Evolutionist Gavin de Beer (1971) has shown that homologous structures
arise from different, nonhomologous genes, which means that they cannot be
derived from common ancestors.
It was 1971 when de Beer made his argument. That was before technology
for manipulating DNA made it possible to examine genes directly, so de
Beer's conclusions (and those of Hardy 1965, making essentially the
same argument) were based on indirect evidence. Since then, many
similar genes have been found to participate in the development of
homologous structures (see, e.g., Carroll 2005).
Granted, some of the examples raised by de Beer have not yet been
explained in detail. For example, some organs considered homologous
arise from different layers of embryological tissues. But although
such cases are not explained, that does not mean they are
unexplainable. We now know that organs can be stimulated to grow in
many parts of the body (such as eyes growing on a fly's wings) simply
by ensuring that the proper signalling chemicals are present. Thus
homologous organs arising from different areas may result simply from
mutations in where signalling proteins are expressed.
The difference in finger development
between
birds and theropod
dinosaurs shows an example of how a small difference in development can
lead to a nonobvious difference in adult form.
References:
Carroll, Sean B. 2005. Endless Forms Most Beautiful. New York:
W. W. Norton.
de Beer, Gavin. 1971. Homology, an unsolved problem. Oxford Biology
Readers, J.J. Head and O.E. Lowenstein, eds., Oxford University Press.
Reprinted (abridged) in Ridley, Mark, 1997. Evolution. Oxford
University Press, 211-221.
Hardy, Alister. 1965. The Living Stream, New York: Harper & Row,
pp. 209-219.
Further Reading:
Carroll, Sean B. 2005. Endless Forms Most Beautiful. New York:
W. W. Norton.
