bio 221 lec 7+
what are complex adaptions
those consisting of modifications to more than one part of the genome that have to work in concert to produce the final phenotypic product.
what is true about the complexity of traits
many genes control most traits and most traits are complex
parts of the genome involved in complex adaptions can be divided into two broad categories:
1) genes that encode proteins
2) regulatory elements (DNA sequence)
what are regulatory elements
they control how and where a gene is activated, control the amount, location and timing of gene expression
gene expression
that gene is actively functioning in that time and space
what do cascades of gene expression need to respond to
signal from environment ( can be genetic or actual environment signals)
what is a cascade responde
intial response an trigger multiple downstream effects
what is an example of a cascade effect.
the formation of a hard coated spore by bacillus bacteria is triggered by molecules indicting environmental degradation that bind to recpetors on the surface of the bacterial cell.
how does the cascade effect work bacillus
enviro triggers: dna damage, dessication, irradiation, nutritional deficiences (survival stress)
signals are integrated at master switches/regulators which regulate that downstream response, the downstream effectors realize the "decision"
master regulator is the main character that affects everything onwards
cis acting element
a DNA sequence that carries info that regulates nearby genes
trans acting elements
dna sequence that regulate the expression of distant genes
what is an example of cis regulating gene cascade
best studied gene cascade in evolution is the hox (homeobox) gene cluster, where we see a gene hierarchies. this gene cluster influences anterior- posterior development (head-tail) and patterns the embryonic segments
what happens if there is muation in a hox gene
can result in major deformations such as legs growing where antenna should be. wt fly X (ant) antennaepedia mutant
what organism in the hox cluster best understod
d.melanogaster
when are hox genes expressed and how did they evolve
hox gene cluster evolved after a series of gene duplication and all show sequence homology, they are expressed during development
what is notable about the arrangment of hox genes in the genome
the hox genes in the genome mirrors position of expression along anterior-posterior axis, hox genes repress other devlopmental genes and body parts are specifiec by developmental genes
what gene is muated when the legs grow out of the head
antp gene which patterns legs on posterior thorax
sequence homology
The similarity between biological sequences (DNA, RNA, or protein) due to shared ancestry, often indicating similar function or structure.
Orthologs
These are genes or proteins in different species that evolved from a common ancestral gene through speciation. They typically retain the same function across species. For example, the hemoglobin gene in humans and chimpanzees would be orthologous genes.
Paralogs:
These are genes or proteins within the same species that have evolved from a common ancestral gene through duplication. They may acquire new functions or sub-functions over time. For example, the two globin genes in humans (alpha and beta) are paralogs, as they arose from a gene duplication event.
what does the hox genes pattern
segments to be eventually built in adult form
How does the Eve gene illustrate the role of regulatory elements in gene expression?
The Eve gene is regulated by multiple enhancer elements, each controlling expression in specific stripes in the embryo.
🧬 Example: Eve has separate enhancers for stripe 2, stripe 3, etc.—each responds to a unique combo of transcription factors. works in embryogenesis before hox genes. The eve gene serves as a model for understanding how cis-regulatory elements like enhancers and promoters control the spatial and temporal expression of a gene, determining its role in development. Changes in regulatory regions (not the coding gene itself) can alter gene expression patterns without disrupting gene function.
🧠 This allows evolutionary changes in body plans through tweaks in gene regulation—Eve stripe enhancers are a model for how this works.
✅ Same gene, different expression = new traits without breaking old ones.
cis regulatory elements
are regions of non-coding DNA that regulate the transcription of nearby genes. These elements include promoters, enhancers, silencers, and insulators.
how does the cis regulatory element(switch) effect the eve gene
it normally turns on the eve gene in alternating embryonic segments
what relationship is between a single gene and different regulatory elements
a single gene can be affected by different regulatory elements depending on what part of the embryo it is in. cause these regulatory elements to drive gene expression of other genes.
differentially regulate expression
The process by which different factors (such as transcription factors or regulatory elements) control the activity of a gene in various cells, tissues, or conditions, leading to varying levels of gene expression.
