Download presentation

Presentation is loading. Please wait.

Published bySophie Alexander Modified over 4 years ago

1
What causes geographic populations to become differentiated? Natural Selection? Genetic Drift? (limited gene flow) http://alex.roedlach.at/Kottak/Map%2009.jpg http://anthro.palomar.edu/adapt/images/skin_color_range.jpg

2
Genetic drift Changes in allele (and genotype) frequencies due to random sampling of breeding individuals in small (finite) populations Leads to: Loss of genetic variation (heterozygosity) increase in homozygosity (inbreeding) Fixation (or loss) of alleles Key Concept:Effective population size (N e ) Number of breeding individuals contributing genes to next generation

3
Genetic drift is STRONG in SMALL populations Genetic drift is WEAK in LARGE populations

4
Experimental demonstration of drift among sub population (demes)

6
Buri experiment: experimental demonstration of genetic drift Average allele frequency does not change Variance among sub-populations increases

7
Loss of heterozygosity (H) H= heterozygosity = 2pq (from p 2, 2pq, q 2 ) = proportion of heterozygotes in population = (1 - sum of all homozygotes) = 1 - x i 2, where x i = frequency of ith allele if there were 1, 2, 3, …i alleles in popln. H decays due to drift (and inbreeding) H t+1 = H t [1 - 1/(2N e )] Inbreeding coefficient = F = H expected - H observed H expected

8
Drift causes: Change in the genetic make up of the population (evolution) Loss of heterozygosity; increase in homozygosity Differentiation among populations Fixation of alleles

9
Probability transition matrix for 2N = 4 Probability of i A 2 alleles in generation t+1 given j A 2 alleles in generation t

12
Large population sizes of marine invertebrates (shrimp)

13
Small(er) population sizes of some mammal species

14
Estimating Ne Inbreeding effective size -increase in inbreeding over time Variance effective size -increase in the variance among demes Eigenvalue effective size -loss of heterozygosity over time

15
http://www.amiq.org/galleries/furseal/big/lion7619.jpg Fur seal lion and harem: Unequal sex ratio of breeding individuals Effective population size is lower than census size

16
Sex ratio effects on effective population size Skewed sex ratio reduces N e below census size N f = number of breeding females N m = number of breeding males N e = 4N f N m / (N f + N m ) Example: sea lions, 1 -male and a harem of females N f = 100, N m = 1 N e = (4 x 100 x 1) / (100 + 1) = 4 sea lions!

18
Examples of population size fluctuation. The low points have a disproportionately large effect on reducing the effective population size below the average census size over the time interval. Population size in Egypt http://www.isl.uni-karlsruhe.de/fallmodule/cairo/Egy-pop-dens.jpg http://www.okc.cc.ok.us/biologylabs/Images/Homeostasis%20Images/lynx-hare.jpg

19
Effect of population fluctuation on effective population size Bottlenecks have a disproportionate effect on reducing N e Generationtt+1t+2t+3 N e 10010020100 Average N e = 320 / 4 = 80 (not correct) Real N e = harmonic mean 1/N e = 1/(# of generations) x ∑ 1/N e = 1/4 x [1/100 + 1/100 + 1/20 + 1/100] 1/N e = 1/4 x [0.08] = 0.02 Ne= 50 LESS THAN THE SIMPLE AVERAGE (arithmetic mean)

20
Variance in reproductive success Poisson distribution

21
Ne/N ratio Effective / census population size

Similar presentations

© 2019 SlidePlayer.com Inc.

All rights reserved.

To make this website work, we log user data and share it with processors. To use this website, you must agree to our Privacy Policy, including cookie policy.

Ads by Google