Evolution of Populations
Evolution of Populations
Additional Criteria of Natural Selection
Natural selection directly affects phenotype, not genes or alleles.
Phenotype: Observable traits.
Genotype: Genetic makeup of an organism.
Understanding Natural Selection and Evolution
Three conditions for natural selection to occur:
Variation: Variation within a population.
Heritability: Selected trait must be heritable.
Differential Survival and Fitness: Differences in survival and reproductive success based on the selected trait.
Mechanisms of Evolution
Five mechanisms to alter allele frequencies:
Selection: Differential survival or fitness.
Mutation: The only source of new alleles.
Genetic Drift: Changes in allele frequencies due to random events.
Gene Flow: Migration; keeps species together.
Nonrandom Mating: Mate choice is not random.
Types of Selection
Directional Selection: One allele is favored over another. Example: Peppered moths during the industrial revolution.
Stabilizing Selection: The mean trait is favored over extreme traits. Example: Mean infant weight at birth has the highest survival rate.
Disruptive Selection: Extreme traits are favored over the mean trait. Example: Black-bellied seed cracker finches with large or small beaks.
Mutations
Mutations can be passed down only if they occur in the germ line.
Somatic mutations typically lead to cancer.
Mutations are the only source of new variation in a species, making them important for evolution.
Genetic Drift
Genetic drift is the change in allele frequency in a population due to random chance.
Two main mechanisms of genetic drift:
Founder Effect: Allele frequencies in a new population differ from the original population.
Bottleneck Effect: Drastic reduction in population size with only some alleles retained in the population.
Founder Effect
A founder population has different allele frequencies from the ancestral population.
Population Bottlenecks
A population experiences a drastic reduction in size, causing a shift in allele frequency.
Northern Elephant Seals
The population went through a bottleneck event at the turn of the 20th century due to overhunting.
The population was reduced to 10–20 individuals by 1892.
Currently, there are over 100,000 individuals.
This bottleneck event significantly affected genetic diversity.
Genotypes of specimens before and after the bottleneck event can be compared using museum specimens.
Gene Flow – Migration
Gene flow counteracts genetic drift.
Alleles move between populations via migration.
Populations are linked via migrants, introducing new or uncommon alleles into a population.
Gene flow upsets the balance of alleles in a population but can be negligible if the population is extremely large and migration rates are low.
Non-Random Mating
Individuals mate with partners with either the same or different phenotypes.
Assortative Mating: Individuals mate with similar genotypes/phenotypes.
Inbreeding depression: Offspring of genetic relatives have decreased fitness.
Disassortative Mating: Individuals mate with different genotypes/phenotypes.
Major Histocompatibility Complex (MHC): Functions in immune system pathogen recognition.
Female Savannah Sparrows are more likely to mate with MHC dissimilar males.
Hardy-Weinberg Model
Independently derived a mathematical model at the same time to predict allele frequencies in a population.
G.H. Hardy (1877 – 1947) and Wilhelm Weinberg (1862 – 1937).
A mathematical model to test against any evolutionary processes that may be occurring within a population.
If true:
Frequencies of alleles do not change over time in the absence of evolutionary processes.
Ability to predict genotype frequencies in a population.
Alleles not in equilibrium will reach equilibrium in a single generation.
Hardy-Weinberg Assumptions
Mathematical model to test against any evolutionary processes that may be occurring within a population.
Sexually reproducing organism – Allele frequency equal between males and females.
Diploid – Two copies of each gene.
Discrete generations – Parents all reproduce at the same time.
Random mating – No mate choice with respect to alleles.
Infinitely large population – Results in no genetic drift.
No evolutionary forces – Mutations, selection, gene flow.
Hardy-Weinberg Equilibrium
Goal: See how evolutionary processes influence allele and genotype frequencies to test for evolution.
Frequency of alleles:
p = f(B)
q = f(b)
p + q = 1 (Allele frequency of the entire population).
Genotype frequency of the entire population: D + H + R = 1
Frequency of genotype:
Dominant homozygote = f(BB)
Heterozygote = f(Bb)
Recessive homozygote = f(bb)
Derivation of Hardy-Weinberg
If all assumptions are met, genotype frequencies are mathematically expressed as: p^2 + 2pq + q^2 = 1
D (Dominant homozygote) + H (Heterozygote) + R (Recessive homozygote).
To determine the probability of two independent alleles occurring together, multiply each allele frequency (basic probabilities!).
If a population remains at this frequency, it is said to be in Hardy-Weinberg equilibrium.
Example Problem
Dominant – Recessive alleles: 720 total cats, 960 black, 320 white.
Assuming Hardy–Weinberg, what are the allele and genotype frequencies?
p + q = 1
p^2 + 2pq + q^2 = 1
This population is not in Hardy-Weinberg equilibrium. If HW assumptions are met, what will be the genotype frequency of the next generation?
Given:
Genotype frequency:
AA = 0.4
Aa = 0.4
aa = 0.2
Allele frequency:
A = 0.75
a = 0.25
Hardy–Weinberg Equilibrium Example
Myoglobin protein alleles in a Japanese population – Oxygen-binding protein in muscles.
Measured allele frequencies:
p = 0.755
q = 0.242
p + q = 1
p^2 + 2pq + q^2 = 1
Measured genotype frequencies:
D = 0.59
H = 0.33
R = 0.08