Lecture 4: Geographic distribution

Question 1

hybrid zones:

Answer 1

where genetically distinct populations meet, mate and reproduce
-challenge our view of ‘species’

Question 2

why are hybrid zones good to study?

Answer 2

• involved in nearly all speciation events

- range of genotypes show genetic differences and selection pressures that separate the taxa

Question 3

hybrid zones: covers the CLINE between

Answer 3

the two populations of alleles

Question 4

ability to study hybrid zones has increased over the years as

Answer 4

massive advances in DNA studies have occurred

Question 5

hybrid zones show historical patterns example:

Answer 5

Hewitt (1999) 
European hedgehog 
-2 sub species 
-- Erinaceus europaeus 
-- Erinaceus concolor 
-used mitochondrial DNA you can see recolonisation pattern after ice age

Question 6

Post glacial recolonisation is known as:

Answer 6

Refugia;
facilitated the persistence of components of biodiversity over millennia and under changing conditions
–>movement of hedgehogs, grasshoppers and bear (& oaks) (all took refugia in south of europe)

Question 7

consequences of hybridisation:

Answer 7

• can remain indefinitely (pop never rlly mix with each other)
• merge (become 1 again)
• reproductive isolation (hybrids rlly unfit)
• third species

Question 8

Cline =

Answer 8

(graph) a change in the allele frequency over a geographical transect

Question 9

we can use cline characteristics to tell us about the mixing of populations

Answer 9

• shape
• co-occurance
• movement

Question 10

things which influence cline shape:

Answer 10

width of cline from one population to another:
dependant on dispersal (how many genes are flowing between the two populations) and selection (how much selection is acting against those alleles in the two population.)

Question 11

things which influence cline shape: dispersal

Answer 11

• no dispersal = rigid line (at right angle)
• more dispersal = more gene flow and cline gets wider and flatter
• ABO blood groups in humans

Question 12

things which influence cline shape: selection

Answer 12

stronger selection = steeper cline

-steeper S

Question 13

high selection against hybrids known as

Answer 13

tension zone

-heterozygotes DISadvantage

Question 14

heterozygote DISadvantage

Answer 14

• lower intrinsic fitness than either parental individual

- normally keeps deleterious heterozygotes from appearing

Question 15

clines appearance in tension zones

Answer 15

v steep

Question 16

primary hybrid zone

Answer 16

-rock pocket mice
change in the environment where the populations simply change with the environments (mice)
-natural selection alters alleles in a continuers population
-neutral alleles stay the same for both pops

Question 17

secondary hybrid zone

Answer 17

• -hedgehogs
• formally allopatric species expand to meet
• pops isolated and then meet again
• neutral alleles vary

Question 18

weird cline behaviour: movement hybrids

Answer 18

-the can move –> if hybrids are less fit, selection will move cline to where density is lower so fewer hybrids produces

Question 19

weird cline behaviour: asymmetry

Answer 19

Clines move in response to strength of dispersal (may not even swap populations)
-neutral genes can pass through without key ones
– hybrid X homozygouse
(A1A2B1B2 XA1A1B1B1) =introgression

Question 20

dispersal (gene flow) influences

Answer 20

the shape and location of the cline

Question 21

introgression=

Answer 21

movement of genes from one species or population into a mother by hybridisation and backcrossing

Question 22

consequences of hybridisation: indefinite

Answer 22

• selection maintains steep clines at some loci
• could be a tension zone
• only if character differences are favoured by different environments
• could move - area of low density

Question 23

consequences of hybridisation: Merge:

Answer 23

• fitness of hybrids not lower than the original populations
• introgression and post zygotic reproductive barrier broken down
• variation and distinction between 2 populations lost

Question 24

consequences of hybridisation: Reproductive isolation

Answer 24

• strengthening of barriers to gene exchange
• large areas of genome protected from introgression
• mechanism controversial –> reinforcement?

Question 25

consequences of hybridisation: third species

Answer 25

hybrids becomes reproductively isolated from original populations
-new species