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July 7, 2000

How do members of similar species avoid mating with one another?

Because hybrids are often infertile or have reduced reproductive fitness, animals that waste their time, energy and resources producing them put themselves at a significant reproductive disadvantage. In order to reproduce to the greatest extent, an individual must find, recognize, and breed only with members of its own species. It is only in this way that it can pass its genes to subsequent generations.

In the critically important matter of recognizing one's own species, vison, hearing, smell, and analysis of behavioral signals may all be used. These signals act both as a means of positive species identification as well as biological barriers to discourage crossing between distinct species. Such signals or cues are known as Îisolating mechanismsâ.

Butterflies which fly by day, recognize potential mates within their own species by distinctive colors and patterns, whereas, moths which fly by night rely more heavily on scent. For humpback whales, it is believed that it is their elaborate songs which used to select or attract a mate. Here in the Galapagos, the Waved Albatross has a courtship ritual that consits of a complex series of repeated and very stereotyped (and very visible and audible displays).

Because species change slightly all of the time, when one population of a species becomes isolated from another (say, by travelling to a different island)differences accumulate between the original and the island populations during the period they are living in isolation from each other. The reason this happens is because mutations are occuring at random in the two population and it is therefore incredibly rare that they change in the same way during the period of their separation. Later, should the descendants of these two population come together (perhaps by a group from the island returning to the original habitat) they will probably encounter descendants from the original population which their ancestors left. If the separation of these two populations has gone on for relatively few generations, they will probably not have changed enough to prevent them from mating and producing offspring. If, however, they have been apart much longer, they may have accumulated so many differences that they are incapable of mating and/or producing viable offspring any longer. At such time they are properly referred to as different species. It is in this way that one species becomes two, reproductively isolated, species.

Listen to Roger Payne's Voice from the Sea piece entitled:
How Finches and Tortoises Contributed to the Theory of Evolution
or
How Finches Explained Tortoises to Darwin
or
How the Galapagos Islands Saved Giant Tortoises From All But Humans

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