Peter Shute wrote:
wrote on Wednesday, 7 February 2007
5:08 PM:
The divergence in mating signals
found between these two taxa seems to have occurred recently and
relatively rapidly.
Forgive my ignorance, but how do they know that?
If it's anything like the grasshoppers mentioned in Richard
Dawkins' book "The Ancestor's Tale", the difference in mating
signals is probably most distinct where the two taxa
overlap(?) than at the extremes of their ranges. Once the
speciation event occurs (for whatever reason), there is
positive selective pressure to drive the two species apart
(different calls, colours, behaviours or
whatever) to avoid unproductive interbreeding.
Yes, but how can they tell it happened recently and rapidly?
As another poster said; by the DNA analysis. Analysis of the DNA from
numerous species of birds and other animals has given estimates of the
rate of change of those sequences based upon other information, usually
the paleontological record. Then similar comparison between the DNA
sequences of the two species give some estimate of how long ago they
were a single species (ie had the same sequence for the regions of DNA
being used in the study).
It is then assumed that all the divergence in mating signals occurred
since the divergence of the two species.
Most of these analyses seem to be based on mitochondrial sequences about
which I have major reservations. There are also other problems with
this type of analysis which mean that it is not as accurate as some
people claim, but it does give a general idea.
Cheers
Andrew
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