lecture 8 - population genetics and speciation Flashcards
What is a population bottleneck?
• A large population is reduced to a few individuals. • Rare alleles are lost. • Change of allele frequencies by chance.
What is the founder effect?
- A few individuals start a new population
* E.g. colonisation of an island
What is the neutral theory?
- Much DNA variation appears to have no effects on fitness and is therefore neutral.
- No selection acts on this variation.
- Genetic drift acts on these alleles and is therefore important in molecular evolution.
What is gene flow?
movement of alleles from one population to
another
• Increases genetic diversity in the receiving population
• Homogenises connected populations
• Can be maladaptive
What is the effect of non-random mating?
Non-random mating does not affect allele frequencies as all individuals still have the same reproductive success.
What is a species?
• Reproductive isolation from other species
• Species form morphological and/or genetic
clusters that are distinct from other such
clusters
• Members of a species share a common ancestor
• Group that is adapted to a particular ecological
niche
A species is a group of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups. Problems with this concept: • Difficult to test for reproductive isolation • Cannot apply to asexual organisms • Cannot be tested in extinct groups
What is the Genotypic Cluster Species
Concept?
A species is a [morphologically or genetically] distinguishable group of individuals that has few or no intermediates when in contact with other such
clusters.
How do species form?
• Speciation requires the evolution of reproductive
isolation.
• Pre-zygotic (before fertilisation):
e.g. behavioural isolation (no mating), incompatibility of reproductive organs
• Post-zygotic (after fertilisation):
some type of genetic incompatibility
• New species form by splitting of existing species.
Cladogenesis (phylogeny):
evolutionary splitting event where a parent species splits into two
distinct species, forming a clade (= speciation (population biology) )
• allopatric: geographically isolated,
• parapatric: adjacent,
• sympatric: without spatial separation (host, time?)
• In reality: a continuum between these geographic extremes
• New species form by change over time
Anagenesis :
cumulative change within lineage
What is allopatric speciation?
Geographic barrier
Populations evolve independently
Reproductive isolation gradually evolves
On renewed contact, new species do not interbreed
Key processes in allopatric speciation:
• Genetic drift acting independently in separate
geographic areas
• Divergent selection if environmental conditions
differ
• Different mutations occur
Speciation is a by-product of gradual processes
causing divergence.
• Allopatry can occur because of
• dispersal
• vicariance (geographic barrier arises)
What is peripatric speciation?
• Speciation due to dispersal from a large area to
smaller areas, e.g. islands
• Differentiation often faster in the smaller populations
because genetic drift has larger effect
What is parapatric speciation?
• No geographic barrier, but extreme change in
environmental conditions
• Parental range limited by environmental conditions
• Range expansion with adaptation to new conditions
What is sympatric speciation?
• Speciation occurring without physical barriers • Key process: disruptive selection • Has been controversial whether it can occur, because gene flow homogenises populations
What is adaptive radiation?
Unusually rapid evolutionary diversification, accelerated
by natural selection
What is cospeciation?
Two groups of organisms speciate in response to
each other at the same time
Describe the links between selection and speciation
• Speciation can occur with or without
selection
• Sympatric: disruptive selection is required
• Allopatric: selection can facilitate speciation
• Direct selection: selection leading to
assortative mating
