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 squee
|Contact: Nancy O'Shea|