Evolution of Sex Flashcards
How does sexual reproduction play out
Plays out in complicated ways
***Must be important
Sexual reproduction norm
Sexual reproduction seems like the norm but hard to explain why this is the case
Example weird reproduction
- Snail – hermaphroditic –> recipircally fertilize
- Weird fish – start as female and when males leaves becomes male
- Clown fish – Start as male and turns female when female dies
- Male angler fish – Male supplies sperm into body
- Parasitic that just adds sperm to body
Commanality of sexual reproduction
Very common in living world
Just because we do it doesn’t mean it needs to be done
Sexual selection
Fitness is determined by variation in attracting males
- Occasionally leads to outlandish phenotypes
Phenotype relates to mate choice – not just survival and # of offspring
Example – Bird + colors in monkeys + elk that has long antlers + Dopsin fly (has mandables that attracts mates)
What are traits of many organsims driven by
Traits of many organisms seem to be driven by sexual selection
- Traits are there because ways selection interacts in sexual reproduction to drive trait evolution
Sexual conflict
Males and females have different optima for fitness
- May be common in nature
Example – 1. Lions –> Males kill all of the females pride that she had with another male to make the female fertile
- Good for fitness of male; bad for fitness of female
- Bed bugs = violent insemination – male stabs sperm into female body
- Weird falas on falcan – corkskrew shapes – female has vaginal track that is corkscrew in the opposite direction –> Males change direction to control reproduction and females will chnage in response to control reproduction
Understanding why organisms have sex
Still major subfield of evolutionary biology
Sex = NS
Sexual reprdouction poses a paradox for adaptation by NS –> it is reproductively costly to maintain two maintaining two mating types
Two fold cost of sex
It is reproductively costly to maintain two maintain two mating types
Each cost = doubling of fitness effect and 2 different aspects that come into play
Maintaining two mating types
Sometimes hard to explain
Females making only females that also only make females = X2 reproduction rate = X2 fitness
Example – Each females makes 4 offsrping –> only the females will make more offspring
IF 1/2 off are female – each female makes 4 offsrping –> end get 8 Grandchildren
IF Females can make female offsrping without males –> each females makes 4 females –> End 16 grandchildren
8 VS. 16 – BIG difference – X2 replication rate in only females = big fitness advantage
- Females making only females = growth rate X2 = fitness X2
Aescual growth rate
Evolve to have aesexual reproduction – #s grow fast = something else is in check
Genes in sexual reproduction Vs. Asexual
Sexual reproduction – only pass 1/2 genes to individuals –> offspring is 50% related to mom
Asexual reproduction – offspring is 100% related to mom
In sexual if have a mutations that leads to asexual –> allelle will be in all = spred fast (because asecual will pass mutation to all)
Sexual –> if have gene that maintains sexual = only pass to 1/2 offspring
Females that only make females
Contribute disproportionately to next genertion –> have higher fitness
***Females making only females = growth rate X2 = fitness X2
First thing to look at in evolution of sex + why it persisted
First need to look at distrubution of sex in nature
Maybe it was impossible to evolove asexual
Loss of sex
Diploid organisms whose ancestors had sex – sex is lost rather infrequently
Evolution of asexual out of sexual does happen enough that we know it can happen
Reprdouction in many dilpid
Parthenogensis
Parthenogenesis
Reproduction by females of other females via unfertilized eggs (females make other females via unfertilized egg)
- Often facultative not obligative –> organisms can repduce in either mode depending on conditions – occurs depending on contexct
- Colony of genetic recombination
Might alternate like clockwork (one generation sexual; 1 generation aesexual)
Example Parthenogenesis
- Daphnia –> can grow a colony so you know they all have the same genotype
- Do sometimes reproduce sexually
- Insects –> reprduce asexual for many generations and then when stressed csan reproduce sexually
Occasional parthinegensis
Parthenogensis can just occur ocasionally happen sometimes – organisms can be largley sexual and sometimes be aesexual
- Seen in complex organisms –> seen in female if away from male
Example – Komoto dragons + bords + sharks –> asexual reproduction of females without sperm
Aesexual from sexual
Some organisms evolove asexual from sexual
Example –
1. Stick bugs –> all female – all parthenogensis (eggs develope to females)
- Worms – Asexual
- Lineages of lizards – some exlusivley female
- Some make male offspring –> gives males to stimulate behavior not to get sperm
SHOWS – asexual can evolove from sexual
bdelloid rotifers
No one has ever seen them have sex BUT genomic data shows they might on occasion
Question in Sexual Reproduction
Given the 2 fold cost of sex why did sex evolve to begin with and why does it persist?
