The Evolution of Complex Organs
By T. Ryan Gregory
(Evolution: Education and Outreach, Oct. 2008)
The origin of complex biological structures has long been a subject of interest and debate. Two centuries ago, natural explanations for their occurrence were considered inconceivable. However, 150 years of scientific investigation have yielded a conceptual framework, abundant data, and a range of analytical tools capable of addressing this question. This article reviews the various direct and indirect evolutionary processes that contribute to the origins of complex organs. The evolution of eyes is used as a case study to illustrate these concepts, and several of the most common misconceptions about complex organ evolution are discussed.
As a career, science would hold very little appeal if all it entailed were the confirmation of existing knowledge or the memorization of long lists of well-established facts. Science thrives on what is not yet known: the more vexing a problem, the more inspiring it is to investigate. With millions of species alive today (and orders of magnitude more thought to be extinct), not only describing but also explaining the diversity, history, and complexity of life is a challenge nearly without equal in all of science. Nevertheless, the diligent accumulation of data punctuated by occasional empirical or theoretical breakthroughs has, over the past two centuries, yielded tremendous advances in the understanding of life's complexities and the historical origins thereof.
There was a time when natural processes capable of producing complex biological features were deemed inconceivable, leading to the conclusion that these, like human artifacts, must be the products of intelligent agency (e.g., Paley 1802). Beginning with Darwin's (1859) description of natural selection, and expanding considerably upon it in the 150 years since, the science of evolutionary biology has assembled a theoretical framework capable of explaining how complex features can arise naturally over time. Still, while few nonspecialists have trouble acknowledging small-scale evolutionary processes such as the evolution of antibiotic resistance within populations of bacteria, they often remain uncertain as to how similar mechanisms could account for complex structures such as eyes or wings (Ayala 2007; Scott and Matzke 2007).
This article provides a general overview of the various processes that play a role in the evolution of complex biological systems. The classic exemplars of organ complexity, eyes, are then used as a case study to illustrate these general mechanisms. Although it is not possible to deliver a comprehensive discussion of eye evolution within the confines of this paper, an extensive (but by no means exhaustive) reference list is provided in order to facilitate further study of the subject, as well as to highlight the rich scientific literature that exists on this topic but which may be largely unknown outside professional biological circles. Finally, some common misconceptions regarding the evolution of complex features are discussed.