Lizard Genome Promises Great Advances in Understanding
Evolution
Posted by Jonathan Losos
National Geographic 
August 31, 2011 
http://newswatch.nationalgeographic.com/2011/08/31/lizard-genome-promises-great-advances-in-understanding-evolution/

The genome of Anolis carolinensis has just been
published in the journal Nature, and most attention is
focusing on how this genome, the first reptile to be
sequenced (not including birds), differs from other
vertebrate genomes, and what these differences may tell
us about genome evolution. No doubt this is interesting,
but the real value of this genome-in my unbiased
opinion-resides in the questions we finally will be able
to address about the evolutionary process, particularly
in one model system of evolutionary study, Anolis
lizards. Chris Schneider published a perceptive article,
"Exploiting genomic resources in studies of speciation
and adaptive radiation of lizards in the genus Anolis,"
on this topic three years ago, and I will briefly expand
on his points here (for more information on anole
biodiversity and evolution, go to Anole Annals).

An anole genome will be useful for evolutionary studies
in two ways. First, a long-standing question in
evolutionary biology concerns the genetic basis of
convergent evolution (i.e., when two or more
evolutionary lineages independently evolve similar
features). Do convergent phenotypes arise by convergent
evolution of the same genetic changes, or do different
lineages utilize different mutations to produce the same
phenotype? In other words, does convergence at the
phenotypic level result from convergent change at the
genetic level, or can different genetic changes produce
the same phenotypic response? In the last few years,
molecular evolutionary biologists have produced a wealth
of studies investigating whether convergent changes in
coat color in rodents, eye and spine loss in fish,
bristle loss in fruit flies and many other changes are
the result of changes in the same gene, even some times
by the very same genetic mutation. Underlying these
questions are more fundamental questions about
constraints and the predictability of evolution.

The anole ecomorphs, habitat specialists behaviorally
and morphologically adapted to use different parts of
the environment. The same set of ecomorphs (with several
exceptions) have evolved independently on each island in
the Greater Antilles. Figure from "Lizards in an
Evolutionary Tree: Ecology and Adaptive Radiation of
Anoles.".

Anolis lizards are, of course, the poster child for
evolutionary studies of convergent evolution. Indeed,
convergence has run rampant in this clade.  Anoles are
famous for the evolution of "ecomorphs," sets of habitat
specialist types that have evolved repeatedly on each
island in the Greater Antilles to occupy different
habitat niches. This convergence is usually studied in
terms of limb length, tail length, and toepad
dimensions: arboreal species have big toepads, twig
species short legs, grass species long tails, and so on,
with these traits independently evolving many times. But
the ecomorphs are convergent in many other traits that
have received less attention: head and pelvis
dimensions, sexual dimorphism in both size and shape,
territorial and foraging behavior, to name a few, and
the more closely we look, the more convergent traits we
find. And, further, anole convergence is not entirely an
ecomorph phenomenon; some traits vary within an ecomorph
class, but are convergent among species in different
ecomorph classes, for example, thermal physiology and
dewlap color.

In other words, there's more convergence in Anolis than
you can shake a stick at, and the availability of the
anole genome sequence will provide the tools to
investigate its underlying genetic basis.Anolis is
already a textbook example of replicated adaptive
radiation; getting at the genetics of this phenomenon
will provide great insight on how adaptive radiation
occurs and perhaps will help explain why anoles
experience such identical adaptive radiations so
readily, whereas most evolutionary lineages do not. In
addition, given the well understood ecological and
selective context for this convergence, genomic tools
may make anoles are an ideal group in which to study the
interplay between selection and developmental processes
in evolutionary diversification. See Thom Sanger's
recent post on the developmental basis of limb
convergence for one potential example.

The anole genome will be useful for evolutionary studies
in a second way. In recent years, a number of
researchers have used anoles to study the process of
natural selection and how it produces adaptation. Such
studies have been conducted by comparing populations of
the same species that live in different environments, by
following populations through time to see how they
change, and by measuring the action of natural selection
directly by following individuals and seeing how long
they live. Some of these studies have even been
experimental, altering selective conditions such as the
presence of predators and seeing how natural selection
changes and how, from one generation to the next, the
population evolves.

The anole genome now gives us powerful tools to study
natural selection and evolutionary change at the genetic
level. For many evolutionists, the holy grail is to
identify the actual genes under selection, and watch
them change in response to selection. Though still not
easy, this now is practical. In addition, the genome
will provide a wealth of material for other related
purposes, such as establishing maternity and paternity
to quantify reproductive success-a key component of
evolutionary fitness-and thus determine whether some
individuals produce more descendants than others.

One could argue that in terms of breadth and depth of
knowledge, Anolis is the best-studied species-rich
adaptive radiation. Other radiations are well known in
some respects, but for few do we know so much about so
many aspects of the ecology, behavior, functional
morphology, and physiology for so many species, not to
mention having a good understanding of phylogenetic
relationships and evolutionary processes. The genetic
basis of trait variation has always been the one hole in
our knowledge of anole evolutionary biology. The anole
genome plugs this hole in a major way, and will make
anoles an even more important evolutionary case study,
allowing us to learn much not only about evolution in
anoles, but the evolutionary process in general.

___________________________________________

Portside aims to provide material of interest to people
on the left that will help them to interpret the world
and to change it.

Submit via email: [log in to unmask]

Submit via the Web: http://portside.org/submittous3

Frequently asked questions: http://portside.org/faq

Sub/Unsub: http://portside.org/subscribe-and-unsubscribe

Search Portside archives: http://portside.org/archive

Contribute to Portside: https://portside.org/donate