Sexual Selection Flashcards
Describe sexual selection
a type of natural selection that is caused by competition between individuals of the same sex for mates
How is sexual selection a useful tool to understand evolution?
it is an explanation for diversity of:
reproductive modes
mating systems
traits for reproduction and mate attraction
traits for defense and fighting (competition)
behaviours related to mating displays
What drives sexual selection?
by differences in reproductive biology
Explain how sexual selection is driven by differences in reproductive biology?
generally
sexual selection favours reproductive biology individuals that produce many, small (biologically inexpensive) gametes (usually males, sperm) which have the capacity to fertilize many eggs and can be eggs from different individuals = traits that increase mating success
vs.
no sexual selection for traits that improve mating success in individuals that produce fewer, larger gametes (usually females, eggs) and can fertilize eggs with a single mating
Why does sexual selection favour male gametes?
male gametes have traits which increase mating success = smaller, more numerous gametes that can fertilize multiple eggs
What is Bateman’s principle?
that sexual selection is driven by differences in reproductive biology between the female and male gametes, where male gametes experience sexual selection for increased mating success
What are some predictions of Bateman’s principle?
male fitness increases more than female fitness with repeated matings
males have more opportunities for sexual selection because they have greater variance in reproductive success than females
What are some exceptions to Bateman’s principle?
females often mate with multiple partners –> more opportunity for sexual selection
sex role reversal = females compete to mate with males –> sexual selection on female traits
What is an example of sex role reversal?
Male seahorses carrying and birthing offspring
this would increase female-female competition because choosing a good/fit mate is essential for the successful birthing of the offspring
What is an example of variation in mating success between sexes?
elephant seals
140 individual seals
paternity of almost all the pups from less than 10% of the males (= a lot of males produced no offspring)
vs.
more than 50% of the females successfully produced at least 1 pup
more females were reproductively successful than males in this population
What are the two major modes of sexual selection?
male-male competition
female choice
How can sexual selection through male-male competition occur?
males may have:
“scramble” traits for early search and quick location of mates
“contests and displays” - traits for size, strength, weaponry, to signal threat
sperm competition
behaviours such as infanticide = a male killing offspring of another male
Explain how Paracerceis sculta is a good example of alternative male mating strategies
this species has evolved 3 genetically different types (morphs) of males
large alpha = guards territories with females
medium beta = mimics female morphology to escape detection of large males guarding
small gamma = sneaks - they sneak into territory, mate, and escape without detection
all 3 have similar fitnesses which is how they are all maintained
How does sexual selection occur via female choice?
different traits in males which represent increased fitness (ex. peacock tail feathers, stalk-eyed fly) that females use to choose mates
What are the two reasons individuals may choose a mate?
direct benefits
indirect benefits
What are the direct benefits individuals may receive from choosing a mate?
a potential mate could offer food or other resources such as territory, parental care (ex. seahorses)
What are the indirect benefits individuals may receive from choosing a mate?
a potential mate could offer increased fitness for offspring via better genes
this would be indicated by larger size, brighter colours, other traits that convey increased fitness
How are sticklebacks a good example of mate selection for good genes?
male sticklebacks have bright red colouration
when they have been infected with a parasite, they have reduced fitness
study compared the redness of father to the infection rates and found that red colouration indicated lower rates of infection = colouration signals fitness to potential mates
also compared redness of father to a trait related to immunity (white blood count of infected fishes) and found that red colouration was related to immunity = colouration signals fitness to potential mates
Describe the case study by Tonnabel et al., (2022) and how it relates to sexual selection in plants
experimental evolution
they induced evolution in lines of Mercury (Mercurialis annua) over 3 generations by growing under high density and low density conditions
then to compare the differences in reproductive traits, they transferred all the experimental plants into a common garden
What were the predictions of the experiment by Tonnabel et al., (2022)?
the prediction was that in conditions of high density, there would be increased sexual selection due to increased competition between pollen donors
What did Tonnabel et al., (2022) measure in their study?
What were the results from the experimental evolution portion of the study by Tonnabel et al., (2022)?
they found that pollen donor plants from the high density conditions evolved faster-growing pollen tubes compared to those from low density conditions
What genetic results did Tonnabel et al., (2022) find?
they used mass spectrometry to study the pollen coat extracts and found that out of 144 identified proteins, 10 were significantly more expressed in plants from the high density treatment
What was one example of a protein Tonnabel et al., (2022) found significantly expressed in high density mercury plants? What is it?
g13338
similar to the quartet protein 2 (QRT2) in Arabidopsis thaliana (a model organism)
involved in breakdown of the cell wall around the pollen mother cell = function required for separation of the 4 haploid gamete products
Explain how Tonnabel et al., (2022) created an experiment which increased sexual selection?
they created conditions in which there was more opportunity for sexual selection because there was increased competition between male plants for fertilization of the ovule
What was the purpose of putting the plants into the common garden after 3 generations in the Tonnabel et al., (2022) study?
after 3 generations of separate evolution, putting the lines from high density and low density conditions into a common garden removed any effects from the parents or previous conditions and allowed the researchers to study just the differences in reproductive traits that may have evolved
What kind of experimental evolution study could remove or reduce sexual selection?
one that manipulates the conditions so that there is decreased competition
ex. Qureshi et al., (2019) and mosquitoes
Describe the 2019 study by Qureshi et al., and mosquitoes and how it relates to reducing sexual selection
they looked at Aedes aegypti
they evolved lines with no male competition (one male to one female) and lines with high male competition (many males and fewer females) over a few generations and then brought each line into conditions with a third standard, unselected group (the source) to compare genetic variation - they measured mating success
What were the results for the 2019 study by Qureshi et al., when there was competition?
