Abstract
Understanding the effects of inbreeding and genetic drift within populations and hybridization between genetically differentiated populations is important for many basic and applied questions in ecology and evolutionary biology. The magnitudes and even the directions of these effects can be influenced by various factors, especially by the current and historical population size (i.e. inbreeding rate). Using Drosophila littoralis as a model species, we studied the effect of inbreeding rate over a range of inbreeding levels on (i) mean fitness of a population (relative to that of an outbred control population), (ii) within-population inbreeding depression (reduction in fitness of offspring from inbred versus random mating within a population) and (iii) heterosis (increase in fitness of offspring from interpopulation versus within-population random mating). Inbreeding rate was manipulated by using three population sizes (2, 10 and 40), and fitness was measured as offspring survival and fecundity. Fast inbreeding (smaller effective population size) resulted in greater reduction in population mean fitness than slow inbreeding, when populations were compared over similar inbreeding coefficients. Correspondingly, populations with faster inbreeding expressed more heterosis upon interpopulation hybridization. Inbreeding depression within the populations did not have a clear relationship with either the rate or the level of inbreeding.
Highlights
The effects of inbreeding, genetic drift and interpopulation hybridization on fitness are relevant for many basic and applied questions in ecology and evolutionary biology, such as metapopulation dynamics (Hanski 1999), evolution of mating and dispersal strategies (Pusey and Wolf 1996), speciation (Coyne and Orr 2004), success of invasive species (Ellstrand and Schierenbeck 2000) and conservation of endangered species (Hedrick et al 2011)
At low levels of inbreeding (f < 0.40), survival was lower in the N2 compared with the N40 populations; survival in the N10 populations was not different from the other population sizes (Fig. 1A; Table S4)
In the N2 populations, egg-to-adult survival decreased with increasing level of inbreeding; in the two larger population sizes, inbreeding level had no significant effect (Fig. 1A; Table S5)
Summary
The effects of inbreeding, genetic drift and interpopulation hybridization on fitness are relevant for many basic and applied questions in ecology and evolutionary biology, such as metapopulation dynamics (Hanski 1999), evolution of mating and dispersal strategies (Pusey and Wolf 1996), speciation (Coyne and Orr 2004), success of invasive species (Ellstrand and Schierenbeck 2000) and conservation of endangered species (Hedrick et al 2011). Inbreeding (mating between close relatives) increases offspring homozygosity and usually results in reduced fitness. Hybridization among genetically differentiated populations, on the other hand, is known to have the potential to alleviate the effects of inbreeding and drift by increasing heterozygosity in the population (Whitlock et al 2000). When population size is small, inbreeding and genetic drift both increase because the number of individuals contributing to each generation is limited (Keller and Waller 2002). Average fitness in a small population is expected to decrease from generation to generation as the level of inbreeding (i.e. homozygosity) increases (Crow and Kimura 1970; Wang et al 1999; Keller and Waller 2002).
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