BI407 Animal Behavior - Sexual Selection

sexual selection as a distinct evolutionary process

a subset of natural selection driven by differential access to mates or fertilizations

traits that enhance mating success even at a survival cost

intrasexual selection and intersexual selection

intrasexual selection

competition for mates

examples of intrasexual selection

competition by combat or display

intersexual selection

mate choice

intersexual selection purpose

direct and indirect benefits

sexual reproduction

defined by the production of gametes, two of which fuse to produce a zygote

anisogamous species

individuals producing larger gametes are females, and those producing smaller gametes are males

anisogamy

sexual reproduction with larger immobile eggs and smaller mobile sperm

isogamy

gametes of similar size

Bateman’s principle

reproduction success for one sex tends to be more influenced by the number of mates, while for the other sex, success is often more limited by factors like the quality of mates and resources, rather than the number of mates

Trivers parental investment theory

the sex that invests more in offspring becomes the limiting resource, results in competition for access and choosiness by the higher-investing sex

differences in parental investment explains

why the other sex competes more, why one sex is choosier, why sex roles may diverge or “reverse”

operational sex ration

ratio of sexually active males to fertilizable females at a given time

OSR often male-biased due to

shorter refractory period in males, grater initial and ongoing investment by females

direct benefits

preferences evolve because mates improve the chooser’s own survival or reproductive output

indirect benefits

choosing mates whose genes enhance offspring fitness

visual courtship displays

colorful/conspicuous morphology, stereotyped display movements

auditory courtship displays

calls/songs

tactile courtship displays

stereotyped touching/drumming

olfactory/chemical courtship displays

release of sexual pheromones

indirect benefit of good genes

traits reflect heritable viability or condition

indirect benefit of sexy songs (fisherian/runaway)

traits are attractive but not necessarily linked to viability

indirect benefit of chase-away selection/sensory bias

preferences arise from pre-existing sensory biases

runaway selection

occurs when traits and preferences coevolve, reinforcing each other across generations

sensory bias

males evolve traits that exploit pre-existing female preferences

chase-away selection

females evolve resistance to avoid exploitation causes sexual arms race

sexual conflict occurs when

one sex evolves traits that increase its reproductive success, these traits harm or reduce the fitness of the other sex, the harmed sex may evolve resistance, triggering a coevolutionary arms race

cryptic female choice

when females influence which male’s sperm fertilizes their eggs

sperm competition

when sperm from multiple males compete to fertilize the same eggs