Inbreeding Depression Flashcards
Inbreeding results in decline in mean reproductive fitness
Depression in mean fertility, depression in mean sperm viability, depression in mean litter/clutch size and depression in mean lifetime reproductive success.
Initial belief that inbreeding depression didn’t exist in the wild
ID was originally thought to be an artifact of captive and experimental populations. Ralls and Ballou (1983), higher mortality in inbred versus outbred progeny of 44 mammal species.
F increases by 25% and effect to mean fitness characters
Human IQ and height decrease, chicken survival rate decreases, turkeys reproductive survival decreases.
Example, pink pigeon
Effect of inbreeding on survival of free-living birds. Highly inbred birds are likely only to survive for half as long as non-inbred birds. In the wild, population is reduced by 50% and even moderately inbred birds show reduced survival. Survival of all released birds is relatively poor, after 1.5 years of age, survival of inbred birds becomes drastically reduced.
Inbreeding depression increases extinction risk
Frankham (1995) Inbreeding and extinction: a threshold effect.
Used captive population of mice and fruit flies. Populations at different levels of inbreeding, threshold effect: F= above 0.3, probability of extinction rapidly increases. Same results in population inbred quickly or slowly.
Evidence that inbreeding depression leads to extinction in wild populations.
Saccheri et al (1998)
monitored 42 isolated populations, estimated mean F from micro satellite data. Inbred populations went extinct sooner, accounted for demographic and environmental variables. Clear evidience that inbreeding had a significant effect on extinction probability, even when demographic factors were accounted for.
Saccheri et al. 1998
Probability of extinction increases as average number of heterozygous loci decreases.
Inbreeding depression is most prominent in characters closely associated with fitness.
Frankel and Soule (1981), for every 10% rise in F, gave 25%reduction in total fitness.
Potentially Vulnerable characters in animals: Sperm quality and quantity, juvenile survival, mating ability and offspring numbers, maternal and competitive ability, developmental time and others.
Variation of IB between individuals and populaitons
Chance- Large random element to accumulation of deleterious alleles in small populations. Replicate lines shoe higher or lower accumulation due to chance. Individuals of same F will therefore have different levels of heterozygosity.
ID = cumulative effects of multiple loci
Lethal alleles will be a mixture of effects from mildly deleterious alleles.
Purging
exposure via expression of deleterious alleles allows them to be removed by selection more effectively, beneficial to the population. Only works: 1) at dominant Loci, over dominant loci harder for deleterious alleles to be flushed out. 2) at the most lethal alleles, mildly del. alleles are selectively neutral in small pop.
Efficiency of purging
1) Proportion of Dominant /Over dominant loci
2) Lethality of the alleles
3) F, inbreeding coefficient of individuals
4) S, selection coefficient of loci
5) N, population size
What to expect in small, purged populations
Lower frequency of near lethal alleles. Slightly lower frequency of moderately deleterious alleles. Higher frequency of mildly deleterious alleles.
Experimental evidience of ineffectiveness of purging
Frankham et al (2001)
Measured proportion of Drosophila populations that survived. Result: No difference between non-purged (outbred) and purged populations.
Recovering populations from inbreeding depression
Need to stimulate heterosis.
Outcross inbred population with outbred/unrelated individuals. introgression increased litter size and fitness.
