quiz #4: 10/30/26

sexual selection

an individual's relative ability to attract, secure and mate with a partner; it favors traits that increase reproductive success

sexual dimorphism

the distinct difference in physical appearance, size or behavior between males and females of the same species; includes traits like body size, coloration, ornamentation or weaponry 

inTERsexual selection

(mate choice) individuals of one sex (usually females) actively choose their mates based on specific attractive traits, displays or courtship behaviors

inTRAsexual selection

(male-male competition) individuals of the same sex (usually males) directly compete or fight with one another for dominance, territory or access to mates 

example of inTERsexual selection

female peahens choosing peacocks with the brightest, most symmetrical tail feathers 

example of inTRAsexual selection

male elephant seals physically fight on a beach to establish dominance over females 

a lek

specialized communal arena where males gather to preform elaborate courtship displays and vocalizations solely to be evaluated by visiting females

how lek and intrasexual selection connect

Since females visit these arenas for the single purpose of choosing their mate, leks serve as a direct, highly concentrated engine for intrasexual selection

anisogamy hypothesis

states that the fundamental asymmetry between large, nutrient-rich, limited eggs (females) and small, cheap, abundant sperm (male) is the ultimate evolutionary root of sexual selection

highly, intense

Females are ______ selective choosers since their eggs are more limited than sperm which drives ______ competition among males to fertilize those limited eggs

Good genes/handicap principle

females choose mates with costly, exaggerated traits because those features act as an honest indicator of underlying genetic quality and disease resistance, passing strong survival genes to their offspring

Fisherian runway (sexy son hypothesis):

An arbitrary female preference for a random trait triggers a positive genetic feedback loop; daughters inherit the preference and sons inherit the trait, driving the ornament to extremes simply because it makes the sons sexually attractive

Sensory exploitation (sensory bias)

Males evolve exaggerated traits that capitalize on pre-existing sensory biases that females originally evolved for survival, such as a preference for a specific color or shape associated with finding food

traits of sexual selection

• increase an organisms evolutionary fitness

• are primary engines behind sexual dimorphism

• favor costly traits that can make an individual more vulnerable to predators or environmental stress

traits of intersexual selection

• evolutionary filter is a cognitive choice or sensory preference by one sex

• usually plays out in courtship settings or specialization arenas like leks

traits of intrasexual selection

• the filter is physical power, dominance or tactical control over competitors

• plays out through physical combat, territorial disease, or post-copulatory mechanisms like sperm competition

gene pool

all copies of every type of allele at every locus in all members of the population

genotype, allele frequency

_______ + ________ = gene pool

phenotype frequency

number of individuals with a given phenotype/total number of individuals (red, pink, white individuals) 

genotype frequency

number of individuals with a given genotype/total number of individuals (C^rC^r, C^rC^w, C^wC^w, individuals)

allele frequency

number of copies of a specific allele/total number of alleles

hardy-weinberg equilibrium

p²+ 2pq + q² = 1

p²

expected frequency of C^RC^R genotype (homozygous dominant AA)

2pq

expected frequency of C^RC^W genotype (heterozygous Aa)

q²

expected frequency of C^WC^W genotype (homozygous recessive aa)

p allele

frequency of allele A

q allele

frequency of allele a

a population is in hardy-weinberg equilibrium

1. Mutations DO NOT occur

2. NO migration

3. Indefinitely LARGE population

4. Natural selection DOES NOT occur

5. Random mating

• good genes/handicap principle

• intersexual selection (it depends on a sex (usually females) choosing a mate (of the opposite sex) based on specific traits between 2 sexes

• peahens choosing peacocks with the brightest/most symmetrical feathers

type of mechanism example (on quiz)

microevolutionary processes

alters allele frequencies within isolated populations over time. As these genetic changes accumulate independently in each group, the populations gradually diverge until they become distinct lineages or species

microevolutionary process examples

• natural selection

• genetic drift

• mutation

species

A group of organisms that can successfully interbreed in nature to produce viable, fertile offspring

reproductive isolation

occurs when biological barriers prevent individuals of 2 different populations from interbreeding and producing viable fertile offspring. (this stops gene flow between groups, allowing them to evolve independently and diverge into distinct species)

pre-zygotic

prevents mating (before zygote is formed):

