Epreuve orale de DNL Sciences Session de juin Baccalauréat 2016 Durée de préparation : 20 minutes Durée de l'épreuve : 20 minutes (10 minutes de présentation orale ; 10 minutes d'entretien)

Theme : EVOLUTION Question: Explain why scientists think that we humans are far more closely related than we should be.

Bottlenecks Made Humans Less Diverse

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Document : The evidence is all around us: Biologists, for example, have long known that measurements of skulls yield far more similarities among Asians, Europeans and Native Americans than among indigenous African populations. As humans migrated out of Africa, humanity lost much of its genetic diversity. But exactly how this happened has become a matter of intense debate. Now, a recent study from University of Cambridge researchers William Amos and J.I. Hoffman suggests that the limited genetic diversity we see among humans today came from two major evolutionary bottlenecks -- or sudden declines in population -- that lead to a loss of genetic diversity. Amos and Hoffman found evidence of these events after they examined genetic samples from 53 distinct world populations. In the study, published in the October Proceedings of the Royal Society: B, the pair studied the frequency of two genetic indicators. When Amos and Hoffman examined their samples, they found evidence of a bottleneck outside of Africa around 50,000 years ago. They also found a second major evolutionary bottleneck that occurred somewhere around the Bering land bridge, a narrow strip of land now submerged beneath the Bering Strait, which many anthropologists believe humans used to populate the Americas from Eurasia. It's tempting to imagine sudden declines in human population at the hands of saber-toothed tigers or from a massive comet strike, but Amos says the likelier scenario is that humans diverged from one large population over and over again, leading to a series of what are known as founder effects. As humans leave big groups for smaller ones, they carry with them only a sample of the genes found in the larger group. "It's almost like if you went into a town and took the first 15 people you met to go found another town," Amos explained. "The new town isn't going to fully represent all the surnames in the original town." And what's true for names is also true for genes, say geneticists. Sources: http://news.discovery.com/human/genetics/human-diversity-bottlenecks.htm