Download Ecology – study of distribution, abundance and relations of

Ecology – study of distribution, abundance and relations of organisms and their interactions with the
environment
Evolution – the change in inherited traits of a successive population of organisms through successive
generations
Ecology and evolution = sister disciplines
- both aim to explain processes operating across different spatial or temporal scales of
organisation
- many concepts are relevant to both  populations, natural selection, development,
adaptation, etc.,
- ecological traits can be mapped on evolutionary trees
Population genetics recap;
Genetic variation
Genotype = DNA sequence
Phenotype = physical expression
- can often infer genotype from phenotype  practice of Mendelian genetics (eg. eye colour, blood
groups)
*often the environment has a lot to do with how a phenotype turns out given a particular genotype
Genotype doesn’t equal phenotype;
- in most cases the genetics underlying phenotypic variation is continuous or quantitative
- the genetics of quantitative variation is;
- polygenic (additive genetic variation)
- influenced by the environment (phenotypic plasticity)
- subject to G x E interaction (different genotypes produce different phenotypes in different
environments)
- therefore phenotypic variation has many sources and the relationship between phenotype and
genotype is usually (very) indirect
*selection acts on phenotypic variation
- but evolution requires heritable variation
- evolution = genetic (ie. genotype) change from one generation to the next (which includes drift as
well as selection)
- but one cannot reliably infer genetics from phenotype  the result is that population theory is
focused primarily on genetic variation
Hierarchy of genetic variation;
- DNA sequence variation
- silent (synonymous)
- replacement (non-synonymous)
- neutral or under selection
- single locus (discrete) trait variation
- continuous trait (quantitative) variation
Single loci and quantitative traits;
- quantitative traits are usually polygenic
- each gene makes a small contribution that are added up to constitute the genetic basis of the
quantitative trait
- theoretically, a quantitative trait can be regarded as being composed of lots of simple Mendelian
single loci
- so measuring the genetic variation at single locus in a population may be relevant after all
- most measures of genetic variation that start with single locus can be summed over many loci to
provide a more complete picture
- this is important as population genetics is in transition from a “data-poor/concept-rich” theory to a
data rich future due to genomic scale analysis of genetic variation
Measurement of variation at a single locus;
▪ heterozygosity = probability that two alleles at a locus drawn at random from a population will be
different
- heterozygosity (H) is a measure of the amount of genetic variation in a population
- 2 ways at looking at H
- HO =proportion of heterozygotes observed in a sample
- HE = expected proportion of heterozygotes under HWE (total no. – expected no. homs)
Hexp = 1 - ∑ 𝑝i2
where pi is the frequency of the ith allele in the population, and pi2 is thus the expected frequency of
i/I homozygotes
what is useful about heterozygosity??
- positive correlation with selection potential of population (more genetic variation present for
selection to act upon  more chance of adapting to environmental change)
- negative correlation with inbreeding
- easy to analyse  insight into population structure/mating system, etc.,
▪ Allelic diversity:
- population mean  total number of alleles divided by number of loci
eg. 4 alleles at 1 locus, 6 alleles at a 2nd locus  A = (4+6)/2 = 5
- need to correct for variation in sample sizes
what is useful about allelic diversity??
- should be related to future selection potential
- closely related to a number of different genotypes at a locus
- good at detecting bottlenecks  low allelic diversity is evidence of a previous population
bottleneck  rare alleles lost more quickly in population bottleneck,
Distribution of variation at a single locus;
- variation exists in a hierarchy;
- alleles within individuals
- individuals within subpopulations
- populations within regions
- regions within species
- species within higher taxa
Within and among individuals;
**Hardy-Weinberg Equilibrium (HWE)  expectations for genotypes (not allele frequencies!)
at HWE;
- there should be p2 genotypes of type AA
- there should be 2pq genotypes of Aa
- there should be q2 genotypes of aa
Implications of HWE;
- rare alleles are found mostly in heterozygotes
- common alleles are found mostly in homozygotes
- maximum HE is 0.5 for a 2 allele locus, rising to 1 with more alleles
Assumptions of HWE;
ideal population – many assumptions, including (but not limited to);
- random mating
- no selection, migration, mutation
- infinitely large population size
- Mendelian segregation of alleles
- non-overlapping generations
* somewhat unrealistic for population to meet all assumptions
Uses of HWE;
- reasonably robust to modest violation of assumptions
 so departures mean that something major is going on
- can use natural history, sampling plan, differences among
loci to work out what that something might be
eg. most loci at HWE but one is not  selection?