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Saturday, September 21, 2013

98. Coevolution of Species



'Just when you think you’ve won the rat race, along come faster rats' (Anon.).

Living organisms interact, not only with the inanimate surroundings, but also with organisms of the same or different species. Therefore, evolution of a species cannot occur in isolation from the evolution of other species with which it interacts. They coevolve. Coevolution of species is an important aspect of biological evolution.

Darwin’s theory of evolution says that a species evolves to become better and better for the task of survival in a given set of conditions. Genes of individuals in the population that are not good enough for the task of survival tend to get eliminated in successive generations. All this happens through an interplay of the blind forces of Nature, but the end effect appears to be as if the genes have the thinking power of being 'selfish' at the job of survival and propagation. Richard Dawkins’ (1989) phrase the selfish gene sums up the situation well.



This may seem to indicate that a species will always evolve strategies (consciously or 'purely chemically') that are entirely selfish when it comes to dealing with other species. In fact, even within a species, it is conceivable that the genotype, and thence the phenotype, of an individual member may exhibit selfishness, without regard for other members of the species. But what happens in reality can actually be far from this simple-minded speculation. There can be evolutionarily stable strategies, involving both competition and cooperation, and there can be evolutionary arms races. There can also be symbioses of species.



Before I discuss some of these processes, it is interesting to take note of an analysis by Douglas Caldwell (cf. Margulis and Sagan 2002), according to which the following terms were never used by Charles Darwin in his Origin of Species: association; affiliation; cooperate, cooperation; collaborate, collaboration; community; intervention; symbiosis. What I am now going to describe in this and the next few posts are some of the post-Darwinian developments.

Coevolution of species is an important ingredient of the succession of turmoil and relative stability (stasis), so that what really unfolds is punctuated equilibrium.

 

Although species tend to evolve towards a state of stable adaptation to the existing environment, it is also true that many of them are extinct, as the fossil records show. This is because there is never a state of permanent, static equilibrium in Nature. There is a never-ending input of energy, climatic changes, terrestrial upheavals, as also mutations and a coexistence with other competing or cooperating species. Rather than evolving towards a state of permanent equilibrium and adaptation, the entirety of species (in fact, the Earth as a whole) evolves towards a state of self-organized criticality (SOC). This state of complexity is poised at the edge of order and chaos. The never-ending inputs just mentioned result in minor or major ‘avalanches’ (catastrophic events of various magnitudes), which sometimes lead to the extinction of species, or the emergence of new ones.

SOC can explain the finding by Eldredge and Gould (1972) in fossil records that there are long periods of stasis, followed by quick bursts of evolutionary change; i.e. there is punctuated equilibrium. Imagine an ecosystem in which the various species have reached more or less a state of equilibrium with one another. Suppose there is a random genetic crossover in one of the species that is beneficial to it and thus survives and gets propagated to other members of the population. The propagation is not linear; it is more likely to be avalanche-like or ‘explosive’ with time, rather like the avalanches in the sandpile experiment I described in Part 82. In due course, things stop changing, but then some other member of the population may mutate. There is thus a steady drizzle of mutations, resulting in periods of avalanches and relative equilibrium.

As I have explained in the previous few posts, in conventional game theory it is usually presumed that each player is a rational being who expects that other players are also rational beings. Each player therefore adopts a strategy to minimize his losses, assuming that other players will work out strategies to maximize the losses of other players as they try to minimize their own losses (the minimax strategy). In the evolutionary context, if each member of a population did this, the end result may be an extinction of that species. The fact that a species has survived can imply that an evolutionarily stable strategy (ESS) must have evolved. In an ESS, the survival of an individual, though not completely subservient to the survival of the species, is determined by a strategy that ensures that contests between individuals, though leading to an improvement of the gene pool of the population, do not result in an excessive annihilation of the contest-losers. The ESS idea, which entailed a modification of conventional game theory, was put forward by Maynard Smith (1974, 1976, 1982), and it can be operative even in the coevolution of two or more species. In fact, the ESS concept is also relevant to the evolution of entire ecosystems.

I shall discuss the ESS notion in the next two posts.