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evolution:phylogenesis as mirror of ontogenesis?

Paul Smith paul.smith at tip.nl
Thu Jan 30 17:41:51 EST 1997


On 29 Jan 1997 07:33:54 GMT, "Sarah at teleport.com" <lfitz at teleport.com>
wrote:

>Biologist used to say that "ontogeny recapitulates phylogeny"  an
>invertebrate taxonomist that I know  says "no more" . But I don't know why.
>
>Ontogeny being the stages in  embryonic development and phylogeny the
>stages in evolutionary development. For instance: a developing embryo
>resembles a fish  at one point because it has gill slits and at another
>stage an amphibian, both susposed stages in the evolutionary development
>chain. 
>
This, Haeckel's "biogenic law," still occasionally gets taught by high
school teachers but it's not in the textbooks because it doesn't
reflect current opinion. (Perhaps the last great biologist to believe
it was the late Sir Julian Huxley, and only then with reservations.)

The problem is that there are too many exceptions for it to count as a
law. Taken seriously, it would mean that modifications to development,
and thus phenotypic change, could only occur by addition to the end of
the process, and this clearly isn't the case. The adult form of a very
remote ancestor of a butterfly may have looked like a caterpillar but
catapillars have complex, not to mention constantly evolving,
adaptations to life as a caterpillar. If, for instance, the
caterpillar  were to develop  a way of neutralising a toxin recently
evolved by its habitual food plant, this would be a mutation affecting
an early stage of development only, and, moreover, it would be a
recent modification and not a primative one as would be the case if
development were a straightforward recapitualtion of ancestoral forms.

Biologists these days prefer to refer to Von Baer's Laws to explain
such things as the "gill slits" you metioned. These "laws" are really
a set of generalisations:

1. The general features of a large group of animals occur earlier in
the embryo than the specialised features.
2. More specialised characters develop from ealier, less specialised,
ones.
3. Instead of passing through the forms of ancestoral species as it
develops, an embryo diverges from them.
4. The embryo of a higher animal is never like the adult of a lower
animal but it does resemble *the lower animal's embryo*.

These laws are thought to result from "The conservative nature of
hereditary," to quote Gould. A mutation affecting early development is
not likely to result in a viable organism; a mutation near the end of
development will much more likely leave the organism viable and may
result in an increase in fitness. This doesn't mean that all
evolutionary change must occur by small increments, just that it's
more likely to. Again, it's a generalisation.

If you've got a spare week when you won't be doing much, you might try
reading "Ontogeny and Phylogeny," Stephen Jay Gould's study of the
history of the biogenic law controversies. It' not his most
approachable book but it has the best exposition I've seen of
heterochrony - the slowing and speeding up of morphological
development, and how this might allow rapid evolutionary change while
circumventing the problem of viability. He also covers the subject in
a more readable form in his first essay collection, "Ever Since
Darwin." Thanks for reading to the end. Hope it helped.

With friendly greetings, Paul Smith.



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