3: Divergence & Hybridisation

Question 1

What are hybrid zones?

Answer 1

• Where genetically distinct pop.s meet, mate and reproduce
• Form between pops that are not yet ‘true species’

Question 2

Why are hybrid zones good to study?

Answer 2

• Tells us about the process of speciation
• Range of genotype show genetic differences and selection pressures that separate taxa

Question 3

Define Refugia

Answer 3

Habitats that are safe and stable during times of big environmental changes e.g ice ages or climate fluctuations
- Allow species to ‘hang on’ when conditions around them become harsh

Question 4

What is a cline?

Answer 4

a change in allele frequency over a geographical gradient

Question 5

Fill in the blank

More dispersal = —— cline

Answer 5

Wider

Question 6

Fill in the blank

Stronger selection = —— cline

Answer 6

Steeper

Question 7

What does high selection against hybrids lead to?

Answer 7

A tension zone
- which in turn leads to heterozygote disadvantage (as they are adapted to neither environ)
= narrow steep cline

Question 8

Define a primary hybrid zone

Answer 8

Natural selection alters alleles in a continuous pop.

Question 9

Define a secondary hybrid zone

Answer 9

Formerly allopatric species expand to meet

E.g expansion out of glacial refugia (pops. come back into contact)

Question 10

define Introgression

Answer 10

Movement of genes from one species or pop. into another by hybridisation and back crossing

Question 11

What does cline shape depend on?

Answer 11

strength of selection and the dispersal of the organism

Question 12

How does asymmetric dispersal affect clines?

Answer 12

• Clines move in response to the strength of dispersal:
• Asymetric dispersal (unequal movement of individuals or genes across a landscape) affects shape and location of the cline

Question 13

How does density affect clines?

Answer 13

Clines tend to congregate in areas of low density and therefore can be barriers to gene exchange