Speciation

Question 1

What is a species according to the biological definition?

Answer 1

A population of reproducing organisms that is isolated from other populations

This definition is attributed to Mayr and Dobzhansky.

Question 2

What are the two main contexts in which species can appear?

Answer 2

• In time along a lineage (e.g. humans/chimps and our common ancestors)
• In space due to geographic isolation (e.g. on an island)

Question 3

What is allopatric speciation?

Answer 3

A mode of speciation that occurs when populations are geographically isolated

Conditions are slightly different in each geographical location

Question 4

What is an example of allopatric speciation?

Answer 4

Two different species of antelope squirrels on either side of the Grand Canyon

Question 5

What is genetic drift?

Answer 5

Random fluctuations in allele frequencies that can cause major changes in small populations, lowering the genetic variation

Question 6

What are founder effects and bottlenecks?

Answer 6

Events that can lead to a loss of genetic variation in a population

Question 7

What is sympatric speciation?

Answer 7

Speciation that occurs without geographic isolation, often debated in animals but observable in plants

Question 8

What is assortative mating?

Answer 8

A mating pattern where individuals mate with others that are phenotypically similar to themselves

Question 9

What role does temporal isolation play in speciation?

Answer 9

It can lead to reproductive isolation based on differences in breeding times

Question 10

What is polyploid speciation?

Answer 10

A type of speciation that occurs in plants due to genome duplication events

Question 11

How long can speciation take?

Answer 11

Up to 5,000,000 years for a single eukaryotic population to become two reproductively isolated populations

Question 12

Why is estimating the number of species difficult?

Answer 12

Due to massive unknowns, especially in the sea, and variability in plants, fungi, and bacteria

Question 13

What is mating isolation in the context of the biological species concept?

Answer 13

Lack of interbreeding is key to defining species

Often called isolating mechanisms - selection may be involved directly or indirectly

Question 14

What are prezygotic barriers?

Answer 14

Factors that prevent mating or fertilization between species

Question 15

What are postzygotic barriers?

Answer 15

Factors that reduce the viability or reproductive capacity of hybrid offspring

At this stage, natural selection can become involved by reinforcing species differences

Sometimes can produce fertile hybrids but reproductive failure at F2 or afterwards (hybrid breakdown)

Question 16

What can hybrid zones lead to?

Answer 16

• Fusion of groups (when lots of gene flow)
• Reinforcement of isolation (when some gene flow)
• Stability of hybrid populations (limited gene flow with negligible fitness loss)

Question 17

What is the role of natural selection in speciation?

Answer 17

Natural selection can reinforce species differences after populations have diverged

Question 18

What are the different types of selection?

Answer 18

1. Directional selection - selects advantageous allele (curve shifts to one side)
2. Disruptive selection - selects both extremes (curve dips in the middle)
3. Stabilising selection - extremes disappear (curve peaks in the middle)

Question 19

How do genetic factors play a role in speciation?

Answer 19

• sampling and statistical effects such as drift and bottlenecks, combined with long periods of isolation can lead to speciation
• these random effects do not undermine the power of natural selection
• speciation is not an adaptation, it is a consequence of particular conditions which can involve the byproduct of subsequent selection

Question 20

What are the types of premating prezygotic barriers?

Answer 20

• Habitat isolation
• Temporal isolation
• Behavioural isolation (mating behaviours e.g. dance, calls)

Question 21

What are the types of post-mating prezygotic barriers?

Answer 21

• Mechanical isolation = sexual organs are incompatible
• Gametic isolation = sperm and egg don’t ‘recognise’ each other

Question 22

What are the types of postzygotic barriers?

Answer 22

• Reduced hybrid viability
• Reduced hybrid fertility
• Hybrid breakdown