Do species become more complex as they evolve? Ever since Darwin, scientists have accepted the fact that plants and animals are more complex today than their ancestors were millions of years ago.
A U-M paleontologist, who has just completed the first empirical test of this evolution-toward-complexity truism, says its not necessarily so.
The notion that morphological complexity increases in evolution is widely accepted in biology and paleontology, says Daniel W. McShea, assistant professor of geological sciences. The belief has never been tested, mainly because nobody knows how to define complexity, much less measure it.
McShea decided to tackle the problem by separating form from function; he measured complexity by seeing organisms as simply a collection of components or parts. Results of his experiment will be published in Evolution this summer.
In his experiment, McShea took detailed measurements in six different dimensions of individual vertebrae and spinal columns from about 30 modern mammalsincluding squirrels, deer, giraffes, whales and camelsand compared these with measurements of their ancestors vertebrae. I wanted an animal with a series of repeating parts, McShea says. The choice of vertebrae was arbitrary.
McShea classified evolutionary lineages as more complex if descendants had more differentiation among vertebrae or more irregularity in the arrangement of vertebrae than their ancestors had.
The data showed no significant increase or decrease in complexity in most cases, McShea says. In the few cases that did show a significant change over time, the descendants were just as likely to be less complex as more complex than their ancestors.
Some scientists, he notes, believe natural selection is a driving force. For example, some argue that because complex organisms have a greater number of more specialized parts, they can be more efficient and thereby increase their chances of surviving and reproducing.
Other scientists see entropythe natural tendency of everything in the universe to become disorganizedas the driving force pushing evolution toward increased complexity.
But McShea, who found no quantitative evidence for any driving force at all, thinks cultural bias and an idealization of progress may be the real reason scientists tend to see increasing complexity in evolution.
People may tend to read into evolution the progress and increasing complexity they see in technology, McShea suggests.
McShea admits his experiment is just one study of one organ within mammals, but believes the fact that he found no evidence for something most evolutionary biologists take for granted calls for additional research.
Financial support was provided by the National Science Foundation and the Searle Foundation. The experiment was designed to build on earlier research by John Cisne of Cornell University.