Wednesday, June 10,
2009
As Darwin observed, natural selection leading to adaptation of
individuals and populations is occurring gradually and all the
time. But over very long spans of time, the major channels of
genetic organization, organism form, and the different ways
organisms develop arose as outcomes of history-dependent variation
that is now channeled, or constrained, within different groups of
organisms. For example, most cats look like cats, develop like
cats, but have a fossil record that begins from less than cat-like
ancestors. So do snails, and crabs, and so on. But what if the
broad evolutionary diversification of one of these groups were
repeated by a few species in a single genus tens of millions of
years after that initial diversification? What would that say about
the roles of contingency, constraint, and adaptation? In other
words, how big is the role of chance in the history of life?
An international team of researchers including Field Museum curator
Scott Lidgard, PhD, has discovered a group of closely related
living species that independently repeated the different step-like
changes that occurred in the major diversification of their kind
during the Cretaceous Period, roughly 100 to 90 million years ago.
But this group of species arose 80 million years later!
The findings of Dr. Lidgard and his collaborators will be published
online this week by the British journal, Proceedings of the Royal
Society B. Dr. Lidgard's research focuses on cheilostome bryozoans,
marine animal colonies whose bodies are made up of many genetically
identical box-like individuals (zooids). In the simplest, most
primitive cheilostomes, the soft feeding organ is squeezed out of
the box by muscles pulling on a flexible membrane. The next step in
diversification was calcified spines around the membrane, then
fusion of the spines, then reduction of the fused spinal shield and
membrane and invention of a water sac inside the box to provide
enough volume to squeeze out the feeding organ. Lineages showing
each of these stages are alive today. Then as now, these steps are
seen as evolved defenses against small predators and parasites on
the colony surface.
What is remarkable is that the molecular genealogy of the living
species shows their origin only 15 million years ago, with the same
trajectory as in the distant past! Evidence suggests that
trajectory has occurred again and again in other groups. The
authors argue that the original trajectory was highly contingent on
a set of initial conditions, but that given the possibilities
afforded by time, a genetic background would arise (like flipping a
coin long enough to achieve 10 heads or tails in a row) that was
visible to natural selection, most likely driven by predation.
Acting together, the eventual realization of a particular genetic
and developmental channel, and natural selection opened the way for
an adaptive solution.
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Field Museum
Thanks to Field Museum for this
article.