Lecture 18-19 Flashcards
Allopatry
non-overlapping distributions
Sympatry
largely overlapping distributions
Parapatry
partially overlapping populations
Disjunct population
physically separated populations
Vagility
the ability of an organism to disperse
Deme
a local population of a species
Cline
a gradient in the genotype or phenotype of an organism that correlates with the direction or orientation of some geographic feature
Extrinsic barrier
a barrier to gene glow caused by factors external to the organism
Intrinsic barrier
a barrier to reproductive isolation caused by reproductive isolating mechanisms
Pleiotropy
when a single gene product effects multiple phenotype characteristics
Introgression
gene flow from one species into another
Incipient species
a population that is on its way to becoming a new species
Cryptic species
a species complex comprised of multiple, similar species
Three steps for speciation
- isolation of the population: physical (allopatric, parapatric, peripatric) or genetic/ecologcial (sympatric)
- divergence in traits: adaptation to different habitats or assortative mating/sexual selection
- establishment of reproductive isolation: reinforcement
Allopatric Speciation
- Reproductive isolation, geographically disjunct
- Extrinsic barriers: geographic
- Intrinsic barriers: reproductive isolating mechanisms
- One or more isolating mechanisms
- Extrinsic barriers over time result in intrinsic barriers
- Three types: vicariance, peripatric, and parapatric
Vicariance Speciation
Speciation between relatively large population that are geographically separated
Isolated populations tend to differentiate in response to local extrinsic factors
Differential selective forces in demes
Given initial amount of geographic variation, disjunction of populations or groups of populations is likely to result in a cessation of gene flow across the geographic boundary and eventual divergence
Factors that influence rate at which new species are formed
- Magnitude of geographic variation
- Organismal vagility
- Magnitude of selection
- Initial variation in population
- Population size
Peripatric Speciation
Founder effect, dispersal
Speciation in peripheral populations founded by a small number of individuals
New species will intially appear in marginal/peripheral habitats
Process of speciation may be relatively rapid
Founder Flush cycle
Founder Flush Cycle
Drift via Founder effect
Sudden increase in numbers of individuals
Strong selective pressure leads to a genetic bottleneck and crash
Repeat
Possible Outcomes when recently diverged populations come into secondary contact
Speciation is complete
Introgression back into a single population
Remain distinct populations that then speciate via reinforcement
Formation of a relatively stable hybrid zone
Formation of a new (third) species in the hybrid zone (homoploid hybrid speciation)
Reinforcement
Development of prezygotic isolation mechanisms that prevent the formation of hybrids with reduced fitness and drive speciation to completion
Why can selection act against hybrids?
Natural selection produces adaptations to distinct habitats
Sexual selection produced changes in the mating system
Genetic drift led to fixation of alleles that do not work well together when heterozygous
Divergence
Interaction between the environment, natural selection, and sexual selection
Homoploid Hybrid Speciation
Formation, in the progeny of a natural hybrid, of a new, true-breeding line that is isolated from parental species
