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Mengapa ada begitu banyak spesies kumbang dan begitu sedikit species buaya ?

Mengapa ada begitu banyak spesies kumbang dan begitu sedikit pada buaya ? Jawabannya mungkin batas ekologis ke nomor spesies , para ilmuwan melaporkan ....read more

Why are there so
many species of beetles and so few crocodiles?

Date:

August 28, 2012

Source:

University of
California - Los Angeles

Summary:

Why are there so many
species of beetles and so few crocodiles? The answer may be ecological limits
to species number, scientists report.




.......................

there are more than
400,000 species of beetles and only two species of the tuatara, a reptile
cousin of snakes and lizards that lives in New Zealand. Crocodiles and
alligators, while nearly 250 million years old, have diversified into only 23
species. Why evolution has produced "winners" -- including mammals
and many species of birds and fish -- and "losers" is a major
question in evolutionary biology.

Scientists have often posited that because some animal and plant lineages
are much older than others, they have had more time to produce new species (the
dearth of crocodiles notwithstanding). This idea -- that time is an important
predictor of species number -- underlies many theoretical models used by
biologists. However, it fails to explain species numbers across all
multi-cellular life on the planet, a team of life scientists reports Aug. 28 in
the online journal PLoS Biology, a publication of the Public
Library of Science.

"We found no evidence of that," said Michael Alfaro, a UCLA
associate professor of ecology and evolutionary biology and senior author of
the new study. "When we look across the tree of life, the age of the group
tells us almost nothing about how many species we would expect to find. In most
groups, it tells us nothing."

Another idea, that some groups are innately better or worse at producing
species, similarly fails to explain differences in species number among all of
the major living lineages of plants and animals, the life scientists found.

"We know that some groups, like flowering plants or cichlid fishes,
have been exceptionally good at producing species during certain periods of
their evolutionary history," Alfaro said. "However, when we look at
the ages of all of the major groups of plants and animals, these differences in
speciation rate are not sufficient to explain the differences in species number
that exist in extant groups."

Alfaro and his colleagues studied 1,397 major groups of multi-cellular
eukaryotes -- including animals, plants and fungi -- that account for 1.2
million species. Working as "evolutionary detectives," they were able
to see whether the groups that split the earliest tended to have the most
species. They assigned a "species richness score" to these 1,397
groups, using novel statistical and computational methods they developed.

If age does not explain species diversity, an alternative idea is that a
lineage will produce species up until the point that it fills an "adaptive
zone" that allows a maximum number of species, Alfaro said. In other
words, a lineage of bats, whales or penguins has a maximum capacity that is
determined by habitat requirements and competitors.

When an adaptive zone is first colonized, the growth of new species will be
rapid, up until that limit has been reached. Once a zone is full, this
speciation rate will level off. New species will not emerge until one of two
events occurs: First, an existing species may go extinct, in which case it may
be replaced. Second, a species within the adaptive zone may evolve a new trait
-- sharp teeth, wings, chemical defenses or camouflage, for example -- that
confers a significant ecological advantage and takes it into a new adaptive
zone, creating opportunities for new species to emerge,Alfaro said.

"Adaptive zones are an old idea in evolutionary biology, but there is
little understanding of whether speciation rates or adaptive zones are more
important in explaining species richness across the tree of life," Alfaro
said. "If adaptive zones control biodiversity at the broadest scales, then
the rate of species growth will be a good explanation of species richness only
right after a lineage has entered into a new adaptive zone. Once the adaptive
zone has filled up, then, no matter how much time goes by, the number of
species will not change much."

Thus, these adaptive zones, which Alfaro also calls "ecological
limits," serve to restrict the number of new species that can emerge.

"Most of the groups that we studied have hit their limits," he
said. "Ecological limits can explain the data we see. What's really
driving things is how many times lineages evolve new innovations that move them
into new adaptive zones.

"The ultimate goal in our field is to have a reconstruction of the
entire evolutionary history of all species on the planet," Alfaro added.
"Here we provide a piece of the puzzle. Our study sheds light on the
causal factors of biodiversity across the tree of life."

Daniel Rabosky, an assistant professor of ecology and evolutionary biology
at the University of Michigan-Ann Arbor is the lead author of the study, and Graham
Slater, a National Science Foundation-funded UCLA postdoctoral scholar in
ecology and evolutionary biology, is a co-author.

The research is funded by the National Science Foundation and by the
University of California's Miller Institute for Basic Research in Science.

In a 2009 study published in Proceedings of the National Academy of
Sciences, Alfaro and his colleagues reported that mammals and many species
of birds and fish are among evolution's "winners," while crocodiles,
alligators and a reptile cousin of snakes and lizards known as the tuatara are
among the "losers." That study also showed that new species emerge
nearly as often as they die off.







Story Source:

The above story is based on materials provided by University
of California - Los Angeles. The original article was written by Stuart Wolpert. Note:
Materials may be edited for content and length.







Journal Reference:

1.    Daniel L. Rabosky, Graham J. Slater,
Michael E. Alfaro. Clade Age and Species Richness Are Decoupled Across
the Eukaryotic Tree of Life. PLoS Biology, 2012; 10 (8):
e1001381 DOI: 10.1371/journal.pbio.1001381

http://www.sciencedaily.com/releases/2012/08/120828171744.htm

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