3: Speciation

Question 1

Define a biological species

Answer 1

Mating occurs with the production of fertile offspring

Question 2

What does the formation of a new species depend on?

Answer 2

• Formation of a new species hinges on this
• Barriers that stop members from interbreeding: Geographic isolation, behavioural isolation, temporal isolation

Question 3

Define stabilising selection

Answer 3

average phenotype favoured (extremes disfavoured)

Question 4

Define directional selection

Answer 4

shift in mean pop. phenotype

Question 5

Define disruptive selection

Answer 5

shift to phenotypic extremes, mean selected against

Question 6

What type of selection is closely associated with speciation and why?

Answer 6

Disruptive selection
- Drives 2/ more peaks in fitness
When the peaks are separated and other conditions met = new species formed

Question 7

What can counter disruptive selection?

Answer 7

Disruptive selection can be countered by gene flow

Question 8

Describe gene flow

Answer 8

exchange of genes between pop.s as a result of movement and interbreeding of individuals

Directional/ stabilising selection + gene flow = holds a species together
Disruptive selection + low gene flow = speciation

Question 9

Describe the 2 changes that occur during speciation

Answer 9

→ Divergence (species adapt to diff envions/ selection pressures)
→ Reproductive isolation - pop.s cannot be interbred

Question 10

define allopatric speciation

Answer 10

geographic isolation & reproductive isolation = then divergence

Question 11

Describe sympatric speciation

Answer 11

Divergence first, then reproductive isolation (may not be any geographic isolation)

→ greek ‘same place’
→ Splitting of ancestral species into 2 or more reproductively isolated groups without geographic isolation
→ Emphasises the importance of disruptive selection

Question 12

Give an example of allopatric speciation

Answer 12

Chaffinch - island endemics
- Many morphs of chaffinch geographically isolated e.g Maderia, Canary Islands
- If you transferred them to diff islands it is unknown whether they could interbreed, speciation currently occuring

Question 13

What is heterozygote disadvantage?

Answer 13

When heterozygotes are poorly adapted for either environ = removed by natural selection

e.g
Selection in 2 diff environs determines fitness:
AxA = fit offspring
AxB = unfit
BxB = fit

Question 14

What is post-zygotic isolation?

Answer 14

(after the zygote) - reduced survival or viability of hybrids
Evolution of post-zygotic isolation

Question 15

Describe pre-zygotic isolation

Answer 15

→ before the zygote, block fertilisation from occurring
- Impeding individuals from attempting to mate
- Preventing an attempted mate from being completed successfully
- Hindering fertilisation if mating is completed successfully

Question 16

What are the 4 requirements for sympatric speciation?

Answer 16

1. Species must be largely sympatric (in same place)
2. The sympatric species must be reproductively isolated
3. The taxa must be sister species (each other’s closest relative)
4. The biogeographic and evo history of the species must make it extremely unlikely that they were ever allopatric
   = Finding robust evidence for all these conditions is hard!

Question 17

Give an example of sympatric speciation

Answer 17

Rhagoletis polmonella - North American apple maggot fly
- Native host plant (mates on) hawthorn
- Colonisers introd apple trees = sympatry with hawthorns (pre- zygotic barrier)
- Apples mature faster so timing of feeding & mating differs (2nd pre-zygotic barrier)
= Created subspecies, can still interbreed
→ can be considered speciation in action

Question 18

Describe an example of sympatric speciation involving sexual selection

Answer 18

Cichlids in North African lakes
→ huge number of species evolved in a short timeframe!

Sexual selection created a barrier to gene flow:
- Pundamilia nyererei, Pundamilia pundamilia → Closely related species, females phenotypically similar, males differ in colour
- Experiment manipulating light condition showed females have a strong pref for colour patterns = barrier to reproduction in closely related but phenotypically diff species