Week 12 - Population Genetics

Population genetics

The study of what changes allele frequencies in populations through time

Population genetics and Darwin

He had blending inheritance

When did population genetics become incorporated

Was incorporated in 1940s to the Theory of Evolution after Modern Synthesis

Modern Synthesis

Understanding of relationship between natural selection and evolution - how it affects the population’s genetic makeup 

Allele and allele frequency

Rate of specific allele appears in population

Gene pool

Sum of all alleles in population

Genetic drift

Random change in allele frequencies in population

Heritability

The fraction of phenotype variation we can attribute to genetic differences in a population

What happens when there is greater heritability?

Greater evolutionary forces that will act on it

Genetic variance

The diversity of alleles and genotypes within a population

Why is genetic variance important?

Understanding genetic variance preserves phenotypic diversity in breeding programs, and helps reduce risks of inbreeding

Inbreeding depression

Occurs when mating of closely related individuals carry deleterious recessive mutations - producing diseases offspring

Habsburg Inbreeding

Inbreeding in the Spanish Habsburg dynasty in 1500-1700s. There were deformations among the members of Habsburg Dynasty along with infant and child mortality increase in progeny. Rare diseases in general population as well. 

Hardy-Weinberg Principle of Equilibrium

States that a population’s allele and genotype frequencies are inherently stable 0 unless some evolutionary force is acting upon the population, the allele and genotypic frequency won’t change. Offers useful model against which to compare real populations

Hardy-Weinberg Principle of Equilibrium Assumptions

Infinitely large population, two alleles at a gene locus, and no mutation migration, emigration, or selective pressure

Allele and genotypic frequencies

Given a single locus with only two alleles (A and B) in a breeding population of diploid individuals.

N =

Total number of breeding individuals in a sub-population

q =

Frequency of a particular allele at locus within a sub-population

p = 1 - q =

Frequency of other allele at locus within a sub-population

Allele frequency equation

A = p, a = q, and p + q = 1

Genotype frequency equation

1 = p² + 2pq + q²

What happens when populations are in equilibrium?

The allelic frequency is stable and distribution of alleles can be determined from the Hardy-Weinberg equation

What happens when allelic frequency differs from the predicted value?

Scientists can make inferences about what evolutionary forces are at play

p²

Homozygous dominant

2pq

Heterozygous

q²

Homozygous recessive

Factors that affect equilibrium of a population

Evolution, change in population, and random-mating interference

Factors that affect equilibrium of a population - Evolution

No natural population is immune

Factors that affect equilibrium of a population - Change in Population

Mutation of an allele, genetic drift, migration of alleles into/out of the population (gene flow), and selection of one allele over the other 

Factors that affect equilibrium of a population - Random mating interference

Assortative mating - similar phenotypes mate more frequently (mate-choice, self pollination)

Genetic Drift

Random change in allele frequencies in population - small populations are more susceptible and can lead to elimination of an allele from a population by chance

Process of genetic drift

Random 

• natural selection is not random and is driven by environmental pressure

Genetic Drift: Bottleneck effect

A chance event or catastrophe can reduce the genetic variability within a population. natural events such as earthquakes for example. 

Genetic Drift: The Founder Effect

Some portion of population is separated from the original population, changing the genetic structure of the new population (founder)

Gene flow

Can occur when an individual travels from one geographic location to another due to migration of individuals or gametes. Flow of alleles in and out of a population. 

Important Drivers of Diversity - Mutation

Changes to an organism’s DNA, driving evolution. Introduces novel genotypic and phenotypic variability. The selective advantage of these mutations can differ. Provides opportunity for introducing new alleles in a population

Important Drivers of Diversity - Nonrandom mating

Mate choice, assortative mating - individual’s preference to mate with partners who are phenotypically similar to themselves, and physical location 

Important Drivers of Diversity - Geographic Separation

Can lead to differences in phenotype between populations

Cline

Refers to a geographical variation where species vary gradually across an ecological gradient

Adaptive Evolution

Natural selection acts on the population’s heritable traits, and acts on entire organisms rather than individual alleles. Evolutionary (Darwinian) fitness.

Natural selection - heritable traits

Selection for beneficial alleles increase their frequency in the population, and selection against deleterious alleles which decreases frequency.

Types of selection

Stabilizing, Directional, and Diversifying. Different types of natural selection cna impact the distribution of phenotypes withing a population.

Types of selection - Stabilizing

An average phenotype is favored

Types of selection - Directional

A change in the environment shifts the spectrum of phenotypes observed

Types of selection - Diversifying

Two or more extreme phenotypes are selected for, while the average phenotype is selected against

What does natural selection act upon?

The population’s heritable traits: selection for beneficial alleles, increasing their frequency in the population, and selecting against unfavorable alleles. Individuals do not evolve, populations evolve. 

Types of selection - Frequency Dependent Selection 

Serves to increase the population’s genetic variability by selecting for rare phenotypes

Positive frequency-dependent selection

Usually decreases genetic variability by selecting for common phenotypes

Types of selection - Sexual Selection (dimorphism)

Males and females of certain species are often physically different beyond reproductive organs. Some are better at fighting off others of the same sex for a mate. 

What can dimorphism lead to?

Variation in reproductive success, strong selection pressure among males. Evolution of desirable traits. Can result in developing secondary sexual characteristics that do not benefit the individuals likelihood of survival, but maximize reproductive success. 

Dimorphism reproduction 

Males and females appear different from one another in ways beyond reproductive organs, arises where there is more variability in male’s reproductive success (some males obtain majority of matings due to being larger or more decorated) 

Evolution

More than natural selection and has no long term purpose or direction. Constraints and tradeoffs are important. 

Evolution - Adaptivity

Not all is adaptive, natural selection selects the fittest individuals, but other forces of evolution (drift, gene flow), can do the opposite

Supergene

When a piece of chromosomal DNA gets inverted which prevents recombination, so the genes in the inversion are almost always inherited as a unit

Chromosome Recombination

Duplicated maternal and paternal chromosomes align and randomly swap pieces of DNA which breaks up existing combination of genes.