how was the hox gene cluster able to evolve such diverse functions
through gene duplication whil allows one copy to supply gene function while the other copy can perform a new function
duplication provides the oppurunity to create
paralogs (selection can act on one of the duplicte genes in novel way)
what does similar duplicate coding sequences allow for
for both coding sequence variation (different proteins) and regulatory element variation
does the coding sequence have to change for their to be a chnage in phenotype
no mutations/ changes to regulation of genes can change phenotypes as well (where/when the gen is expressed)
gene duplication is a way to allow for gene novelty, what is another method
horizontal gene transfer
horizontal gene transfer
one cell transfers DNA to another species/strain/individual. the fastest way to get genetic variation without sexually reproducing
sometimes the new gene is similar to one the host already has so the process behaves like a gene duplication event, how does this proceed? and involved what?
transduction, transformation, conjugation. most mechanisms of horizontal gene transfer involve bacteria.
there are some rare examples of hgt in multicellular eukaryotes though, what are these (2)
1. antifreeze production in some vertebrates
2. carotenoid production in arachnids
in other cases, the new gene is completly novel (neomorph), what is an example
gut bacteria in humans getting algae digesting genes from marine bacteria associated with seaweeds used in sushi.
what comes first selection or variation
In evolution, genetic variation within a population occurs first, and natural selection acts on this variation, favoring traits that enhance survival or reproduction.
case study example of neomorph
e.coli in 12 flasks, way more cloudies, bacteria multipled greatly --> bacteria acquired additonal method of getting nutrients: abiloty to grow on citrate which they usually dont metabolize. went back and looked at frozen culturals, found mutation rose around 33 K years ago, sequences genomes before, at, and after citrate eating. saw a very fast rise in citrate metabolism.
if they repeated this experiment would the novel ability come in the same generation
no, although it doesnt happen, they could reproduce the acquistion of evolutionary novelty randomly
how did this citrate metabolism mutation happen
found that mutation invovled a duplication of several genes associated with the usual anarobic digestion of citrate (the citT complex). citrate promoter was duplicated and the mk promoter is active when O2 is avaible, citT genes are now under control of a different promoter when O2 is available
who is it harded to observe emergence of adaptions in unicellular or multicellular and why
harder to observe adaptions in multicellular organisms because of longer generation times.
what are the harder adaptions to target with fossils? what is an example of this?
chemical or behavioural rather than morphological. snake venom.
did the genes for making venom arise within snakes or before them?
researches looked at a phylogeny of genee associated with production of venom in snakes. there are many genes that follow the one that makes a crotamin, a muscle destroying venom
what is a crotamin simialr to
a immune fxn gene (defensin) in vertebrates used in bacterial immune defense. defensin is expressed on the skin of vertebrates. the defensin genes in snakes are expressed in the mouth of venomous snakes
in snakes the defensin gene underwent a duplication, what did this cause?
caused a change in regulatory network and the location of expression
even snakes we consider non venomous produce these same venoms that are found in snakes that have hollow fangs/injection systems, what an example
garter snakes the produce mild venom, and some lizards, some lizards that do are more related to snakes than other lizards.
what does this relationship between snake a lizard show
that the venom may have orignally evolved in a cmmon ancestor of modern snakes, and these lizards whihc include gila monsters and komodragons. though these venoms do vary.
evo-devo
evolution of development. new field dating from mid -1990s goals of evo devo reasearch range from mechanistic (how does that structure develop) to phylogenetic (where did that structure originate) to theoretical (does development constration evolution)
single celled organism lack
development
morphologcial differences among multicellualr organisms come from
chnages in developmental processes over evolutionary time
ontogeny recapitulates phylogeny- haeckel, means
developmental steps reflect evolutionary hisotry
flies and mice are sepearted by >570 MY yet how is there developent related
development of embryos of both taxa is controlled by homolgous hox genes. though in basal vertebrates the entire hox cluster was duplicated twice, resulting in 4 sets.
can a fly hox gene work in mice
yes, since there was gene duplication that happened in vertebrates
fossil records of transition of fish like fins to tetrapod digit-bearing limb, can we hypothesize what developmental mutation might have been involved?
hox 13a= expressed in limbs of extant fishes and tetrapods when expressed produces proteins along the outer rim of fin early in development and patterns fin margin. in fish active for quite a whole and then turns off
difference (temporal difference-time); in tetrapod Hox13a becomes active early in development then turned off then turned on again to allow more complex appendages creating digits
there is hox genes in plants what are these called and what plants
ABC complex in dicot plants, infleunce floral structure through various combinations of A-B-C gene expression that determines the face of plant regions. independently evolving equivalents.
