Lectures by Juan Paritsis

• Genetic diversity is the variation at the level of individual genes

• In a population, genetic diversity, means that the population contains most of the possible alleles for each particular gene locus

Provides a mechanism for populations to adapt to their changing environment

The more variation, the better the chance that at least some of the individuals will have an allelic variant that is suited for the new environment, and will produce offspring with the variant that will in turn reproduce and continue the population into subsequent generations.

• Loss of genetic diversity can imply lack of evolution and premature extinction
• Fitness decreases with reduced genetic variation
• Populations of endangered species are small and tend to lose genetic diversity
• When genetic variation disappears the basis for life on earth becomes impoverished

• Locus: physical location of a gene
• Allele: one of the genes at a particular locus
• Homozygous: two of the same alleles at a given locus

• Heterozygous: two different alleles at a given locus
• Fitness: contribution of an individual’s genotype to the next generation

1. Within individuals
2. Among individuals within the same population
3. Among populations

Every diploid organism has duplicated genetic information (from its mother and father)

It is the same type of information (locus) but the specific forms may differ (alleles)

Different individuals carry different genetic information

The sum of the variation of an interbreeding population is called Gene pool

• Different gene pools among populations.

• Isolation is and important factor responsible for among population genetic diversity (natural or man made barriers). Can lead to local adaptation

• Natural local adaptation: Example of mistletoe species in Argentina and Chile

• Ultimate effect of among population variability:
• Speciation

• In an individual organism, is the state of possessing different alleles at a given locus in regard to a given character

• At the population level: "The fraction of individuals in a population that are heterozygous for a specific locus"
• Often positively correlated with fitness
• Often positively correlated with population size (declines in small populations)

When an organism is referred to as a heterozygote, or being heterozygous for a specific gene, it means that the organism carries two different versions of that gene on the two corresponding chromosomes. Heterozygosity refers to both the state of being a heterozygote, but more commonly in population genetics to the fraction of individuals in a population that are heterozygous for that locus.

Two ways of loosing it…

1. Loss of genetic diversity in small populations
2. Changes in the natural distribution of genetic diversity among populations (artificial isolation and mixing)

Loss of genetic diversity in small populations

• Population size critical factor
• Census population size (Nc) vs. Effective population size (Ne)

• Usually Ne is much smaller that Nc (10 to 30%)

Four factors responsible for genetic diversity loss in small populations:

1.Genetic drift

2.Founder effect

3.Demographic bottleneck

4.Inbreeding

• " Random change in gene frequency within a population" (Meffe et al.)

• Not necessarily adaptive

• Stronger in small populations

"The establishment of a new population by a few original founders which carry only a small fraction of the total genetic variation of the parental population." (Ernst Mayr)

• Alleles may be lost

• Differentiation from the parental population

Example: Irish Potato Famine

• Population suffers reduction in size and then recovers
• Random losses of genetic diversity

• Usually associated with catastrophic events or diseases
• Genetic variation doesn't rebound from a decrease as quickly as population size.

• Inbreeding is breeding between close relatives

• Leads to a reduction in genetic diversity (inbreeding depression: reduced health and fitness)
• The primary problem with inbreeding is that two closely related individuals are likely to have very similar genomes, and if one individual has a gene for a given negative trait, then the other is likely to have it as well.
• Inbreeding increases homozygosity (therefore decreases heterozygosity)

The other way to loss genetic diversity:

Related with the geographical distribution of the species and therefore with landscape management issues

Avoids genetic flux among populations due to barriers such as highways, dams, etc

Enhances genetic flux among populations where that flux was not possible due to natural barriers (examples: bridges, tunnels)

• Is the number of alleles in a sample (population)

• Rare alleles are important during extreme environmental events

• Loss of allelic richness is perhaps more serious than loss of quantitative variation because alleles are lots forever while quantitative variation can be recovered
• Rare alleles are more important that their frequency in the population

Example: Peppered moth and pollution in Manchester

• Small sized populations are more prone to lose allelic richness

• Useful in (some examples):

–Endangered small populations

–Captive breeding

–Translocation of individuals

–Determining dispersal patterns

• Recognize limitations and scale of application

• It’s easy to lose the sight of the larger picture and try to apply a genetics approach when it is not appropriate to do so.

Conclusions…

• Very powerful tool
• Usually useful to solve specific problems
• Rarely useful if it is not combined with other type of approaches