It’s been known for a while that ants evolved from a wasplike ancestor, but the relationships among living wasps and ants have been unclear. An new online paper in Current Biology by Brian Johnson et al. (the senior author is my old friend Phil Ward at UC Davis), using a huge amount of sequence data, seems to have resolved the situation pretty definitively. The group they studied were the “aculeate” Hymenoptera, which includes all the “stinging” Hymenoptera.
Earlier workers suggested that ants evolved from ectoparasitic wasps that laid their egg on other insects, and whose larvae then hatched on the host or in its nest. These wasps were obviously not “eusocial” (i.e., having reproductive queens with a nonreproductive female worker caste), leaving open the question of what factors prompted the evolution of that marvel of behavior, eusociality (mostly found in insects but also in naked mole rats and a few other species). What factors could prompt the evolution of a sterile worker caste? That was a question that puzzled Darwin in The Origin.
The new phylogeny, based on 308 to 5,214 genes, is strongly supported and is given below. It shows that the sister group of ants are the speciform wasps and bees (“Apoidea”), which is something nobody suspected. That group includes the familiar mud-dauber wasps, which you might have seen making their cylindrical nests on your house.
That novel finding gives one clue to the origin of eusociality in ants, for the Apoidea is a group characterized by two things: like ants, they build nests to contain their eggs, and they provision those nests by bringing prey back to feed the offspring. The outgroups, scoliid wasps and their relatives, as well as the tiphioids and some pompiloids , retain their ectoparasitic habits, and none of them are eusocial. (Eusociality is indicated by asterisks in the diagram below.):The results suggest that the common ancestor of ants and Apoidea exhibited two behavioral traits not shared with the ectoparasitoid Scolioidea, namely the building of a defensible nest and transport of prey to the nest; and that these were important prerequisites for the evolution of eusociality.
The offspring in such nests, of course, contain the genes of their mothers, and all of these species (including every group in the diagram above) are haplodiploid: the males are haploid, with a single set of chromosomes, while females have two sets. The reproductive queen produces males from unfertilized eggs, and females from fertilized ones. It’s been hypothesized that this situation also favors the evolution of eusociality, as sterile workers share 3/4 of their genes with the sisters they help rear, which might give an evolutionary leg up to becoming sterile—using your efforts not to produce your own offspring, but to make more copies of your genes by helping your mother rear your sisters.
Now haplodiploidy is not sufficient to promote the evolution of eusociality, as most of the non-eusocial species above are also haplodiploid. And termites, which are not haplodiploid (both males and females have two sets of chromosomes), are also eusocial, with a reproductive queen and sterile workers. But previous work that I’ve discussed elsewhere suggests that haplodiploidy is nevertheless a factor important in promoting eusociality.
Rather, it may be the combination of haplodiploidy and the construction of a defensible and provisioned nest, which allows you to keep your offspring around to help you, that was pivotal in promoting the evolution of the eusocial lifestyle. This paper, at least, supports that two-factor hypothesis, and that’s why it’s important.
Edward O. Wilson and his colleagues have suggested that haploidiploidy and the “kin selection” it can cause was irrelevant in the evolution of eusociality, and the “nest” hypothesis was the important factor, along with selection among groups. The Johnson et al. paper doesn’t speak to that hypothesis, except to support the idea that defensible, provisioned nests are important.
Rather, Wilson’s idea of group selection, and of the irrelevance of kin selection, seems unlikely for other reasons. Those include previous work showing that eusociality evolved only in hymenopteran groups whose females mated only once (that causes increased relatedness in offspring, promoting kin selection), as well as the general unlikelihood of (and failure to demonstrate) group selection in any species. Finally, although the Nowak et al. paper in Nature claimed that, in their models, kin selection did not promote eusociality, they showed no such thing, for their models did not vary the level of relatedness among individuals.
You can read my posts earlier posts on the “group selection/eusociality” controversy here.
Carl Zimmer had a piece on this work in last Tuesday’s New York Times.
Johnson, B. R., M. L. Borowiec, J. C. Chiu, E. K. Lee, J. Atallah, and P. S. Ward. 2013. Phylogenomics resolves evolutionary relationships among ants, bees, and wasps. Current Biology, online, in press. http://dx.doi.org/10.1016/j.cub.2013.08.050