how do mutations effect the ABC complex (hox genes in plants)
in arabidopsis thaliana mutation affecting B results in petals connverting to sepals and stamens to pistils
there are homologues to ABC system present in monocots and maybe gymnosperms what does this imply?
that precusor to ABC present in ancestor of seed plants before flowers evolved
what is the ancestral form of flowers
radial symmetry but bilateral symmetry evolved at least 70 times in the angiosperms.
what is more advatnageous bilateral or radial
bilateral symmetry has more effcient pollen transfer
what two genes are important playwers in networks that control plant asymmetry
DICH and CYC
what happens when DICH and CYC are mutated?
if both are mutated in snapdragon flowers then radial symmetry is developed. these pathways control floral bilateral asymmetry in other angiosperm lineages (flowering plants)= same pathway has been recruited repeadeatly independently multiple times
bilaterl and radial symmetry is an example of what
parallel evolution- The process where two or more species independently evolve similar traits or characteristics, often in response to similar environmental pressures, even though they are not directly related.
irreducibly complex
a design that is so complicated that it implies a designer BUT evolutionary history shows the simple steps that make up the whole
what is the first step of explaning the evolution of the vertebrate eye
comparative morphology, almost all phyla have some form of light sensing structure (exceptions- sponges and placozoans
some single celled eukaryotes have light sensing organisms (ex. nematodinium), what are different about these
can detect presence of light without forming images
all "eyes" use what to direct and capture light
crystallins to direct light and opsins to capture light. different specific proteins do these jobs in different categories of organisms
opsins vary widley (insects vs octupus vs humans) what does this indicates
suggests that image forming eyes may have evolved independent multiple times
what are opsins
molecules that react to being struck by a photon, triggers a chemical rxn that produces an electrical signal that can be detected by neurons
vertebrate opsins are called? what about other animals?
vertebrates: ciliary or c opsins (stored in extenstions of retina)
other animals: rhabdomeric or r opsins (stored in infoldings of photosenstive membranes)
though maybe c opsin and r opsin evolved independently BUT...
all taxa have evidence of both types, just used differently by each group (not creating new genes)
origin of opsin shows what
phylogeny of genes that encode opsins have found that they are ancestrally codes receptor for melatonin
why melatonin? what happened to the melatonin receptor gene.
key regulator in our circadian rhythym ( sleep/walk cycle) which is regulated by sunlight . several duplications, modification, sorting events which is the reason for distribution and variations of opsins found today.
when and where did opsin originate
650 mya- ( evolution of light capturing opsins) from a common ancestor. one copy evolved into melatonin receptors others duplicated again into opsins.
what is the evidence for deep homology reflected in the developmental genes associated with eye development.
two genes, called "eyeless" in insects and "pax6" in mammals exist as homologs in almost all animals. both resulting in lack of eyes/ eye development issues.
ecotpic eyes come from
some alleles of eyeless in drosophila, can cause eyes that develop on parts of the body where they arent supposed to be. basic function of pax6/eyeless --> "put pigment here"
mutations can also alter visual pigment development
in humans and mice pax6 mutations can cause underdeveloped eyes and in fruit flies no eyes.
what other structure is detrimental to the eye
the evolution of the lens to focus light on receptor cells so it can be ordered into an image.
what creates lenses in vertebrates
a protein called a-crystallin, some of the most stable proteins in animals
what did phylogenetic analysis show about alpha cystallin
that a-crystallin shows ancestry with heat shock proteins (molecular chaperones that help other proteins fold after heat denaturation)
heat shock proteins are generally very stable, but what can happen later in life
cataracts are formed by denaturation of crystallins which clump and form opaque films
all organisms have to cope with constraints to evolution, phylogenetic constraints are:
features that are inherited from ancestral species that appear to limit current species to life history options
even if we have light sensing pigment and a lens= how do we make an eye
if we look at the simplest light sensing structure --> we can see the "steps" in extant animals. light sensing spot, evolved with a bump like structure, proto lens, involution of light sensing surface, then much more spherical accurate lenses and retina (inside out cellular organization of retina)
physical constraints example
more obvious, example. adult mayflies lack functional mouthparts, so egg production depends on energy gathered at larval stages
if they had mouthparts? too heavy to fly and find mates
pleiotropic constraints
antagonistic pleiotropy occurs when a mutation with beneficial effects for one trait also causes detrimental effect on other traits
pleiotropic constraint example
mammals have 7 vertebrae because different values may cause some deletrious effects. giraffe neck has vertebrae therefore same # as humans just much bigger. adding more is deletrious.
