Gene Expression: Notes on Sewall Wright: the Adaptive Landscape

Apparently inspired by Razib’s epic post on faith and historical dynamics, DavidB over at gnxp writes his own epic post on fitness peaks. Again, there is too much to summarize, but this analogy to why there may be many (or none) alien civilizations should catch some interest:

Gene Expression: Notes on Sewall Wright: the Adaptive Landscape
In his original 1932 presentation Wright used a simple probabilistic argument for the existence of numerous peaks. The number of possible genotypes is vast, so even if only a tiny proportion of them are local optima, the number of local optima would still be very large: ‘With something like 10^1000 possibilities it may be taken as certain that there will be an enormous number of widely separated harmonious combinations. The chance that a random combination is as adaptive as those characteristic of the species may be as low as 10^-100 and still leave room for 10^800 separate peaks….(ESP p.163)’.

This is a dubious argument. It may be compared to a common argument for the existence of intelligent life elsewhere in the universe. There are around 10,000 billion billion stars in the universe, so even if the proportion of stars with planets supporting intelligent life is tiny – say, 1 in 10,000 billion – there would still be an enormous number of such stars. But consider the following counter-argument. It is plausible that the emergence and survival of intelligent life requires a moderately large number of conditions – say, at least 100 – to be met. It is also plausible that these conditions are largely independent, and individually quite improbable – say, with a probability of only 1 in 100. But with these assumptions, the probability that all of the necessary conditions are met in any given case is less than 1 in 1/100^100. This is vastly less than 1 in 10,000 billion billion, so rather than expecting there to be a large number of stars with planets supporting intelligent life, it would be a miracle if there are any at all. In reality, neither argument goes much further than establishing the bare possibility of the conclusion. Similarly, in the case of selective peaks, the sheer number of possible genotypes is in itself not a strong argument for the existence, rather than the bare possibility, of numerous different peaks.

Razib’s and DavidB’s posts are higher on both velocity and depth than most journal articles, though I guess one could say they may far away from the ‘fitness peak’ of blog posts!

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