Ontogeny and Phylogeny

Xuhua Xia xia at cc.umanitoba.ca
Thu Apr 7 21:21:34 EST 1994

This is not for veterans. I am only popularizing the idea about ontogeny
and phylogeny that may be interesting to some netters. The idea is not
found in any evolutionary biology textbook I know of.

	     Developmental Biology and Evolutionary Biology

Traditionally, developmental biology deals with differential expression
of genes during ontogeny, whereas evolutionary biology deals with
differential transmission of genes over generations. There are some
interesting links between the two.

The ontogeny of an individual can be schematically represented below, 
with maternal mRNA transcripts translated first in the fertilized egg.
This triggers the expression of the earliest embryonic genes (say Group 1
genes), which in turn lead to the expression of Group 2 genes and so on,
until the last genes, Group N genes are expressed and the individual 
gets old and dies:

Egg with
Maternal --------Group 1----Group 2...Group n...Group N---death
Gene Product      genes      genes      genes    genes

                            Fig. 1

(This depiction is of course an oversimplification. Some genes are
expressed for a long time while some others may express for a brief
period. There are also side chains. etc. These complications will be
dealt with much latter)

Obviously, genes expressed in adult stage are located at the
Group N-terminal. Mutations on these Group N-terminal genes are not
expected to have major effect because few other genes have expression
patterns dependent on them. These mutations are the source of micro-
evolution and are the genetic variation most population biologists
are currently measuring. Selection on alternative alleles of these
genes leads us to believe gradualism.

If a mutation occurs at the early embryonic stage (towards the Group 1-
terminal in Fig. 1), then the expression pattern of many genes expressed
subsequently would be altered, and a hopeful (or hopeless) monster
would result. These mutations are the source of macroevolution and
should have been the subject of those studying speciation. Evolutionary
patterns caused by these genes leads us to punctuated equilibrium.

Several points can be made from this view:

1. Nucleotide substitution should be most frequent for genes at the
Group N-terminal, but least frequent for genes at the Group 1-terminal,
everything else being equal. This implies that different genes expressed
at different time during ontogeny can have very different molecular
clocks (evolutionary rates).

2. The rate of nucleotide substitution can change for a gene during
	1) if several groups of genes are added after the Group N-
	terminal so that the chain continues to Group N+1, Group
	N+2 ......, then Group N genes would become more conserved 
	than before. For example, in some species the sex determination
	genes are expressed in adult stage (e.g., some sex-switching fish
	species), these genes should be more variable than their counterparts 
	in species where sex determination genes are expressed much earlier.

	2) if several groups of genes at the Group N-terminal are deleted,
	which may occur with neoteny, then the opposite would occur.

  (Because neoteny could alter the rate of nucleotide substitution of 
  many genes, and because human evolved from ancestral apes through 
  neoteny, the timing of Eve and Adam is unlikely reliable)

3. If genes at the Group N-terminal only contribute to intraspecific
variation in adult stage, then the study of these genes may not be
relevant to speciation. It is those genes that are closer to the 
Group 1-terminal will allow the species to jump the trough in Wright's
adaptive landscape and result in speciation.

4. Darwin need not have worried about the lack of intermediate fossil
record because mutations towards the Group 1-terminal would affect
many genes expressed subsequently, leading to morphological jumps.

(to be continued)

Xuhua Xia

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