| In the early 1890s, the American paleontologist Henry Fairfield Osborn lamented that "after studying Evolution for a century we are in a perfect chaos of opinion as to its factors." This "perfect chaos" was no small matter: a few years earlier, the famed philosopher Herbert Spencer had declared in his book Factors of Organic Evolution that determining the primary causal factors in evolution should demand, "beyond all other questions whatever, the attention of scientific men." By the 1890s, a full-fledged debate over the importance of various factors was underway among philosophers, psychologists, and biologists: long-defended factors were viciously attacked, and several new factors were proposed. Fifty years later, however, the debate seemed to be settled. Neo-Darwinism, which rose to prominence in the 1890s, had won the day: natural selection, acting on heritable variation due to genetic mutation and recombination, was judged to be the primary factor in biological evolution. But as the decades wore on, many biologists came to question the so-called Modern Synthesis of genetics and natural selection, and in biology today, one again encounters debates over the operation of factors other than selection at several hierarchical levels.;In this dissertation, I analyze historical and modern debates about the relative importance of a subset of these causal factors. I demonstrate that closer attention to the details of both historical arguments and recent experimental work leads to a clearer conception of each factor, making it possible for scientists to address the 'factors of evolution' question empirically. The different factors that I examine -- constraints, convergence, and ecosystem engineering -- stem from different characterizations of how organisms interact with their environments. Constraints on variation are internal factors that bias the production of variants, thus limiting the power of the external selective environment (Chapter 3). Convergence, or the independent evolution of the same traits in unrelated lineages, is often taken to show the opposite -- the omnipotence of environmental 'forcing' (Chapter 4). Finally, ecosystem engineering complicates the internal-external divide by exploring how organisms modify their physical environments (Chapter 5). Two key historical episodes provide an introduction to many of the conceptual distinctions of the later chapters: the rise of the idea of organism-environment interaction in the 1850s (Chapter 1), and the seminal debate over the factors of evolution in the 1890s (Chapter 2). This dissertation is not, in Charles Darwin's words, "one long argument." Instead, it is a series of thematically linked studies, each of which contributes to ongoing historical, biological, and philosophical debates about organism-environment interaction and the causal factors of evolution. |
