RE: Tiktaalik, Great Danes and speciation

[gerarus@westnet.com.au]

>as such be used to distinguish clades. Or in other words: ability to
>interbreed will never be a result of convergence.

     Some examples of where interbreeding is indeed the result of
convergence:

a) Polyploidy. When a polyploidy event occurs it can mean instant
genetic isolation from the parent species (while not so important in
vertebrates, polyploidy is quite an important factor in the evolution
of other organisms such as plants). However, if polyploidy events
happen more than once within a single species, they will give rise to
progeny that are capable of interbreeding with each other, but not
with members of the parent population.

b) Sticklebacks (and a few other fish) generally have two varieties
present in lakes, a deeper-bodied benthic form and a more slender
pelagic form. The two varieties generally maintain identity and don't
interbreed with each other. Genetic studies of sticklebacks from
different lakes around North America have shown that most lake
populations derive from single colonisations, which have then
differentiated sympatrically into the two varieties. When examples of
both forms from two lakes have been mixed experimentally, pelagic
forms from the different lakes will mate with each other, and benthic
forms with other benthics, rather than benthics mating with pelagics
from the same source lake, despite the latter being more closely
related in ancestry.

c) Isolation mechanisms are often stronger between closely-related
species than distant species. For instance in grasshoppers, where
different species perform different songs to attract mates, the same
(or very similar) song may be performed by two more-distantly related
species occupying different geographic ranges (no selection pressure
for distinction from taxa which aren't normally encountered). If an
irruption of one such species into the range of another occurs, there
is the possibility for genetic intermingling.

    Cheers,

        Christopher Taylor