Biodiversity IV: genetics and conservation

Bio 415/615

Questions

1. What is heterozygosity, and how does it represent genetic diversity?

2. What is the 50/500 rule for conserving populations?

3. What factors influence effective population size?

4. How is genetic diversity used for descriptive vs. functional goals in conservation?

Florida panther

• Lowest variation of 31 panther spp in N and S America– Allozyme polymorphism

• 4.9% vs. 7.3-17.1% at 41 loci

• Vs. museum specimens of Florida panther– 1.2 vs. 2.3 alleles per locus

Florida panther

• N = 80-100?

Allozyme DNAHe

He

– Main population 1.8 10.4– Introgressed population 1.8 29.7– W’rn US 4.3 46.9– Other cats 3-8

• Inbreeding depression: abnormal sperm, cardiac abnormalities, susceptibilty to infectious diseases, cowlicks

Florida panther update

http://floridapanther.org 35 new kittens, 27 mortalities (2010-2011)

Heterozygosity• Genes can exist in many forms called

alleles (big B, little b; dominant vs. recessive)

• Alleles are specific to genetic loci = region of DNA

• Heterozygosity is the proportion of diploid genotypes in a population composed of two different alleles

• If alleles are identical in an individual, that individual is homozygous

How is genetic diversity measured?

• At sampled loci (by allozymes, DNA)– Polymorphic vs. fixed (after fixation),

Polymorphism• Because of rare mutations, employ arbitrary

cut-off (e.g., most dominant allele <99% or <95% frequency)

– Heterozygosity, Ho (observed) and He (expected heterozygosity)• Average heterozygosity• Proportion heterozygosity retained

– Allelic diversity, Average allelic diversity, Effective number of alleles

Genetics are used in two basic ways in conservation biology

• Descriptive genetics– A tool for understanding ecological and

evolutionary patterns

• Functional genetics– Used as a proxy for endangerment (e.g., low

heterozygosity is a problem)

Descriptive Genetics• Resolve taxonomy

– Phylogeny in conservation value– Evolutionarily Significant Units (ESUs)

• Detect mating systems, parentage, migration, gene flow, hybridization

• Understand historic distribution and potential for reintroduction from museum specimens

• Choose material for reintroduction, including geographic pattern and genetic diversity

• Forensics• Understand pattern: genetic diversity

within and between population

Functional genetics

• As Area , spp• As N , genetic diversity

– Loss on sampling– Ongoing loss (genetic drift)

• Function for fitness, evolutionary adaptation

Genetic diversity is said to be functional in conservation

because:

1. Genetic diversity is correlated with short-term fitness.

2. Genetic diversity is correlated with long-term evolutionary potential.

Fitness is defined as the “lifetime reproductive success of a genotype relative to other genotypes”.

Effective population size (Ne)

• N = number of individuals in a population• Are inds equal in their capacity to increase

population size/heterozygosity?– too old to reproduce– can’t find a mate– UNEQUAL GENETIC UNIQUENESS

• Ne/N ranges from 0.02 to 0.4 with mean of ~0.1 (Frankham 1995); thus genetic changes are contributed by only about 1/10 of the individuals!

What affects Ne?

What affects Ne?

1. Unequal sex ratio

Ne = 4(Nm)(Nf) / (Nm + Nf)

Nf = number of females; Nm = number of males

What affects Ne?

2. Variance in family sizeVariation in family size occurs when some pairs have

no or very few offspring and others have large family sizes.  The formula tells us that the effective population size decreases as the variance in family size increases above 2

Ne = (4N – 2)/(Vk + 2)Vk = variance in family size

Why does family size variance matter?

What affects Ne?

3. Variance in population size through generations

Ne = t/Σ (1/Nei)Nei = effective population size for generation I;

t = number of generations

What affects Ne?

3. Variance in population size through generations

For example: what is Ne for a population with 10, 100, and

25 individuals over the course of three generations?Average value = 45 (wrong answer)

Ne = 3 / (1/10 + 1/100 + 1/25) = 20

Genetic diversity = Ne?

• Because we can estimate how genetic frequencies should change with certain Ne, we can flip the equation around and estimate Ne from the observed change in heterozygosity:Ht/Ho = e-t/2Ne

Ht/Ho = ratio of current (Ht) to prior (Ho) heterozygosity t = number of generations

For example, the northern hairy wombat had retained 41% of its original heterozygosity over 12 generations.  From this data, Ne was calculated as 6.7

Genetic drift

• The genes of one generation are a sample of the genes of the previous generation.  If that sample is small, it will deviate from the previous generation by chance. 

• Given time, small populations will become homozygous—fixed for particular alleles. 

Genetic drift

• Rare alleles are more likely to be lost.

• If population size is small, drift may play a bigger role than selection in allele frequencies, so that the traits that become fixed are not necessarily the ones that convey highest fitness.

50/500 Rule – Soule and Frankel (1980)

(based on mutation rates in Drosophila)

50 reproductive individuals necessary to prevent losing much genetic variation over the short term

500 necessary to prevent loss over the long term

Lande (1995) suggests that 5000 individuals may be necessary

Why is loss of heterozygosity bad?

• Inbreeding depression = higher mortality, lower reproduction in individuals sharing alleles by common descent

• Inbreeding depression = higher mortality, lower reproduction in individuals sharing alleles by common descent

Why is loss of heterozygosity bad?

Florida panther malformities: kinked tail, cowlick, abnormal sperm

Deleterious alleles

Johnson et al. 2010

Johnson et al. 2010