Genetic Diversity Occurs at 4 Levels

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?