Genetic diversity
Chapter 5
Genetic diversity occurs at 4 levels:
 Among species
 Among populations
 Within populations
 Within individuals
Diversity between species
Diversity within species
Diversity within populations
Diversity within individuals
Measuring genetic diversity
 Protein electrophoresis (indirect method)
 Restriction Fragment Length Polymorphism (RFLP)
 Random Amplification of Polymorphic DNA (RAPDs)
 Microsatellites or Simple Sequence Repeat (SSR) Polymorphisms
 Amplified Fragment Length Polymorphisms(AFLP)
 DNA sequencing (direct method)
Genetic variation in proteins
 Enzymes are run through electrophoresis
 Electrophoretically distinguishable forms of an enzyme are called allozymes
 Not all amino acid substitutions alter electrophoretic mobility
Allozyme gel
Table 5.1
Polymorphism (P)
 Proportion of genes that are polymorphic (frequency of common allele is less
than 95%)
 Table 5.1 – Bison example
 24 different genes, only 1 polymorphic
 MDH -1 has gene frequency of 0.6 for one allele and 0.4 for second allele
 Estimated polymorphism is 1/24 = 4.2%
Heterozygosity (H)
 Proportion of genes at which the average individual is heterozygous
 In Bison example, 2/5 individuals were heterozygous, so H = 0.4 for this gene
 Sum heterozygosity across all genes and H = 0.4/24 = 0.017
Use of H
 Compare actual heterozygosity versus expected heterozygosity
 Partition variability to determine how much variability is due to variability
among populations (Dst) and between populations (Hs)
 Ht = Hs + Dst
 Dst is related to Fst
FST
Which will have a higher Dst/Fst?
Quantitative variation
 Continuous characters are polygenic and environmentally affected
 Complicates the picture
Why is genetic diversity important?
 Evolutionary potential
 Loss of fitness
 Utilitarian values
Evolutionary potential
 Genetic variation is the raw material for natural selection
 Greater diversity means greater ability to evolve with changing conditions
 Humans are top agent of change
Selection by overharvesting
Greater diversity means greater ability to colonize wider range of habitats
 Greater genetic diversity in amphibians living in forests than in aquatic habitats
Genetic diversity is related to reproductive fitness
Loss of fitness
 Inbreeding depression
Isolated populations have more inbreeding
Illinois prairie chicken
Disease susceptibility in
California sea lions
Inbreeding increases extinction risk
Heterosis
 Heterozygote advantage or hybrid vigor
Evolutionary potential of heterozygous population
 A population dominated by heterozygotes will also have greater genetic
variation
Anomaly – Outbreeding depression
 Local adaptation
Gene flow can reduce adaptiveness
Eichornia paniculata
 Outcrossing Brazilian population and self-fertilizing Jamaican population
 Inbred each for 5 generations then outcrossed the inbred lines
No evidence of heterosis in self-fertilizing population
Utilitarian values
 Importance in domestic species
Crop diversity
What causes reduction in diversity?
Small populations are prone to loss of genetic diversity through genetic drift
Loss of heterozygosity in small populations
 Loss of heterozygosity (H)
Genetic drift
 Random fluctuation in allele frequencies over time by chance
 Important in small populations
 Founder effect
 Bottleneck effect
Genetic drift
 When a few individuals remain in a population (or found a new population),
genetic constitution depends on the genes of the small population
 A low number of individuals may lead to low genetic diversity
 Gene frequency may not represent that of the population that founders
came from
Bottleneck effect
 Drastic reduction in population and gene pool size
Founder Effect
 Effect of drift when a small number of individuals start a new population
 Effect is pronounced on isolated islands
Contribution of 21 founders to the captive Guam Rail population
Table 5.2
Table 5.3
Genetic drift
 Random change in allele frequencies due to sampling error in small
populations
 Mathematically, genetic drift represents a chronic bottleneck that results in
repeated losses of variability and eventual fixation of loci
Table 5.4
Regeneration of diversity
 Loss of heterozygosity (H)
Regeneration of diversity
 Mutation takes over 100 generations to regenerate diversity at a single locus
Genetically effective population size (Ne)

Variation from idealized population

1:1 sex ratio

Sexually-reproducing

Non-overlapping generations

Even distribution of progeny among females

No selection

No mutation

Population size relevant for determining genetic effects OR the number of
individuals contributing genes to the next generation

Ne is typically 1/3 – 1/4 of Nc
Reasons why Ne<Nc
 Unequal numbers of progeny
 Unequal breeding sex ratio
 Fluctuating population size
 Assortative mating
Unequal numbers of progeny
Unequal breeding sex ratio
Fluctuating population size
Fluctuating population size
Inbreeding
 Mating of close relatives leads to:
 Reduced heterozygosity
 Reduced fecundity
 Increased mortality
Estimate inbreeding coefficient (F)
Gathering genetic information
 How do you get DNA samples without disturbing the animals?