Academic journal article Genetics

Population Genetic Consequences of the Allee Effect and the Role of Offspring-Number Variation

Academic journal article Genetics

Population Genetic Consequences of the Allee Effect and the Role of Offspring-Number Variation

Article excerpt

(ProQuest: ... denotes formulae omitted.)

THE demographic Allee effect, a reduction in per-capita population growth rate at small population sizes (Stephens et al. 1999), is of key importance for the fate of both endan- gered and newly introduced populations and has inspired an immense amount of empirical and theoretical research in ecology (Courchamp et al. 2008). By shaping the population dynamics of small populations, the Allee effect should also strongly influence the strength of genetic drift they are ex- posed to and hence their levels of genetic diversity and evolutionary potential. In contrast to the well-established ecological research on the Allee effect, however, research on its population genetic and evolutionary consequences is only just beginning (Kramer and Sarnelle 2008; Hallatschek and Nelson 2008; Roques et al. 2012). In this study, we focus on the case in which the average population growth rate is negative below a certain critical population size. This phenomenon is called a strong demographic Allee effect (Taylor and Hastings 2005). Our goal is to quantify levels of genetic diversity in introduced populations that have suc- cessfully overcome such a strong demographic Allee effect. Of course, the population genetic consequences of the Allee effect could depend on a variety of factors, some of which we investigated in a companion article (Wittmann et al. 2014). Here we focus on the role of variation in the number of surviving offspring produced by individuals or pairs in the population.

There are several reasons why we hypothesize offspring- number variation to play an important role in shaping the population genetic consequences of the Allee effect. First, variation in individual offspring number can contribute to variability in the population dynamics and this variability influences whether and how introduced populations can overcome the Allee effect. In a deterministic model without any variation, for instance, populations smaller than the critical size would always go extinct. With an increasing amount of stochastic variability, it becomes increasingly likely that a population below the critical population size establishes (Dennis 2002). Depending on the amount of variability, this may happen either quickly as a result of a single large fl uctuation or step-by-step through many gen- erations of small deviations from the average population dy- namics. Of course, the resulting population-size trajectories will differ in the associated strength of genetic drift. Apart from this indirect influence on genetic diversity, offspring- number variation also directly influences the strength of genetic drift for any given population-size trajectory. In offspring- number distributions with large variance, genetic drift tends to be strong because the individuals in the offspring gener- ation are distributed rather unequally among the individuals in the parent generation. In distributions with small vari- ance, on the other hand, genetic drift is weaker.

In Wittmann et al. (2014), we have studied several aspects of the population genetic consequences of the Allee effect for Poisson-distributed offspring numbers, a standard assumption in population genetics. However, deviations from the Poisson distribution have been detected in the distributions of lifetime reproductive success in many natural populations. Distribu- tions can be skewed and multimodal (Kendall and Wittmann 2010) and, unlike in the Poisson distribution, the variance in the number of surviving offspring is often considerably larger than the mean, as has been shown, among many other organ- isms, for tigers (Smith and Mcdougal 1991), cheetahs (Kelly et al. 1998), steelhead trout (Araki et al. 2007), and many highly fecund marine organisms such as oysters and cod (Hedgecock and Pudovkin 2011).

The number of offspring surviving to the next generation (for example the number of breeding adults produced by any one breeding adult) depends not only on the sizes of individual clutches, but also on the number of clutches produced and on offspring survival to adulthood. …

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