This sounds like a stupid question. I have an interest in science generally but I'm no biologist. I've been reading up about evolution and species etc. But I can't quite understand how or why distinct species come into being, and why different species can't interbreed.
For example, a lion and a tiger evolved from a common ancestor. What causes the two distinct versions? Why can't they breed? Why is there nothing in between a lion and a tiger? Why isn't there a constant spectrum of variations of cat? Why isn't there a constant spectrum of variations of all living things?
It's something to with gene pools and unique sets of alleles, but could someone please explain this to me?
For example, a lion and a tiger evolved from a common ancestor. What causes the two distinct versions? Why can't they breed? Why is there nothing in between a lion and a tiger? Why isn't there a constant spectrum of variations of cat? Why isn't there a constant spectrum of variations of all living things?
It's something to with gene pools and unique sets of alleles, but could someone please explain this to me?
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Genes in a given population are co-adapted; that is, the genes reinforce each other to produce individuals that will leave behind a maximum number of offspring. When two populations first start to diverge from each other, crosses between the two can produce fertile offspring. But as divergence continues, the mixing of genes from two different populations produces offspring with genes that are not co-adapted. Consequently, these offspring are not as fertile as their ancestors were. The crosses that produce the most offspring are those between two individuals in the same population. Furthermore, there is selection in each population for individuals that do not interbreed with individuals in a different population. This selection gives rise to isolating mechanisms of different types--behavioral, temporal, chemical, and mechanical.
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"...By Means of Natural [and sexual] Selection" What's not to get?