2 fold cost – decrease in reproductive rate + Lowers chance of passing on allele to offspring
Potential Answer: maybe because once evolved –> can’t go back
Loss of sex
Diploid ancestors who had sex lose sex rather infrequently BUT there are exceptions
Exception – Loss of sex to have parthenogenesis –> Shows that there must be ways for asexual to evolove
Widespread Parthenogesis
Can occur in widespread way
Example:
1. Can occur in normal part of lifecycle
2. Can be facultative way
3. Can be rare/spontaneous way in complex organisms (can be occasional ion some species)
- Organisms that are otherwise sexually reproducing –> Shows that might be way for asexual to evolove from sexual
4. Species can be fixed for asexual reproduction
What does parthenogensis show
Shows that asexual can evolve from sexual (Have sexual –> asexual where you stay asexual)
New question = why doesn’t it sweep through given fitness advantage
Obligate Parthenogens
Fixed for asexual reproduction (need parthenogenesis) – tend to be single species or very small clased (NOT large raduioations of diversity) + They tend to be young
- NOt very diverse
- One or a handful of species
- Lineages are young –> NOT long standing groups of aninals that reproduce asexually for long periods –> suggests that there are constraints that asxueal is not good long term
Expectations to long term asexual lineages rule
Bdelloid rotoifers
- Several hundred soeces going back 100 million years –> NO one has ever seen them have sex BUT the latest genomic data shows that they might on occasion
- Expectations to diverges long lineages evoloving asexual
- Very diverse –> thought that they were the most diverse and longest standing asexual organism
Sequencing the genome –> shows that they likley have sexual reproduction sometimes
How can genomic data reveal that sex has taken place?
Recombination between lineages – shows that sexual reproduction goes on
If a chromosome is the resulyt of reocmbination = evidencve that not purley clonal
When have recombination = varaition from 2 parents that recombine = evidence for sexual reproduction
Why does sex persist
Recombination
Benefits of recombination
The 2 fold cost of sex can be made up for by the benefits of recombination acting to break up linkage disequilibrium that inevitabley forms in purely asexual popultions
- Cost is counteracted by the benefit of recombination
Process = Leads to ineviatble accumulation of deletrious mutations in asexual species
Benefit in recombination = reshuffle variation to get new geentic combinations
Issue in clonal organisms
All new mutations = have linkage disequilibrium with other variation = have accumulations of deleterious mutations
Process = Leads to ineviatble accumulation of deletrious mutations in asexual species
Inevitable accumulations of deleterious alleles = Muller’s ratchet
Ratchet
A mechanism that can only rotate in one direction
Muller’s ratchet – names that way because it only goes in one direction –> unidirectional increase in deletrious mutations in asexual organisms
Muller’s Ratchet
Start: Well adapted high fitness genome – well adapted asexual organism
OVER time – some mutations will arise
- Some of the mutations will be good BUT most are nuetral or bad –> Most will make fitness decrease
Mutations in lineage of asexual –> All offspring will have the mutations –> once mutation pops up it is there for good
- The deletrious mutation that affects fitness a little but not enough to kill you –> Over time lose high fitness alleles until all that is left is deletrious mutations
If only slightly deleterious = not bad enough that NS can act on it
OVER time – initial high fitness genotype can drift out of existence
What drives the ratchet fowards
Combination of mutation and drift drives the ratchet forward
Why doesn’t NS act on deletrious mutations in ratchet
Because the mutations are only slightley deleterious = not bad enough that NS can act
- - Mutation occurs in copy of high fitness genotype BUT only sloightly deletrious so NS can’t act well = deeleyeious affect can build up
Over time = accumulate deleterious mutation – THEN get individuals with enough deleterious mutations that you weigh down the fitness = get individuals that NS can work on
Fitness change in Ratchet
Start with high fitness genotype –> get copies of self = have high fitness offsrping THEN have a mutation that decreases fitness a little but not a lot –> THEN pass mutation to offspring because clonal –> THEN have another mutations
- We expect mutation in almost all generations
HAVE more mutations in the background of the first mutations
Each round the new mutations that occur stay for good in all decendents of mutated indiviauls –> the orginal high fitness genotypes become rare
End = lose the zero mutation genotype – NOW only have 2 mutation vs. 1 mutation genotype –> ability of NS to seperate mutations get weaker
End = the best fitness is 1 mutation (no longer have 0 deleterious) = have one click on ratched
NS acting in ractch over time
Lose high fitness = ability of NS to purge deletrious is weaker over time because NS is weaker with less varaition (when lose highest fitness genotype lose varaition)
Over time = accumulate deleterious mutation – THEN get individuals with enough deleterious mutations that you weigh down the fitness = get individuals that NS can work on
Perhaps NS can favor the no mutation high fitness genotype gainst the lineages with 4 mutations BUT if the high fitness genotype is lost due to drift OR of another mutations occurs in an individual with that genotype –> The high fitness genotype is gone for good and the best genotypes all have at least one deletrious mutations = the ratchet has moved foward a click = less varaition = NS can’t act
