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Transcript
Chapter 2 -- Genetics & Extinction Main Point -- Inbreeding & loss of genetic diversity are inevitable in small populations. These factors reduce reproduction & survival in the short-term and diminish capacity of population to evolve in response to environmental change in long-term. Inbreeding -- Production of offspring by individuals related by descent. Genetic Diversity -- Extent of heritable variation in a population, or a species, or across a group of species and include: heterozygosity allelic diversity haplotype diversity nucleotide diversity Rates of Inbreeding & Extinction Risks Slower inbreeding allows natural selection more opportunity to remove genetically compromised individuals (an therefore deleterious alleles). For fitness components such as survival & fecundity, slower rates of inbreeding generally lead to less inbreeding depression than fast inbreeding for the same total amount of inbreeding. Even slow rates of inbreeding though increase risk of extinction. The inbreeding coefficient (F) on an individual refers to how closely related its parents are. Parents Unrelated Brother-sister; mother-son, Or father-daughter Half-brother - half-sister First cousins Self-fertilization (or selfing) Offspring F 0 0.25 0.125 0.0625 0.5 Response of populations to environmental and demographic stochasticity and the impact of catastrophes are not independent of inbreeding and genetic diversity. Inbreeding, on average, reduces birth rates, increases death rates, and may distort sex ratios. Inbreeding therefore interacts with the basic parameters determining population viability, such as population growth rate. Adverse effects of inbreeding on population growth rates probably occur in most naturally outbreeding species. Experimental populations of mosquito fish founded From brother-sister pairs showed 56% lower Growth in numbers than populations founded from Unrelated individuals (Leberg 1990). Strong reductions in pop. Growth were also found In flour beetle populations inbred to small numbers. They even detected adverse effects at F = 0.1 (McCauley & Wade 1981). If populations become small for any reason, they become more inbred and less demographically stable, further reducing population size and increasing inbreeding. This feedback between reduced population size, loss of genetic diversity, and inbreeding is referred to the “Extinction Vortex”. Habitat loss Pollution Over-exploitation Exotic species Small, fragmented Isolated populations Environmental variation Catastrophes Reduced N Extinction Vortex Demographic stochasticity Reduced adaptability, Survival & reproduction Inbreeding Loss of Genetic diversity Smaller populations are expected to be more extinction prone than larger ones for demographic, ecological, and genetic reasons. Since the 1600s, the majority of recorded extinctions have been of island forms even though island species represent a minority of total species of all groups. Human factors have been the major recorded causes of extinctions on islands over the past 50,000 years. The mechanisms underlying susceptibility of island populations to extinction are controversial. Ecologists stress the susceptibility of small island populations to demographic and environmental stochasticity. However, this susceptibility is also predicted on genetic grounds. Island populations are expected to be inbred due to low number of founders and subsequent small population size. There is little evidence to separate the effects of non-genetic factors from the effects of inbreeding and loss of genetic diversity. Inbreeding can certainly diminish the resistance of a population by reducing its reproductive rate and survival such that it is more susceptible to non-genetic factors. Island populations typically have less genetic diversity and are more inbred than mainland populations. Analysis of 202 island populations revealed that 82% had lower levels of genetic diversity than their mainland counterparts. Inbreeding in many island populations is at levels where captive populations show an elevated risk of extinction. Relationships between loss of genetic diversity & Extinction Natural populations face continuous assaults from Environmental change including: New diseases Pests Parasites Competitors & predators Pollution Climatic cycles Human-induced global change Species must be able to evolve to cope with these new conditions or face extinction!! To evolve, species require genetic diversity. Naturally outbreeding species with large populations normally possess large stores of genetic diversity that confer differences among individuals in their response to such environmental changes. Due to sampling of alleles in the parental generation to produce offspring, small populations typically have lower levels of genetic diversity than large populations. Due to this random sampling process, some alleles increase in frequency, others decrease and some alleles may be lost entirely. The smaller the population the more change there will be between the parental and offspring gene pools. Over time, genetic diversity will decline, with Loss being more rapid in smaller than larger Populations.