Lecture 5: Reproductive Isolation

Question 1

allopatric speciation

Answer 1

2 species aren’t in same geographical area

Question 2

sympatric speciation

Answer 2

speciation when species is in same geographical areas

Question 3

prezygotic reproductive isolation:

Answer 3

BEFORE egg and sperm meet

• habitat
• behavioural
• morphological
• temporal
• gametic
• mechanical

Question 4

postzygotic reproductive isolation

Answer 4

AFTER egg and sperm meet

• hybrid inviability
• hybrid infertility
• hybrid breakdown

Question 5

postzygotic isolation:

you have to have ______ mating to complete reproductive isolation

Answer 5

assortative -> genes must not flow between

Question 6

postzygotic isolation: assortative mating. Direct system

Answer 6

the same gene that is adaptive also affects mate choice
-example cichlids:
males change in colour to depth in water (deep = red, high = blue)

Question 7

postzygotic isolation: assortative mating. INdirect system

Answer 7

the gene that is adaptive is LINKED to one which affects mating preference (adaptive and mate choice are on 2 linked loci)

Question 8

linkage disequilibrium :

Answer 8

alleles appearing together more often than you would expect by chance

Question 9

reinforcement:

Answer 9

• was a post zygotic barrier (hybrids weaker) selection for prezygotic isolation due to selection against hybrids
• the process ‘completes’ speciation
• can only happen in sympatry
• often most obvious in sexually selected trait

Question 10

reproductive character displacement

Answer 10

caused by reinforcement

• decrease of fitness of hybrids
• populations in sympatry have different morphologies to reduce prezygotic
• pied flycatchers = when in sympatry one male changes to brown

Question 11

what is the genetic basis of a barrier? Dobzhansky-Muller model 1934

Answer 11

without selection opposing any intermediate step, can arise from background mutations

• if genotype split, mutate in both, hybrid from these potential for incompatibility
• over time chances of incompatibly increases (as more alleles mutate)

Question 12

how quickly do these barriers evolve?

can even non species individuals produce viable offspring?

Answer 12

yea, 8% of genera can

Question 13

how quickly do barrier evolve?

Answer 13

• greater diverge of genetics less chance of viable offspring
• converted to time using cytochrome beta clock
• species that produce infertile hybrids last shared a common ancestor on average about 7mya
• 5my-17my to produce infertile offspring
• gradual process
• variable

Question 14

how quickly do barrier evolve? Haldane’s rule

Answer 14

most of first half of post zygotic isolation due to male sterility or inviability

• when a cross produces inviable or sterile offspring, the heterogametic sex is more strongly affect
• xy MALES in humans
• -> as only have one copy of sex chromosome

Question 15

pre zygotic isolation evolves faster in allopatric/sympatric pairs

Answer 15

SYMPATRIC

–> reinforcement?

Question 16

reinforcement can only happen in

Answer 16

sympatric populations

Question 17

a current understanding of speciation:

Answer 17

1) small (adaptive) divergence in 2 genomes in 2 populations
2) divergent areas from via linkage
3) genes diverge so much that interbreeding very reduced
4) isolation
- -gene flow decreases 1-4–

Question 18

search at the minute for alleles which cause ___ to occur v v quickly

Answer 18

speciation

Question 19

‘speciation genes’

Answer 19

• genes which cause reproductive isolation on their own

- OdsH in 2 closely related drosophila causes sterility when imported in another