Lecture 4 - Conservation genetics

how is genetic drift enhanced in genetically declining populations?

because chance changes in gene frequency increases and there is a loss of genetic variation cause alleles fix randomly

how does inbreeding become more common in genetically declining populations

because heterozygosity is reduced, inbreeding depression from exposure is rare due to recessive alleles and there is a loss of heterozygote advantage for fitness

colonial breeders

all gather in one location and compete for mates

low densities

low reproductive rates

what are reproductive rates dependent on?

density

allee effect

small population size or density can negatively impact individual fitness or population growth rate

• smaller population might struggle more to survive and reproduce than larger populations

what are the demographic effects of declining populations?

• potential for allee effects is enhances

• increased risk for demographic stochasticity

demographic stochasticity

random differences among individuals in survivorship and reproduction create variability in population growth rate even if individuals have the same average survival and reproduction

• population demography analogue of genetic drift

genetics

influence of population decline on Ne (effective size) and its consequences should predominate in leading to extinction

demography

the effects of demographic stochasticity should predominate and lead to extinction before the effects of genetics can manifest according to Lande

cheetahs

generally lack genetic variation

• proportion of polymorphic loci in population is 2 to 4 %

• proportion of heterozygous loci per individual is 0.04 to 1.4%

MHC

major histidine compatibility

MHC locus in cheetahs

there is a striking lack of variation between them and they dont reject skin grafts from other cheetahs because they are so genetically similar

captive cheetahs

low sperm counts and high disease load which might indicate inbreeding

how was N and Ne lowered in cheetahs?

• bottleneck (lower Ne)

• inbreeding while N and Ne was low caused further reductions

• disease and MHC symptoms

what observations were seen in dead Namibian cheetah populations

no signs of disease, high viral load or infectious disease induced mortality suggesting immune system loci might be lacking variation

leading cause of mortality in cheetah cubs

75% due to predation when mother is hunting so low genetic variability is unlikely to affect predation effects

genetic diversity estimates

• MHC lower than other animals

• nucleotide diversity similar to other cats but higher than dogs

• amino acid variation at MHC is similar to other cats

modern view (cheetahs)

• inbreeding depression will affect demographic parameters (survival and reproduction)

• demographic stochasticity will speed up decline in Ne

how is genetic diversity estimated?

by allele number, nucleotide diversity or amino acid sequence diversity

Florida scrub jays

threatened non-migratory species that are declining in population due to human mediate habitat destruction

• individuals don’t move as a breeding adult

female Floridian scrub jays

tend to disperse farther than males so they might have more genetic structure

cooperative breeders

if parents are having trouble, children will stay to help rather than seek out reproduction of their own (florida scrub jays)

male observed dispersal patterns and genetic consequences

they disperse at shorter distances than females so identity by descent is stronger in males indicating that dispersal is female-biased dispersal

• so at a given distance male scrub jays are more related to each other than females

identity by descent

identical stretch of DNA inherited from a common recent ancestor

female chromosomes

ZW

male chromosomes

ZZ

expected chromosome patterns of IBD

since the Z chromosome spends 2/3 of their time in a male and females disperse further greater identity by descent is expected for the Z chromosome at small distances

what happens when identity by descent goes up?

immigration declines because inbred birds from other populations are homozygous at different stretches of the genome than resident birds

what is the impact of inbreeding depression on the scrub jays?

• reduced immigration in population increasing inbreeding

• inbreeding depression negatively impacts hatching success

what is the advantage that comes with small inbred populations?

they can bring new hybrid superiority back into the main large population

Florida panthers

population declining in numbers and developmental abnormalities like males having one testicle and 100-degree tail end indicating inbreeding depression

how was genetic rescue attempted in the florida panther?

8 female pumas were translocate from texas to florida in hopes of increasing genetic diversity, improving population numbers and reversing indications of inbreeding depression

genetic rescue results

N and Ne numbers increase and the number of mixed individuals increased increasing genetic diversity

how did genetic rescue fail?

it only temporarily increased population because 10% of the Texas panthers are killed yearly by car accidents

how does genetic rescue work as a time buying strategy?

temporarily increase the population size making the population still viable in order to buy time to fix environmental and other factors that were the primary drivers of endangerment in the species