Chapter 2Genetics and Ecology课件.ppt

Fraction of initial genetic variation 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 N=20 N=100 N=300 N=1000 0 100 200 300 400 500 Generations 精品文档 Genetic Drift Probability of the failure to mate Loss of possible rare gene Loss of genetic information for subsequent generations resulting in a loss of genetic diversity. 精品文档 Genetic Drift Probability of the failure to mate Small populations more susceptible to drift. The rate of loss of original diversity over time is approximately 精品文档 Genetic Drift Probability of the failure to mate equal to 1/2N per generation. Example: 1. N = 500 then 1/2N = 0.001 or 0.1% genetic diversity lost per generation. 精品文档 Genetic Drift Probability of the failure to mate equal to 1/2N per generation. Example: N = 50 then 1/2N = 0.01 or 1% genetic diversity lost per generation. 精品文档 Genetic Drift Probability of the failure to mate Example: (cont.). Over 20 generations, the population of 500 will still retain 98% of the original variation, but the population of 50 will only retain 81.79%. 精品文档 Genetic Drift Probability of the failure to mate Example: (cont.). 50/500 Rule: Need 50 individuals to prevent excess inbreeding and 500 is the critical size to prevent genetic drift. 精品文档 Genetic Drift Effects of immigration on genetic drift (Figures 2.11 and 2.12). Often immigration of only one or two individuals into a population can counteract genetic drift 精品文档 Number of immigrants per generation 5 2 1 0.5 0.1 None 10 20 30 40 50 60 70 80 90 100 Generation Percentage of initial genetic variation remaining 50 60 70 80 90 100 精品文档 Percentage of populations persisting 0 20 40 60 80 100 10 20 30 40 50 Time (years) N = 101 or more N = 51-100 N = 31-50 N = 15 or less N =16-30 精品文档 Neighborhoods and Effective Population Size Effective population size is determined on mating range. Individuals may only mate within their neighborhood. 精品文档 Neighborhoods and Effective P