[asa] homology and analogy

From: Bill Powers <wjp@swcp.com>
Date: Fri Oct 09 2009 - 10:19:14 EDT

Evolutionary theory differentiates between homologous features and
analogous ones. The former are those features that can be traced to a
common ancestor, the latter are said to have arisen independently.

I am not concerned here with being able in practice to differentiate the
two, but rather with the argument that homologous features support an
evolutionary theory.

We might attempt to define homologous features as those wherein the
probability of them having a common origin is greater than that they might
have arisen independently.

So, for example, that so many mammals have 5 digits might be judged to
proabablistically suggest that they have a common origin, rather than that
they arose independently.

Given this definition, however, it does not unambiguously support

Consider for example two well-known processes. The first, we'll call it
Darwinism, is one in which processes obey certain physical laws, but is
otherwise blind. The second, we'll call it ID, is one in which there is
an intentional designer.

What homology supports, we'll say, is a common origin. But homology alone
cannot differentiate Darwinism from ID. What homology supports is, in
these two instances, evolutionary common descent or a common designer, and
opposes evolutionary independent descent or multiple designers.

There may be many more possibilities than Darwinism and ID. In all these
cases, homology simply argues for a common origin. But the nature of that
origin is unspecified.


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Received on Fri Oct 9 10:19:46 2009

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