Biology Reference
In-Depth Information
l
Loss of organs does not imply loss of the developmental potential to form
those organs.
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Life-history stages (embryos, larvae, adults) can develop and evolve
separately, providing opportunities to modify and modulate embryonic
development, for specialization or diversification of adult structure, and for
the evolution of novel structures.
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Homology is now seen as hierarchical, with homologous genes initiating
development of structures that are not homologous (such as arthropod and
vertebrate eyes) and homologous structures developing by processes that are
not homologous.
For example, studies seem to confirm that there was an inversion of
the dorso-ventral axis during animal evolution, with the ventral region of
Drosophila
homologous to the dorsal side of vertebrates (
De Robertis and Sasai
1996
). Thus, developmental systems that control patterns from eggs to adults
are remarkably similar across a wide range of phyla despite at least half a bil-
lion years of evolution since their origin from their last common ancestor (
Erwin
et al. 1997
). Despite their genetic similarities, the developmental systems pro-
duce very different body plans. The basic aspects of the developmental control
systems of long-extinct animals can now be reconstructed and the diversification
of animal form and the evolution of the genetic controls that regulate it are
becoming understood (
Erwin et al. 1997
).
The field of evo-devo uses the concept of homology. However,
homology
is
a difficult and, sometimes, fuzzy term (
Bolker and Raff 1996
). There are at
least nine homology concepts in current use in the comparative biology litera-
ture, which makes evo-devo a particularly contentious topic (
Janies and DeSalle
1999
). The concept of homology originated from classical studies of comparative
adult morphology and one classical definition is “the same organ in different
animals under every variety of form and function.” Another is “derived from
an equivalent characteristic of the common ancestor.” Homology has been used
recently to be “shared patterns of gene expression,” a controversial use of the
term (
Bolker and Raff 1996
), and more often is limited to “similarities due to
descent from the same ancestral source.” Because most genes play multiple roles
in development, it is difficult to resolve homologies. For example, arthropods,
annelids and chordates all possess segments. It remains unclear whether the seg-
ments evolved independently or are derived from a common ancestor (
Davis and
Patel 1999
). A review of the successes of evo-devo by
Brakefield (2011)
noted
“the evolution of endless forms is more about the evolution of the regulatory
machinery of ancient genes than the origin and elaboration of new genes.”
