reproductive isolation can be: (1) pre-zygotic (before sperm and egg fuse) usually governed by...
TRANSCRIPT
Reproductive isolation can be:
(1) pre-zygotic (before sperm and egg fuse)
usually governed by mating behavior
(2) post-zygotic: sperm meets egg, but things go wrong
a) complete: embryo fails to develop
b) incomplete: embryo develops, but is sterile (at least in one sex)
Reproductive isolation
Reproductive isolation can evolve in two ways:
(1) by genetic drift, gradually + randomly fixing different traits controlling aspects of mating behavior (pre-zygotic) or genetic compatibility (post-zygotic)
not adaptive; happens over time as different alleles fix
(2) as a response to selection against hybrids, often between populations or species in ecologically different habitats
adaptive:
Reproductive isolation
Hybrids that have intermediate phenotypes will not be as fit in either habitat
Selection against hybrids
forest-adaptedspecialists
desert-adaptedspecialistshybrids,
not fit in eitherenvironment
Reproductive isolation often evolves very rapidly between populations adapted to ecologically different habitats
Hybrids have intermediate phenotypes and thus lower fitness in both habitats, compared to their specialized parents
Selection against hybrids will thus favor traits that contribute to assortative mating (this is termed reinforcement)
- Why? Because individuals that do not produce hybrid offspring will have higher fitness (their kids do better)
- therefore, traits that favor assortative mating will confer a fitness advantage
- any trait that stops you from hybridizing is good
Selection against hybrids
Pre-mating isolation is likely to be favored by selection when recently diverged species encounter each other
- maybe a barrier has disappeared and species now overlap
- maybe they speciated in sympatry to begin with As predicted, sympatric pairs of Drosophila species have higher levels of pre-zygotic isolation than allopatric pairs
amount ofpre-zygoticisolation
relatedness
Allopatricpairs
Sympatricpairs
amount ofpre-zygoticisolation
genetic difference
Allopatricpairs
Sympatricpairs
“Sympatric” pairs live in the same place NOW – they did not evolve in sympatry (at least, not that we know)
-
genetic difference
amount ofpre-zygoticisolation
genetic difference
Allopatricpairs
Sympatricpairs
The issue is, what does selection do when two distinct species run into each other and potentially try to interbreed?
Selection AGAINST hybrids favors “mate with your own kind” individuals – i.e., assortative mating
Therefore, pairs that bump into each other a lot have been under selection for assortative mating; they ignore each other pre-zygotic isolation is high, no matter how similar they are
genetic difference
amount ofpre-zygoticisolation
genetic difference
Allopatricpairs
Sympatricpairs
Selection AGAINST hybrids favors “mate with your own kind” individuals – i.e., assortative mating
Pairs that have NEVER seen each other have also never been under selection for assortative mating
- if they have it, they got it via genetic drift – this takes time
- therefore, only the very different allopatric pairs (the ones that were separated for a very long time) have it
genetic difference
genetic difference genetic difference
amount ofPOST-zygoticisolation
Allopatricpairs
Sympatricpairs
POST-zygotic isolation is NEVER favored by natural selection
-
genetic difference genetic difference
amount ofPOST-zygoticisolation
Allopatricpairs
Sympatricpairs
POST-zygotic isolation is NEVER favored by natural selection
Post-zygotic isolation is not a trait like mating preference;
- it’s the inevitable by-product of divergent gene pools getting so different, their alleles can no longer work together to build a fully fertile zygote
Many traits can potentially contribute to pre-zygotic isolation, if they determine...
A) I never see you
(1) habitat choice, when it causes direct access to mates
(2) time shifts in adult mating behavior or gamete release
At what time of night do you... - release your sperm and eggs?- open your flowers to pollinators?
- just plain feel sexy?
(3) time/space shifts in adult presence
Pre-zygotic reproductive isolation
Many traits contribute to pre-zygotic isolation by determining...
B) I see you... and find you repulsive
(1) mating rituals – songs, dances, pheromones
(2) size-assortative mating (big likes big, small likes small)
C) We totally did it... but nothing happened
(1) sperm didn’t find the egg (navigational problems)
(2) gamete recognition proteins blocked sperm-egg fusion
Pre-zygotic reproductive isolation
Monastraea annularis
Monastraea franksi
Monastraea faveolata
Time of spawning
Coral genus Monastraea has 3 similar species that broadcast gametes one night a year- are they reproductively isolated?
M. faveolata (grey) spawns at the same time of day as other two species, but their gametes do not hybridize in the lab
Pre-zygotic isolation: Time shifts
Coral genus Monastraea has 3 similar species that broadcast gametes one night a year- are they reproductively isolated?
coral colony releasing eggs
Pre-zygotic isolation: Time shifts
Monastraea annularis
Monastraea franksi
Monastraea faveolata
Time of spawning (hr after sunset)
8 9 10 11 PM
Coral genus Monastraea has 3 similar species that broadcast gametes one night a year- are they reproductively isolated?
M. faveolata (grey) spawns at the same time as other two species, but their gametes do not hybridize in the lab
Monastraea annularis
Monastraea franksi
Monastraea faveolata
Time of spawning (hr after sunset)
8 9 10 11 PM
Coral genus Monastraea has 3 similar species that broadcast gametes one night a year- are they reproductively isolated?
