The new mutation theory of phenotypic evolution

Abstract

Recent studies of developmental biology have shown that the genes controlling phenotypic characters expressed in the early stage of development are highly conserved and that recent evolutionary changes have occurred primarily in the characters expressed in later stages of development. Even the genes controlling the latter characters are generally conserved, but there is a large component of neutral or nearly neutral genetic variation within and between closely related species. Phenotypic evolution occurs primarily by mutation of genes that interact with one another in the developmental process. The enormous amount of phenotypic diversity among different phyla or classes of organisms is a product of accumulation of novel mutations and their conservation that have facilitated adaptation to different environments. Novel mutations may be incorporated into the genome by natural selection (elimination of preexisting genotypes) or by random processes such as genetic and genomic drift. However, once the mutations are incorporated into the genome, they may generate developmental constraints that will affect the future direction of phenotypic evolution. It appears that the driving force of phenotypic evolution is mutation, and natural selection is of secondary importance.

Fig. 1.

Nucleotide sequences of the cis-elements for five bicoid (bcd), three hunchback (hb), and one giant (gt) transcription factors in the regulatory region of the even skipped (eve) enhancer 2 gene in six Drosophila species. N/A, no homologous sequence identified. –, nucleotide deletion. mel, D. melanogaster; sim, D. simulans; yak, D. yakuba; ere, D. erecta; pse, D. pseudoobscura; pic, D. picticornis. Adapted from Ludwig et al. (74). The phylogenetic relationships of these species are shown at the left-hand side of the diagram.

Fig. 2.

Schematic representation of human and chimpanzee evolution. The branches of the human and chimpanzee lineages are the species or subspecies that have become extinct in the past. The retrospective view of evolution is depicted by the smooth lines aiming at the current morphologies of the two species. It is assumed that the morphology of chimpanzees is similar to that of the common ancestor of the two species, whereas the human morphology has changed substantially. In the prospective view, the future evolution is unpredictable, and therefore the evolutionary process might have been deviated considerably from the smooth lines.