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recapitulation




On Wed, 21 Jan 1998, Ray Stanford wrote:

> Your transmission on feather development is valuable. Thanks.
> But, you say in the other communication that ontogeny's recapitulation
> phylogeny is "...merely disproved"! I could almost think some bible-belt
>fundamentalist is talking "creation science" here; but, granting the
>benefit of a doubt, CAN YOU PROVIDE A RELIABLE REFERENCE FOR YOUR
>ASSERTION, i.e., FROM A QUALIFIED RESEARCHER WRITING IN A REPUTABLE,
>PEER-REVIEWED PUBLICATION? If entirely true (and not just a personal
>revelation), your 'information' could be of major importance and a major
>area of embriologic research can now retire!

If I may step in on Ron's behalf here, it is accepted by modern
evolutionary biologists that ontogeny can recapitulate phylogeny but it
doesn't necessarily have too. Some good books written by eminant
scientists on this subject are:

Gould, S. J. 1977. Ontogeny and Phylogeny. Harvard University Press:
Cambridge, Mass.

McKinney, M. L. & McNamara, K. J. 1991 Heterochrony.The evolution of
ontogeny. Plenum Press: New York.

The references in these could lead you into the primary literature.
In the meantime you could try this thought experiment. For recapitulation
to occur any evolutionary novelty has to be added to the end of the
ontogeny of the ancestral organism. But there is no mechanism that
constrains new genes (mutations) from exerting there effects at the end of
ontogeny. Imagine a juvenile stage of an organism that was undergoing
strong selection pressure for a particular trait. Any mutations that cause
that trait to be expressed during the juvenile stage will be favoured and 
this new trait will be 'injected' early in ontogeny not at the end of it. 
In short I think it boils down to "juveniles need to survive just as much 
as adults". A real world dinosaurian example of this would be the teeth of
dromaeosaurids. The juvenile skulls of Velociraptor bear distinctive
teeth, they are unserrated and have a constricted base, whereas those of
adults are typical blade-like serrated theropod teeth. Now if ontogeny was
recapitulating phylogeny we would have to assume that the unserrated tooth
with constricted base was the ancestral condition. By outgroup comparison
we can see that the adult tooth morphology is the ancestral one. 

whew! that took longer than I thought!

Adam Yates