Re: Species diversity through time

On Thu, 15 Nov 2007 05:57:38 -0800, John Harshman
<jharshman.diespamdie@xxxxxxxxxxx> wrote:

Robert Maas, see http://tinyurl.com/uh3t wrote:

From: John Harshman <jharshman.diespam...@xxxxxxxxxxx>
Speciation is the evolution of reproductive isolation between
populations. Generally this happens when two populations become
geographically isolated from each other.


You seem to be totally dismissing the proposed mechanism of the
other poster, in which case I respectfully disagree.

Which proposed mechanism of which other poster? You really need to
retain some context if you want to be understood.

Consider for
example this hypothetical example: A mating pair of finches wanders
to an island that is devoid of birds. It isn't adapted to either of
the two flower types on the island, but there are so many flowers
and so few birds that even poorly adapted birds can get enough
nectar to survive and flourish and exponentially grow in
population. Over time, some individuals become better adapted to
getting nectar from one type of flower, while some other
individuals become better adapted to getting nectar from the other
type of flower. Matings between two individuals with the same
specialized adaption generally yield offspring with that same
adaption, which can get nectar very easily and raise large
families, whereas cross-breeds tend to be adapted to neither and
while they can get nectar from either type of flawer they can't do
either well so they tend to have smaller families. Thus there's a
partial reproductive barrier between the two adaptions in regard to
fitness of offspring to rear grandchildren of the original
mismatched pair. Over time pre-mating barriers arise to make the
whole process more efficient (there's selection pressure to not
mate at all with mismatched partners, thereby avoiding wasting your
energy rearing offspring that will be less fit, "stick to your own kind, Maria").

What you have just described is called disruptive selection. While it's
certainly conceivable, mathematical modeling suggests that the
conditions under which this can cause actual reproductive isolation are
rare, perhaps nonexistent in nature. Nor do we appear to have any good
examples of speciation actually resulting from such a thing.

It does maintain balanced polymorphism, though, no? I am asking for
real, I don't know the results of modeling or observations, only
trying to verify what I used to say in class.

.

Relevant Pages

  • Re: Species diversity through time
    ... Speciation is the evolution of reproductive isolation between ... Generally this happens when two populations become ... getting nectar from one type of flower, ...
    (talk.origins)
  • Re: Species diversity through time
    ... Speciation is the evolution of reproductive isolation between ... Generally this happens when two populations become ... the two flower types on the island, but there are so many flowers ... getting nectar from one type of flower, ...
    (talk.origins)
  • Re: Species diversity through time
    ... Speciation is the evolution of reproductive isolation between ... Generally this happens when two populations become ... getting nectar from one type of flower, ...
    (talk.origins)
  • Re: Species diversity through time
    ... groups with high speciation rates also have high ... I'm starting with the assumptions that populations ... tend to adapt to their environments, ... precisely because it would lead to reproductive isolation. ...
    (talk.origins)
  • Re: Species diversity through time
    ... groups with high speciation rates also have high ... I'm starting with the assumptions that populations ... tend to adapt to their environments, ... precisely because it would lead to reproductive isolation. ...
    (talk.origins)