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
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
How Finches Explained Tortoises to Darwin
How the Galapagos Islands Saved Giant Tortoises From All But Humans
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