RESEARCH ARTICLE Morphological Integration, Evolutionary Constraints, and Extinction: A Computer Simulation-Based Study Brian Villmoare Received: 9 September 2011 / Accepted: 28 April 2012 / Published online: 20 May 2012 Ó Springer Science+Business Media, LLC 2012 Abstract It is well known that there is a strong rela- tionship among the environment, selection, and extinction, but the underlying role of genetics and genetic constraints in contributing to extinction is less appreciated. Integration of characters may enhance survivability for species, pro- viding that selective pressure is parallel with the patterns of morphological integration. However, we hypothesize that, if the direction of selection shifts, integration may also prevent populations from responding quickly enough to the new directions of selection. This would lead to the inability to find a successful adaptive solution, causing downward pressure on the population, and ultimately, extinction. We test this model with a computer simulation, using an adaptive landscape model. We generate populations of varying levels of multivariate integration and generate selection pressures to test the ability of the populations to respond to selection both parallel and orthogonal to the axis of maximum variation. In these simulations, more highly integrated populations survived longer when selection was in the direction of maximum variation. However, when selection was closer to orthogonal to the axis of maximum variation, extinction was more rapid in highly integrated populations. These results suggest that integration may play a strong role in both survivability and extinction. Tightly integrated populations are highly persistent when selection pressure is close to the axis of maximum variation, which is expected to frequently be the case since integration is likely often a product of selection. However, these highly integrated taxa are more susceptible to extinction when the direction of selection shifts, and is closer to orthogonal to the axis of maximum variation. Keywords Integration  Evolutionary constraints  Extinction  Selection  Environment  Computer simulation Introduction That environmental change plays a role in evolutionary transitions has been long known (Simpson 1953; Wright 1949). However, the complex interplay of environment, developmental mechanisms, and phenotypic expression, all of which play a role in extinction, is less well-understood (Berger and Lynch 1995; Lande and Shannon 1996; Gomulkiewicz and Holt 1995; Chevin et al. 2010). One of the key elements in biology on which evolutionary change rests is population variation (Darwin 1859; Maynard Smith 1976; Gomulkiewicz and Houle 2009). Selection requires sufficient variation in a population so that some part of the gene pool can survive to reproduce. This is especially the case when strong selection is acting on a species and only particular anatomical solutions may exist to the environ- mental circumstances. If the variation in a population is too limited, there may not be a sufficient number of individuals with phenotypes that successfully respond to the environ- ment, and the species may face extinction (Maynard Smith 1976; Kellermann et al. 2009). Morphological integration, which constrains elements of population variation by warping the overall pattern so that it falls along particular axes, also serves to facilitate B. Villmoare (&) Department of Anthropology, Center for the Advanced Study of Hominid Paleobiology, George Washington University, Washington, DC, USA e-mail: bav@gwu.edu B. Villmoare Department of Anthropology, University College London, London, UK 123 Evol Biol (2013) 40:76–83 DOI 10.1007/s11692-012-9186-3