RE: Re: [RML] D.G inbreeding

Harro Hieronimus (Harro.Hieronimus at t-online.de)
Sat, 8 Feb 97 17:03 +0100

On Fri, 7 Feb 1997, George R. Trumbull IV wrote:

>> But, given all of the talk on
>> hybrids, I thought I'd mention that perhaps there are two genetically
>> different populations found sympatrically. Or maybe one of the scientists
>> can tell us that this is impossible? My feeling is it probably is, but I
>> thought we could bring it up.

Peter U. wrote:

>It is certainly not impossible, just unlikely. Stranger things have
>happened!

According to the presently most accepted definition of species it is
impossible that two different populations which don't hybridize occur
sympatrically. If this happens it are different species. However, I think
George wanted to construct another point which is nevertheless interesting:
May there be two phenotypically similar species in one habitat, thus found
sympatrically? Yes, maybe. Similar means not to distinguish from each
other by normal methods (our eyes). How could that happen?
If a part of a population is separated over a longer time it not necessarily
has to develop morphological differences but may develop genetic,
behavioural or other differences.
One important point hasn't been mentioned yet, the ecological niches. That's
also part of an an answer on the question why four species live in the same
habitat and don't hybridize while they do as soon as they are in
aquarium - they maybe live in different niches.
But back to the population which has been separated for some time. Let's think
a river has flown into a lake and by a natural reason (maybe earthquake) it now
takes another way. Then we have a population in the lake and one in the river.
Even if they are separated for 10,000s of years they not necessarily may have to
develop differences. However, they may. And let's think the two populations come
together again by another earthquake or so and live together in the river or lake
again.
There may be several reasons why these two former populations may now be
different species although - as a proposal, otherways it's easier to understand -
they look identically. Just to give a few of these reasons.
Genetic barriers. We have a natural gene flow in all populations. By chance
through this flow the genetic diffences may develop.
Behavioural barriers. The water in the river may have been turbid, the water
in the lake clear. There may have been no plants in the river but in the lake.
Maybe it has been necessary according to the stronger temperature changes
in the smaller water body of the river to spawn in the warmer evening times, while
the larger waterbody of the lake made the lake population still spawn in the
morning.
Maybe there has been different food, which the species are now specialised in and
thus live in different niches. and so on, and so on, and so on.
There are so many possible ways of species differentiation as you ever want.

Back to the point why the sympatric species which nearly never produce hybrids
(what surely would cause the vanishing of one or both species soon) do so in aquarium.
Under the very small conditions (and even a 1000 gallon tank is small in regard to
nature) of aquarium the fish are maybe under sexual pressure, maybe they can't find
their ecological niche, and so on.

Formerly the species definition was built up on the result if a species produces fertile
hybrids or not. This definitely is an ancient thesis which doesn't explain anything.
If two species hybridize they may be rather closely related but they not necessarily really
are. Let's give a classic example. All red colour bearing canary cage birds are results of
crossbreeds between canary birds from the Canaries (or yellow cage birds) and Latin
American birds which aren't closely related. Scheel for the killies did an outstanding
work trying to crossbreed all the species but the results:
If they crossbreed: Okay, they may be rather closely related.
If they not crossbreed: Okay, they may be not rather closely related. But there is
a chance as great as this hypothesis that they are very closely related and under
other circumstances would hybridize and you just made the mistake in the aquarium
setup when you tried breeding them.

Last word for Peter U.: Do you really seriously believe that M. eachamensis is more
closely related to M. s. australis than to M. s. splendida? I don't. I explained before, why,
but may do it again if anybody wants. Don't be a slave to socalled genetic results which
force you to bend e.g. the geological history around some corners, we also have other
methods, luckily.

Harro