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Hammerhead shark study shows cascade of evolution affected size, head shape
Date:5/18/2010

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.

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."

A paper on the subject was published in this month's issue of Molecular Phylogenetics and Evolution. Led by former CU-Boulder ecology and evolu
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Contact: Andrew Martin
andrew.martin-1@colorado.edu
303-492-2573
University of Colorado at Boulder
Source:Eurekalert  

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