After Darwin at AMNH: Rob DeSalle
Monday, December 14 10:19 am
Curator Rob DeSalle Explains the Limitations of Genetic Ancestry Tests
This year, scientists at the American Museum of Natural History are celebrating the 200th anniversary of Charles Darwin’s birth and the 1859 publication of On the Origin of Species with a collection of vignettes describing expeditions and ideas with links to Darwin’s seminal work.
An annotated academic poster from a conference held at AMNH in 1932 shows the inheritance of two genetic sequences commonly used in ancestry testing. On top of Charles Darwin’s pedigree, DeSalle plotted mitochondrial DNA in red and Y-chromosome in blue. Darwin is outlined by a blue square. - AMNH
When Henry Louis Gates Jr., Harvard University professor and host of the PBS series African American Lives, learned that his Y-chromosome—a gene popularly used to trace male ancestry—“goes back to Europe,” he joked that he was having an identity crisis. And in his best-selling book The Seven Daughters of Eve geneticist Brian Sykes suggests that 95% of all Europeans are descended from just seven Stone Age women. He used mitochondrial DNA, inherited directly from mothers and employed by companies for ancestry testing of the maternal ancestry, for this work.
These are dramatic moments, but they are based in part on misleading assumptions, according to geneticists like Rob DeSalle, Curator in the Sackler Institute for Comparative Genomics at the Museum. “Ancestry testing is pushed as simple and straight-forward,” he says. “But it is not. Often the verbal history passed down in families is more accurate than ancestry DNA testing done now.”
DeSalle, along with Museum colleague Ian Tattersall, turned to the family history of the most famous of evolutionary thinkers to illustrate their point. By simply mapping basic inheritance onto Charles Darwin’s pedigree, it is easy to see how quickly a specific Y-chromosome or sequence of mitochondrial DNA (mtDNA) can be lost in a lineage. Darwin’s Y-chromosome, for example, is probably found in only two of his living descendents. Only three of his four sons reproduced and one of these sons only had daughters. Ultimately, the number of male descendents of Darwin’s two sons who had male offspring dwindled to a very small number. There are many direct male descendents of Charles Darwin living today, but most of them do not have his Y chromosome.
For mtDNA, the story is similar. Darwin inherited his mtDNA from his mother, Susannah Wedgwood. But because Darwin’s children received their mtDNA from their mother, Emma (Darwin’s first cousin, who was also a Wedgewood, but from a completely different mtDNA lineage), and because Darwin’s four sisters (Marianne, Caroline, Catherine and Susan) did not have children, this sequence was not passed on to the Darwin’s direct descendents. A further wrinkle is that more distantly-related relatives like the female descendents of his maternal grandmother (Elizabeth Allen, from yet another completely different mtDNA lineage) could have an exact copy of Darwin’s mtDNA, even though they are not his direct descendents.
“If any descendent of Darwin’s did these genetic tests to trace their roots, they probably would not have found their most famous ancestor,” says DeSalle. “Genetic tests that primarily rely on the Y-chromosome and mtDNA offer only a small window onto the genome of a particular family, and these tests alone skew evolutionary history.”
This After Darwin at AMNH story is based on an excerpt from an upcoming book on race and genetics by Tattersall and DeSalle.
When the HMS Beagle sailed from England just after Christmas of 1831, Darwin was on board as the ship’s naturalist and geologist. Over the next five years, he made scientific observations and collections in Cape Verde, tropical Brazil, Tierra del Fuego, the pampas of Argentina, Andean Chile, marsupial-dominated Australia, Mauritius, the tip of Africa, and, of course, the Galápagos Islands that so influenced his thinking about evolution.
Darwin’s observations on this voyage must have been central in his thoughts as he walked the sand path around a thicket in Kent, where he spent the next decades crafting On the Origin of Species by Means of Natural Selection, the foundation of evolutionary biology.
The After Darwin at AMNH series explores links between the scientific research work being done at AMNH today and Darwin’s influential work.
Charles Darwin’s route aboard the HMS Beagle from 1831 to 1836. AMNH/David Rumsey Map Collection (www.davidrumsey.com)
