Evolution
Lecture 22, Chapter 15
Sexual selection (Darwin)
 Differences in reproduction resulting from differences in the ability to obtain
mates
Why do males compete more than females?
Males produce many small gametes (sperm)
 Mate with many females
 Little negative consequence to his fitness if one mate has low fitness
Females produce few large gamete (eggs)
 Fewer matings
 Large negative consequence to her fitness if her single mate has low fitness
Fundamental drivers of sexual selection
o Males compete for limited reproductive opportunities (eggs)
o Females are choosey
Males vary in reproductive success
 Previously thought that male seals dominate females in harem
 Only 23.5% of pups are from the closest male
 New study shows females move to find a good mate (35 m)
 Males rarely move more than a body length (2 m)
Females compete more when gender roles are reversed
 Males care for young
 Females can produce more offspring than males can care for
Examples
1. Red phalarope
2. Seahorses and pipefish
What drives sexual selection in males?
1. Contests produce selection on traits that improve success in confrontations
 Large size
 Strength
 Weapons
 Threat signals
 Dove behavior around superior males
Examples
 Canines of a tropical Asian pig
 Antlers of Red deer
Fig. 14.6 old only
Directional selection can cause trait exaggeration
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Ultimately balanced by the other costs
More exaggerated than if there was selection only from other ecological
factors (no sexual selection)
Lack of these traits in females suggests that they are ecologically
disadvantageous
Example
 Túngara frog
Fig. 14.8 old only
2. Mate preference by the opposite sex selects for attractive and stimulatory
features
o Larger, more intense, more exaggerated features
o Attractive territory
Sexual selection on males
 Sperm competition selects for mechanisms of assuring paternity
 Defend female from other males after mating
 Clasp females after mating until no longer receptive
 Males insert copulatory plug
 Seminal fluid itself reduces female attractiveness
3. Sperm competition selects for mechanisms of assuring paternity
 Produce more sperm to increase their chances of fertilization when there are
multiple matings
Fig. 11.22 new 11.20 old
Sperm competition selects for mechanisms of assuring paternity
 Produce more sperm to increase their chances of fertilization when there are
multiple matings
 Genitalia adapted to remove sperm of previous mates
Fig. 14.7 old only
What drives female preferences?
Traits may be harmful to male but selected for anyway because:
1. Direct benefits to female
 Nourishment
 Superior territory
 Better parental care
Example
 House finch color varies
 Bright red males bring food to nestlings at a higher rate
 Females chose more brightly colored males
2. Sensory bias
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Traits are intrinsically stimulating
Stimuli outside normal range often attractive
Female preference may evolve before male trait
Fig. 15.22 and Fig 15.25 new 14.9 old
3. Indirect benefits
 Male trait doesn’t directly increase female’s fitness
 Only contributes genes
If there’s no benefit to the female, why is she selecting male traits?
Two ways indirect benefits can evolve
1. Runaway sexual selection (sexy son hypothesis)
 Females express a preference for a male trait
 No benefit to her
 Sons express the trait that mothers preferred increasing the son’s
reproductive success
 Linkage disequilibrium develops between genes that affect female
preference and genes that affect male expression of the trait
Evidence for sexy son hypothesis
Fig. 14.12 old only
Sand fly
 Females prefer certain males
 There is no fitness advantage to the offspring associated
with male attractiveness
 But sons of attractive males also have higher reproductive
success
2. Good genes hypothesis
 Females chose males with high genetic quality
 How can females detect good genes?
 Some male traits only expressed if they are in good physiological
condition
Hoffman et al. 2007 on web
Example
Evidence for good genes hypothesis
 Stickleback Gasterosteus aculeatus
 Females prefer redder males
 Females mated to “red” fathers and “dull” fathers
 Offspring of “red” fathers had higher resistance to tapeworms
 Red males eat food with higher carotenoid concentrations
which is involved in immune system development