University:
Amherst College
Course:
BIOL 321 | Evolutionary Biology with Lab
Academic year:

2021
Views:

94

Pages:

14
Author:

weitergabdl0

Founder effects - the loss of genetic diversity following the establishment of a new population by a few individuals! Mainland 20 n oo Recent colonizations 15 - CD t > mam 73 u 10 t 0) ML = Australian mainland T = Tasmania SI = South Island New Zealand Cl = Chatham Island PN = Palmerston North, south end of North Island A = Auckland, north end of North Island Nl = Norfolk Island < 5T ML T SI Cl PN A Nl Population Natural populations! When we describe the genetic structure of natural populations, the data usually are not based on experimental manipulations, nor do we know the history of the populations, but...! we can infer the causes of evolution (e.g., genetic drift or selection) by interpreting the patterns of variation that we observe.! Molecular variation @ 2 loci in 42 populations of mice from widely separated barns in TX !– small & large populations – Mean allele frequency & variance among populations Estimated population size Small (median = 10) Large (median = 200) Source: After Selander 1970. Number of populations sampled Mean allele frequency Variance of allele frequency Es~ 3b Hbb Es - 3 b Hbb 29 0.418 0.849 0.0506 0.1883 13 0.372 0.843 0.0125 0.0083 Natural history of genetic drift ! (2)! Effective population size < actual population size! e.g., !sexually immature individuals! !restrictive sexual systems! !skewed sex ratios! !population size fluctuations! What is population size?! !Ecological sense, count # individuals! What is population size?! !Evolutionary sense, what matters is the number that contribute !! !genes to the next generation! effective population size, Ne! (= every individual reproduces)! … the number of individuals in an ideal population in which the rate of genetic drift would be the same as it is in the actual population.! 1)! Equal numbers of males and females! 2)! No sexual or natural selection (i.e., all individuals have an equal probability of contributing gametes to next generation)! 3)! Population size remains the same in each generation! Effective population size, Ne! Breeding structure of population! !number of sexually mature individuals! !unequal sex ratios! Unequal variance in mating! restrictive sexual systems that limit individuals from mating! Population fluctuations in size! ! 8N Ne " Vm +V f + 4 Natural history of genetic drift ! (1)! Small populations! e.g., !naturally small populations (ephemeral pools ! !or edaphic specialists)! !species introductions (founder effects)! (2)! Effective population size < actual population size! e.g., !sexually immature individuals! !restrictive sexual systems! !skewed sex ratios! !population size fluctuations! (3)! When historically large populations are reduced in size! e.g., !bottlenecks or habitat fragmentation! Population bottlenecks - !the loss of genetic diversity following a !reduction in population size! Northern elephant seal, Mirounga angustirostris! •! historically distributed along Pacific coast (NA); > 150,000 seals! •! 1800’s hunted almost to extinction (by 1890, < 20 individuals)! •! recovery efforts began 1900’s! •! current population ca. 100,000 individuals, but...! Hoelzel et al. (1993 J Heredity 84:443)! 41 allozyme loci, 67 individuals, 2 locations ! Bonnell & Selander (1974 Science 184:908)! 24 allozyme loci sampled in 159 individuals across 5 populations ! ~ all loci monomorphic ! Population bottlenecks - !the loss of genetic diversity following a !reduction in population size! -^ TsUi A A ccanpanscn of iMe frtqucnoe* for rhe n*o p *ilanor* a* torn loci lv' u< r»iancc of drfrmK« in alick fmfumo knwtn rwc pcfuliom » a» onnd onr Umrcaygoycy r *: r \ i %y"*v •! Drought, salt tolerant •! Worldwide distribution Lycium (~80 species) s~ \ ~ •! Long-lived, woody shrubs •! Three genera in Lycieae - ( Ar ux A “N N / . / 0 I i Ii r \ \ Taxon sampling ~88% of species td —. r I | jt * Grabows waplus L/oum spp. * OAl » 72 Lycieae (all 3 genera) 89 taxa total Nolana, Sclerophylax, Jaborosa (outgroups) Old World Lycium C V GBSSI (waxy) Aligned length = 1968 bp . 4 1 L ML analysis TVM + I -lnL = 6589.81 Tribe Lyoeae L sect Schistocatyx L < » l \ <% - rt 200 ML bootstrap replicates \ \ rJ L * -AY Phfodus mK-roptryihis

Genetic drift Part 2

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