Evolution by selection
-Changes in allele frequency
-Natural selection typically takes many generations to have a substantial impact.
-Rate of change is a function of selection intensity
patterns of selection
- recessive vs dominant alleles
- heterozygous favoured
- homozygous favoured
- frequency dependent selection
evolution by mutation
-Mutation is the source of all new alleles and genes and provides the raw material for evolution
-Mutation can cause substantial changes in allele frequencies, but only over really long periods of time
-Mutation-selection balance
-Many mutations are deleterious
- Selection may remove deleterious alleles but mutation may re-introduce them, hence they persist.
- When the rate at which new copies of a deleterious
allele are produced by mutation equals the rate at
which selection removes them, there is a mutation-
selection balance
gene flow
May be caused by:
-Dispersal- one-way movement of a juvenile individual away from the place of their birth.
-Transport of pollen, seeds, spores by any means (e.g., wind, animals, water, etc.)
-Tends to homogenize allele frequencies among populations.
-Therefore, tends to prevent evolutionary divergence of populations.
-May function to decrease the population level impact of natural selection and/or other mechanisms of evolution
-FST (the fixation index) is a measure of variation among populations in allele frequencies at a locus
-Gene flow reduces Fst among populations
genetic drift
-Random, unpredictable changes in allele frequencies from one generation to the next.
-“blind luck”
-Mechanism of evolution resulting from random sampling of gametes from one generation to the next
-Genetic drift is sampling error across generations
-From Hardy-Weinberg perspective, it results from a violation of the assumption of infinite population size.
-Incidence and rate of genetic drift increases as population size decreases
-Does not lead to adaptation (as does natural selection).
-Natural selection happens for a reason: i.e., alleles that produce certain phenotypes are more successful (i.e., fit) in particular environments, and therefore increase in frequency
-For genetic drift allele frequency changes happen by ‘chance’
-typically leads to fixation (loss of alleles) and a decline in heterozygosity
-Demographic events that can cause genetic drift:
1. Founder events – may be considered a form of genetic drift.
-Occur when a new population is founded by a small group of individuals
2. Bottleneck– a sharp decline in population size followed by population recovery
non-random mating
- inbreeding
-mating among closely-related individuals.
-Results in increase in the frequency of homozygotes in a population, relative to what is expected based upon H-W assumptions
-Inbreeding alters genotype frequencies.
-But it does not alter allele frequencies.
-Therefore, on its own non-random mating is NOT a mechanism of evolution - outbreeding
-mating among highly unrelated individuals - positive assortative mating
-between individuals with similar phenotypes (large individuals mate with large individuals) - negative assortative mating
-between individuals with dissimilar phenotypes (large individuals mate with small individuals)
-does not, on its own, change allele frequencies.
-It does impact genotype frequencies, so in concert with natural selection, it can have important evolutionary consequences
agents of evolution
- Selection- can change allele and genotype frequencies
- Mutation- weak evolutionary force but provides the raw variation for evolution
- Gene flow (i.e., ”migration”)-when source and recipient populations differ in allele frequencies,
migration can cause recipient population to evolve
-Tends to homogenize allele frequencies among
populations - Genetic drift-random shift in allele and genotype frequencies
- Non-random mating- does not change allele frequencies, but does change genotype frequencies.
- Can effect evolution in concert with natural selection.
what is phylogenetics
the study of ancestor descendent relationships. The objective of phylogeneticists is to construct phylogenies
what is phylogeny
A hypothesis of ancestor descendent relationships
what is a phylogenetic tree
a graphical summary of a phylogeny
-Also called an evolutionary tree
what is plesiomorphy
refers to the ancestral character state
what is apomorphy
character state different than the ancestral state, or DERIVED STATE
what is a synapomorphy
a derived character state (apomorphy) that is SHARED by two or more taxa due to inheritance from a common ancestor: these character states are phylogenetically informative using the parsimony or cladistic criterion
what is autapomorphy
a uniquely derived character state
what is homoplasy
Homoplasy describes similarity of character state state due to independent evolution
evolutionary ways homoplasy can occur
- parallel evolution
-independent evolution of same feature from same ancestral condition - convergent evolution
-independent evolution of same feature from different ancestral condition - secondary loss
-reversion to ancestral condition
what is parsimony
the simplest scientific explanation to fit the evidence is preferred (Occam’s razor)
molecular clock
-Some types of DNA sequences change in a regular ‘clock-like’ fashion.
-Neutral changes in DNA should accumulate in populations at a rate equal to the mutation rate.
-Therefore, if mutation rate stays reasonably constant, and generation times remain similar, then the number of neutral molecular differences between two taxa should be proportional to the age of their most recent common ancestor
what is an adaptation
-A trait that increases the fitness of the individuals that possess it relative to those that do not
-A product of natural selection
methods to test hypothesis
- Experiments
-Powerful
-Permits the testing of one factor
-May be impractical - Observations
-Requires careful monitoring of the study
environment. - Comparative method
-Test the patterns in the frequency of traits
among species
phenotypic plasticity
- no plasticity
-environment does not affect the trait - plasticity
-traits are affected equally by the the environment - genotype-by-environment interaction
-lots of variability of traits effected by the environment in different ways
trade-offs
-only so much energy can be invested in traits
-some traits have to be prioritized over others
