Chimps are our closest relatives… but not for all of our genes
But there was one very cool result to come from the analysis of the gorilla genome. About 15% of our genes are more closely related to their counterparts in gorillas than they are to the same genes in chimps.
That sounds suprising. People are always going on about how humans and chimps are ninety-nine-point-some-magic-number percent identical, and there are exactly two scientists in the world who think chimps are not our closest relatives (Grehan and Schwartz, 2009 doi: 10.1111/j.1365-2699.2009.02141.x). Have we been wrong? And how can 15% of a genome show one pattern while the rest shows another?
To understand what’s going on, we need to remember where species come from. Species start forming when populations stop sharing genes which other. When genetic changes in one population can’t filter through to another, those two populations are capable of evolving apart from each other and so can become distinct and take on the various characters that we use to tell species apart. So, new species only become different as they start to evolve apart, but they start of with a more or less random sampling of the genes in the ancestral population from which they descend. If we want to understand what’s going with the gorilla genome, we need to understand the history of those genes.
In most populations at least some genes come in distinct “flavors” (technically called alleles) . So, for instance we all have a gene called MC1R, but some of have an MC1R allele that is associated with red hair, and others have alleles that usually lead to dark hair. We inherit our genes from our parents, so each allele has a history that stretches back through time. If we look at modern populations we can use genetic differences between alleles to reconstruct that evolutionary history. Here’s a simplified history of four alleles, in a very small population (if you re-trace the lineages you see they fit the tree to the right):


So called “incomplete lineage sorting” is a problem for people like me who aim to reconstruct the evolutionary history of species using genetic data. Although we’ve always known this problem existed, we’ve only recently been able to extend population genetics theory to actually infer the history of species for gene trees even when those gene trees are unsorted. It’s important we have these methods, because it’s actually predicted that most genetic lineages will be unsorted for about 1 million years after speciation starts – often all we have are unsorted genes and it’s nice to be able to extract some information from them.
0 Responses to “Chimps are our closest relatives… but not for all of our genes”
I recall this being a particular problem for the African Great Lake cichlids, one of the best-known examples of a stupidly fast-speciating group (last I heard, there were some who suspected that cichlid species might separate over only a few generations). There are species that are clearly morphologically and reproductively distinct, but are absolutely impossible to resolve in a gene tree.
The first line in my comment was supposed to be blockquoted. Sorry.
Hi Christopher,
Yeah, rapid radiations are the hardest thing for molecular phylogeny to deal with – even if the lineages sort we have to hope that some of the genes actually picked up enough unique mutations between one speciation and the next for us to be able to infer the relationships. Since we usually use selectively neutral markers, there’s very little chance such mutations are fixed and we end up with the “star topology” problem.
I don’t know if anyone has used the species-tree methods on the cichlid species flocks – would be a good test of their power