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Plant Evolution: Adaptation or Historical Accident?

by Dr. Karl Niklas
Professor, Plant Biology, Cornell University

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Nothing in evolutionary biology has inspired more debates than adaptive evolution - whether natural selection is typically strong or comparatively weak. If the former, then most species are adapted in the sense they have been selected on the basis of environmental sorting. If not, then most species evolutionarily survive because their differences go largely undetected by natural selection. This debate continues with most agreeing that adaptive evolution has occurred, but there is no consensus regarding its prevalence. In large part this is because traditional disciplines of evolutionary biology offer limited insight when taken in isolation.

In order to resolve the debate, some biologists have turned to modeling the interactions between random and non-random evolutionary forces to gain insight into the a priori effects that different assumptions about the roles of different evolutionary forces have on the history of life. One such model, which will be discussed in detail, deals with the early evolution of land plants.

The evolution of land plants model is based on a computer that generates a "universe" containing billions of possible early land plant architectures. These plant models are quantitatively explored in terms of their abilities to perform various biological functions essential to plant growth,  survival, reproduction. This "universe" is used to predict how plant architecture must change in order to assure evolutionary success as the environment changes, and to draw comparisons between the predicted and observed evolutionary changes in plant architecture during the colonization of land. Our model indicates that adaptive responses to the physical environment were likely to be the dominant theme during the evolution of land plants and adaptive evolution became more easily accomplished, as the functional obligation of plants became more complex. If true, then the model for early land plant evolution provides an exceptionally powerful tool with which to teach the dynamics of the evolutionary process because it immediately exposes the biological implications of our a priori assumptions about how the process works.

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