speciation Flashcards
what does the process of speciation lead to
-Species
-Genera
-Families
-Orders
-Classes
-Phyla
definition of biological species
-a species is a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring—but do not produce viable, fertile offspring with members of other such group
whats reproductive isolation
-“the formation of a new species hinges on reproductive isolation—the existence of biological factors (barriers) that impede members of two species from inter- breeding and producing viable, fertile offspring”
-existance of some barrier meaning 2 different species can’t reproduce fertile offspring
what are the 3 types of selection
-Stabilizing selection:
the mean phenotype is favoured (extreme phenotypes are disfavoured).
-Directional selection:
evolutionary change with a shift in the mean population phenotype.
-Disruptive (or diverging or diversifying) selection:
two or more phenotypes are favoured, the mean phenotypes is selected against.
whats disruptive selection
-Speciation is expected to be associated with disruptive selection
-Disruptive selection drives two, or more, peaks in fitness.
-When the peaks have separated and other conditions are met then you might consider that new species have been formed.
-Disruptive selection pushes means or groups of populations apart causing divergence in phenotypes
-Disruptive selection can be countered by gene flow
what does gene flow mean
-exchange of genes between populations as a result of movement and interbreeding of individuals
-no physical barrier
what happens with gene flow and disrupt selection
-If individuals within a population move in space and can interbreed then the effects of disruptive selection will be reduced.
-Dummy example: imagine two islands with potential for disruptive selection, perhaps selection favours camouflage of yellow butterflies in location A and orange butterflies in location B
-But individuals move freely between the islands and interbreed (yellow butterflies are just as likely to breed with orange butterflies as they are with other yellow butterflies). The outcome would be a mixed population, not divergent populations.
whats selection and gene flow
-To understand speciation we need to think about natural selection and gene flow
-Directional or stabilizing selection + gene flow = holds a species together
-Disruptive selection + low gene flow = speciation
what are the 2 changes during speciation
1) Divergence – species adapt to different environments or selection pressures
2) Reproductive isolation – populations cannot interbreed
what are the 2 main theories of speciation
-Two major hypotheses, supported with data, for how speciation occurs.
-Focus on the interplay of gene flow & disruptive selection as drivers of reproductive isolation and phenotypic divergence.
-Major difference is in the order of isolation and divergence
what are the 2 types of speciation
-Allopatric speciation:
Geographic isolation & reproductive isolation first, then divergence.
-Sympatric speciation:
Divergence first, then reproductive isolation
whats allopatric speciation
-geigraphic isolation
- Two mountain ranges separated by a large valley
-A species of squirrel lives on both mountains and the valley inbetween
-The population mixes freely (there is gene flow)
- Over geological time the valley becomes flooded.
-The mountains are now isolated islands and the squirrels on each side of the mountain are geographically isolated.
As a result they are also reproductively isolated with no gene flow between the mountains.
why can long term isolation result in phenotypic divergence in allopatric speciation
-Different climatic or ecological conditions and therefore different selection pressures favouring different phenotypes.
-Random genetic drift (change in genotype frequency caused by random variation in individual reproduction).
what happens in allopatric speciation if the barrier is removed and species can once again overlap geographically
-If populations have diverged enough they continue to be reproductively isolated (they don’t interbreed), even in the absence of a geographical barrier.
-But, the geographic barrier could be the only thing stopping interbreeding – brown and red squirrels are happy to breed with each other. Then the partially divergent population collapses back into a single species
example of island endemics
-chaffinch divergence in isolation
-gene flow is low
-shows process of speciation in action so we dont know if speciation is finished
barriers and dispersal
-Allopatric speciation depends on geographic isolation
In the tropics speciation depends on the existence of physical barriers across the landscape and the organisms ability to move across that landscape
whats sympatric speciation
-Sympatric speciation, from the Greek ‘same place’, involves the splitting of an ancestral species into two or more reproductively isolated groups without geographical isolation of those groups. The key aspect of sympatric speciation is that it occurs when incipient species are in physical contact with each other, potentially able to interbreed and exchange genes.
How is sympatric speciation supposed to occur? disruptive selection
-Disruptive selection drives two, or more, peaks in fitness.
-When the peaks have separated and other conditions are met then you might consider that new species have been formed.
-Disruptive selection pushes means or groups of populations apart causing divergence in phenotypes
-Disruptive selection can be countered by gene flow
-Imagine a polygenic trait (a trait determined by multiple genes)
-Selection in two different environments determines fitness, e.g.
AxA = fit offspring
AxB = unfit offspring
BxB = fit offspring
-Heterozygote disadvantaged (poorly adapted to either environment) and removed by natural selection
How is sympatric speciation supposed to occur? post-zygotic isolation
-Hybrids (heterozygotes) disadvantaged (poorly adapted to either environment) and removed by natural selection
-Higher fitness if you mate with individuals with similar phenotypes
-Post-zygotic isolation (after the zygote) – reduced survival or viability of hybrids
-Evolution of post-zygotic isolation
-higher fitness if you mate with someone with a similar phenotype
whats Pre-zygotic isolation
-Prezygotic barriers (“before the zygote”) block fertilization from occurring by
-impeding members of different species from attempting to mate
-preventing an attempted mating from being completed successfully
-hindering fertilization if mating is completed successfully
What evidence is required to make a compelling case for sympatric speciation
S1)pecies must be largely sympatric.
2)The sympatric species must be reproductively isolated.
3)The sympatric taxa must be sister species (each others closest relative).
4)The biogeographic and evolutionary history of the species must make it extremely unlikely that they were ever allopatric.
example of sympatric speciation
1) Rhagoletis polmonella
-North American apple maggot fly
Native host plant is hawthorn
-Usually mate on or near host plant
-200 years ago colonised introduced apple trees that occurred in sympatry with hawthorn
-Pre-zygotic barrier (habitat isolation) between apple feeders and hawthorn feeders
-Apples mature faster than hawthorn so the timing of feeding, and therefore mating differs
-Time of mating is a second pre-zygotic barrier
-Rhagoletis polmonella considered as subspecies (hawthorn and apple) and may be on the way to full speciation
-Mechanism may also apply to other Rhagoletis
-Close relative, Rhagoletis mendax, feeds on blueberries
-Sympatric speciation by habitat and temporal isolation reduce gene flow
2)sexual selection
-Lake Victoria in East Africa was home to >600 species of cichlid fish
-More than double the total freshwater fish species in all of Europe
-All originated in the last 100,000 years
-Pundamilia nyererei and Pundamilia pundamilia: Researchers manipulated light environment in two aquarium tanks and compared female mate choice in normal and monochromatic light conditions
Females strongly preferred males of the same species in normal conditions but showed no preference in monochromatic light
how can we test the idea that sexual selection drives reproductive isolation
-Colour perception depends on the light environment and the visual sensitivities of the organism
-Under monochromatic orange light males of P. nyererei and P. pundamilia appear almost identical
-Hypothesis: females are only attracted to males of the same species under normal light (when colour differences can be perceived)
