Lecture 13 - evolution above the species level

Question 1

What is the origin of species?

Answer 1

-splitting of one species into two

Question 2

Over time what does the process of speciation lead to?

Answer 2

NEW:
- species
- genera
- families
- orders
- classes
- phyla

Question 3

What is a species?

Answer 3

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

Question 4

What is reproductive isolation?

Answer 4

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

Question 5

What are the other isolation types which can lead to speciation?

Answer 5

1. Geographic isolation
2. Behavioural isolation
3. Temporal isolation

(alongside reproductive isolation)

ALL PREVENTING REPRODUCTION OCCURING

Question 6

What does disruptive selection push?

Answer 6

groups of populations apart causing divergence in phenotypes

Question 7

How can disruptive selection be countered?

Answer 7

Gene flow

Question 8

What is gene flow?

Answer 8

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

Question 9

What is an example of gene flow?

Answer 9

• 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

Question 10

What do we need to consider to understand speciation?

Answer 10

natural selection and gene flow

Question 11

What holds a species together?

Answer 11

directional or stabilising selection + gene flow = holds a species together

Question 12

What leads to speciation?

Answer 12

disruptive selection + low gene flow = speciation

Question 13

Describe the main theories of how speciation occurs

Answer 13

• 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

Question 14

What are 2 changes during speciation?

Answer 14

Divergence - species adapt to different environments or selection pressures

Reproductive isolation - populations cannot interbreed

Question 15

What are the 2 theories of speciation?

Answer 15

• Allopatric speciation
• Sympatric speciation

Question 16

What is allopatric speciation?

Answer 16

geographic isolation & reproductive isolation first, then divergence

Question 17

What is sympatric speciation?

Answer 17

divergence then reproductive isolation

Question 18

How can long term isolation result in phenotypic divergence?

Answer 18

• 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

Question 19

What occurs if the barrier is removed and species can once again overlap geographically?

Answer 19

• 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

Question 20

What are island endemics?

Answer 20

• chaffinch divergence in isolation
• we don’t know if different birds in different countries interbreeding would lead to fertile offspring

Question 21

What are barriers & dispersal?

Answer 21

• allopatric speciation depends on geographical isolation
• geographic isolation depends on the existence of physical barriers & the organisms dispersal ability

Question 22

What are the allopatric take home messages?

Answer 22

• physical barriers lead to geographical isolation of populations
• lack of gene flow leads to reproductive isolation
• populations in allopathy diverge due to disruptive selection

Question 23

Describe sympatric speciation

Answer 23

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

Question 24

What are incipient species?

Answer 24

groups of organisms that rarely interbreed and are genetically distinct from their ancestral species

Question 25

How is sympatric speciation supposed to occur?

Answer 25

• imagine a polygenic trait
• selection in 2 different environments determines fitness
• heterozygote disadvantaged (poorly adapted to either environment) and removed by natural selection

Question 26

What happens to hybrids?

Answer 26

• 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

Question 27

What is pre-zygotic isolation?

Answer 27

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

Question 28

What evidence is required to make a compelling case for sympatric speciation?

Answer 28

1. species must be largely sympatric
2. the sympatric species must be reproductively isolated
3. the sympatric tax’s must be sister species (each other’s closest relative)
4. the biogeographic & evolutionary history of a species must make it extremely unlikely that they were ever allopatric

Question 29

What reduces gene flow?

Answer 29

sympatric speciation by habitat & temporal isolation

Question 30

Does sexual selection drive reproductive isolation?

Answer 30

• 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

Question 31

What are the take home messages of sympatric speciation?

Answer 31

• 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