Researchers have put forward a simple model of development and gene
regulation that is capable of explaining patterns observed in the
distribution of morphologies and body plans (or, more generally,
phenotypes). The study, by Elhanan Borenstein of the Santa Fe Institute
and Stanford University and David Krakauer of the Santa Fe Institute
was published in this month’s issue of PLoS Computational Biology.
Nature truly displays a bewildering variety of shapes and forms.
Yet, with all its magnificence, this diversity still represents only a
tiny fraction of the endless ‘space’ of possibilities, and observed
phenotypes actually occupy only small, dense patches in the abstract
phenotypic space. Borenstein and Krakauer demonstrate that the
sparseness of variety in nature can be attributed to the interactions
between multiple genes and genetic controls involved in the development
of organisms – a much simpler explanation than previously suggested.
Borenstein and Krakauer further integrated their model with
phylogenetic dynamics, allowing developmental plans to evolve over
time. They showed that this hybrid developmental-phylogenetic model
reproduces patterns that are observed in the fossil record, including
increasing variation between taxonomic groups, accompanied by
decreasing variation within groups. This pattern is consistent with the
Cambrian radiation associated with a rapid proliferation of highly
disparate, multicellular animals, and suggests that much of the
variation seen today is as a result of simpler genetic controls dating
from much earlier in evolutionary time.
The findings presented in this study also bear directly on issues of
convergence (when very different organisms independently evolve similar
features). By including a model of development, rather different
genotypes can produce very similar phenotypes. Consequently, convergent
evolution, which the vast space of genotypes would suggest to be rare,
is allowed to become much more common.
One of the paradoxical implications of this study has been to show
how innovations in development that lead to an overall increase in the
number of accessible phenotypes, can lead to a reduction in selective
variance. In other words, while the potential for novel phenotypes
increases, the fraction of space these phenotypes occupies tends to
contract. They concluded that "The theory presented in our paper
complements the view of development as a key component in the
production of endless forms and highlights the crucial role of
development in constraining (as well as generating) biotic diversity."
Source : Public Library of Science. October 2008.