Speciation

Question 1

What is the morphological species concept?

Answer 1

Groups of organisms that appear identical by morphological criteria
Oldest concept
Sometimes highly misleading

Question 2

What is the biological species concept?

Answer 2

Species = groups of interbreeding individuals that are reproductively isolated from one another
Hard to classify populations in different places
Doesn’t apply to extinct species/asexual organisms

Question 3

What is a conundrum of speciation?

Answer 3

How can reproductive barriers arise given that a reproductively isolating trait would be selected against when rare?

Question 4

What is a null model of speciation?

Answer 4

Divergence in allopatry - one population separated into 2

Over time they will diverge, different selection pressures, genetic drift, rapid change most common in small populations

Question 5

What is expected under the null model?

Answer 5

Each population accumulates mutations at different loci, fixation by genetic drift inevitable, selection increases substitution rate
Hybrids expected to be less fit than either parental species - Bateson Dobzhansky Muller incompatibilities (BDMI), negative epistasis

Question 6

How many BDMIs are expected?

Answer 6

Each mutation fixed in species 1 has some probability of being incompatible with a mutation in species 2
Expected no. incompatibilities grows quadratically with no. substitutions

Question 7

How do you find the number of BDMIs?

Answer 7

Best evidence from genetic mapping in Drosophila
Requires incomplete hybrid incompatibilities
D. melanogaster and D. santomea (island endemic)
Use balancer chromosomes (suppress recombination) & stocks with deletions to count BDMIs
Shows a very large no. genes involved in hybrid fitness

Question 8

What is the snowball effect?

Answer 8

Wanted to test is we see quadratic dependence on time
D. melanogaster - D. santomea: 12.8 MY
D. melanogaster and D. simulans: 5.4 MY
Correlate no. BDMIs with genetic divergence at synonymous sites

Question 9

How do we find out which genes are involved in BDMIs?

Answer 9

Kosswig cross:
2 loci, 2 alleles, Oncogene Tu, repressor R
Backcross of homozygous for Tu and R
F2 Tu/- ; -/- develops malignant melanoma, reduced viability
Hybrid necrosis in Arabidopsis:
Diff. strains A. thaliana
BDMI involving 2 loci on diff chromsomes
Incompatibilities in F1 so dominant-dominant BDMI

Question 10

What is Haldane’s Rule?

Answer 10

If in a species hybrid only one sex is inviable or sterile, that sex is more likely to be the heterogametic sex (two different sex chromosomes)

Question 11

How do you explain Haldane’s rule?

Answer 11

Faster-male evolution - male-specific genes evolve faster because of sexual selection - greater variance in reproductive success in males (doesn’t explain when females heterogametic)
Faster-x evolution - genes on X evolve faster as exposed to selection when recessive (some evidence)
Dominance theory - heterogametic F1 suffer from BDMIs between X-linked & autosomal genes regardless of whether they’re dominant or recessive, homogametic hybrids only affected by dominant BDMIs (well supported from mapping studies)

Question 12

What are the problems with the null model?

Answer 12

Requires long period of allopatry
BDMIs continue to accumulate long after speciation complete
Many have very little to do with initiating the process
Reproductive isolation entirely postzygotic
Most reproductive isolation is actually prezygotic

Question 13

What is the consequences of ice ages?

Answer 13

Many sister species with overlapping ranges originally diverged in allopatry - ice ages
Meet again following range expansion during intergalacial

Question 14

What are some examples of species that form hybrid zones?

Answer 14

Grasshoppers - Iberian, central Europe
Artificial crosses result in sterile males (Haldane’s rule)
Narrow hybrid zone through Pyrenees
Maintained by strong BDMIs

Toads - fire bellied & yellow bellied
Interbreeding in narrow hybrid zone in central Europe
Adaptation to different environments, strong BDMIs, no sex chromosomes

Question 15

What types of prezygotic isolation are there?

Answer 15

Habitat isolation - common in herbivores eg. ladybird beetle, 1 lives on thistles and one that lives on a bush and so don’t meet
Temporal isolation - eg. North American Magicicada, 17 and 13-year life cycles
Premating - meet but choose not to mate, pollinator isolation
Postmating - male gametes transferred but egg not fertilised, cryptic female choice

Question 16

Do prezygotic barriers evolve faster than BDMIs?

Answer 16

Pre and postzygotic isolation accumulate v slowly and at similar rates in allopatry
Prezygotic isolation evolves much faster in sympatry

Question 17

Why does prezygotic isolation evolve faster in sympatry?

Answer 17

Reinforcement:
Selection should favour reproductive isolation to prevent individuals wasting effort on unfit hybrids
Assortative mating to avoid cross-breeding through, behaviour, habitat preference, timing, reinforces postzygotic isolation

Question 18

How has reinforcement been shown?

Answer 18

Flycatchers:
Pied flycatcher & collared flycatcher
Males colouration more pronounced in sympatry (brown vs black)
Studied the choice of sympatric females - can ID conspecific males in sympatric populations more easily
Allopatric females prefer allopatric males

Question 19

What is a model of hybrid speciation?

Answer 19

2 diverged ancestors hybridise
Generates new hybrid species
Compatible combos of BDMIs
New combos of ancestral genomes leads to large genetic variation
Eg. Lake Victoria superflock

Question 20

What is a case of hybrid swarm speciation?

Answer 20

Compared 3 scenarios with genome-wide
Lake Ejagh colonised by cichlids more than once
Speciation was complex and involved repeated gene flow from stream populations

Question 21

What are ‘bad species’?

Answer 21

Closely related species not often 100% reproductively isolated
10% animal, 25% plant species hybridise with at least 1 other species

Question 22

How does hybrid speciation work in plants?

Answer 22

Allopolyploid speciation: 2+ sets of chromosomes derived from different parental species
V common in plants
Selfing gives advantage to the rare type

Question 23

What does the genic view of speciation tell us?

Answer 23

Divergence initiated at a few barrier loci
‘Divergence hitch-hiking’ neutral divergence builds up in the vicinity & facilitates fixation of locally adaptive variants
‘Islands of speciation’
Geneflow becomes reduced across entire genome - ‘congealing’

Question 24

What does recombination with migrants do?

Answer 24

Prevent establishment of BDMIs
Break up locally beneficial combos of alleles
Create positive selection for inversions that lock together co-adapted alleles

Question 25

What are examples of inversions being selected for?

Answer 25

Drosophila - ecological speciation with different cactus hosts in Sonora and Baja California
Hybridise in lab but not in nature
3 overlapping inversions on chromosome 2 cause differences between host races, became established during onset of divergence