Tuesday, March 16, 2010

On the Evolution of Cooperation

A recent paper has been published in the journal of Evolution that shows the evolution of cooperation between two separate species of bacteria. The importance of this paper is that it provides experimental evidence to the previously theorized factors that need to be in place for cooperation to evolve.
(i) directed reciprocation--cooperation with individuals who give in return; (ii) shared genes--cooperation with relatives (e.g., kin selection); and (iii) byproduct benefits
An intriguing hypothesis brought forth by (Sachs et al. 2004) suggests that the "excretion of waste products may provide a mechanism for the initiation of reciprocation." Which the paper experimentally shows and if the model holds true in other cases it could explain much of the symbiosis that we see in organisms (Rhizobium, Corals, etc...). It is not difficult to understand the relationships set by Trivers reciprocal altruism where costs and benefits are weighted against the likely-hood of repayment. ( W > C/B )
"Individuals that pay a cost to help their partners will only spread in a population if they get more of the benefits from the partner than do individuals that do not pay the cost of helping."
Individual selection is used to evolve cooperation and symbiosis. It is a whole other level to propose the selection of cooperative groups as being anyhow a part of symbiosis. It does not to me seem like the case of the fixation of alleles within a group of cooperators can benefit the group without first benefiting the individual. Although an interesting model proposed as fits my own criteria and deserves further notice to see if it is actually valid for the evolution of cooperation.

A simple model of group selection works as follows (51). A population is subdivided into groups. Cooperators help others in their own group. Defectors do not help. Individuals reproduce proportional to their payoff. Offspring are added to the same group. If a group reaches a certain size, it can split into two. In this case, another group becomes extinct in order to constrain the total population size. Note that only individuals reproduce, but selection emerges on two levels. There is competition between groups because some groups grow faster and split more often. In particular, pure cooperator groups grow faster than pure defector groups, whereas in any mixed group, defectors reproduce faster than cooperators. Therefore, selection on the lower level (within groups) favors defectors, whereas selection on the higher level (between groups) favors cooperators. This model is based on "group fecundity selection," which means that groups of cooperators have a higher rate of splitting in two. We can also imagine a model based on "group viability selection," where groups of cooperators are less likely to go extinct.

In the mathematically convenient limit of weak selection and rare group splitting, we obtain a simple result (51): If n is the maximum group size and m is the number of groups, then group selection allows evolution of cooperation, provided that ( b/c>1 +(n/m) )

Martin A. Nowak (8 December 2006)
Science 314 (5805), 1560. [DOI: 10.1126/science.1133755]

But for now I am happy that individual selection can potentially explain a significant proportion of the cooperation and potential symbiosis that we observe in the wild.

Cooperation violates the view of "nature red in tooth and claw" that prevails in our understanding of evolution, yet examples of cooperation abound. Most work has focused on maintenance of cooperation within a single species through mechanisms such as kin selection. The factors necessary for the evolutionary origin of aiding unrelated individuals such as members of another species have not been experimentally tested. Here, I demonstrate that cooperation between species can be evolved in the laboratory if (1) there is preexisting reciprocation or feedback for cooperation, and (2) reciprocation is preferentially received by cooperative genotypes. I used a two species system involving Salmonella enterica ser. Typhimurium and an Escherichia coli mutant unable to synthesize an essential amino acid. In lactose media Salmonella consumes metabolic waste from E. coli, thus creating a mechanism of reciprocation for cooperation. Growth in a spatially structured environment assured that the benefits of cooperation were preferentially received by cooperative genotypes. Salmonella evolved to aid E. coli by excreting a costly amino acid, however this novel cooperation disappeared if the waste consumption or spatial structure were removed. This study builds on previous work to demonstrate an experimental origin of interspecific cooperation, and to test the factors necessary for such interactions to arise.

P.S. It seems to be the case that poop is oddly an important player in the evolution of many species CoEvolution. (Jerry Coynes Blog: Pitcher plant evolves to be shrew loo)

William Harcombe (2010) NOVEL COOPERATION EXPERIMENTALLY EVOLVED BETWEEN SPECIES. Evolution. 21 Jan 2010. DOI: 10.1111/j.1558-5646.2010.00959

Tuesday, March 2, 2010

Why is this in Nature?

Sure this is great science, but does this really deserve to be in Nature? This concept is almost universally accepted within the biological community and has been since the 70's.

Antagonistic coevolution accelerates molecular evolution

The Red Queen hypothesis proposes that coevolution of interacting species (such as hosts and parasites) should drive molecular evolution through continual natural selection for adaptation and counter-adaptation1, 2, 3. Although the divergence observed at some host-resistance4, 5, 6 and parasite-infectivity7, 8, 9 genes is consistent with this, the long time periods typically required to study coevolution have so far prevented any direct empirical test. Here we show, using experimental populations of the bacterium Pseudomonas fluorescens SBW25 and its viral parasite, phage Φ2 (refs 10, 11), that the rate of molecular evolution in the phage was far higher when both bacterium and phage coevolved with each other than when phage evolved against a constant host genotype. Coevolution also resulted in far greater genetic divergence between replicate populations, which was correlated with the range of hosts that coevolved phage were able to infect. Consistent with this, the most rapidly evolving phage genes under coevolution were those involved in host infection. These results demonstrate, at both the genomic and phenotypic level, that antagonistic coevolution is a cause of rapid and divergent evolution, and is likely to be a major driver of evolutionary change within species.

Paterson, S., Vogwill, T., Buckling, A., Benmayor, R., Spiers, A.J., Thomson, N.R., Quail, M., Smith, F., Walker, D., Libberton, B., Fenton, A., Hall, N., Brockhurst, M.A. (2010) Antagonistic coevolution accelerates molecular evolution. Nature. 2010 Feb 24. [Epub ahead of print] [doi:10.1038/nature08798]