derived structures are often built on the scaffold of ancestral forms, how does this relate to the giraffe neck.
the derived giraffe neck for example has to work around the structure of nerves and arteries originally selected for in gilled ancestors, and shows imperfections in 'design'
land veretebrates lost gills but did not lose..
blood vessel and neural architecture
homoplasy
sometimes, innovation indepedently appear or are lost independently
convergent evolution? example?
occurs when similar traits are evolved independently in distantly related taxa ( no shared ancestry)
many marsupials converged on similar body plans to eutherian/placental mammals, they diverged from placental mammals 130 mya = more similar to parallel evolution than true convergent evolution
what is another example of convergent evolution
evolution of flight in both butterflies and birds, they are very distatnly related but they both use wings to fly
parallel evolution
similar phenotypes that independently arose with closely related taxa often via independent occurence of similar mutations
What is a simultaneous hermaphrodite species?
A species in which each individual has both male and female reproductive organs at the same time.
🔁 Can often self-fertilize or exchange gametes with others.
🧠 Think: both sex roles in one body, at the same time.
🔗 Examples: earthworms, some snails, flatworms
where is sperm and oocytes produced in these worms
sperm- in spermatheca
oocytes- produced continously
What is parthenogenesis?
A form of asexual reproduction where females produce offspring from unfertilized eggs—no male gamete (sperm) is involved.
👶 Offspring are usually genetic clones or near-clones of the mother.
What are the evolutionary consequences of parthenogenesis?
✅ Fast reproduction, no need for mates
❌ Low genetic diversity → reduced ability to adapt to changing environments
🧠 Great for stable environments, risky for changing ones.
do animals only use parthenogenesis when necessary
no there has been virgin briths with males present recorded. even though it was previously though to be a hail mary pass
benefits of sex
combining benefical mutations and generation of novel genotypes
combining benefical mutations
provides opportunity to get good genes which could mean offspring are fitter than parents
generation of novel genotypes
Shuffling the decks in meiosis: fertilization can create unique combinanations of alleles for offspring
Why is sexual reproduction thought to lead to faster evolution?
Because it creates genetic variation, giving more raw material for natural selection to act on.
🧠 Variation = flexibility in adapting to new challenges.
What ecological pressure may select for sexual reproduction?
Host–parasite coevolution
🦠 Parasites evolve quickly, so hosts must also evolve rapidly to defend themselves—sexual recombination helps by increasing genetic diversity.
What is the Red Queen Hypothesis?
An evolutionary idea that species must constantly evolve just to keep up with their enemies (like parasites). evolutionary pressure that is never escaped because of the rapid evolution of the parasite
🧠 Sex evolved to help hosts "run faster" by producing varied offspring better able to resist parasites.
How does sexual reproduction help clear deleterious mutations?
through genetic variation and outbreeding, sex allows recombination and selection against harmful mutations, helping populations purge them over time.
🧠 Healthy genotypes can be restored by mixing genes.
What is Muller’s Ratchet?
A process where asexual populations accumulate deleterious mutations over time—because they can’t recombine or remove them.
🔁 Each generation "ratchets up" the mutation load.
🧠 No sex = no reset button on bad mutations.
Why is sexual reproduction beneficial according to Muller's Ratchet?
Sex recombines genotypes, allowing natural selection to weed out harmful mutations and maintain healthy lineages.
✅ Prevents the irreversible buildup of genetic damage.
What are the major costs of sexual reproduction?
Two-fold cost of males
❌ Males don’t produce offspring directly, so sexual populations grow half as fast as asexual ones.
🧠 Asexual females double the reproductive rate.
Search cost
🕵️♀️ Males and females must find each other—this takes time, energy, and increases risk of predation or injury.
🧠 More effort = more danger.
Breakup of good gene combinations
🔀 Recombination can disrupt advantageous gene combinations.
Why do species still reproduce sexually despite these costs?
Because sexual reproduction creates genetic variation, which:
✅ Helps purge deleterious mutations (Muller’s Ratchet)
✅ Speeds up adaptation (Red Queen Hypothesis)
✅ Increases resilience to changing environments
What is the "reduced relatedness" cost of sex?
Sexually reproducing parents pass on only 50% of their alleles to each offspring.
How does this compare to asexual reproduction?
