population
randomly mating group of individuals of the same species
theory
set of principles, based on large body of expectations and observations accepted by scientists
gene
physical entity transmitted from parent to offspring in reproduction that influences hereditary traits
allele
different states of a gene
locus (loci)
position of a gene / marker on a chromosome
Hardy-Weinberg principle
model to predict genotype frequencies under ‘random’ mating in an ‘ideal’ population
With two alleles (A and B) the frequencies of genotypes AA, Ab and B B are p^2, 2pq and q^2
A population that is under hardy-weinberg equilibrium if there isn’t a significant difference between the genotype numbers observed than expected
Non random mating?
assortative mating - (positive assortative mating) similar individuals mate more often than expected under random mating (reduction of heterozygotes)
disassortative mating - (negative assortative mating) dissimilar individuals mate more often than expected under random mating (increases heterozygotes)
mutation (u)
- origin of new genetic variants (alleles)
- spontaneous heritable changes in genes
- weak force for changing allele frequencies
- generally increases diversity
- types: point mutations, insertions/deletions (indels), translocations, inversions
- mutation rate per locus (gene) = 1 x 10^-5
- mutations are often lost due to drift later
migration
- movement of individuals with their alleles between (sub) populations
- new alleles in population because introduced from a different one
- generally increases diversity
- Migration of a few individuals per generation between populations prevents accumulation of high levels of differentiation between populations
random genetic drift
- the random undirected changes in allele frequencies that occur by chance in all populations but particularly in small ones (eg. bottlenecks, small founder populations, endangered species)
- chance process
- generally lowers diversity
- fixation of alleles within populations (loss of rare allele)
- causes random differentiation between populations
- reduces variation within a population but increases variation between populations
natural selection
driving force for adaptive evolution
differential survival and reproduction of individuals due to differences in phenotype.
1. in all organisms more offspring than can survive and reproduce
2. organisms differ in their ability to survive and reproduce, some of these differences due to the genotype
–> survival of the fittest
It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations.
selection?
- balancing selection - heterozygote superiority
maintains diversity in the population - directional selection - leads to much lower frequency of allele that causes the low fitness in the genotype
- may lead to fixation or loss of an allele
- selection increases frequency of advantageous allele
effective population size (Ne)
the size of the genetically ideal population which has the same rate of loss of heterozygosity as an actual , non-ideal, wild population
metapopulation
collection of interacting populations (sub- pops) of the same species
- populations of many species occupy patches of high quality habitat and only use the in between land for movement
- no of isolated populations (lack of resources)
- low levels of variation within sub-pops
- as a whole either have same or higher variation than one large pop
genomic conflict
genes that affect the same trait, but different pressures
- selfish cytoplasmic gene
meiotic drivers
gene that distorts meiosis to produce gametes containing themselves over half the time (intragenomic)
- selfish drive elements gain trainsmission advantages through diverse genetic mechanisms
- wide range of consequences; extinction - changes in mating system
- can be natural / synthetic
- (t haplotype in mice)
selfish cytoplasmic gene
gene located in an organelle, plasmid or intracellular parasite (endosymbiont) that modifies reproduction to cause its own increase at the expense of the cell / organism (intergenomic)
Heteroplasmy
presence of more than one mitochondrial (or chloroplast) genome variant in an individual
genotypic fitness
- usually expressed as relative fitness, w = 1 - S
- S = selection coefficient
- compared to a genotype with no selection (neutral)
micro-satellite?
- a micro-satellite marker is a repetitive DNA sequence
- satellite DNA = tandemly repeated
- micro-satellites are highly polymorphic because the mutate more frequently
- short tandem repeats (microsatellites) evolve very fast
- mutations occur because polymerase makes mistakes during replication
monomorphic?
1 allele is fixed in the population
polymorphic?
both alleles are still present in the population
Bottlenecks?
The population size is reduced dramatically by:
- regular events (seasonal variability in resources) - results in death of many in resource poor times
- irregular random events (storms / disease outbursts)
- sustained pressure (habitat destruction / human hunting)
EXAMPLE: Northern elephant seals have reduced genetic variation most likely due to being hunted. Hunting reduced their population size to as few as 20 individuals at the end if the 19th century
Founder effect?
a new population is founded by a few individuals: 2 consequences:
1. loss of variation - an allele may be lost so the population may be genetically monomorphic
2. diversification by drift - in small pops, frequencies of alleles may drift from the parental population –> high frequencies of otherwise rare alleles
EXAMPLE: Amish population in Pennsylvania:
- closed pop origin from a small number of german immigrants (200)
- carry unusual concentrations of rare disorders (Ellis-van Creveld syndrome - causes dwarfism, polydactyly etc.)
- 50% have a hole in their heart
- traces back to Samuel King and wife came to area 1744 and mutated gene passed on to offspring
