Mid-term Flashcards
4 Primary genetic factors in the extinction of small populations are
1) loss of genetic diversity
2) inbreeding depression
3) mutational meltdown
4) hybridisation
Loss of genetic diversity
Loss of diversity (i.e. heterozygosity) due to drift is greatest in small populations
But loss of heterozygosity is also affected by population growth rate and the presence of overlapping generations
Loss of heterozygosity each generation equation
Ht+1 = Ht[1 - (1/2Ne)]
Loss of diversity is common in threatened populations
Threatened species have significantly lower genetic diveristy (Ht) than non-threatened species (Hnt), across all taxa
e.g. Kakapo
Loss of alleles
- although moderate population size reduction may not have a large effect on heterozygosity, it will still have a large effect on allelic diversity because of genetic drift will rapidly eliminate low frequency alleles
- long term response to selection depends on allelic diversity
- short-term persistence may be affected if loss of alleles at major histocompatibility complex loci, due to increased susceptibility to disease
Inbreeding Depression
- inbreeding depression is a decrease in fitness due to mating between related individuals
- results from the presence of partially recessive deleterious mutations maintained by selection - mutation balance
- mating between relatives increases homozygosity and therefore the deleterious effects become fully expressed and fitness of inbred individuals decreases
- increased homozygosity may also reduce the opportunity for expression of over-dominance which will also lead to reduced fitness
Mutational meltdown
- mutational meltdown is the decline in reproductive rate and downward spiral towards extinction due to chance fixation of new mildly deleterious mutations in small populations i.e. it is the accumulation of mildly deleterious mutations due to genetic drift
- two phases: 1) fitness drops but populations are still able to more than replace themselves 2) fitness is less than required for replacement and the population declines towards extinction. Mutational meltdown strictly refers to the second phase
Hybridisation
Abthropogenic hybridisation (i.e. resulting from human activities) can result in extinction where hybridisation is extensive and 'pure' individuals no longer exist - complete admixture leads to extinction
Two common measures of genetic diversity
- Allelic diversity
- average number of alleles (A = total alleles / # loci)
- effective number of alleles (ne = 1/∑pi2) - Expected heterozygosity ( expected under Hardy-Weinberg equilibrium). Expected heterozygosity reported rather than observed because it is less sensitive to sample size.
He =1-∑pi2
Hardy-Weinberg equilibrium
Uses in conservation genetics:
1. Null model to test real populations against. Describes genotype frequencies when there is no evolution (i.e. no mutation, no migration, no selection, no genetic drift) and is randomly mating with no population subdivision.
2. Estimation of recessive allele frequencies.
3. Estimation of frequency of carriers (heterozygotes).
Hardy-Weinberg Equilibrium (HWE) for single locus with only 2 alleles:
p2 +2pq+q2 =1.
HWE & Sex-linked loci
- Homogametic sex (e.g. XX in mammals) expected genotype frequencies = same as for autosomal loci (p2 + 2pq + q2 for 2 alleles).
- Heterogametic sex (e.g. XY in mammals) expected genotype frequencies for locus on X = p + q (i.e. the allele frequencies).
HWE & Polyploids
Polyploids – more than 2 copies of each chromosome (common in plants).
e.g. tetraploid – 4 copies of each chromosome (4n).
Deviations from HWE
Usually seen as either:
- Deficit of heterozygotes
1. Non-random mating (inbreeding / assortative mating)
2. Selection against heterozygotes
3. Population subdivision
4. Null alleles - Excess of heterozygotes
1. Selection for heterozygotes
2. Non-random mating (disassortative mating)
Effective inbreeding coefficient
Effective inbreeding coefficient (Fe) can be calculated from deviation from HWE:
Fe = 1 – (Hobs/Hexp) Inbreeding can be compared:
1. Within a population at one time (i.e. to assess non-random mating).
2. Between 2 populations (compare founder pop to source): Fe = 1 – (Hobs founder/Hobs source)
3. Over time (compare population today to same population several generations ago). Fe = 1 – (Hobs time 2/Hobs time 1)