M. franksi (black) and annularis (white) gametes do hybridize in the lab, but they spawn a few hours apart in the field
Coral genus Monastraea has 3 similar species that broadcast gametes one night a year- are they reproductively isolated?
When related species can still hybridize, selection against hybrids drives corals to spawn at different times of night
Less related species can’t hybridize (more fully isolated), so they aren’t under selection to spawn at different times
Reproductive character displacement = change in a reproductive trait like spawning time when two similar species come into contact with each other; a response to selection
Time of spawning
Broadcast spawning in abalone
Males & female abalone snails free spawn into sea water at the same time along our coast (no time shifting)
- there are 7 co-occurring abalone species
what stops them from hybridizing back into one species?
Sperm navigation can be species-specific
Red and green abalone sperm only navigate towards eggs of their own species; ignore eggs of the other species
Genes involved in reproduction are often the fastest evolving
- gamete recognition proteins allow sperm to dock with egg Alleles can be “matched”, meaning one sperm protein allele fits into a corresponding egg receptor allele of the right shape
males with one allele can fertilize eggs with matching allele
Pre-zygotic isolation: Gamete recognition
egg
eggreceptor
alleleA
spermdocking protein allele B
eggreceptor
allelea
spermdocking protein allele b
sperm
Genes involved in sexual reproduction are often fast evolving
- gamete recognition proteins allow sperm to dock with egg
- different alleles often have high % of amino acid changes compared with alleles of non-reproductive genes
Adaptive evolution results when natural selection promotes amino acid divergence through positive selection
Differences between alleles fixed by speedsilent substitutions drift slow
non-synonymous changes positive fast(alter amino acid sequence) selection
Pre-zygotic isolation: Gamete recognition
Compare genes encoding the same protein in 2 different species
- compare the # of sites where they differ in amino acids
If there are more non-synonymous than synonymous changes, positive selection has likely favored divergence of that gene between the two species
Example: the sperm protein sp18 is up to 73% different in amino acid composition between Californian abalone species - sp18 exons (coding regions) are evolving 20 times faster than introns (non-coding regions) !
Pre-zygotic isolation: Gamete recognition
Male seminal proteins in Drosophila are also evolving rapidly
- the fast change in these proteins is partly responsible for maintaining species barriers
- both sperm and “helper” seminal proteins are required for successful mating in Drosophila; prevent hybridization
Regions of sperm proteins that directly contact egg receptors are the fastest evolving parts of the protein
Pre-zygotic isolation: Gamete recognition
2 models for why sperm-egg proteins might evolve so fast:
#1) egg receptors evolve by drift, and sperm proteins quickly evolve changes to “catch up” and dock with mutant receptors due to selection (match egg, or no reproduction)
#2) egg receptors evolve away from common sperm alleles, because selection favors eggs with rare (hard to match) receptor alleles
Pre-zygotic isolation: Gamete recognition
Why would it be beneficial for eggs to have receptors that few sperm can stick to?
Eggs face 2 problems:
1) they might fail to get fertilized, due to:
A) low density of males
B) bad match between your egg receptor allele and the common sperm docking protein allele
you’re too hard to fertilize
rare egg receptor allele..
no males nearby..
Why would it be beneficial for eggs to have receptors that few sperm can stick to?
Eggs face 2 problems:
1) fail to get fertilized, due to low density of males, or a bad match between your egg receptor allele and common sperm docking protein allele (i.e., you’re too hard to fertilize)
2) polyspermy: too many sperm dock with you dead egg (i.e., you’re too easy to fertilize)
1 sperm polyspermy no sperm
When could it benefit eggs to have receptors that few sperm can stick to?
- selection favors common egg receptor alleles when male density is low (common = matches most sperm)
- favors rare egg alleles when too many males are around (rare = few matches, avoid polyspermy)
reproductivesuccessas a pair
low maledensity
high maledensity
no match
partial match
full match
When could it benefit eggs to have receptors that few sperm can stick to?
conditions of high male density therefore impose selection favoring rare female egg receptor alleles
- if rare alleles fix, then sperm will be under selection and evolve new or better matching alleles; keep changing to keep up with ever-evolving egg receptors
reproductivesuccessas a pair
low maledensity
high maledensity
no match
partial match
full match
Pink salmon fish (Oncorhynchus) have a 2-yr breeding cycle:- mate in streams- travel downstream to ocean- return to birth stream 2 yrs later as adult to spawn
Alternate-year brood lines are genetically:
- more different from other-year brood in the same stream
- more similar to same-year broods from different streams along the coast
Differentiation proceeds as different, random adaptations to the same average environment (same stream) over time in different brood-lines (= groups going upstream in a given year)
Pre-zygotic isolation: Adult presence
Cicadas insects (Genus Magicicada) - 3 pairs of species - spend many years underground as larvae - emerge as adults for a 1-month mating frenzy
3 species emerge every 17 years - each closely related to a species emerging every 13 yrs
Each 17-yr species likely evolved from its sister 13-yr species, by adding a 4-yr delay period during the larval stage
Pre-zygotic isolation: Adult presence