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Unformatted text preview: a pop’n • Variation in number of progeny among individuals
– Stochastic differences in the genetic contribution of individuals • Effective size (Ne) = number of adults that breed • Unequal sex ratio (i.e. not 50m : 50f) – Effective size usually << Census size
– Size of an ideal population (every adult reproduces) in which
genetic drift equals the rate in the real population • Overlapping generations
– Mating between offspring and parents • Rate of drift = rate of decline in heterozygosity over time • Fluctuations in population size • Effective size is most important for evolutionary analysis – Population bottlenecks
– Ne is a harmonic mean across generations • Example: pop’n with 10,000 adults
– But only 1000 of them mate at random and reproduce
– N = 10,000 … Ne = 1000 Consequently, the number of individuals making genetic
contributions to the next generation is almost always lower than
the total census number of individuals.
19 20 -Not everyone has the same number of offspring
-Individuals making genetic contribution is ALWAYS lower!! Two views of domestication Ne << N
Past • This happens even in the absence of selection Ne at T1 Ne at T1 Ne at T1 + Versus...
TIME • Natural or artificial selection reduces Ne even further
– Deterministic differences in the genetic contributions of
individuals • Ne can be measured as a “species average” across the
generations t t Present
Ne at T2 Ne T1 > Ne T2 • Ne can also be measured for each gene separately Bottleneck affects the entire
genome. All loci are affected – Selection on a given gene will lower its Ne
21 Ne at T2 Ne at T2 Ne T1 >>> Ne T2 Ne T1 > Ne T2
reduction in Ne for
all loci. Selected loci show
even greater reduction
in Ne compared to the
rest of the genome. 22 -Ancestral population experiences a bottleneck that affects ALL
genes and reduces the Ne of ALLLLLL loci
-Loci that are selected for or against (via NS) show even MORE
reduction in Ne Results for maize & teosinte Key idea
• Purely demographic consequences of bottleneck
have genome wide effects of reducing Ne
• Artificial selection should lead to addit...
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This document was uploaded on 01/27/2014.
- Winter '14
- Annotated Lecture S