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What is "Social Evolution and Symbiosis"?
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What is "Social Evolution and Symbiosis"?
The study of social evolution addresses questions about conflict and cooperation between individuals of the same species. In particular, we are interested in how cooperation can evolve and how it can be evolutionary stable in a natural world where natural selection normally rewards competition and selfishness. Cooperation is found everywhere in nature. We study insect societies (ants, termites, social bees and social wasps). These insects are special because cooperation has evolved to such levels that most individuals have given up their personal reproduction to become “worker helpers” in colonies where someone else (the queen) lays all the eggs. Less advanced forms of “helping” behaviour are known in some birds and mammals, in other groups of insects, spiders and shrimp, and even in microorganisms. Today we understand almost all aspects of natural cooperation by evolutionary models derived from W.D. Hamilton’s inclusive fitness principle. This evolutionary theory of cooperation is also commonly referred to as “kinship theory” or “kin-selection” theory, because it is through kinship with breeders that helpers realize copies of their own genes in the next generation even if they do not reproduce themselves. Remarkably, queens and workers in the same colony are often each others close relatives, but express greatly different phenotypes. In many tropical ants, also workers come in different castes (small ones, large ones and even soldiers). It has long been believed that these differences were exclusively determined by the way in which larvae were raised by their older sister workers. However, CSES has recently shown that there is a significant genetic component to this variation as well.
Usually, we can measure a significant degree of kinship in social insect colonies, but there are exceptions. Several ants species have evolved societies with zero relatedness, i.e. where any individual is no more related to a nestmate than to a random individual in the population at large. This lack of relatedness is usually compensated by an enormous ecological dominance and many of these ant species are therefore pests. As ecological cost-benefit ratios of cooperation are the other decisive variable next to kinship in Hamilton’s theory, it seems that these “hypersocial” ant societies represent an evolutionary unstable extrapolation of kinship theory. However, very few details about these anomalous insect societies are in fact known and CSES actively studies several such ant species.
The analysis of reproductive conflict is central when studying social evolution. This is because cooperating individuals “agree” that it is in their joint interest to secure as many resources as possible for the family group or society, but they often at least partially disagree about the best way to use these resources for reproduction. A central paradigm in of kinship theory is that cooperation between individuals in a society can be evolutionary stable provided the reproductive interests of individuals are at least partly aligned. Over evolutionary time, kin-selection will then usually find ways to regulate he remaining conflicts, so that their expression does not harm the functioning of the society as a whole. However, this does not mean that these conflicts are no longer there. Even the most advanced insect societies have variation in the ways in which latent conflict is expressed and these societies can often be manipulated to express their conflicts in a more visible way. Also the mating biology of social insects has many latent conflicts because the interests of young queens and males are far from parallel. CSES therefore explicitly studies mating systems of ants and bees. The interface between mating biology, chemical recognition processes and reproductive allocation ir an area where CSES has recently made important contributions.
Cooperation in nature persist in spite of individuals being also under continuous selection to pursue selfish interests. Over evolutionary time, the outcome is not always “a happy one” and many examples are known where “cheater morphs” ultimately evolved to be social parasities, exploiting the societies of other social insect species as some kind of disease. In reality, however, the exploitation aspects does not come out of the blue as a nasty evolutionary innovation. In fact, it is most logical to consider cooperative societies as cases of mutual exploitation where the partial alignment of reproductive interests keeps the interaction cooperative. If, for whatever reason, this alignment diseappers the exploitation quickly becomes unilateral, i.e. parasitic.
CSES studies various forms of social parasitism in societies of ants and bees to gain more insight into their ecological functioning and evolutionary dynamics. Social parasites always live in tight symbiosis with their victims, the host societies and have evolved a series of special adaptations to fool the hosts into acting as though they have a joint reproductive interest with them. Quite frequently, the larvae of social parasites mimic larvae of their ant hosts, so that their hosts “believe” that they do the right thing while feeding these larvae and raising them. CSES also studies fungal diseases of insect societies, the defences that social insects have evolved to minimize their effect and the genetic variation in these defences. Where living in large groups has long been believed to increase disease pressure on insect societies, CSES has recently shown that these negative effects can be more than compensated by positive effects of mutual grooming.
Not all interactions of social insects with other organisms are hostile and parasitic. Spectacular examples of mutualistic cooperation between insects and microorganisms have also arisen in the social insects and CSES studies particularly the single groups of ants and termites that have evolved an agricultural symbiosis with fungi. CSES has recently published the phylogenetic relationships between the fungus-growing termites and their symbionts, complementing a similar analysis for the fungus-growing ants, that was completed in the USA a few years ago. These mutualistic insect-fungus interactions are clearly beneficial for both parties involved and are obligate, i.e. no ants or termites in these groups have given up the symbiosis and the domesticated fungi can no longer survive without the care of an insect society. However, also here latent reproductive conflicts between the social insects and their symbionts still persist and CSES studies the expression of these conflicts.
The modern meaning of the term symbiosis is broad and ranges from mutualistic symbiosis with maximal cooperation (via bilateral exploitation) to non-cooperative parasitism (with unilateral exploitation). Some symbioses are always mutualistic, some are always parasitic and some may switch over evolutionary time. The ecological details usually determine the extent of cooperation or conflict, just as in social interactions between individuals. Recent evolutionary theory recognizes the parallels between understanding conflict and cooperation at the individual level and the species level and models of symbiosis work in approximately the same way as kin-selection models.
CSES is also actively involved in conservation projects. The socially parasitic Maculinea bufferflies that CSES studies are threatened both in Denmark and in Europa as a whole. Also the dark (brown/black) north-west European race of the honey bee is an explicit conservation issue. Both butterflies and black honey bees have stronghold populations on the island of Læsø, which has now become the major Danish field site for CSES research.