Salit Kark Department of Evolution, Systematics and Ecology

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Conservation Biology (Ecology) Lecture 4 November 2009. Salit Kark Department of Evolution, Systematics and Ecology The Silberman Institute of Life Sciences The Hebrew University of Jerusalem. Loss of genetic variability has multiple aspects. - PowerPoint PPT Presentation

Text of Salit Kark Department of Evolution, Systematics and Ecology

  • Salit Kark Department of Evolution, Systematics and EcologyThe Silberman Institute of Life SciencesThe Hebrew University of JerusalemConservation Biology (Ecology)Lecture 4November 2009

  • Loss of genetic variability has multiple aspectsspecific alleles will either be lost or retained (maintained)genetic variance (or heterozygosity) will be lost

  • Probability that alleles are lost in a founder population can be described by the following equation:

  • E = m - (1 - Pj) # of alleles leftat a locusafter foundation2N# of original allelesat a given locus# of foundersfrequency ofeach alleleLoss of Alleles

  • 4E = 4 - (.0081 + .6561 + .6561 + .6561) = 2.0236 alleles leftE = m - (1 - Pj)2N# of original allelesat a given locus# of alleles leftat a locusLet m be 4allele freq =p1= 0.70 p2 = p3 = p4 =0.10 N = 2 (two founders)


    1 1.48 1.12 2 2.02 1.23 6 3.15 1.64 10 3.63 2.00 50 3.99 3.60 >>50 4.00 4.00

  • Two things are clear from this example:1. More alleles are lost in populations founded by small numbers of individuals2. Alleles with a high frequency have relatively little influence, while alleles with low frequencies have considerable influence

  • Heterozygosity (H) Approximation of the proportion of heterozygosity remaining following the sudden reduction of a large population can be described by the following equation:

  • Hf = (1 - ) Hor 12N# foundersHeterozygosityremainingOriginalheterozygosity

  • # % of original percentagefounders heterozygosity lost remaining 1 50% 50% 2 75 25 6 91.7 8.3 10 95 5 20 97.5 2.5 50 99 1 100 99.5 0.5For any size of HOriginal

  • The expected proportion of variation remaining after t generations can be calculated by:Ht = (1 - ) Hor 12N tHeterozygosityretained aftert generations # generations# individualsoriginalheterozygosity

  • % Genetic Variance H remaining after t generationsPopSize (N) 1 5 10 100

    2 75 24 6

  • So, the following conclusions can be drawn:Small populations of constant size will lose heterozygosity through timeThe smaller the population is, the more rapidly heterozygosity will declineThe higher the number of generations a population of small size is bred the more heterozygosity is lost

  • During Bottlenecks the loss of alleles, especially rare ones, is much greater than the loss of heterozygosity

  • Rare allele freq. is 10%

    q2 = .012pq = .18

    Rare allele freq. is 1%

    q2 = .00012pq = .02

  • Changes following the foundation (or reduction in size)When population sizes are low, a population is, in effect, going through a serious bottleneck every generation, and the effects are cumulative

  • Factors affecting population genetic diversityPopulation structure, size, sex ratio etc

    Dispersal and gene flow in or out of the population

    Rates of various processes, (e.g., mutation)

    Recombination (creates new combinations of existing diversity)


    Genetic Drift


  • Various genetic variability estimates and markers can be used, such as:AllozymesSequencing (genes and others)mDNA, nuclear DNAMicrosatellitlesand many morewhich show different patterns of diversity

  • Clegg, S.M., S.M. Degnan, J. Kikkawa, et al. 2002. Genetic consequences of sequential founder events by an island-colonizing bird. PNAS 99:8127-8132FOUNDER EFFECTSsilvereye (Zosterops lateralis)

  • They chose to work with allelic variation at six microsatellite loci They found that allelic diversity gradually declined with repeated colonizations of new islands.The individual reductions are small, but the cumulative changes are large. From first to last in the sequence of recent colonizations, the mean number of alleles per locus dropped by almost half.

  • Because the last population in the sequence is the youngest, one cannot explain this result by long-term genetic drift. Instead, the pattern seems to reflect a small loss of alleles at each colonization, although hardly on the scale envisaged in the original formulation of the founder effects model.More in paper..

  • Effective Population SizeUP to now we made the assumption that the number of males and females contributing to each subsequent generation is the same

  • If the sex ratio is not 1:1 for each generation then the population loses genetic variability more rapidlyThis is because the effective number of individuals is smaller than the actual number of individuals in the population

  • Effective Number can be calculated as follows:Ne = 4Nm * Nf Nm + Nf# breeding females# breeding malesEffective Number

  • For a sex ratio of 1 male : 9 females in a population of 100 animals 4(10 X 90) 10 + 90= 36Ne =

  • Which means that a population of 100 individuals, consisting of 10 breeding males and 90 breeding females would lose genetic variability as rapidly as a population consisting of only 18 males and 18 females or 36 individuals

  • When do we want to include population genetics in conservation considerations?

  • Many possible inferences from population genetic studies that are important for conservation: Effective population size Inbreeding/selfing Mating success Bottlenecks Time of isolation Migration/dispersal

    *Good morning, thank you for inviting me to present my proposal to establish a MJRG on...***************