Asexual parents pass on 100% of their genes, making offspring fully related.
How can recombination reduce offspring fitness?
It can break up adaptive gene combinations, lowering the offspring's fitness.
How is sex linked to risk of STIs/STDs?
Sex increases exposure to infectious agents during mating—this is a cost.
Are STDs a problem only in humans?
No—STDs affect animals and even plants. It's a widespread evolutionary issue.
Example of STI transmission during mating?
The podapolipid mite spreads between beetles during mating—uses sex as a transfer route.
Why does sex still evolve despite the risk?
Sex is favored only if its benefits outweigh the costs.
It must:
✅ Produce more offspring, or
✅ Produce more successful offspring
What is anisogamy?
Individuals produce gametes of different sizes—e.g., small sperm and large eggs.
🧠 Leads to male and female sexes.
What is isogamy?
Both partners produce same-sized gametes—no distinction between sperm and egg.
❌ No true sexes—just mating types or strains.
What defines biological sexes?
sexes are based on different intraspecific strategies for relative investment in gametes.
How does gamete size difference lead to conflict between the sexes?
It creates different reproductive strategies and priorities:
Females: few, costly gametes → quality matters
Males: many, cheap gametes → quantity matters
Why is it usually easier for males to mate than females?
Because sperm are small and numerous (“sperm are cheap”), while eggs are large and few (“eggs are expensive”).
🧠 Males can mate often with little cost; females must be selective.
Why do females benefit less from frequent mating?
Their reproductive success is limited by egg number, not mate number.
❗ Mating with a low-quality male can reduce offspring viability.
What limits male reproductive success?
Access to eggs.
✅ Mating with more females generally increases male success, even if some are low quality.
How do reproductive strategies differ between males and females?
Females: Reproductive success tied to fecundity (number of eggs); eggs are costly, so mate quality matters.
Males: Reproductive success often tied to access to females, not egg production; sperm are cheap, so quantity of mates matters.
How do males invest in reproduction?
hrough intrasexual competition (e.g., fighting, displays)
Mate attraction (intersexual competition)
In some species, by producing huge amounts of gametes (e.g., sperm in broadcast spawners, pollen in plants)
What is intrasexual selection?
Competition within one sex (usually males) for access to mates
What traits are favored when females can be monopolized?
Aggression, large body size, horns, or other weapons
Often used more as deterrents than actual weapons
🧠 These traits help males outcompete rivals for female access.
What traits are favored when females cannot be monopolized?
Increased sperm output
Traits that improve sperm competition (e.g., fast-swimming sperm, larger testes)
🧠 This strategy is favored when females are rare, dispersed, or dominant.
How does variance in reproductive success affect sexual selection?
The greater the variance, the stronger the selection on that sex.
🧠 More variation = more opportunity for traits to give an advantage.
Which sex usually has greater variance in reproductive success?
Males, especially in species where a few dominate mating (e.g., harems).
Example of extreme variance in male reproductive success?
Elephant seals—a few large males control harems and father most offspring, while many males mate rarely or not at all.
What did studies on southern elephant seals reveal about male reproductive success?
In one breeding season, most males sired no offspring, while a few sired many pups.
How does female reproductive success compare in elephant seals?
Female success is more even—most reproduce, and variance is much lower than in males.
What is the consequence of this variance difference between sexes?
Males show greater lifetime reproductive variance, meaning stronger sexual selection acts on them.
🧠 More variance = more “fuel” for natural selection.
How does sexual competition work in broadcast spawning or wind-pollinated species?
Males can’t control which females they fertilize—fertilization is like a lottery.
What is the main male strategy in these species?
Produce as many gametes as possible to increase the odds of success.
🧠 Quantity over control—just flood the field and hope for hits.
What additional strategies can males use if fertilization is internal?
Males can use post-mating adaptations to increase their chances of paternity.
What are copulatory plugs, and what do they do?
In species like rats, males produce a chemical plug that blocks the female’s genital tract after mating.
🔒 Prevents other males from mating with her.
What are antiaphrodisiacs, and what is their function?
In species like Drosophila, males transfer chemicals that reduce the female’s sexual attractiveness after mating.
🧠 Discourages other males from trying to mate with her.
Example of sperm removal in insects?
In seed beetles, males have spined genitalia that sweep out sperm from earlier males.
How do spine traits affect fertilization success in seed beetles?
Longer spines increase the chance of removing rival sperm, boosting fertilization success.
