Traditional values in bees
The beehive was an early Mormon icon, symbolizing hard work and cooperation. To an evolutionary biologist, however, a beehive could symbolize reproductive skew, a situation where some individuals reproduce much more than others. Extreme reproductive skew is one of the defining characteristics of eusocial species, of which honey bees are a prime example. Reproductive skew can differ between the sexes. In honey bees, the queen lays most of the eggs, and most females don't reproduce at all. Polygamous species and groups show the opposite pattern: males vary much more in reproductive success than females do. Maybe an inverted beehive would have been a better symbol. Note that the cells in our bodies behave somewhat like a eusocial bee colony; any children we have are directly descended from a few sex cells, while brain cells and skin cells play the supporting role of worker bees.
This week's paper, "Ancestral monogamy shows kin selection is key to the evolution of eusociality" was published in Science by William Hughes and others. Like humans, some bees are monogamous, meaning that the queen mates with only one male, so her daughters (the workers) are all sisters. In other bee species, the queen mates with several males, so her daughters are half-sisters. Relatedness generally favors cooperation, although there are some possible complications, discussed below.
This week's paper asks how mating behavior affects the evolution of eusociality. They reasoned that, if mating system doesn't matter, then today's eusocial species could be descended from either monogamous, polygamous, polyandrous (each female has multiple mates), or promiscuous ancestors. Alternatively, eusociality may evolve more easily with one of these mating systems than with the others.
The authors used the method of ancestral state reconstruction, applied to nine groups of eusocial insects (bees, wasps, and ants). If a reader can suggest a web page with a clear explanation of this method, preferably with diagrams, I will add a link to it here. The basic idea is to work through the known family tree of a group of related species, trying to infer the traits of ancestral species from the traits of their descendants.
For example, consider the evolution of diet in a group of related animals. Suppose most species descended from X eat insects and only a few eat seeds. It seems more likely that there were a few switches from insects to seeds, over the course of evolution, rather than many switches from seeds to insects. So maybe X ate seeds. But what if, since X's time, insects have become more common where these species live, or seeds have become scarce? Then maybe eating insects could have evolved independently in multiple species. If insect-eating had evolved independently in each species, however, you might expect different species to catch insects in different ways. If the insect-eaters descended from X all use tubular tongues to catch insects, then maybe X also caught insects with a tubular tongue. The more arbitrary the similarities (in the sense that other methods would have worked equally well), the more likely it is that they are inherited from a common ancestor, rather than having evolved independently.
The authors used information on 267 species of eusocial insects. Most were monogamous and their analysis suggested that the exceptions, which include honey bees, represent an evolutionary change from the ancestral state of monogamy. In those cases when polyandry evolved, they found that "totipotency [ability of workers to lay eggs] was lost prior to or concurrently with the evolution of polyandry."
This last result seemed somewhat surprising to me, because I had thought that polyandry was a strategy by the queen to prevent reproduction by workers. In some species, workers can lay male eggs without mating. The more mates the queen had, the more likely a fellow worker is to be a half-sister rather than a sister. Therefore, multiple mating by the queen decreases relatedness of one worker to another worker's egg (relative to an egg from her mother, the queen). So, in polyandrous species, workers tend to eat eggs laid by other workers. In monogamous species "it is often the queen who does the policing, unless worker reproduction seriously decreases colony efficiency" (Beekman and Oldroyd 2008 Annual Review of Entomology 53:19). So if workers had already lost the ability to reproduce, what drove the evolution of multiple mating by queens? I hope we humans haven't been a bad influence!
Also this week: