BIO 360   Ecology

Lecture 18:  Conservation genetics of small populations
 

I.  What is Conservation Genetics?

A.  Major issues for conservation of small populations
            1.  Inbreeding depression
            2.  Loss of genetic diversity
            3.  Fragmentation of populations and loss of gene flow
            4.  Genetic issues with captivity and  reintroduction




Important observations about genetics diversity

        1.  Genetic diversity is the fundamental component of biodiversity in conservation

        2.  High genetic variation in individuals or populations (heterozygosity) is positively correlated with lifetime reproductive success, or fitness

        3. Superiority of heterozygotes (overdominance)

        4. Genetic diversity may permit selection in response to environmental change

Note: 2-4 not true for all species


II.  Loss of genetic diversity in small populations

        1.  Founder effects: when a few individuals establish a new population, the genetic makeup of the population depends on the founder’s genes

        2.  Demographic bottlenecks, caused by severe, temporary reduction in population size
 
 

        3.  Genetic drift
            A.  Definition: random changes in allele frequecies caused by chance variation in survival and reproduction
            B.  Effects small in large populations, can be large in small populations
            C.  In the absence of selection, drift will cause an allele to become lost or fixed
 

        4.  Inbreeding
            A. Mating of individuals of common ancestry
            B. More likely in small population

            C.  Is often the result of a population bottleneck
            D.  May result in reduction in offspring size, growth, survivorship, or fecundity and changes in age at maturity, and physical deformities

            E. Consequences of inbreeding depression
                           1.  Greater probability of deleterious recessive alleles appearing in homozygous form
                           2.  Reduction of fitness of homozygotes

             Inbreeding avoidance through behaviors, including mate choice and dispersal.
             Plants are often self-incompatible.
 

              D.  Note: possibility of outbreeding depression, when:
                    1) individuals with different gene complexes mate and produce sterile offspring

                    2) individuals adapted for particular environments produce offspring that are not adapted for their current environment
 

III.    What is effective population size Ne?
            A.  Effective population size:  Number of individuals that contribute genes to future populations

            B.  Ne is usually less than census size in most populations
                    estimated from demographic data
                    also from genetic data (short, long term)


            C.  What factors influence estimates of Ne?
                1.  Unequal proportions of females and males
                2.  Sex ratios distorted by mating system (e.g. harem)
                3.  Stochastic variation in small populations
                4.  Anthropogenic effects
 

IV.    Measures of diversity 

            1. Allelic diversity (A), Mean number of alleles per locus
            2. Heterozygosity (H), Sum of proportions of heterozygotes at all loci divided by number of loci
            3. Percent loci polymorphic (P), Percent of loci with >1 allele


        A.  Measuring genetic diversity
                1.  Molecular marker: defined segments of DNA in genome
                2.  Allows precise estimates of genetic diversity

                3.  Heterozygosity: 2 alleles are different (for diploid individuals)

V.  Genetics and conservation
        A.  Recent example, Westemeier et al.  1998
            1.  Long-term study of prairie chicken populations
            2.  Populations have declined...
            3.  ...and so has fitness
            4.  Introduced new individuals from other populations
            5.  Fitness increased

        B.  Guidelines
            – Large population size better than small
            – Concern about losses of genetic variability (not absolute amount)
            – Encourage rapid population growth after collapse
            – Prevent homogenization