🧠 Better tools = better odds.
Can males within a species have different mating strategies?
Yes—males can be polymorphic in both morphology and behavior, using different strategies to reproduce.
What are some common male mating strategy types?
Large-bodied males: compete physically (e.g., guarding, fighting)
Small-bodied males: use sneaky tactics or rely on sperm competition
Example of a species with multiple male mating strategies?
The bluegill sunfish (non-native in Alberta) has three types of males:
Parental males: build nests, guard eggs
Sneaker males: dart in to fertilize eggs covertly
Satellite males: mimic females to sneak past dominant males
🧠 Different paths to reproduction depending on size, timing, and behavior.
What are the 3 male mating strategies in species like the bluegill sunfish?
Large males (parentals):
🛡️ Build nests, defend territory, protect eggs
🧠 Invest in size and physical defense
Intermediate males (satellites):
🎭 Mimic females to sneak into nests undetected
🧠 Avoids aggression by deception
Small males (sneakers):
🏃♂️ Hide nearby, then dart in to quickly release sperm
🧬 Have large testes to boost sperm output
🧠 Strategy depends on sperm competition
How does female mating behavior affect sperm competition?
It determines how intense sperm competition will be:
Monogamous females → little or no sperm competition
Polygamous females → high sperm competition
How do males respond to high sperm competition?
By investing in sperm competition strategies, such as:
Larger testes
More sperm production
What is one way scientists measure sperm competition intensity across species?
By comparing testis size relative to body size—larger testes-to-body ratios are found in species where females mate with multiple males.
🧠 More risk of competition = more investment in sperm.
Is monogamy common in nature?
No—true monogamy is rare. Even in socially monogamous species, extra-pair copulations (EPCs) and extra-pair fertilizations (EPFs) are common.
Why might females engage in extra-pair matings?
To increase genetic diversity or offspring quality, even while maintaining a social bond with a partner.
example of a socially monogamous species with frequent EPFs?
Superb fairy-wrens:
Form pair bonds
But females often mate with neighboring males
EPF rates decrease with distance between territories
🧠 Stay paired, but still hedge genetic bets.
What is intersexual selection?
When one sex (usually females) selects mates based on certain traits in the other sex.
🧠 “Mate choice” selection.
When can intersexual selection occur?
Before mating: based on displays, ornaments, calls, etc.
After mating: via cryptic female choice
What is cryptic female choice?
When a female influences which male’s sperm fertilizes her eggs after mating, often by manipulating sperm in her reproductive tract. It allows females to favor high-quality or genetically compatible mates, even if mating with multiple males.
What does pre-mating intersexual selection lead to?
The evolution of elaborate courtship traits in males, used to attract females.
What determines the type of courtship trait that evolves?
The species' sensory abilities:
Good vision → visual displays (e.g., color, movement)
Good hearing → acoustic signals (e.g., songs)
Why do these traits evolve so rapidly?
Because female preferences can change quickly and are often species-specific, driving rapid trait evolution and even speciation.
How do males invest under pre-mating intersexual selection?
Males invest energy not in gametes, but in being chosen—through ornaments, displays, etc.
What’s a classic example of an exaggerated trait due to intersexual selection?
The peacock’s tail—Darwin’s famous puzzle—explained by runaway selection: female preference for the trait drives its exaggeration over generations.
Why might females prefer exaggerated traits like long tails or bright colors?
According to the good genes hypothesis, these traits are costly to maintain and signal high genetic quality or health.
🧠 A long, heavy tail = male is strong enough to survive despite the handicap.
Why do females invest effort in choosing mates?
Because choosing the right male can provide direct benefits that improve her own survival or offspring success.
What are some examples of direct benefits from mate choice?
Protection from harassment
🛡️ Ex: In some species, a strong male can guard a female from other aggressive males.
Chemical protection for offspring
💀 Ex: Rattlebox moths—males give females toxic compounds that are passed to eggs to deter predators.
Resources like nests or territories
🏠 Ex: Weaverbirds—males build intricate woven nests, and females choose the best builders.
Can food be a direct benefit of mate choice?
Yes—some females gain nutritional benefits from males during or after mating.
What is sexual cannibalism?
When the female eats the male during or after mating, gaining nutrients that may help her produce more or healthier offspring.
Is sexual cannibalism always intentional?
