Speciation II Flashcards
What are the evolutionary forces involved in speciation?
Drift, gene flow, mutation, selection
Modes of speciation:
1) allopatry
2) peripatry
3) parapatry
4) sympatry
What types of empirical evidence do we collect to assess the speciation continuum?
- palaeontology
- biogeography
- phylogenetics
- genetics
Mutation
The ultimate source of genetic variation
Random genetic drift
Fluctuations in allele frequencies that occur by chance and can have particularly strong effects in small populations
Gene flow (gene migration)
Movement of alleles among populations
Random genetic drift and gene flow
Speed up or constrain development of adaptations, but cannot cause it
Natural selection
- inherited differences in the ability of organisms to survive and reproduce
- leads to adaptation
Vicariant allopatric selection
- no gene flow
- drift present
- divergent selection in different habitats
- different mutations fixed in similar environments
Peripatric allopatric speciation
- no gene flow
- strong drift in small population
- divergent selection in novel habitat
Stepping stone parapatric speciation
- limited gene flow among discrete populations
- drift and selection more effective
- selection for local environment
- results in neighbouring sister species
Clines parapatric speciation
- less limited gene flow along environmental cline
- selection for local environment results in neighbouring sister species
Sympatric speciation
- free gene flow
- requires very strong divergent selection
For speciation to occur, you need some combination of
1) isolation
2) divergent selection
Why do you need isolation for speciation to occur?
- reduce/stop gene flow
- drift alone can cause speciation
Divergent selection can result in speciation even
In the face of gene flow
If there is no gene flow, the speciation is
Allopatric
Behavioural isolation can develop
quickly (e.g. 5 generations of Diptera fly)
Describe the speciation of snapping shrimp Alpheus
- emergence of the Isthmus of Panama created barrier ~3Mya: separated marine life
- different abiotic and biotic environments
- créâtes sister species pairs either side of the Isthmus
- strong reproductive isolation
- mtDNA divergence
- morphological divergence
Describe the case of Pinaroloxias inornata
- sole representative of a genus on an island post-colonisation
If gene flow is low - medium, speciation is
Parapatric
If divergent selection is strong, the speciation is considered
Ecological
Describe M. guttatus (Wright et al. 2013)
- widespread
- adjacent populations on copper line waste evolve copper tolerance
- partial gene flow, strong selection
- reproductive isolation: hybrid lethality
It’s not an allopatry event but an allopatry
Phase
What speeds up speciation?
- selection against immigrants
- selection against hybrid intermediates
- reproductive isolation as a byproduct of ecological divergence
- byproduct of adaptive divergence
Give examples of adaptive divergence
1) different flowering times
2) different body sizes
Give the criteria of sympatric speciation
1) species must be largely or completely sympatric
2) substantial reproductive isolation, preferably genetically based
3) sympatric taxa must be sister groups (not the result of hybridisation)
4) biogeographic and evolutionary history of the groups must make existence of an allopatric phase very unlikely
Describe the Lord Howe island palms (Papadopulos 2011)
- molecular based phylogenies differentiate between sympatric speciation and double colonisation
- 4.5-8.2% of flora arose by sympatric speciation
- 24.8% arose by allopatric speciation
- the rest did not spéciate
Why must double colonisation be ruled out
To ensure group is monophyletic
Describe the speciation of the apple maggot fly Rhagoletis pomonella
- sympatric speciation via host shift
- from hawthorn to apple in New York State
- only occurred once in last 150yrs
- assortative mating
- maladapted hybrids
- genetic divergence between host races creating allozymes
Describe the assortative mating R. Pomonella (Bush, 1969)
- habitat preference: adults mate on the fruit they fed on as larvae
- incomplete temporal isolation: apple fruits earlier than hawthorn
- adaptation to respective hosts: which howl diapause length
What are the cues for mate choice in Darwin’s finches?
Song and bill morphology
Describe the “Big Bird” lineage (Lamichhaney, 2017)
- began with G. fortis (resident) female and G. conirostris (immigrant) male
- successful ingroup breeding occured (despite intense inbreeding)
- reproductive isolation and ecological segregation occurs in 3 generations
Polyploidy
- Individuals carry 3 or more complete sets of chromosomes
- common in plants, rare in animals
An increase in chromosome sets can
Produce nearly instantaneous reproductive isolation
What percentage of speciation events in ferns involve polyploidy?
7
What percentage of speciation events in angiosperms involve polyploidy?
2-4
Evolutionary inertia
Co-adapted gene complexes constrain further adaptation
What events might break evolutionary inertia
Severe drift (e.g. during a founder event)
True allopatry means
No gene flow
At a global level, new variation can only be created by
Mutation
At a local level, new variation can be created by
Migration, and mutation
Drift shows
No directionality
What changes evolutionary trajectory?
Interaction between different microevolutionary processes
Drift is a divergence by
Chance
Cline
- gradation from one population to the other
- isolation by distance
True sympatry means
Completely congruous
Theoretically,
- drift can result in speciation alone
- but reduced gene flow rlly helps
Parapatric speciation occurs with
Relatively high frequency
Ecotones
Two different vegetation forms come together and mix in a relatively narrow strip relative to the overall habitat size
Reconstructing speciation is
Tricky
Strong selection overcomes
Gene flow
To be a ring species, you can’t have
Any deep biogeographic/vicariant breaks or an allopatric phase
In a ring species, rather than flowing, genes
‘Trickle’
