Re: Fwd: Re: interbreeding

[wrisch@mendel.berkeley.edu (lisa wrischnik)]

Hi all,
In regards to different aggie populations interbreeding, Bob said:

>I remember from my killie days that there are several species of Aphyosemion
>whose various populations and subspecies were occasionally incompatible as
>far as fertility, while other species in the genus were crossable with
>similar species.  Go figure.  Over time, isolated populations can develop
>differences in chromosome count, or differences in gene arrangement on the
>chromosomes.  How this happens, I don''t have a clue.  Maybe Dr. Uwe can
>handle the technical part of this answer.

Ed responded with:
>Some one on an earlier post mentioned that this is where the definition
>of "species" comes into play.  The definition of species has appeared to
>be a lot less clear-cut than my days in high school biology (long ago)
>when it was than members of the same species can produce fertile
>off-spring.  In recent years, I have heard that members of different
>species can conceivable produce fertile offspring but maybe not carry
>the fertility to the 2nd or 3rd generation (don't remember where I heard
>this).  Anyway--does anybody know what is the currently accepted
>definition of species?

Sorry Ed, there really isn't a definitive answer here. I'm certainly no
expert, but I can tell you what I know.

What you are referring to is called the 'biological species concept', which
pretty much defines a species as a group of actually or potentially
interbreeding individuals. Note
the "potentially" here - it allows for separate populations that might not
ever come into contact to be considered the same species. The problems are,
of course, things like cross-breeding, so where do you draw the line? Some
people define species strictly on the basis of morphology (ie. a type
specimen), but this ignores natural variation in a population and has its
own problems. There are several 'schools' with their own way of defining a
species.

So, species are often not clear-cut entities -
like the above killie examples. Look at what Sven Kullander has done in
splitting up the Cichlasomines - different taxonomists can classify
organisms in different ways depending on the
morphological/behavioral/ecological critera they use to do so or what
"camp" they fall into. So defining a species is somewhat at the whim of the
person doing it.  It is a very tricky situation sometimes.

Bob was fairly correct on his take on speciation, but it certainly doesn't
have to require the kinds of drastic chromosomal rearrangements he
mentioned (although it can - many plants speciate by changes in chromosome
no.). Accumulation of point mutations or small deletions/insertions within
the DNA will do, as long as it keeps the two populations reproductively
isolated (
mechanically, behaviorally, whatever). Mutation is occurring all the time,
from cosmic radiation, mistakes in replication (your DNA replication and
repair mechanisms are certainly NOT foolproof. Skin cancer is one example
where mutaions in a skin cell cause that cell to replicate indefinitely
into a tumor), chemical mutagens (again, not only artificial, but present
in high concentrations in 'organic' food - plants have to protect
themselves somehow), among other things.

Anyway.......
I guess the gist of this is that populations of aggies could be speciating
or not, depending on gene flow between the various pops. It's certainly
possible  that while aggies that are 100s of miles apart are not directly
interbreeding, they are still sharing genetic information throught the
intervening aggie populaitons (sort of a bucket brigade, if you will). I
could imagine that
there are local differences in mate choice that affect what color male a
female aggie prefers, and this could slow down hybridization between color
morphs without precluding it. Will specific aggie color morphs only
interbreed with certain related colors or what? Has anyone seen
interbreeding between the different "locations" of
Pelvicachromis taeniatus, as another example? I bet most of us try so hard
to keep our similar apisto species (and other fish) separated that we don't
see much of this.

Well, I better stop before everyone hates me.....
Lisa