3: Speciation

Question 1

Define Allopatric speciation

Answer 1

No gene flow between spatially separated populations

Question 2

Define allopatric speciation by vicariance

Answer 2

Range splitting
- Where a once continuous range is split into 2 by a geopgraphic barrier

no gene flow

Question 3

Describe Peripatric speciation

Answer 3

(founder effect)
long distance dispersal and colonisation event to a geographically isolated area

no gene flow

Question 4

Define Parapatric speciation

Answer 4

Neighbouring pop.s of a continuously distributed species, diverge into diff species with gene flow!

Question 5

Define sympatric speciation

Answer 5

A pop splits into 2 repro isolated species whilst being in geographical contact with one another at all times

Question 6

What happens if reproductive isolation is incomplete? 3 options:

Answer 6

1. Known as speciation collapse/failure/reversal (9)
2. Stable hybrid zone forms, where some genetic differentiation persists (7)
3. New selection to avoid hybrid maladaptation = reinforcement of reproductive isolation, driving it to completion

Question 7

Why is sympatric speciation more rare than allopatric?

Answer 7

Because gene flow and recombination breaks down genetic combinations that might form new species

e.g linkage between ecological trait (under disruptive selection) and mating trait is difficult in sympatry because linkage is blocked by interbreeding

Question 8

What can make sympatric speciation easier to arise?

Answer 8

The presence of a speciation trait that causes both ecolological divergence and reproductive isolation between developing species

e.g Apple maggot fly
- Ancestor lays eggs on hawthorn tree
- Later, fly started to infest apple trees in same area
- These flys mate on host plant and are now genetically divergent pops that differ in host pref and mating season

Question 9

Why is parapatric speciation expected to be more common than sympatric speciation?

Answer 9

Because it involves less gene flow between diverging populations

Question 10

What is usuallt involved in parapetric speciation?

Answer 10

Usually involved adaption of pop.s to an environ or ecological gradient producing a cline in allele frequencies

Question 11

(Lizards at White Sands)

Describe an example of Parapatric speciation

Answer 11

Adaptation to environ extremes has resulted in repro isolation.
- Colour morphs have evolved in three lizard species, in at least 1 species coloration effects mating behaviour and has led to partial sexual isolation between colour morphs

Question 12

Causes of speciation

Define Ecological speciation

Answer 12

the evolution of reproductive isolation between pops. as a result of ecologically based divergent natural selection

Question 13

Causes of speciation

Define Sexual selection

Answer 13

Differential reproduction as a result of variation in the ability to obtain mates
- Divergent sexual selection can cause the rapid evolution of phenotypic traits and also generate really strong repro isolation between pops.

Question 14

Describe the reinforcment of sexual selection

Answer 14

When organisms evolve traits that mate it less likely for them to make with individuals from different groups
→ happens because hybrids may be less fit

Question 15

Describe how prezygotic isolating mechanisms can evolve by reinforcement

Answer 15

Alleles that strengthen prezygotic isolation gain an advantage because individuals with them have higher fitness than those who hybridise

Question 16

Describe why postzygotic isolating mechanisms (generally) cannot evolve by reinforcement.

Answer 16

Stronger postzygotic isolation usually cannot evolve by reinforcement because an allele that lowers hybrid fitness cannot increase in frequency

Question 17

Under what conditions is reinforcement likely?

Answer 17

• When pop.s are well differentiated (phenotypic traits and mating prefs …)
• Low hybrid fitness

= when pops. exhibit a high degree of pre AND postzygotic isolation

Question 18

Fill in the blanks

We would expect: Populations that overlap geographically to show ——- mating preference differentiation (——— ———) than geographically separated species that have never experienced the cost of maladaptive ————.

Answer 18

We would expect: Populations that overlap geographically to show greater mating preference differentiation (prezygotic isolation) than geographically separated species that have never experienced the cost of maladaptive hybridisation

Question 19

Define Reproductive character displacement

Answer 19

The accentuation of differences between sympatric pops of 2 species as a result of reproductive interactions between them

Question 20

Define Autoploidy

Answer 20

Duplication of chromosomes of the same species

Question 21

Define alloploidy

Answer 21

Duplication of chromosomes in hybrids between species

Question 22

Describe how tetraploids can build a population (if they are unable to produce fertile offspring with diploids)

Answer 22

By reducing hybridisaion with diploids they gain an initial foothold, can be done by:
- Self fertilisation
- Vegetative propagation
- Habitat segregation

Question 23

Define hybrid speciation

Answer 23

hybridisation between 2 or more distinct lineages that contributes to the origin of a new species

Question 24

Define Homoploid hybrid speciation

Answer 24

Speciation via hybridisation without a change in chromosome (ploidy) number
- Hybrid form is stabilised and remains distinct when in contact with either parent

Question 25

What is the difference between ‘normal’ hybridisation and homoploid hybridisation?

Answer 25

• In normal, hybrid forms dont stabilise and remain and kind of gradation between 2 parental species
• In homoploid, hybrid form is stable and remains distinct when in contact with either parent species

Question 26

What is Random genetic drift?

Answer 26

Random changes in frequencies of 2 or more allele or genotypes within a pop

Question 27

Speciation by genetic drift

Describe how Dobzhansky-Muller incompatibilities occur

Answer 27

Long-term geographic isolation and drift
- Mutations that have arisen and fixed because of drift, may be incompatible when brought together = postzygotic isolation

Question 28

Speciation by genetic drift

Describe peripatric (founder effect) speciation

Answer 28

Speciation by the evolution of reproductive isolation in small peripheral pops as a consequence of genetic drift and natural selection
- debated

Question 29

List the 6 main causes of speciation

Answer 29

1. Ecological speciation
2. Sexual selection
3. Reinforcement of reproductive isolation
4. Polyploidy
5. Hybrid speciation
6. Random genetic drift