Rise of the Animal Kingdom and Epigenetic Mechanisms of Evolution - Building the Most Complex Structure on Earth - page 287

expression. Indispensable as it is to develop and maintain animal structures, the
binary control must have been ready to go from the dawn of the Cambrian biota.
The Biological Demon 2—Selective Recruitment of Genes
The existence of the genetic toolkit and its high conservancy across the animal taxa
indicates that evolution employs basically the same genes for producing most dif-
ferent animal structures. Modern biology has demonstrated that the evolution of the
animal phenotypes depends on gene expression patterns and in the incorporation of
genes in new GRNs rather than on the evolution of genes themselves. This mecha-
nism of gene recruitment liberated metazoans from the need to evolve new genes for
new structures and functions. From this view, gene recruitment restrained the evolu-
tion of animal genes in the animal kingdom and can partially explain the discrepan-
cies or absence of a clear correlation between gene number and animal phenotypic
complexity.
Two questions arise in relation to the nature of gene recruitment. The first consid-
ers the mechanism of recruitment and the second relates to the mechanism of main-
taining the gene “recruited” in the next generation.
Modern biology has no clear-cut answer for any of these questions. However,
attempts to answer the first question on the mechanism of gene recruitment have
been made. Among these attempts, the most intriguing is Sean Carroll and John
True's hypothesis ( Carroll, 2005; True and Carroll, 2002 ).
They believe that gene recruitment may be a result of gene mutations, gene dupli-
cation, and mutations in the regulatory sequences of genes, especially in the Hox
genes that are important in the development of body plans. Not only are observations
on a relationship between mutations in Hox genes and changes in their functions
scarce but also, more importantly, they do not show whether the change in the func-
tion or recruitment for a new function occurred as a result of mutations or preceded
the occurrence of mutations. Later on, Carroll admits that these events are too rare to
account for the observed morphological diversification in the animal world ( Carroll,
2005 ). Relating to the possible role of gene duplication in gene recruitment, he also
acknowledges that:
the frequency of duplication events is not at all sufficient to account for the continu-
ous diversification of lineages.
Carroll (2005)
Hence he believes that the primary sources of morphological evolution are
changes in regulatory sequences. His argument is based on the example of the
human FOXP2 gene, which differs from that of the chimp in only two coding
sequences and that mutations in this gene can lead to speech disorders in humans
( Lai et al., 2001 ). But these facts in themselves do not hint, let alone prove, that
changes in the regulatory sequences of the FOXP2 gene may be related to the evolu-
tion of speech in humans. Even if such changes in regulatory sequences are found
down the road, this will not prove that the change caused the evolution of speech in
humans.