Selection Dan Graur
Conditions for maintaining Hardy-Weinberg equilibrium: 1. random mating 2. no migration 3. no mutation 4. no selection 5. infinite population size
2 mathematical approaches to studying genetic changes in populations: Deterministic models Stochastic models
Deterministic models assume that changes in allele frequencies from generation to generation occur in a unique manner and can be unambiguouslypredicted from knowledge of initial conditions. Strictly speaking, this approach applies only when: (1) the population is infinite in size, and (2) the environment either remains constant with time or changes according to deterministic rules.
Stochastic models assume that changes in allele frequencies occur in a probabilistic manner, i.e., from knowledge of the conditions in one generation one cannot predict unambiguously the allele frequencies in the next generation, but can only determine the probabilities with which certain allele frequencies are likely to be attained.
Stochastic models are preferable to deterministic ones, since they are based on more realistic assumptions. However, deterministic models are easier mathematically and, under certain circumstances, they yield sufficiently accurate insights.
Selection The deterministic approach
Natural selection The differential reproduction of genetically distinct individuals (genotypes) within a population. Differential reproduction is caused by differences among individuals in such traits as (1) mortality, (2) fertility (offspring), (3) fecundity (gametes), (4) mating success, and (5) viability ofoffspring.
Is the fitness of slim men higher than that of fat men? ? Dixson et al. 2003. Masculine somatotype and hirsuteness as determinants of sexual attractiveness to women. Archives of Sexual Behavior 32:29–39.
Non-Genetic Genetic Variability
Arashnia levana Non-genetic variability.
Helix aspersa Genetic variability.
Non-Genetic Genetic Fitness-related Fitness-unrelated Variability
Genetic? No Fitness related? Yes Hair color Does selection operate?
Sperm morphology Genetic? Yes Fitness related? Yes Does selection operate?
Genetic? Fitness related? Wealth Does selection operate?
Darwinian selection requires variation. Lamarkian selection does not require variation.
Natural selection is predicated on the availability of genetic variation among individuals in characters related to reproductive success (variation in fitness).
Evolutionary Success Ryan Kremer Carlos Slim Helú (richest person on earth) 6 children Linus Pauling (Only person to win 2 unshared Nobel prizes) 4 children
The fitness(w) of a genotype is a measure of the individual’s ability to survive and reproduce. The size of a population is constrained by the carrying capacity of the environment. Thus, an individual’s evolutionary success is determined not by its absolute fitness, but by its relative fitness in comparison to the other genotypes in the population.
In nature, the fitness of a genotype is not expected to remain constant for all generations and under all conditions. However, by assigning a constant value of fitness to each genotype, we are able to formulate simple models, which are useful for understanding the dynamics of change in the genetic structure of a population brought about by natural selection.
For simplicity: • We assume that fitness is determined solely by the genetic makeup. • We assume that all loci contribute independently to fitness (i.e., the different loci do not interact with one another in a manner that affects fitness), so that each locus can be dealt with separately.
A very simple model (1): One locus = A Two alleles = A1 & A2 The old allele = A1 The new allele is = A2 Three genotypes = A1A1, A1A2 & A2A2 Each genotype has a typical fitness (w) We are interested in the fate ofA2
A very simple model (2): The fitness of the old genotype (A1A1)is set at 1. The relative fitnesses of the two new possible genotypes (A1A2 & A2A2) are defined comparatively as 1 + s or 1 + t, where s and t are the selection coefficients.
In comparison with A1, A2 may deleterious, neutral, or advantageous, and it will be subject to purifying selection, no selection, or positive Darwinian selection, respectively.
Genotype A1A1 A1A2A2A2 Fitness w11w12w22 Frequency p2 2pqq2
These are the variables we fiddle with Genotype A1A1 A1A2A2A2 Fitness w11w12w22 Frequency p2 2pqq2
Dominance & Recessiveness At the phenotypic level At the fitness level
A1 dominance Genotype A1A1 A1A2A2A2 Fitness w11w11 w22 Frequency p2 2pqq2
A1 dominance Genotype A1A1 A1A2A2A2 Fitness 1 1 1 + s A2
A2 dominance Genotype A1A1 A1A2A2A2 Fitness w11w22 w22 Frequency p2 2pqq2
A2 dominance codominance Genotype A1A1 A1A2A2A2 Fitness 1 1 + s 1 + s A2
Codominance (genic selection) Genotype A1A1 A1A2A2A2 Fitness w11 (w11 + w22)/2 w22 Frequency p2 2pqq2
codominance Genotype A1A1 A1A2A2A2 Fitness 1 1 + s 1 + 2s A2
Directional Selection codominance A2 dominance A1 dominance A1 = old mutant A2 = new mutant
Selection against recessive lethal alleles b-hexosaminidase A is a dimeric lysosomal protein consisting of two a-subunits. It is encoded by a gene on chromosome 15.
Selection against recessive lethal alleles b-hexosaminidase-A catalyzes the removal of N-acetylgalactosamine from GM2 ganglioside, thereby degrading and removing it from the nervous system.