Lecture 9 Genetic Consequences of Invasion Low at the beginning of many invasions may have negative, neutral, or sometimes positive (at least in the short term) effects. A lack of genetic diversity: - Can limit responses to environmental stochasticity. - Can limit ability of a population to adapt to novel environments. - Can contribute to . However, low genetic diversity can facilitate invasion in some species In social insects, low genetic diversity allows for higher population densities as individual colonies fail to recognize other unrelated colonies as distinct, preventing the aggression that maintains lower population density where the species are native. Some species have been enormously successful despite very low genetic diversity. Hemlock woolly adelgid appears to have been founded largely by a single female lineage. Since it reproduced clonally in North America, the only is that accrued through . How important is the founder effect (bottleneck) for Invaders? Important - Reduced genetic diversity - Less for selection to act on, thus less responsive to variable environments. - Expression of deleterious through inbreeding – leads to Not important - Genetic ‘purging’ - Advantage of relatedness - Many invasive species are not really bottlenecked Inbreeding Depression acts on deleterious alleles in populations § dominant deleterious alleles expressed, ↓carrier's , 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 is healthy and passes on the deleterious allele. Large populations – recessives rarely expressed. Small populations - close relatives mate, and likely carry the same recessive deleterious alleles. Thus offspring inherit two copies of a recessive deleterious allele which is then expressed Inbreeding depression – only when closely related individuals mate are the deleterious recessive alleles exposed in the homozygous state.

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. Harmonia axyridis (Harlequin lady beetle) underwent a bottleneck of intermediate intensity when it invaded Europe Related individuals had an increased probability of mating, increasing the homozygosity of the population. This exposed deleterious recessive alleles to natural selection and the fitness of the population decreased in the short term. Deleterious recessive alleles were then purged from the from population after ~15 generations because once exposed (as a homozygous recessive), natural selection eliminated them.

Overcoming Negative Effects of Low Genetic Diversity Many populations of invasive species do not remain genetically depauperate for very long. Multiple introductions: • May rapidly increase genetic variability • May be the norm rather than the exception for accidentally introduced species Many taxa (such as birds and insects) are relatively tolerant of inbreeding and the effects of low genetic diversity may not be very severe and the consequences to fitness, at least over short periods of ecological time, are not great.