Sometimes it’s accidental—the male just doesn’t escape fast enough (e.g., mantises, many spiders).
Other times, it's deliberate—the male offers himself as food (e.g., redback spider).
🧠 Ultimate “nutritive gift” = himself.
How can paternal care be a direct benefit of female choice?
Choosing a male who provides care for offspring increases their chances of survival—a direct payoff for the female.
Examples of species with male parental care?
🐦 Male cassowaries: incubate eggs and raise chicks.
🪲 Giant water bugs: females lay eggs on male’s back; he carries and protects them until they hatch.
How else does choosing a healthy male benefit the female?
It may reduce disease risk for her and her offspring—strong, healthy males are less likely to carry infections.
What are indirect benefits of mate choice?
Benefits that don’t help the female directly, but improve the fitness of her offspring.
What are two main types of indirect benefits?
Good genes → offspring have higher viability, e.g., parasite resistance
Sexy sons → sons inherit traits that make them more attractive to future mates
Example of good genes in sticklebacks?
Female sticklebacks prefer redder males.
Redder males = healthier (better parasite resistance)
Offspring of red males were more resistant to tapeworms
🧠 Red is an indirect cue—a “genetic hitchhiker” signaling good genes
How does mate choice work in grey tree frogs?
Female frogs prefer males with longer and more complex calls
Why are long calls considered an honest signal?
They are costly—longer calls increase predation risk, so only high-quality males can afford to produce them.
What do longer calls indicate?
Possibly good genes—call length is correlated with offspring performance (e.g., growth, survival).
🧠 Female preference may be based on traits that signal genetic quality—even if they’re risky for males.
When is sexual selection strongest?
When there is greater variation in reproductive success—usually in males.
🧠 More variance = more room for selection to act.
Why is sexual selection usually stronger in males?
Males often have higher variance in mating success—some mate a lot, others not at all.
✅ Selection favors traits that increase male success.
How does variance in reproductive success shape sexual selection?
The sex with greater variance in reproductive success experiences stronger sexual selection
Usually males → leads to more male-specific traits (when females are choosy)
In rare cases (e.g., sex-role reversed species), if females have more variance, then males are choosy, and females evolve sex-specific traits
🧠 Whoever has more to gain (or lose) evolves more traits for competition or attraction.
What is the operational sex ratio (OSR)?
The ratio of sexually active males to sexually active females.
Often male-biased, since females may be pregnant or caring for young and temporarily unavailable to mate.
Why is the typical sex ratio still ~1:1 in most species?
Because of Fisher’s Principle: if one sex becomes rare, it has higher reproductive value, so selection balances the ratio back to 1:1.
Why is it surprising that most species have a 1:1 sex ratio?
Because a population with more females would grow faster, since males can fertilize multiple females
What is Fisher’s Principle (1930)?
In a sexually reproducing, biparental population, males and females must have equal total reproductive success—because each offspring has one mother and one father.
What happens if females produce more sons than daughters?
Daughters become rarer → greater reproductive value → selection favors producing more daughters.
Why does the sex ratio stabilize at 1:1?
Any deviation gives a reproductive advantage to the rarer sex. So natural selection drives the ratio back to equal investment in sons and daughters.
🧠 The average wins—balance is the evolutionary stable strategy.
What do fitness curves show about sex ratio bias?
If the population is male-biased, individuals producing more daughters have higher fitness (red line).
If the population is female-biased, individuals producing more sons have higher fitness (blue line).
✅ Selection favors whatever restores balance → stabilizing at a 1:1 sex ratio.
When can male availability limit female reproductive success?
When males invest heavily in parental care, they become the limiting sex—leading to a female-biased operational sex ratio (OSR).
Example of sex-role reversal in insects?
In Rhamphomyia spp. (dance flies):
Males prefer “fat” females, assuming they have more eggs
Females evolve traits (feathered legs, inflated air sacs) to fake fatness and attract males—sexual deception
🧠 Even females can evolve flashy traits when selection flips!
What is sequential hermaphroditism?
When an individual starts life as one sex and later switches to the other, producing both types of gametes at different times.
What determines when sex change occurs?
It depends on how body size affects reproductive success:
If larger males gain more mates → change from female to male (protogyny)
If larger females produce more eggs → change from male to female (protandry)
Example of protandry?
Clownfish: start as males, but if a dominant female dies, a male switches to female to take her role.
🧠 Sex change maximizes fitness based on social and size dynamics.