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Chapter 13
Evolution and Development
Chapter Summary
Early ideas on what today is called evo–devo can be found in the work of the ancient Greek philosophers and the concept of the "great chain of being." They noted a parallel between what would one day be called the
scala naturae
, which involves the relationships between species, and the
developmental stages
of organisms.
J. F. Meckel hypothesized that the developmental stages of an organism step through all the animal species that came before it on the
scala naturae.
Ernst Haeckel expanded on Meckel’s ideas with his
biogenetic law,
which states "ontogeny is a precise and compressed recapitulation of phylogeny."
von Baer’s law states that embryos in closely related species resemble each other, and not the adult form of some ancestral species. The most general characteristics that are shared among embryos from closely related species appear early in embryonic development, while specialized traits appear later in development.
Gavin de Beer coined the term
heterochrony
to describe changes in the rate of development, and he focused on whether the time at which a trait was first expressed in a given species was accelerated or decelerated relative to that of an ancestral species.
One of the key players associated with the dynamic rules that govern development are the
homeotic genes
that specify position within an embryo.
The position of
Hox
genes on a chromosome corresponds to the position on the anterior–posterior axis of the body part that the
Hox
gene
regulates. This phenomenon is known as colinearity. Homologous
Hox
genes are ordered on chromosomes in a similar way across vertebrates and invertebrates.
The expression patterns of regulatory genes (such as
Hox
genes) in their local cellular environment are responsible for the very different sorts of body plans that we see in vertebrates and invertebrates.
Homeotic genes display deep homologies.
Hox
genes have been uncovered in polyps, mollusks, earthworms, and octopuses. In each case, the genes are involved in constructing the anterior, central, and posterior body parts of these creatures.
Ontogeny is also guided by regulatory enhancers. A regulatory enhancer of a gene is a section of DNA that lies outside of that gene but is involved in
regulating
the timing and level of that gene’s expression.
A gene can have numerous regulatory enhancers associated with it. A gene with multiple regulatory enhancers can be expressed differently in different parts of the body and at different points in time. Regulatory enhancers increase morphological variation, and hence the amount of variation that natural selection has to act on.
Duplicated genes can evolve into
paralogs
. Paralogs of homeotic genes allow new developmental pathways to emerge, and these new pathways may help explain the diversity of forms that we see in nature.