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Distribution of variation : Variation is present within individuals among individuals within populations among populations.

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Presentation on theme: "Distribution of variation : Variation is present within individuals among individuals within populations among populations."— Presentation transcript:

1 Distribution of variation : Variation is present within individuals among individuals within populations among populations

2 Source of variation Populations withinbetween AA, AA, AA AA, AA, AA AA, AA, AA nonenone all individuals homozygous all populations monomorphic locus fixed for A allele in all populations

3 Source of variation Populations withinbetween AA, AA, AA AA, AA, AA AA, AA, AA nonenone AA, AA, AA BB, BB, BB DD, DD, DD noneall all individuals homozygous all populations monomorphic locus fixed for different alleles in each population

4 Source of variation Populations withinbetween AA, AA, AA AA, AA, AA AA, AA, AA nonenone AA, AA, AA BB, BB, BB DD, DD, DD noneall AA, AB, CD AA, AB, CD AA, AB, CD all none individuals are variable all populations polymorphic all populations have same allelic and genotypic frequencies

5 Source of variation Populations withinbetween AA, AA, AA AA, AA, AA AA, AA, AA nonenone AA, AA, AA BB, BB, BB DD, DD, DD noneall AA, AB, CD AA, AB, CD AA, AB, CD all none AA, AB, AD AB, BC, CC DD, BB, AC presentpresent individuals are variable all populations polymorphic all populations have different alleles and allelic and genotypic frequencies

6 Factors that reduce variation within populations

7 Short-term small population size –founder effect – a very small number of colonists that originate a new population –genetic bottleneck – a dramatic collapse in numbers

8 Factors that reduce variation within populations Short-term small population size –founder effect –genetic bottlenecks N TIME new population established

9 Factors that reduce variation within populations Short-term small population size –founder effect –genetic bottlenecks Huntington’s chorea - neural function decay, leading to death - frequent in South Africa, and near Lake Maracaibo, Venezuela - single gene, dominant allele, rapidly mutating - weak selection – does not manifest until after maturity - founder effect – single woman with 10 children bearing the allele

10 Factors that reduce variation within populations Short-term small population size –founder effect –genetic bottlenecks N TIME RECOVERY CRASH bottleneck

11 Factors that reduce variation within populations Short-term small population size –founder effect –genetic bottlenecks TIME POPULATION SIZE

12 Factors that reduce variation within populations Short-term small population size –founder effect –genetic bottlenecks Elephant seals: N = unknown (thousands) 20 30,000 late 1800s s 24 loci examined all monomorphic David Smith, UCMP Bonnell and SelanderBonnell and Selander Elephant seals: genetic variation and near extinction Science 184:

13 Factors that reduce variation within populations Short-term small population size –founder effect –genetic bottlenecks Retention of genetic variation in a small population of constant size: # generations N << << <

14 Factors that reduce variation within populations Short-term small population size –founder effect –genetic bottlenecks Retention of genetic variation in a small population of constant size: # generations N << << <

15 Factors that reduce variation within populations Average number out of 4 alleles retained: original allele frequency before founder/bottleneck event N 0.7, 0.1, 0.1,

16 Factors that reduce variation within populations Average number out of 4 alleles retained: original allele frequency before founder/bottleneck event N 0.7, 0.1, 0.1, , 0.02, 0.02,

17 Factors that reduce variation within populations Short-term small population size –founder effect –genetic bottlenecks THE BAD NEWS: Effects of small population size are cumulative – a population is, in effect, going through a serious bottleneck every generation – perennial low numbers erode genetic variation

18 Factors that reduce variation within populations Short-term small population size –founder effect –genetic bottlenecks THE BAD NEWS: Effects of small population size are cumulative – a population is, in effect, going through a serious bottleneck every generation – perennial low numbers erode genetic variation THE GOOD NEWS: A single bottleneck generation will not eliminate most of the genetic variation in a population Crucial issue is whether the population remains small or grows to a relatively large size

19 Factors that reduce variation within populations Short-term small population size –founder effect –genetic bottlenecks Long-term small population size –genetic drift –inbreeding

20 Factors that reduce variation within populations genetic drift: random loss of variation due to stochastic events

21 Factors that reduce variation within populations genetic drift Qualitatively - genetic variance (or heterozygosity) will be lost Quantitatively – alleles will be lost

22 Small populations of constant size always lose heterozygosity through time More alleles are lost in populations founded by small numbers of individuals –the smaller the population is, the more rapidly heterozygosity is lost Alleles which have low frequencies in the original population tend to be lost much more easily in the founder population than alleles with high frequencies

23 How to avoid the consequences of bottlenecks: increase population size rapidly Issues with intrinsic rate of increase taxonomic biases age at maturity fecundity

24 Factors that reduce variation within populations Short-term small population size –founder effect –genetic bottlenecks Long-term small population size –genetic drift –inbreeding

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