Inbreeding is mating between close relatives and can depress components of reproductive fitness thus having detrimental effects on the populations survival, a phenomenon known as inbreeding depression. There are two principal theories for the mechanism of inbreeding depression. The partial dominance hypothesis (Charlesworth and Charlesworth, 1987) suggests that inbreeding increases the frequency of homozygous combinations of deleterious recessive alleles due to the increased chance of offspring inheriting alleles identical by decent from both heterozygous parents. This is shown in figure 1 and results in a reduction of population fitness. Figure 1 Adapted from Madsen (1996)
However the overdominance hypothesis suggests that because inbreeding increases homozygotes it reduces the overall frequency of the superior heterozygote 's relative to Hardy Weinberg ratios if the population was randomly mating. This results in the loss of the heterozygote advantage and in a decrease in fitness (Charlesworth and Charlesworth 1987).
Both hypotheses are likely to be correct and that a combination of the two causes an overall increase in homozygotes and decrease in heterozygotes of the offspring relative to that of the population as a whole if it were mating randomly and in Hardy Weinberg frequencies. As a result there is a loss of genetic diversity, superior heterozygotes and increase in recessive mutations which leads to the loss in the populations ' evolutionary adaptability to cope and evolve to environmental change. This is indicated by the following equation for loss of neutral genetic variation in a random mating population:
Hg / H0 = [1 1 / (2Ne) ]g = 1 F (Frankham 2002)
Where Hg is the heterozygosity at generation g, H0 the initial heterozygosity, Ne the long-term effective population size and F the inbreeding coefficient. As stated by
References: 1) Bijlsma, R., Bungdaard, J. and Borerema, A.C. (2000) Does inbreeding affect the extinction risk of small populations?; predictions from Drosophila. J. of Evol. Biol. 13 502-514 2) Caughley, G 3) Charlesworth, D. & Charlesworth, (1987). B. Inbreeding depression and its evolutionary consequences. Annu. Rev. Ecol. Syst. 18, 237-268 4) Caro, T.M 5) Crnokrak, P. & Roff, D.A. (1999). Inbreeding depression in the wild. Heredity 83: 260–270. 6) Frankham, R. Inbreeding and extinction: a threshold effect. (1995). Conserv. Biol. 9, 792-799 7) Frankham, R., and Ralls, K 8) Frankham, R., Ballou, J. D. and Briscoe, D. A. (2002) Introduction to Conservation Genetics (Cambridge Univ. Press, Cambridge, UK) 9) Frankham 10) Greenwood, P. J., Harvey, P. H. and Perrins, C. M. (1978) Inbreeding and dispersal in the great tit. Nature 271, 52−54 11) Keller, L.F., Grant, P.R., Grant, B.R., Petren, K 12) Lande, R. (1988) Genetics and Demography in Biological Conservation. Science. 241: 1455- 1460 13) Madsen, T., Stille, B 14) Merola, M. (1993) A reassessment of homozygosity and the case fore inbreeding depression in the cheetah, Acinonyx jubatus: implications for conservation. Conservation Biology 8(4); 961-971 15) Nevo, E. (1978) Genetic variation in natural populaions ; patterns and theory. Theoretical Population Biology. 13 ; 121-177 16) Newman, D., and Pison, D 17) O 'Brien, S.J., Wildt, D.E., Goldman, D., Merril, C.R. and Bush, M. (1983) The cheetah is depauperate of genetic variation. Science. 221: 459-462 18) O 'Brien, S.J et al 19) Pray, L.A., Schwartz, J.M., Goodnight, C.J. and Stevens, L. (1994) Environmental dependency of Inbreeding depression: Implications for conservation biology. 20) Ralls, K. and Ballou, J. (1988) Estimates of lethal equivalents and the cost of inbreeding in mammals. Conservation Biology. 2: 185-193 21) Ralls, K 22) Reed, D.H., Lowe, E.H., Briscoe, D.A., Frankham, R. (2003) Fitness and adaptation in a novel environment: effect of inbreeding, prior environment, and lineage. Evolution Int J Org Evolution. Aug; 57(8):1822-8. 23) Roff, D.A. (2002) Inbreeding depression: tests of the overdominance and partial dominance hypotheses. Evolution Int J Org Evolution. Apr; 56(4):768-75 24) Saccheri, I 25) Spielman, D., Brook, B.W., and Frankham, R. (2004) Most species are not driven to extinction before genetic factors impact them. Proc Natl Acad Sci U S A. Oct 19; 101 (42): 15261-4. 26) Westemeier, R.L., et al (1998) Tracking the long-term decline and recovery of an isolated population. Science 282: 1695-1698