Chapter 2, Dynamics of Genes in Populations

 

Chapter 2, Dynamics of Genes in Populations

 

CHANGES IN ALLELE FREQUENCIES

 

Locus - the chromosomal or genomic location of a gene

Allelels - alternative forms of the gene at a given locus

Allele frequency (gene frequency) - the relative proportion of an allele

Gene pool - the set of all alleles existing in a population at all loci

Deterministic model - chnages in the frequencies of alleles in a population from generation to generation occur in a unique manner

Stochastical model - changes in allele frequencies occur in a probablistic manner

 

NATURAL SELECTION

Natural selection - the differential reproduction of genetically distinct individuals or genotypes within a population

Fitness (ω)-  a measure of the individual's ability to survive and reproduce

Absolute fitness, Relative fitness

Deleterious mutation - reducing the fitness of carriers, Negative (purifying) selection

Neutral mutation - fit as the best allele in the population

Advantageous mutation - increase the fitness of carriers, Positive (advantageous) selection

Hardy-Weinberg equilibrium - a population in which p^2, 2pq, q^2 genotypic ratios are maintained

p = p^2 + 1/2(2pq) = p^2 + pq

q = 1/2(2pq) + q^2 = q^2 + pq

Selective advantage - increase in fitness

Selective disadvantage - decrease in fitness

Selective neutral - s = 0

 

Codominance

Codominant (genic selection) - mean of the fitnesses of the two homozygous genotypes, directional selection

 

Dominance

Dominant selection - the fitness of the heterozygote is the same as the fitness of one of the two homozygous genotypes

 

Overdominance and underdominance

Overdominant selection - the heterogygote has the highest fitness

-> stable equilibrium (balancing or stabilizing selection)

Underdominant selection - the heterogygote has the lowest fitness

-> unstable equilibrium

 

RANDOM GENETIC DRIFT

 

Random genetic drift - the process of change in allele frequency due solely to chance effects

Loss (extinction) - the frequency of an allele reaches 0

Fixation - the frequency of an allele reaches 1

 

EFFECTIVE POPULATION SIZE

 

Census population size (N) - the total number of individuals in a population

Effective population size (Ne) - the population size that matters in the evolutionary process (reproducible)

 

GENE SUBSTITUTION

 

Gene substitution - the process whereby a mutant allele completely replaces the predominant or wild type allele in a population

Fixation probability - the probability that a particular allele will become fixed in a population

1. allele frequency

2. allele selective advantage/disadvantage (s)

3. the effective population size (Ne)

Fixation time - the time required for the fixation or loss of an allele

1. allele frequency

2. allele selective advantage/disadvantage (s)

3. the size of population

> Conditional fixation time - the mean fixation time of mutants that will eventually become fixed in the population

Rate of gene substitution (K) - the number of fixations of new alleles per unit time

 

GENETIC POLYMORPHISM

 

Monomorphic - only one allele at the locus

Polymorphic - two or more alleles coexist in the population

 

Gene diversity

Mean expected heterozygosity (gene diversity)

Single-locus expected heterozygosity (gene diversity at a locus)

 

Nucleotide diversity

Nucleotide diversity (Π) - the average number of nucleotide differences per site between any two randomly chosen sequences

 

THE DRIVING FORCES IN EVOLUTION

 

Mutationist - an evolutionary phenomenon is explained by the effects of mutational input and random genetic drift

Neutralist - stressing the effectrs of mutation, random genetic drift and purifying selection

Selectionist - the effects of the advantageous and balancing modes of selection as the main driving forces in the evolutionary process

 

The neo-Darwinian theroy and the neutral mutation hypothesis

Synthetic theory of evolution (neo-Darwinism)

Pan-selectionism - selection is the only force capable of driving the evolutionary process

Neutral theory of molecular evolution - the majority of molecular changes in evolution are due to the random fixation of neutral or nearly neutral mutations