Saturday, 13 April 2013

Convergent evolution as relatedness

When organisms appear to be similar to one another there are several possible causes: The first category in this list represents kinship - or relatedness. It could be one organism copied from the other, or that both copied from a shared source. This post will argue that convergent evolution can be usefully seen as a type of kinship or relatedness - that the first two categories are fundamentally similar - and that the theory of kin selection applies to both of them.

In some cases, it is obvious that similarity between non-kin is due to copying. For example, many unrelated insects resemble wasps. Bees are the best-known example - but many other creatures have adopted the same colouring scheme. This is widely known as "mimicry".

This "convergence" of appearance seems fairly clearly due to information being copied via vision systems of predators and enemies into the genes of insects.

Similarity involving camouflage colouration also plainly involves copying. Many female birds are a mottled dirty brown colour. They are copying colours from environmental backgrounds - in order to blend in. They often look similar because they are copying from similar environments.

So: some convergent evolution plainly involves copying - in the sense described in detail here. However, the thesis here is that all convergent evolution involves copying. Let's look at a few more-challenging cases:

Birds and bats both have wings. They resemble each other - without being closely related. Their last common ancestor had no wings and could not fly. What's being copied here? Here, the copied information involves some aspects of a shared environment, and some things derived from physical laws.

Marsupials and mammals share many common features - and it is widely agreed that many are the result of convergent evolution. However, they evolved on different continents - making copying difficult. What was copied here? Again, shared environments resulted in similar selection pressures - and ultimately similar morphologies. The environments were similar because they were on the same planet - with much the same fauna and flora - due to shared descent.

Even convergent evolution based on the uniformity of nature can be regarded as being likely to involve copying. Why are the laws of physics uniform? The answer involves identity copying. Either the laws of physics were copied from a shared source when now-distant parts of the universe were once closer together - or physical uniformity is due to something like a state machine that iteratively deals with all locations in the universe at each moment in time. The latter case also qualifies as copying - since by definition, copying involves information in one place being later found in multiple places.

The applicability of kin selection theory to cases of convergent evolution is in line with Price's approach to the issue - which is based on correlations between traits. Convergent evolution produces such trait correlations - it is appropriate that kin selection also deals with these.

Sunday, 7 April 2013

Clade selection

"Kin selection" and "group selection" are common terms for the way that organisms help organisms which are similar to themselves. I've proposed the term "similarity selection". Another contender is "clade selection". This term came from George C. Williams (1992).

Dawkins said of this:

Williams efficiently disposed of “group selection,” which never recovered (except as a muddled version of kin selection). But in Natural Selection: Domains, Levels and Changes (1992), where he gathered many threads of thought, he developed the important and superficially similar idea (foreshadowed in Adaptation and Natural Selection) of “clade selection” to explain, not “altruism” but macroevolutionary patterns of diversity and - as I would put it - “the evolution of evolvability.”

"Clade selection" seems like better terminology than "group selection" - in some respects. Like the term "kin selection" it puts an explicit emphasis on relatedness. Group selection only works - in the sense of producing group-level adaptations - when relatedness is involved. So: why not use the term clade selection?

Alas, I think there's a good reason not to use the term "clade selection". A clade consists of an organism and all its descendants. The problem is with the "all". You can still have group selection that works without it being clade selection - by the fitness delta involved not affecting all the descendants.

The "all" in "clade" makes "clade selection" a confusing term. Not worthless, perhaps, but I don't think I can endorse the term.