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Lecture 9 (Feb 12) Take Home Points • the Biology of Small Populations

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Lecture 9 (Feb 12) Take Home Points • the Biology of Small Populations Lecture 9 (Feb 12) Take Home Points • The Biology of Small Populations Small Population Paradigm • Small populations must contend with a series of forces that acting alone or in synchrony can push the population into an extinction vortex. • The vortex is a feedback loop: as the population gets smaller, the influence of these forces becomes larger, further reducing population size • Allee Effects Environmental Stochasticity • Temporal changes in vital rates (births and / or deaths) that affect all individuals of a given age or stage similarly; the sampling variances of the vital rates are nearly independent of population size • = largely a density independent process • Random environmental variation that affects a population • Can affect even large populations • May result in population extinction Demographic Stochasticity • Density dependent – the smaller a population gets, the more important this becomes. • Random fluctuation in vital rates (births deaths, sex ratio, etc) that is an intrinsic function of the population (not driven by external factors) • Variance increases with smaller populations – the larger fluctuations in the vital rates over time lead to a significant increase in extinction risk. Genetic Stochasticity -Random fluctuation in allele frequencies can lead to loss of particular genotypes and fixation of particular traits, independent of selection. Genetic drift is a key process in the loss of genetic diversity and increases in importance as the population gets smaller (= density dependent). -Small founder populations inherently contain limited genetic diversity, and inbreeding and genetic drift can enhance the loss of diversity. -Inbreeding: Populations that have a high frequency of closely related individuals can suffer from inbreeding. In inbred populations, deleterious alleles can be exposed in homozygous individuals, which may reduce fitness. The importance of inbreeding varies with species. Potential Genetic Consequences of Invasion Genetic Bottleneck and Founder Effect ◦ Constrained genetic diversity / small population sample Genetic Drift ◦ Random loss of alleles in small populations Inbreeding Depression ◦ Consequences of deleterious recessive genes becoming apparent through inbreeding Genetic Hotspots ◦ Repeated introductions lead to hyperdiversity and the potential for novel invasive genotypes Inbreeding Depression § Natural selection removes deleterious alleles from populations § dominant deleterious alleles expressed, ↓ carrier's fitness, and fewer copies wind up in the next generation. § recessive deleterious alleles are "hidden" from natural selection by dominant non-deleterious counterparts. An individual carrying a single recessive deleterious allele is healthy and passes on the deleterious allele. § Large populations – recessives rarely expressed. § Small populations - close relatives mate, and each individual likely carries the same recessive deleterious alleles. § Thus some offspring inherit two copies of a recessive deleterious allele which is then expressed. In newly established invasive populations, inbreeding is a potential problem. The long lagtime between establishment and invasion evident in some species may be inpart due to the exposure of deleterious alleles reducing the fitness of the population. Are Bottlenecks Bad for Invaders? Yes Reduced genetic diversity Less for selection to act on, thus less responsive to variable environments Expression of deleterious alleles through inbreeding Inbreeding depression and No Genetic ‘purging’ Advantage of relatedness Many invasive species are not really bottlenecked Genetic purging – moderately inbred populations can shed deleterious alleles over several generations as selection acts against them. After a number of generations, the fitness increases to that of outbred populations. Genetic Consequences of Invasion • Low genetic diversity • May have negative, neutral, or positive (short term) effects • Low genetic diversity – can limit responses to environmental stochasticity. Can limit ability of a population to adapt to novel environments. May contribute to inbreeding. • In some social insects, low genetic diversity can facilitate much higher population densities as individual colonies fail to recognize other independent colonies as distinct preventing the aggression that maintains lower population density where the species are native. Overcoming Negative Effects • Many populations of invasive species do not remain genetically limited for long. • Multiple introductions – may rapidly increase genetic variability • May be the norm rather than the exception for accidentally introduced species • • The consequences of reduced genetic diversity are variable • Effects may not be very severe and the consequences to fitness, at least over short periods of ecological time, are not great. • In some cases, a lack of genetic diversity may amplify the effects of invasions • Phenotypic plasticity can overcome low genetic diversity .
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