Mutation in asexual
Once mutations pops up it is there for good
Ratchet moving foward a click
Perhaps NS can favor the no mutation high fitness genotype gainst the lineages with 4 mutations BUT if the high fitness genotype is lost due to drift OR of another mutations occurs in an individual with that genotype –> The high fitness genotype is gone for good and the best genotypes all have at least one deletrious mutations = the ratchet has moved foward a click
Genetic load
The accumulation of deletrious mutations weighs doen the fitness of a popultions
- Over time popultion fitness decreases – decrease in fitness = genetoc load
- Decrease in optimal fitness
- Decrease fitness based on accumalation of deletrious alleles
Major probelm in asexual organisms –> even driving them to extsiction
What breaks the ratchet
IN sexual popultions recombination breaks the ratchet
- Sexual reproduction breaks ratchet (can go back and forth) –> can get high fitness genotype back
If have a popultion that lost the high fitness genotyoe –> Can have crossing over in meiosis can give back the best genotype
Example – if the highest fitness genotype is 1 mutations
- IN asexual all mutations go to offspring
- In sexual – by combining gametes in sexual reproduction + recombination –> we can get back the fitness with 0 mutations
Recombination + Novel phenotypes
Recombination via sex might speed up the rate at which novel phenotypes appear
Ability of recombination to make novel genotyoes = important
Benefical mutations in Asexual
Purley clonal popultions have to wiat for benefical mutations to occur in sequntial order in the same lineages to get novel phenotyopes
What happens when lose high fitness genotype in Asexual
High fitness is gone for good unless have exact mutation back
Mutation rate = 10^-8 –> chance of exact mutation to high fitness is small
2 mutations + 5 mutations Vs. 0 mutations + 5 mutations
Harder to week out differencve between 5 mutations and 2 mutations than weeding out with 5 mutations and 0 mutations
NS can act on 0 vs. 5 mutations = allow high fitness to be maintained in popultion
Reason most asexual lineages are young
Because of genetic load
As mutations occur = fitness decreases over time
Adaptation in more than 1 gene
Adaptations often require more than one gene
In asexual – need to wait for mutations to ccur in sequntial order in the same lineagese to get high fitness
- To get high fitness need mutation in one indiviudal –> need to occur in occsrping in the background of the other mutations
Sexual – Do not need mutation to occur in offspring in background of othe rmutations –> in sexual you can get the beneficial mutation in one organism and one beneficial in another organism and then they can combine through sex = get high fitness
IMAGE – AB = high fitness
- Top – Sexual – can get mutation in different lineagse and then mix lineages = get high fitness –> OCCURS much more rapidly – get more AB (more high fitness) – increase probability of polygenic adpatation
- Bottom = Asexual – aB = dead end –> Need A and B –> Need ab THEN Ab –> THEN AB – need B mutation to occur in backgroun of A – need mutation in same genetic line
Sex + rapid evolution
Sex might be important beyond steady accumulation of deleterious mutations – might be needed for rapid evolution
Other use of sexual reproduction
Might be needed for constanty adapations that organisms need all of the time
Organisms need to constantly adapt each generation – constantley need new genetic combinations
- NEED to reshuffle alleles to get new geneotypes
To deal with threat from other popultions = ability to evolve fast is important
What is constant evolution driven by
Driven by co-evolution – evolove to envirnmemnta + evolove to threats from other evolving organsims
Overall – popultions don’t evolove in static envirnmmnets – have major evolutionary pressures coming from other species (predators + Prey + parasites + Competators)
Example – evolove to parasites or pathogens (evolove fast)
To deal with threat from other popultions = ability to evolve fast is important
Red Queen Hypothesis
Popultions are constantley evoloving to chnaging biologic chalelemnged and sex is crucial way for popultions to keep pace woth their eneminies
To deal with defeinsive stredegy –> get niovel genetics in all generations (only can occur in sexual)
Overall – popultions don’t evolove in static envirnmmnets – have major evolutionary pressures coming from other species (predators + Prey + parasites + Competators)
- Threats to fitness from othe rorganisms that themsleves chnage (other organisms have chnaging adaptive topographies) –> Change in one organsims = have shift in other organisms adaptive topography
- Sexual reproduction = gives organims ability to keep pack in co-evolutionary arms race
Co-evolution
Reciporcal evolution across species interactions
PLays a role in maintaining sexual reprodyction
Sexual vs. Asexual experiments
Studies = done with organisms that can do both
Low predator or parasites = asexual is maintained at higher rate
Add predators –> Sexual reproductions = sweeps through population
Example – Snails + fish –> vary in degree of sexual reproduction
- High predatores = increase rate of sexual
- Low predators = maintain asexual
SHows red queen hypothesis
Slides:
A number of natural and
experimental systems appear to back this up- asexual populations thrive in the absence of parasites, but sexual populations do better with intense coevolution
More than making up for the 2-fold cost
