Fw: [leps-talk] Darwin Scores a Point in DNA Test of Fish

[Ron at]

I am sure many of you are already aware of this.   This is precisely my
working concept for many years - one reason I am big on subspecies.   There
is a world of difference between isolation and seperation in the
evolutional process. One or both may be working and we can't just put
everything into a tidy conceptual box and then run with that as our
predetermined paradigm.  Thus, the tweener forms should always be studied
and where the subjective opinion of a worker is that such entities exist as
recognizable populations they should be described as a subspecies - which
may or may not one day continue to evolve into a species (in the biological
concept).  I see the genetic makeup of "subspecies" as the most fertile
ground for the study of current evolutional shifts.  Of course, this has to
be in conjunction with all the other factors at work at the same time on
the organisms.

Ronald R. Gatrelle, president
The International Lepidoptera Survey
gatrelle at tils-ttr.org
http://www.tils-ttr.org


----- Original Message -----
From: "Ed Reinertsen" <ereinertsen at iprimus.com>
To: <TILS-leps-talk at yahoogroups.com>
Sent: Saturday, April 09, 2005 11:33 AM
Subject: [leps-talk] Darwin Scores a Point in DNA Test of Fish



Darwin Scores a Point in DNA Test of Fish

Fri Apr 8, 2005

Michael Schirber
LiveScience Staff Writer
LiveScience.com

A new study of tropical reef fish calls into question a modern theory
for how species develop and supports Darwin's original idea of ecology
as the driving force of speciation.

Natural selection by itself can cause an advantageous gene - say a
longer neck on a giraffe - to spread among interbreeding organisms.
But in order for evolution to explain the emergence of two distinct
species, some sort of "wedge" needs to exist that drives populations
apart.

Darwin thought that the distinct survival pressures in different
habitats were enough to cause new species to form. But in 1942, Ernst
Mayr developed a theory that populations had to be isolated
geographically - such as by a mountain range or a glacier - in order
to form separate gene pools, where random genetic mutations would
generate divergent species.

"People started to think that isolation is more important than natural
selection in creating species," said Luiz Rocha from the Smithsonian
Tropical Research Institute.

But Rocha and his colleagues have found that genetic differences in
wrasses, a type of fish living in coral reefs, are best explained by
the unique selection mechanisms of particular habitats, as opposed to
some kind of physical barrier.

Sea of possibilities

The ocean, which has no mountain ranges or rivers, would seem a hard
place to isolate populations.

"There are not many clear barriers in the ocean, and yet there are
plenty of distinct species," Rocha told LiveScience.

The wrasses, in particular, spend 30 days in a free-floating larval
stage, which allows them to spread over a large region. However, where
the Amazon River dumps into the Atlantic, the amount of sediment and
freshwater is too much for coral reefs to form and fish larvae can't
easily spread across this 1,200-mile gap.

Rocha's team decided to look at wrasses on both sides of this apparent
barrier.

"Since all of these species have similar abilities to disperse, I
expected to find genetic differences that corresponded to the barrier
in all of them, but, surprisingly, that wasn't the case," Rocha said.

The DNA of wrasses living in different habitats separated by just 200
miles were more distinct than the DNA from wrasses with similar
habitat-preference living more than 2,000 miles apart across the
Amazon barrier.

Some of the wrasse species are specialists - they do best in a
particular coral reef habitat. According to Rocha, the larvae of a
specialist will easily travel 200 miles and wind up in a different
habitat, but because of natural selection, "they will lose out to the
locals," he said.

There have been other studies with lizards and freshwater fish that
also support an ecological, or habitat-based, origin of species. But
Rocha said the idea is still not well accepted. He thinks, in the end,
a more comprehensive theory of speciation will be a mixture of both
geographic and ecological speciation.

Researchers at Harvard University, the University of Florida and the
University of Hawaii contributed to the study, which will be detailed
in the journal Proceedings of the Royal Society B.

Glossary

Natural selection Survival of the fittest - the trend in a population
toward traits/genes that provide a survival or reproductive advantage.

Species Group of interbreeding natural populations.

Speciation The production of new, reproductively isolated populations.

Ecological speciation Species form as certain populations adapt -
through natural selection - to a specific environment, or niche.

Geographic speciation Species from when populations are physically
isolated.


[Non-text portions of this message have been removed]



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