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quarta-feira, maio 19, 2010

Hammerhead Shark Study Shows Cascade of Evolution Affected Size, Head Shape

ScienceDaily (May 19, 2010) — The ancestor of all hammerhead sharks probably appeared abruptly in Earth's oceans about 20 million years ago and was as big as some contemporary hammerheads, according to a new study led by the University of Colorado at Boulder.

A new study led by the University of Colorado at Boulder indicates several species of small hammerhead sharks living today stemmed from one large ancestor living some 20 million years ago, evolving twice at different times and places. (Credit: Terry Goss 2008 Marine Photobank)

But once the hammerhead evolved, it underwent divergent evolution in different directions, with some species becoming larger, some smaller, and the distinctive hammer-like head of the fish changing in size and shape, said CU-Boulder Professor Andrew Martin of the ecology and evolutionary biology department.

Sporting wide, flattened heads known as cephalofoils with eyeballs bulging at each end, hammerhead sharks are among the most recognizable fish in the world. The bizarre creatures range in length from about 3 feet up to 18 feet and cruise warm waters around the world, Martin said.

In the new study, scientists focused on the DNA of eight species of hammerhead sharks to build family "gene trees" going back thousands to millions of generations. In addition to showing that small hammerheads evolved from a large ancestor, the team showed that the "signature" cephalofoils of hammerheads underwent divergent evolution in different lineages over time, likely due to selective environmental pressures, said Martin.

The team used both mitochondrial DNA passed from mother to offspring and nuclear DNA -- which is commonly used in forensic identification -- to track gene mutations. The researchers targeted four mitochondrial genes and three nuclear genes, which they amplified and sequenced for the study.

"These techniques allowed us to see the whole organism evolving through time," Martin said. "Our study indicates the big hammerheads probably evolved into smaller hammerheads, and that smaller hammerheads evolved independently twice."

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Molecular Phylogenetics and Evolution
Volume 55, Issue 2, May 2010, Pages 572-579

Phylogeny of hammerhead sharks (Family Sphyrnidae) inferred from mitochondrial and nuclear genes

Douglas D. Lima, Philip Motta b, Kyle Mara b and Andrew P. Martin a, ,

a Department of Ecology and Evolutionary Biology, University of Colorado, Boulder, CO 80309, USA

b Department of Biology, University of South Florida, Tampa, FL, USA

Received 27 July 2009;
revised 25 January 2010;
accepted 30 January 2010.
Available online 4 February 2010.


Hammerhead sharks (Family Sphyrnidae) get their name from their laterally expanded, dorsal–ventrally compressed head, a structure referred to as the cephalofoil. Species within the family vary for head size and shape and for body size in ways that are functionally significant. Here we infer the phylogeny for all species within the family based on analysis of mitochondrial and nuclear genes amounting to 6292 base pairs. Mixed model Bayesian analysis of the concatenated data and Bayesian estimation of the species tree (BEST) converged on the same topology of the relationships. Shimodaira–Hasegawa tests revealed that all previously proposed hypotheses could be refuted by the data. The new hypothesis for the group suggests that the ancestor of all extant sharks was large (>200 cms) and that small body size probably evolved twice at different times and places. Moreover, the results suggest that once the cephalofoil evolved, it underwent divergent evolution in different lineages presumably in response to unique selective regimes.

Keywords: Hammerhead sharks; Phylogeny; Head shape; Body size


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