All living things on earth possess similarities and differences in appearance. Similarities allow us to group relatives together. Differences allow us to distinguish them apart. For example, perhaps you have been told, “You look just like your brother” or “you have your mother’s eyes.” These are similarities. Meanwhile, your sister may be the only one in your family with red hair, which perhaps her grandfather may have had when he was young. Heritable features that group relatives together and distinguish individuals apart are based upon genes passed down along our family tree generation after generation. The blueprint for these genes is coded by our DNA.
In the ocean, many fish species like tuna and sharks swim great distances across much of the world. Sadly, many of these species are suffering from overfishing and habitat loss. At the TRCC, marine geneticists are working with DNA to identify genetically unique groups within these very large, highly dispersed species so that fishery managers can better conserve and protect them from overexploitation.
For example, white sharks (Carcharodon carcharias) are found around the world. Individuals grow to large size and travel great distances in their lifetime. In recent years, white shark populations have dropped dramatically, especially in some areas, due to overfishing, habitat degradation, and decline in abundance of prey species. In the State of California and other regions around the world, these sharks have been given protected status to encourage their conservation. Unfortunately for us, all white sharks look pretty much the same. So it becomes difficult for conservationists to enforce protection of white sharks on a regional scale if they cannot tell the different groups apart.
The DNA of all individuals contains differences we call mutations. They can be easily counted when comparing DNA data between different individuals as seen below. Arrows show a mutation between two different white sharks along their DNA sequences.
DNA sequences from two white sharks showing a G-A mutation along the strand at position 249.
Geneticists know that brothers and sisters born from the same parents will have fewer DNA mutations between them than their cousins or their cousins’ cousins. They know that two completely unrelated families will be the most different of all and will therefore have the most mutations between them. As generations come and go and time passes by, more mutations are acquired and our DNA becomes increasingly different from our ancestors.
This is the power of genetics which we used at the TRCC to determine that white sharks sampled off California are indeed genetically unique from other white sharks found in the regions of South Africa and Australia. Because California sharks have fewer mutations with sharks from Australian than South African waters, we recognize Australia as the closest ancestor to white shark population living off our Californian shores.
DNA tree of white shark populations from three regions. California white sharks share their closest ancestry with sharks from Australia.