1. Habitat isolation→ live in different areas

2. Temporal isolation→ breed at different times of day, year, etc

3. Behavioral isolation→ unique courtship rituals

4. Mechanical isolation→ differences in reproductive organs (can’t mate)

5. Gametic Isolation→ sperm and egg are chemically incompatible

post-zygotic

after zygote is formed

1. Reduced hybrid viability→ dies during development

2. Reduced hybrid fertility→ develops but can’t reproduce

3. Hybrid breakdown→ next generation is weak/sterile

mechanical isolation

only evolves when structural changes to reproductive organs physically prevent different populations from successfully mating

Pleiotropism

Natural selection may favor a change in an organism's general body shape or size to help it survive but because those same genes also influence genitalia development, reproductive structures change on accident

cryptic female choice

Females can internally screen or reject sperm based on physical stimulation or structural cues provided by the male’s genitalia during sex. (male anatomy will evolve to match preferences, creating mechanical mismatch)

adaptive locks and keys

Natural selection favors male “keys” that perfectly match female “locks” to prevent damage during mating or to ensure efficient sperm transfer

sexual imprinting

type of behavioral learning where offspring learns the characteristics of a desirable mate by observing its parents

sexual imprinting in strawberry frogs

they imprint on the physical appearance– specifically the skin color– of their mother (red)

sexual imprinting

High parental care

genetic preferences

no parental care

connection between sexual imprinting and degree of parental care

Sexual imprinting requires high parental care because offspring can only imprint who is actively present. In species with no parental care, offspring never interact with their parents so they can’t learn their traits, so their mate preferences are entirely genetic

assortative mating

sexual imprinting can cause speciation by driving _________ which cuts off gene flow between different groups and builds a reproductive barrier

steps of sexual imprinting generating reproductive barrier

1. A new trait appears (the trigger)

2. Behavioral isolation (barrier forms)

3. The halt of gene flow (no mating in different groups)

4. Genetic divergence/speciation (two groups diverge genetically)

allopatric speciation

populations are completely geographically separated, entirely eliminating gene flow 

vicariance

either results in pre- or post-zygotic RI as a byproduct of divergence OR gene flow will reverse divergence if the geographic barrier is temporary

peripatric

(founder effect) results in either small number of individuals colonize a new, isolated habitat OR dilutes the effect of drift and prevents local adaptation

parapatric speciation

Populations are continuously distributed in adjacent geographic areas with a shared narrow border

parapatric speciation results in…

Individuals adapt to their specific local niches and selection favors reinforcement OR alleles mix across the boundary, breaks up advantageous gene combinations

sympatric speciation

Populations overlap geographically and live in the same local area, with no physical barriers to mating

sympatric speciation results in…

strong disruptive selection combined with assortative mating OR sex constantly shuffles alleles, instantly erasing any nascent genetic differences between the diverging groups

strong selection

if both populations faced intense, different environmental pressures, they would have developed vastly different adaptations. Upon contact, they are likely to remain distinct because they are optimized for different lifestyles

weak selection

Their traits will remain similar, making them highly likely to fuse back into a single population

environment heterogeneity

High heterogeneity: (diverse environment) if the contact contains distinct, patchy habitats, the two populations can partition the landscape. Each will stick to the habitat it is best adapted to, maintaining 2 distinct species

Low heterogeneity: (uniform environment) one population will likely outcompete the other, or they will merge completely

drift and population size

Small populations: (strong drift) small isolated groups fix random genetic mutations quickly via genetic drift. This rapid, random genetic divergence makes it highly likely that incompatibilities evolved, preventing them from successfully interbreeding upon contact.

Large populations: (weak drift) large groups change slowly and predictably. Unless selection was very strong, they will still be genetically compatible and likely to merge.

length isolation and mutation rates

Long isolation + high mutation rates: massive buildup of genetic differences. Upon contact, they will likely behave as completely distinct species because they have already achieved full reproductive isolation. 

Short isolation + low mutation rates: genetic changes will be minimal. When they meet, they will easily interbreed and fuse back together

competition

High competition: (overlapping niches): if they rely on the same resources, they can’t coexist as they are. This leads to either competitive exclusion (one drive the other extinct) or character displacement (selection forces them to evolve differences quickly to use different resources)

Low competition: If they utilize different resources, they can peacefully coexist in the same area as two distinct species

hybridization potential and hybrid fitness

Low hybrid fitness: (reinforcement): if they can mate but their hybrid offspring are sterile, sickly, or poorly adapted, natural selection will favor individuals that choose mates of their own population. This process (reinforcement) quickly solidifies them into two permanently distinct species 

High hybrid fitness: (hybrid zone or fusion): If hybrids survive and reproduce well, the two populations will either merge back into one or form a stable, permanent hybrid zone where they continuously exchange genes.