Lecture biology Conservation genetics

Conservation genetics: The goal is to maintain as much genetic diversity as possible for long-term evolution -how to measure genetic diversity in populations?. -% loci polymorphic popul

Conservation genetics:

The goal is to maintain as much genetic diversity as possible for long-term evolution

-how to measure genetic diversity in populations?

-% loci polymorphic (population measure)

-% loci heterozygous (average individual measure)

-what influences the maintenance of genetic diversity?

-population size and effective population size

-sex ratio

-inbreeding depression versus purifying selection

-two contrasting cases:

-cheetah

-northern elephant seal

Why worry about genetic diversity?

•  Population’s ability to respond to future environmental change

•  Genetic resources – useful to humans (crops, biochemicals, source for

transgenics)

•  Genetic diversity (individual or

population) often is directly proportional

to fitness

–  inbreeding depression if heterozygosity lost

Vertebrates 648 species surveyed

Invertebrates, 370 species surveyed

Plants, 785 species surveyed

How to measure genetic diversity in pop’n? Per cent of loci that are polymorphic

1/3 to 1/2 of all enzyme loci are polymorphic a measure of what populations look like

Per cent of loci that are heterozygous

Average heterozygosity = 4 to 15% = average % of an individual’s loci that are heterozygous or % of heterozygous genotypes averaged over all loci a measure of what individuals look like

[enzyme electrophoresis data; many sources, SJ O’Brien et al 1985, Freeman & Herron 2004] Group Loci Polymorphic Average

exam’d loci (%) heterozygosity (%) Drosophila (43 species) 24 43.1 14

Mus musculus 46 20.5 8.8

Sources of genetic variation

-  mutation

-  immigration

Processes that cause Losses

–  genetic drift = random loss over time

–  selection = deterministic

[some selection increases diversity

such as negative frequency-dependent

selection; most decreases diversity]

•  Population heterozygosity

‒  Measures variation in genetic makeup (what average individual looks like)

‒  Loss over time can be estimated: Heterozygosity is inversely related to population size

Loss of variation due to drift

Heterozygosity

in the future “effective” population size

(worry about what this means

later)

t

Ne

⎠

⎞

⎜

⎝

⎛ −

=

1 1

1

Effective Population Size (Ne)

= a measure of how many individuals are contributing their genes to the next generation

Ne << N in wild populations

Ne /N = 0.3 – 0.6, usually

Factors that affect Ne

1   Sex ratio of breeders

2.  Fluctuations in population size over time

1.  Sex ratio of breeders, if it is

than N

or

e

e

= +

4

4

1 4

Mating (breeding) system affects Ne

by changing the sex ratio of

breeders

•  More polygamous = lower Ne

•  Example: walrus 1Males : 20Females

2 Changes in population size over time

à main point is that average Ne is driven by smallest population size ∑

=

i

e

e

N

t

N

,

1

à If you have several good years & then a

bottleneck, the bottleneck reduces average Ne a lot

[sudden reduction in population size]

à  Example, 4 years: Ne = 100, 100, 10, 100 à the 10 has most influence on Ne

Ne = 30.8

Small Population Problem = Inbreeding Potentially confusing – 2 meanings

1 verb = mating with relatives (= consanguineous mating, 2 mates share common ancestry)

2 inbreeding coefficient = a measure of relatedness - how many genes are shared in common between 2 individuals

= the probability that two alleles drawn at random from a population are identical by descent from a common ancestor

à Lose genetic diversity through genetic drift

Inbreeding Depression

= decreased individual fitness due to the expression of deleterious alleles

↓ reproductive success & survival (↓ sperm production, ↑ developmental instability, etc.) Lethal equivalents = = number of loci in heterozygous state that

if homozygous would be lethal

Counted across loci – 2 alleles with 0.5 probability of causing death if homozygous = 10 alleles each with a 0.1 probability of causing death if homozygous

human average = ~15 (we are highly outbred, would

have very severe inbreeding depression)

Yet another context for genetic drift

Genetic diversity among cheetahs & others

[enzyme electrophoresis data; many sources, SJ O’Brien et al 1985, Freeman & Herron 2004]

Group Loci Polymorphic Average

exam’d loci (%) heterozygosity (%) Drosophila (43 species) 24 43.1 14

Mus musculus 46 20.5 8.8

Acinonyx jubatus 47 0.00 0.00

Individual cheetahs are so genetically similar that they accept skin grafts from each other à immune systems see the skin graft

as the same as “self”

Contrasting case:

Why did elephant seals fare better than cheetahs? Both: small pop’n (due to bottleneck) à inbreeding

Elephant seals: prior sexual selection for male size > few territories, only ~10% males mate >male genetic contributions come from few males >inbreeding

[the females MATE RANDOMLY with these few males]

à It is hypothesized that there are zero lethal equivalents in elephant seal population because the mating system allowed natural selection to purify the gene pool by purging it of alleles that are good when in heterozygotes, but lethal when homozygous

Is inbreeding always bad?

No

Some species regularly inbreed à wolves

à lots of plants & some animals

Purging the deleterious alleles (ok when heterozygous, lethal when homozygous)

through inbreeding

Speke’s Gazelle

à Had severe inbreeding depression

(low birth weight, low survival)

à Produced as many young as possible

(AA – good, Aa – medium,

aa – bad, some died)

à Bred the good ones

à Greatly reduced deleterious allels – inbreeding no longer

a problem

Is there hope for cheetahs? Maybe We hope yes