete/cell to transfer its DNA into the bacteria via any possible routes.
ReplyDeleteFrank is definitely correct in pointing out the different amount of chromosomes in different species. Andrew also makes a good point about the unnatural concept of allopolyploidy.
ReplyDeleteHowever, both of these ideas are focused on animals. We should not forget that plants undergo evolution as well. Plants have some mechanisms to prevent cross-pollination. For example, Coyne discusses the speciation of plants, and describes a geographic barrier separating two portions of an ancestral species of plants. After a while, the two portions will develop different flowers and different types of nectar to attract the pollinators in that area. When the two different portions are returned to the same area, the species cannot cross-pollinate because the two types of plants have developed different methods of pollination (p175-176). This divergent evolution is one process that results in a mechanism to prevent cross-pollination between species of plants. When plants have different types of DNA or have different types of pollination, the plants will not fertilize each other (Campbell 804-805).