Does selection intensity increase when populations decrease? Absolute fitness, relative fitness, and the opportunity for
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Does selection intensity increase when populations decrease? Absolute fitness, relative fitness, and the opportunity for selection John O. Reiss
Received: 12 September 2012 / Accepted: 25 October 2012 Ó Springer Science+Business Media Dordrecht 2012
Abstract The variance in relative fitness, commonly called the ‘‘opportunity for selection,’’ is a measure of the maximum amount of selection that can occur in a population. I review the relation between fitness variance and population growth, showing that fitness variance is higher during periods of population decline. This is true both for survival and for commonly used models for variable descendant number (Poisson, negative binomial, generalized Poisson). Empirical evidence suggests that not just the opportunity for selection but also the actual selection occurring is commonly greater during such periods of population reduction. Keywords Individual fitness Reproductive success Population growth Fitness variance Demography
Introduction As we look toward an extended evolutionary synthesis (Pigliucci and Muller 2010), achieving a closer integration of ecological and evolutionary theory is a key challenge (Metcalf and Pavard 2006; Kokko and Lo´pez-Sepulcre 2007; Pelletier et al. 2009; Coulson et al. 2010; Schoener 2011). Studies of ‘‘eco-evolutionary dynamics’’ have focused on such issues as the role of phenotypic evolution in influencing ecological traits (e.g., Hairston et al. 2005; Matthews et al. 2011; Turcotte et al. 2011) and on the role of ecological change in driving evolution (e.g., Lavergne et al. 2010). However, there is as yet no generally recognized link between the population growth and decline that is critical to ecologists, and the intensity of natural selection that is critical to evolutionists. This disconnect arises because evolutionary analyses involve the relative fitness of members of the population, while ecological analyses involve absolute fitness and its relation to population persistence (Schoener 2011). The idea that increased intensity of selection might be tied to periods of population decline extends back to Wallace’s initial proposal of a theory of natural selection (1858). It J. O. Reiss (&) Department of Biological Sciences, Humboldt State University, Arcata, CA 95521, USA e-mail: [email protected]
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is of special relevance in the context of adaptation of populations to global climate change (Bell and Collins 2008; Visser 2008; Lavergne et al. 2010) and invasive species (Mooney and Cleland 2001; Strauss et al. 2006; Kinnison et al. 2008), under the general concept of ‘‘evolutionary rescue’’ (Gomulkiewicz and Holt 1995; Bell and Gonzalez 2009, 2011). Here, I review the simple and long-known mathematics showing that selection can be much more intense when populations decline than when they are stable or increasing. I focus on the relation between the variance in relative fitness (opportunity for selection) and mean absolute fitness (population growth rate). I investigate some common models for the distribution o
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