Lecture 20: Speciation

Question 1

Biological Species definition

Answer 1

Group of interbreeding populations, which are reproductively isolated from other populations/species.

Question 2

Allopatric speciation

Answer 2

speciation defined by the absence of gene flow - due to geographic separation
- occurs via vicariance or peripatric (founder effect)

Question 3

Allopatric Speciation

Answer 3

Vicariance
physical barrier inserted - divides population range in 2 - populations then diverge overtime

Question 4

Allopatric Vicariance example

Answer 4

Isthmus of Panama
- land mass extending from North America into South America which took 12 million years to form. (completed 3 MYA)
- Created a physical barrier: cutting off the ocean which originally flowed between the continents

• Divergence of ocean organisms
  Reproductive Isolation established between several taxa
• e.g. sea urchins

Question 5

Mountain range & Bird diversity
Vicariance example

Answer 5

Climate fluctuations cause a once continuous population to separate to opposite ends of the mountain
- when they come back into contact they are reproductively isolated

(the emergence of mountains themselves is also an example of a physical barrier).

Question 6

Partula land snail

Answer 6

‘micro geographic’ separation
- actual geographic distance is tiny but due to the v low dispersal ability of snails its significance is magnified.

(allopatric speciation depends on the organism as well as the physical barrier).

Question 7

Peripatric/Dispersal speciation (founder effect)

Answer 7

Form of allopatric speciation
- a smaller colony separates from the main population
- this distance prevents gene flow and the colony then diverges.

Question 8

Dispersal Example (allopatric)

Answer 8

Island colonisation by birds
- occurs in many taxa
- long distance (no 2 sister species found within 10,000km)

Question 9

Secondary Contact- Test of allopatric specitiation

Answer 9

• When the separated populations come back into contact if they’re unable to hybridise then they have reproductively isolated & diverged into separate species.

IF Hybridisation Occurs
1) The hybrid zone is maintained and the 2 populations persist.
2) Hybrid zone collapses and the 2 populations merge back into the original state.

3) Reinforcement of reproductive Isolation: Hybrids are maladaptive and have lower fitness. Selection favours the 2 populations to continue diverging.

Question 10

Speciation WITH gene flow

Answer 10

Parapatric- a continuous population begins to diverge into 2 distinct populations isolating themselves geographically at opposite ends of their environment

sympatric- genetic differences arise, with no geographic separation, but still speciation occurs.

Question 11

Sympatric Speciation

Answer 11

Speciation without and geographic isolation
- less likely as gene flow & recombinations breaks down divergence
… unless selection for divergence is v high.

Question 12

Example= bird beak size

Answer 12

Intermediate beak size has reduced fitness (Heterozygote disadvantage) selection causes the trait to diverge further.

If the beak size trait becomes genetically linked with a mating trait e.g. colouration

Individuals will be able to easily identify mates with increased fitness so will select for individuals with big/small beaks.
- this establishes reproductive isolation. (females only select colorful mate as is indicative of fitness (due to linkage with beak size gene))

Question 13

Example= Apple maggot fly

Answer 13

Ancestor of the fly laid eggs only on the fruit of Hawthorn Trees
- some flies began infesting apple trees in the same geographic area.

Species consist now of genetically divergent populations - differing in host preference:
- different mating seasons to match the fruiting times of apples v hawthorns.

Question 14

Parapatric speciation

Answer 14

Neighbouring populations diverge but continue to interbreed
- often due to adaptation to an environmental gradient -> creating a cline in allele frequencies.
- more common than sympatric speciation as there’s less gene flow between the populations.

Question 15

Parapatric example= Lizards of the White Sands

Answer 15

• White sand dunes arose recently – they contrast strongly to the surrounding dark soil.
• provides a novel selection environment (for camouflage).
• 3 species have evolved colour morphs

Colouration differences impact mating –> leading to sexual isolation between morhps.

Question 16

Speciation in reality is More Complex

Answer 16

• Often combinations of these 4 types of speciation act together in the process of divergence.