According to the evolutionary theory, homologous features are
programmed by similar genes. Gene sequence similarity would indicate
common ancestry since such similarities are unlikely to originate
independently through random mutations. If the bones of the human arm
evolved from the same precursors as the wing of a bat and the hoof of a
horse as evolution teaches, then we should be able to trace these
alleged homologies to the DNA that codes for them. Some geneticists
thought this knowledge would allow them to find the chemical formula
needed to produce an arm, leg, or other structure. But once biologists
acquired a greater understanding of genetics, they found that what are
labeled as homologous structures in different species often are produced
by quite different genes.
Homology predicted that features produced by similar
genetic sequences are phylogenetically homologous. There are now so many
exceptions to this prediction that the concept of genetic homology
cannot now be said to be a rule, but the exception. The classic example
is mutations in certain homeotic genes41 which can cause wholesale
changes in morphology such as producing two pairs of wings instead of
the normal single pair, or replacing a fly’ antenna with a leg (or can
even cause eyes to develop on the fly’ leg). Genes that produce results
similar to the homeotic genes for flies’ wings have been found in most
other animal kinds, including mammals and humans.
In another example, the gene that controls mouse eye
colour also happens to control the mouse’ physical size; but the gene
that controls the fruit fly’ eye colour controls not the fruit fly’
size, but female sex organ morphology.43 Although mice and flies share a
similar gene (called eyeless) which functions to control their eye
development, the fly’ multifaceted eye is profoundly different from a
mouse’ mammal eye. In both the fly Antennapedia and mouse eyeless,
similar homeotic genes control development of structures which are not
homologous by either the post-Darwinian phylogenetic or the classical
morphological definition.
The finding that similar genes regulate such radically
different structures strongly argues against the concept of homology.
So many genes used in higher organisms have multiple effects that Ernst
Mayr once suggested that genes which control only a single
characteristic are rare or nonexistent. The finding that a consistent
one-gene/one-characteristic correspondence does not exist has been a
major set back to the Darwinian interpretation of homology. Because
evolutionary biologists have failed to provide a biological basis for
their homology research findings, Roth concluded ‘that the title of de
Beer’ 1971 essay|--|Homology, an unsolved problem|--|remains an accurate
description … . The relationships between processes at genetic,
developmental, gross phenotypic and evolutionary levels remain a black
box’.45 Research at the molecular level has failed to demonstrate the
expected correspondence between gene product changes and the organismal
changes predicted by evolution.
Evolution by DNA mutations ‘is largely uncoupled from morphological
evolution’.46 An example of this is the large morphological
dissimilarity that exists between humans and chimpanzees despite a high
similarity in their DNA. In short we now know:
“ … in general the homology of structures such as
organs or modules cannot be ascribed to inheritance of homologous genes
or sets of genes. Consequently, organ homology cannot be reduced to gene
homology. Van Valen recognizes this too and therefore suggests, as an
alternative, to reduce homology to a continuity of [developmental]
information. Information is not the same as genotypic nucleic acid. But
what it is exactly, and how it is continuous, is still an unsolved
problem.”
Evolutionary Naturalism or an Intelligent Designer?