Males that evolved in the high competition lines had higher mating success when brought together than those from the no competition lines
What were the results for the 2019 study by Qureshi et al., when there was no competition (single mating)?
Males that evolved in the high competition lines had lower harmonic convergence and mating success in single pairings
What was the recent follow up study to the Qureshi et al., (2019)?
they sequenced the genomes of the selected lines and looked at the genetic changes
What is coevolution?
when two interacting lineages experience reciprocal evolutionary change
What is conflict?
when there is a difference between the evolutionary interests of the sexes and one sex is experiencing selection to maximize mating success whereas the other isn’t
What is the general prediction for conflict?
there should be more conflict when there is more matings = polygamy»_space;> monogamy
What is antagonistic coevolution?
coevolution between males and females of the same species in which the reproductive traits in one sex evolve over time to counteract the opposite sex’s traits to maximize reproductive success (basically an arms race between sexes)
How are the studies by Locke Rowe on water striders an example of antagonistic coevolution?
first, he found that mating for water striders increases risk of predation, especially for females (more often captured by predators)
in a follow up, found that in species (ex. G. incognitus) where the male has grasping morphology adapted to being better at grasping, the female has evolved counter-adaptations to prevent being locked in = to decrease risk in mating
in species where the male hasn’t adapted grasping morphology, the females have not either = they are nearly monomorphic
T or F: traits involved in sexual reproduction evolve slowly
false! super rapidly
What are some examples of how rapidly sexual reproduction traits evolve?
male plumage and songs in Capachino seedeaters - traits associated with male sexual selection evolved rapidly compared to females
male genitalia in Japanese ground beetles - male reproductive organs evolved rapidly compared to females
antagonistic coevolution water strider - males and females evolved rapidly to keep up with the other
What is a good measure to determine what evolutionary forces are acting?
comparing synonymous and nonsynonymous mutations
(looking at KA and KS)
What kind of evolutionary force is at play when nonsynonymous «_space;synonymous (KA «_space;KS)?
purifying selection = most new mutations are deleterious (NS) so if we are observing less NS mutations, selection must be acting to remove these
What kind of evolutionary force is at play when nonsynonymous ~ synonymous (KA ~ KS)?
When nonsynonymous mutations and synonymous mutations are balanced, the gene is evolving relatively neutrally - no strong purifying or positive selection
What kind of evolutionary force is at play when nonsynonymous»_space; synonymous (KA»_space; KS)?
positive selection, when there are more nonsynonymous mutations, NS must be acting to favour these
What kinds of genes or gene regions might have KA ~ KS?
Genes where there’s not much fitness difference
What kinds of genes or gene regions might have KA»_space; KS?
genes that are evolving rapidly
ex.
genes involved with sexual reproductive traits
genes involved with pathogen-host coevolution
What proteins are some of the most rapidly evolving in mammals?
reproductive proteins
When looking at the evolution of proteins in mammals, what did the data tell us?
there is not much divergence in many of the proteins found between humans and mice = 948 proteins do not have divergence
but, the ones that are most divergent are reproductive proteins = most rapidly evolving proteins are reproductive
What is an example of a protein involved in human reproduction that is evolving very rapidly? what part of it is evolving?
Zonadhesin - mediates fertilization
exons evolving more rapidly
What is interesting about the exon evolution in human zonadhesin proteins?
usually introns are the part of genes which are evolving, but this protein is changing in its exon (branch length of exons was way longer)
What did a comparison of primates conclude about the rate of molecular evolution of the SEMG2 (seminal protein gene)?
the rate of evolution in this gene, expression of which can be measured by size of male testis, was positively related to the degree of polygamy in each species
ex. chimpanzees (lots of polygamy and competition) had the highest rate of evolution whereas gorillas had the lowest (no competition - 1 male mates with multiple females)
What is one of the most rapidly evolving gene in Drosophila?
Pkd2 = a calcium-activated ion channel that transfers and activates sperm
What is a result of Pkd2 mutations?
they are sterile = they can still produce and transfer sperm, but the sperm cannot fertilize the egg (can’t swim)
What kind of mutations are there in Pkd2 mutants?
a lot of nonsynonymous mutations = positive selection
T or F: a gene that is essential can also be rapidly evolving
true
What is an example of a gene that is essential but rapidly evolving?
Pkd2 in Drosophila
What are 2 hypotheses for why Pkd2 is evolving if it’s essential?
sperm competition hypothesis = to avoid competition
coevolution with females hypothesis = antagonistic coevolution
