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)