| At some finite time in the past, life began somehow. (How it began is beyond the scope of the theory, but the observational evidence strongly suggests that only one such beginning on Earth has left descendants to the present day.) As life reproduces and multiplies, mutations occur with small but finite probabilities, causing new genes to be added, and creating new alleles of existing genes. The different alleles confer different traits upon their owners, rendering them more or less successful in coping with their environments. The organisms that are more successful in coping with their environments consequently have a slightly greater probability of passing their genes to the next generation of organisms than do the less successful organisms. This causes allele frequencies to change over time. Because mutations are random according to their probabilities, there is essentially a zero probability that two non-interbreeding populations will get the same set of mutations. (Even if they somehow do, there is essentially a zero probability that the frequencies of the alleles will end up the same in both populations.) The alleles and new genes available in each population will therefore diverge, with the result that the populations become genetically more distant from each other over time. Eventually, the two populations will become genetically so distant that they lose the ability to produce viable hybrids between them. This is the cause of the origin of species.
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