Population genetics
Chapter 13
Few definitions...• Population: group of the same species living in the same
region at a given time.
• Variation: exists in members of a population– Structural - number of fingers, length of tail– Biochemical - Produce a protein/enzyme or not. Leads to blood
types or pigment of fur etc– Physiological - Ability to differentiate between tastes, smells,
colours– Behavioural - difference in dogs; retrieval, herding or response to
training– Developmental- berries green to red to black– Geographic - figbirds in the north have yellow breast & belly,
those in the south are green
... and some more
• Cline: trait changes gradually in members of a population across its rangeeg: brushtail possum increases in body mass as you head from Qld to Tas
• Monomorphic : one form (all galahs are pink & grey)• Polymorphic: many forms (only needs to be two or more
variations in a particular trait)Continuous Variation Discontinuous Variation
variation occurs across a continuum variation can be organised into a few discrete groups
eg: human height eg: flowers that come in 2 colours
Causes of variation1) Environmental Variations (p.472)
Identical twins have identical genotypes but can grow to different heights (nutrition etc)Soil pH, temperature, altitude and light can all influence traits in plant development
• Internal Environment (eg: hormones) can produce variation such as plumage in birdsAlso, substances in the mothers blood can cross from maternal circulation and affect a fetus
2) Inherited variationSome inherited variations don't affect physical or mental functioning (eg: hair colour & blood type)Some do - these are known as genetic disorders
Inherited variation• Monogenic Traits: due to the action of different alleles of one
geneeg: ABO blood type and Wing stripe colour in budgerigar populations
• Variation is all due to Meiosis which leads to recombination (re-assortment of genes) and segregation of alleles
• The more alleles, the greater the variation (ABO --> more variation than Rhesus + or -)
• Polygenic Traits: due to the action of many genes (polygenes)• Number of possible variations: 2n + 1
eg: height or fat content in cow's milksee skin pigmentation example in text (p.475)
• Variation due to changes in Chromosome number (p.476)
Mutation (gene mutation)• Gene mutation: production of new alleles of
genes• If this occurs in a germline cells, they are the
sources of new alleles in a population• Spontaneous or Induced
• If a mutation confers a selectiveadvantage on individual organisms, thenew allele would be expected to increase in frequency oversuccessive generations
Once again some definitions...
• Genotype: individuals genetic information• Gene pool: genetic information in a
population
Allele frequencies• Alleles frequencies can be calculated when all genotypes in a population
are known• Any value between 1 and 0• If the value is 1, all organisms in that population are homozygous• (= gamete frequencies)
See fig 13.24 p.479
• Count the number of each allele• Work out the probability of each of these alleles occurring in this population
See working on p.480
• p is used to represent the frequency of the dominant allele• q is used to represent the frequency of the recessive allele
Calculating allele frequencies:1) Calculate no of alleles: no of alleles x
no of animals in population2) Count no of 1st allele in population3) Count no of 2nd allele in population4) Divide the number in 2) and 3) by the
number you calculated in 1)
Hardy-Weinberg principle• Allele frequencies in populations remain constant
throughout generations* (conditions apply)– large population– random mating– equal fertility producing equal numbers of viable
offspring– closed population (no migration in or out)
• If this occurs, population is in Hardy-Weinberg equilibrium until an agent of change acts on the population.
Hardy-Weinberg equation
p2 + 2pq + q2
homozygous heterozygous homozygousdominant recessive
• The Hardy-Weinberg principle can be used to estimate genetic frequencies in a population where it is not possible to calculate these values directly.
• Read p.481 & 482 to see calculations– Estimating allele frequency in a population– Estimating genotype frequencies in a population
Change agents in populations
• Changes in allele frequency over time• Agents that can cause this change include:– Selection– Migration– Chance
Selection• Members of a population compete for food,
habitat, mating partners etc.
• Takes longer to occur than migration or chance
• A phenotype that makes the greater contribution to the gene pool in the next generation has a higher fitness value and is at a selective advantage
• Selecting agent: the agent that causes differences to occur between phenotypes
Who‘s fittest?
• Genetic Fitness depends on the phenotype that makes the greatest contribution to gene pool in the next generation.
• This often means that the weak one who can hide & then reproduce is 'fitter' than the one who fights & dies
Genetic fitness can vary
• Genetic fitness varies in different environments:– Sickle-cell anaemia is an advantage in countries
where malaria is prevalent– Malarial parasite cannot survive in RBCs with
Haemoglobin S– Therefore being heterozygous produces the 'most fit'
phenotype (see p.485)
• Read the case studies (p.485 & 486)
Two kinds of selection
• Natural selection (Biozone p. 323)Any environmental agent that results in Differential Reproduction
• Artificial selection (Biozone p. 335 - 338)Selection of particular organisms from a population to be the parents
of the next generation
Complete Selection = cannot reproduce (die)
Partial Selection = mating involving a particular phenotype produces fewer viable & fertile offspring
Migration
• Migration = gene flow
• Changes due to migration can occur very quickly
• Emigration changes allele frequency if the emigrant group varies „strongly“ from the original popuilation
Chance• Chance events can cause allele frequencies in a population to
change over time
• Genetic Drift (Biozone p.330)– It is random and the direction of change is unpredictable
• Bottleneck Effect (Biozone p.329)– A population 'crash' that may not be selective (eg: fire or humans
destroying habitats)
• Founder Effect (Biozone p.328)– The small colonising 'founder' population which often has a non-
representative sample of alleles. This may lead the colonising population to evolve differently from the parent population.
Evolution within a species• New alleles in a gene pool are created by gene mutation• Action of change agents may cause some change in
phenotype frequencies
• Populations may be separated– different selecting agents (eg: due to enviro) – Genetic drift produces different changes by chance – different alleles in each gene pool due to mutation – over time, some phenotype frequencies increase & some
decrease– extended period of time, populations may become distinguishable– races or sub-species
Studying populations using mtDNA
• mtDNA is structurally identical to nuclear DNA• descent via maternal line (only mother's kind)• lack of recombination (no meiosis)• high copy number (several hundred per cell as opposed to
2)
• Distinctive mtDNA sequences in different populations are known as haplogroups and each is designated by a capital letter.
• Single Nucleotide Polymorphisms (p.366)