Download Butterfly unlocks evolution secret

Butterfly unlocks evolution secret
By Julianna Kettlewell
BBC News science reporter
Why one species branches into two is a question that has haunted
evolutionary biologists since Darwin.
Given our planet's rich biodiversity, "speciation" clearly happens regularly, but
scientists cannot quite pinpoint the driving forces behind it.
The butterflies' wings offer clues to speciation
Now, researchers studying a family of butterflies think they
have witnessed a subtle process, which could be forcing a wedge
between newly formed species. A Russian-US team discovered
that closely related species living in the same geographical space
displayed unusually distinct wing markings. These wing colours
apparently evolved as a sort of "team strip", allowing butterflies to
easily identify the species of a potential mate. This process, called "reinforcement",
prevents closely related species from interbreeding thus driving them further apart
genetically and promoting speciation.
Although scientists have speculated about this mechanism for years, it has rarely
been witnessed in nature. "The phenomenon of reinforcement is one of the very few
mechanisms that has natural selection playing a role in speciation," said Nikolai
Kandul, who co-authored the Nature paper with Vladimir Lukhtanov and colleagues.
"It might be very widespread but it is hard to find good evidence of it," the Harvard
University researcher added.
Geographical isolation
For speciation to occur, two branches of the same species must stop breeding with
one another for long enough to grow apart genetically. The most obvious way this
can happen is through geographical isolation.
Butterflies choose mates with similar wing patterns.
If a mountain range or river divides a population of
animals for hundreds of generations, they might find
that if they meet again they are no longer able to
breed. But geographical isolation is not enough to
explain all speciation. Clearly, organisms do
sometimes speciate even if there is no clear river or mountain separating them. The
other mechanism that can theoretically divide a species is "reproductive isolation".
This occurs when organisms are not separated physically, but "choose" not to breed
with each other thereby causing genetic isolation, which amounts to the same thing.
Reproductive isolation is much hazier and more difficult to pin down than geographic
isolation, which is why biologists are so excited about this family of butterflies.
Butterfly clue
The Harvard team made the discovery while studying the butterfly genus
Agrodiaetus, which has a wide ranging habitat in Asia. The females are brown while
the males exhibit a variety of wing colours ranging from silver and blue to brown. Dr
Kandul and his colleagues found that if closely related species of Agrodiaetus are
geographically separate, they tend to look quite similar. That is to say, they do not
display a distinctive "team strip".
But if similarly closely related species are living side-by-side, the researchers
noticed, they frequently look strikingly different - their "teams" are clearly
advertised. This has the effect of discouraging inter-species mating, thus
encouraging genetic isolation and species divergence. "This butterfly study presents
evidence that the differences in the male's wing colouration is stronger [when the
species share a habitat] than [when they do not]," said the speciation expert Axel
Meyer, from Konstanz University in Germany. "This pattern would therefore support
the interpretation that it was brought about by reinforcement, hence natural
selection."
The reason evolution favours the emergence of a "team strip" in related species, or
sub species, living side-by-side is that hybridisation is
not usually a desirable thing. Although many of the
Agrodiaetus species are close enough genetically to
breed, their hybrid offspring tend to be rather weedy
and less likely to thrive. Therefore natural selection
will favour ways of distinguishing the species, which is
why the clear markings exist. "For me, this is a big
discovery just because the system is very beautiful,"
said Dr Kandul. "As much as we can we are showing
that [reinforcement] is the most likely mechanism."
Scientists are excited about
this new research
This research was published in the latest edition of
Nature magazine.
Source: http://news.bbc.co.uk/1/hi/sci/tech/4708459.stm