Lecture 13 - evolution above the species level Flashcards
What is the origin of species?
-splitting of one species into two
Over time what does the process of speciation lead to?
NEW:
- species
- genera
- families
- orders
- classes
- phyla
What is a species?
a group of populations whose members have the potential to interbreed in nature & produce viable, fertile offspring - but don’t produce viable, fertile offspring with members of other such groups
What is reproductive isolation?
the formation of a new species can be due to reproduction isolation - the existence of biological factors (barriers) that impede members of 2 species from inter-breeding and producing viable, fertile offspring
What are the other isolation types which can lead to speciation?
- Geographic isolation
- Behavioural isolation
- Temporal isolation
(alongside reproductive isolation)
ALL PREVENTING REPRODUCTION OCCURING
What does disruptive selection push?
groups of populations apart causing divergence in phenotypes
How can disruptive selection be countered?
Gene flow
What is gene flow?
the exchange of genes between populations as a result of movement & interbreeding individuals
- individuals within a population move in space and interbreed then the effects of disruptive selection will be reduced
What is an example of gene flow?
- 2 islands with potential for disruptive selection, perhaps selection favours camouflage of yellow butterflies in location A & orange butterflies in location B
- more 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
What do we need to consider to understand speciation?
natural selection and gene flow
What holds a species together?
directional or stabilising selection + gene flow = holds a species together
What leads to speciation?
disruptive selection + low gene flow = speciation
Describe the main theories of how speciation occurs
- focus on the interplay of gene flow & disruptive selection as drivers of reproductive isolation and phenotypic divergence
- the major difference is the order of isolation & divergence
What are 2 changes during speciation?
Divergence - species adapt to different environments or selection pressures
Reproductive isolation - populations cannot interbreed
What are the 2 theories of speciation?
- Allopatric speciation
- Sympatric speciation
What is allopatric speciation?
geographic isolation & reproductive isolation first, then divergence
What is sympatric speciation?
divergence then reproductive isolation
How can long term isolation result in phenotypic divergence?
- different climatic or ecological conditions and therefore different selection pressures favoring different phenotypes
- random genetic drift (change in genotype frequency caused by random variation in individual reproduction
What occurs if the barrier is removed and species can once again overlap geographically?
- if populations have diverged continue to be reproductively isolated (no interbreeding), even in the absence of a geographical barrier
- BUT, the geographical barrier could be the only thing stopping interbreeding - brown & red squirrels are happy to breed with each other. Then the partially divergent population collapses back into a single species
What are island endemics?
- chaffinch divergence in isolation
- we don’t know if different birds in different countries interbreeding would lead to fertile offspring
What are barriers & dispersal?
- allopatric speciation depends on geographical isolation
- geographic isolation depends on the existence of physical barriers & the organisms dispersal ability
What are the allopatric take home messages?
- physical barriers lead to geographical isolation of populations
- lack of gene flow leads to reproductive isolation
- populations in allopathy diverge due to disruptive selection
Describe sympatric speciation
sympatric speciation involves the splitting of an ancestral species into 2 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 others potentially able to interbreed and exchange genes
What are incipient species?
groups of organisms that rarely interbreed and are genetically distinct from their ancestral species
How is sympatric speciation supposed to occur?
- imagine a polygenic trait
- selection in 2 different environments determines fitness
- heterozygote disadvantaged (poorly adapted to either environment) and removed by natural selection
What happens to hybrids?
- hybrids (heterozygotes) disadvantaged (poorly adapted to either environment) and removes by natural selection
- higher fitness if you mate with individuals with similar phenotypes
- post-zygotic isolation (after the zygote) - reduced survival or viability
- evolution of post-zygotic isolation
What is pre-zygotic isolation?
Prezygotic barriers block fertilisation from occurring by:
- impeding members of different species from attempting to mate
- preventing an attempted mating from being completed successfully
- hindering fertilisation if mating is completed successfully
What evidence is required to make a compelling case for sympatric speciation?
- species must be largely sympatric
- the sympatric species must be reproductively isolated
- the sympatric tax’s must be sister species (each other’s closest relative)
- the biogeographic & evolutionary history of a species must make it extremely unlikely that they were ever allopatric
What reduces gene flow?
sympatric speciation by habitat & temporal isolation
Does sexual selection drive reproductive isolation?
- colour perception depends on the light environment and the visual sensitivities of the organism
- under monochromatic orange light, 2 species appear almost identical
Hypothesis: females are only attracted to males of the same species under normal light (when colour differences can be perceived)
- 2 fish look similar when the former is put in monochromatic
- researchers manipulated light environment in 2 aquarium tanks and compared to 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
- strongly suggests that female preferences for certain colour patterns is a barrier to reproduction in closely related sympatric species
- this prezygotic barrier can be breached in the lab
SO YES
What are the take home messages of sympatric speciation?
- splitting of an ancestral species into 2 or more reproductively isolated groups without geographical isolation
- divergence occurs first
- divergence causes reduction in gene flow, leading to reproductive isolation
