Evolution predicts change over time, not stasis; so researchers continue to search for an organism that shows significant evolutionary change. A recent analysis of marine creatures, however, revealed the unexpected: not the shift in body sizes predicted by evolutionary theory, but rather, an optimal body size unchanged for millions of years.
Biologists from the University of California at San Diego (UCSD) and the University of Chicago (UC) analyzed the range of body sizes for bivalves (two-shelled mollusks such as clams, oysters and mussels) found in the northeastern coastal waters of the Pacific Ocean (Kaustuv Roy et al., “Invariant Size - Frequency Distributions Along a Latitudinal Gradient in Marine Bivalves,” Proceedings of the National Academy of Sciences, USA 97 : 13150-55). The study characterized four groups of bivalves based on their general location: Panamic, Californian, Oregonian, and Arctic. The four groups of bivalves vary in several key ways: they reside in different ecological systems with different temperatures and oxygen contents; they experience different types and degrees of predatory attack; and they include different species.
The scientists from UCSD and UC chose to study body size (which correlates to body mass) since this characteristic impacts nearly every aspect of an organism’s biology and plays a major organizing role in ecological systems. They expected to find varying body sizes resulting from environmental adaptation. Instead, the biologists discovered that the range, peak, and median of body size distribution were the same for all four groups of bivalves.
In an attempt to account for these unexpected results, the scientists turned to a previously established theoretical model that incorporates resource use and likelihood of reproductive success. This model predicts that the peak of body size distribution will be at an intermediate body size. A larger body allows for more effective use of resources, whereas a smaller body size allows for increased reproductive success. An intermediate body size balances the two factors. The findings of the scientists agree with this model. This means that the nearly identical body size distribution possessed by bivalves living under diverse conditions is a perfect adaptation to their respective environments.
Because the peak body size distribution of bivalves maximizes both resource use and reproductive success, evolutionary theory predicts that organisms should have evolved toward this peak. The biologists tested this idea by comparing the body size distribution of current bivalves with Miocene bivalves (found in the fossil record from up to 15 million years ago). Contrary to evolutionary theory, the Miocene bivalves and current bivalves displayed the same body size profile. The UCSD and UC scientists could not detect any evolutionary movement either toward or away from optimal body size. They observed only random microevolutionary changes in body size that, over a 15-million year time span, were insignificant.
The body size profile observed for Miocene bivalves contradicts the evolutionary paradigm. Because the evolutionary model demands nonrandom movement toward an ideal body size, evolutionists must explain these two observations: random changes in body size, and an optimal body size distribution that is stable through time. The creation model, however, readily accommodates these observations. It predicts an ideal body size distribution from the point of creation, with random, microevolutionary drifts away from optimality over time.
As more and more observations fail to satisfy predictions made by the evolutionary model, the paradigm becomes less viable. These recent bivalve observations, which run contrary to the predictions of the model, add to the long list of problems encountered by the theory of evolution.