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How Big A Role Does Chance Play In The History Of Life?

Date:
June 23, 2009
Source:
Field Museum
Summary:
Researchers have 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 remarkably, this group of species arose some 80 million years later.

Zooids of four living species of the bryozoan genus Cauloramphus showing the stepwise evolution (left to right, top to bottom) from a species with simple, flexible frontal membrane with spines (more primitive), to a species with a cage of spines not yet fused over the membrane, to a species with a frontal shield made of fused spined and a large frontal membrane, to a species with a reduced frontal shield of tighly fused spines. A water sac (the ascus, hidden below the shield) has evolved in the last species, as the frontal membrane is so reduced that simplu pulling on it would not change the internal volume enough to squeeze out the feeding organ (polypide).
Credit: Micrographs by Matthew H. Dick

If the broad evolutionary diversification of a group of organisms were repeated by a few species in a single genus tens of millions of years after the group's initial diversification, what would that say about the roles of contingency, constraint, and adaptation?

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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.


Story Source:

The above story is based on materials provided by Field Museum. Note: Materials may be edited for content and length.


Journal Reference:

  1. Matthew H. Dick, Scott Lidgard, Dennis P. Gordon, Shunsuke F. Mawatari. The origin of ascophoran bryozoans was historically contingent but likely. Proceedings of The Royal Society B Biological Sciences, 2009; DOI: 10.1098/rspb.2009.0704

Cite This Page:

Field Museum. "How Big A Role Does Chance Play In The History Of Life?." ScienceDaily. ScienceDaily, 23 June 2009. <www.sciencedaily.com/releases/2009/06/090609220721.htm>.
Field Museum. (2009, June 23). How Big A Role Does Chance Play In The History Of Life?. ScienceDaily. Retrieved January 27, 2015 from www.sciencedaily.com/releases/2009/06/090609220721.htm
Field Museum. "How Big A Role Does Chance Play In The History Of Life?." ScienceDaily. www.sciencedaily.com/releases/2009/06/090609220721.htm (accessed January 27, 2015).

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