Module 9: Sexual Selection

the selective elements that contribute to a sexually reproducing organism’s fitness

1. mortality or viability selection

2. mating success or sexual selection

3. family size or fecundity selection

fitness or darwinian fitness is a measure of

reproductive success (contribution to the next generation)

some characteristics contribute to the survival of the organism example

thick white coat on polar bear

sexual dimorphism

refers to phenotypic differences ebwteen males and females, IN many cases sexual dimorphisms cannot be explained by viability selection - if a trait results in an increased probability of survival why wouldnt both sexes possess it

sexual selection

differential reproductive success resulting from differential abilities to find a mate

parental investment

refers to the energy, time, and resources devoted to mating, gestating, and caring for offspring. parental investment is typically much greater for females. eggs are expensive, sperm are cheap

daily female egg production requires 3x energy needed for daily basal metabolism compared to daily male sperm production requires

4/1000 of the energy needed for daily basal metabolism

females make a much greater parental investment than males and as a result

their reproductive success is limited by the number of eggs that they can produce and rear

male reproductive success is limited by the

number of mates that they can obtain

although more common in nature, the pattern observed in newts

where male reproductive success was much more dependent on the number of mates procured is not universal

asymmetry in reproductive potential predicts that differences in mating behaviour will exist between the two sexes

1. males: will usually be competitive (combat, sperm competition, infanticide) strong sexual selection

2. females: will usually be selective or choosy weak sexual selection

intrasexual selection

interactions between members of the same sex

intersexual selection

interactions between members of opposite sexes

antagonistic selection in iguana example se

sexual selection favors large males that win more mates, while viability selection favors smaller body sizes that maximize survival, resulting in opposing selection pressures on body size

direct combat is not the only type of male-male competition

come in the form of sperm competition, and infanticide

Sperm competition

occurs when multiple males mate with the same female in a short time, and their sperm compete to fertilize her eggs. Males can increase fitness by evolving traits like more sperm, faster sperm, or behaviors like mate guarding to ensure paternity.

Infanticide

is a strategy where a male kills offspring that are not his own, often in species with extended maternal care. This causes the female to return to fertility sooner, giving the male an opportunity to mate and pass on his genes.

the relationship between male fitness, sperm competition, and infanticide

the ultimate goal is to increase the male’s share of reproduction, but while sperm competition is a post-copulatory strategy within a mating context, infanticide is a pre-mating strategy to eliminate rival genes and gain mating opportunities

Bruce effect

pregnancy termination in the presence of unfamiliar males

in many species, male contribution to the next generation is limited by the number of mates they obtain, but they are not able to control females themselves, or resources and habitates required by females

males will advertise for males and females will choose a male based on the quality of the advertisement, resulting in the evolution of courtship displays as well as embellished morphological features

In the case of the widow birds, be sure that you understand how viability selection (as it relates to index of body condition) and sexual selection oppose each other (antagonistic selection)

In widowbirds, antagonistic selection occurs because sexual selection favors long tails for attracting mates, while viability selection favors shorter tails to maintain better body condition and survival.

In the case of the gray tree frog, is it reasonable to hypothesize that males with longer calls might experience higher mortality, and potentially provide a case of antagonistic selection? Why or why not?

Yes, it is reasonable to hypothesize that male gray tree frogs with longer calls might experience higher mortality because longer calls, while more attractive to females (sexual selection), can also attract predators or require more energy (reducing survival), thereby creating a potential case of antagonistic selection, where sexual and viability selection oppose each other.

females with a preference for a weak display mate with males that have a weak display

females with a preference for a strong display mate with males that have a strong or extreme display - positive assortative mating with respect to female preference and male display

stabilizing selection

where average male display matches average female preference, and individuals at extremes are less likely to reproduce.

positive assortative mating

where females with weak display preferences mate with weak-display males, and vice versa for strong displays—offspring inherit both traits, creating a genetic correlation between preference and display.

How does the model for the evolution of female choice explain the development of male display traits, and what role do viability and sexual selection play

Positive assortative mating between females and males with matching preference and display creates a genetic correlation; stabilizing selection aligns average display with preference, but when viability selection opposes sexual selection, antagonistic selection occurs—potentially leading to runaway selection if preference and display evolve away from survival-optimal traits.

The second way in which female preference may evolve is

that they may be responding to males as a result of a pre-existing sensory bias, which makes them susceptible to certain sensations that mimic cues in their environment.

The third way that selection may act on females to exhibit a certain preference is that the preference

may result in the acquisition of resources.

polygyny

males frequently mate with more than one female

polyandry

sexual selection can also act on females: males can be choosy, and females can mate with more than one male

sexual dimorphism in humans

the physical and behavioral differences between males and females of the species, beyond just reproductive organs.

what is sexual selection

differential reproductive success resulting from different abilities to procure a mate.

in some species, females abort ongoing pregnancies int he presence of unfamiliar males this is

• thought to have evolved to avoid wasted reproductive costs in females

• called hte bruce effect

a species with choosy females might evolved because

• the preferred male trait is associated with better genes for the offspring

• the preferred male trait is associated with more or better resources for the female and her offspring

• for no reason, the female’s preference is originally random

female preference is arbitrary

There is positive assortative mating between females with a preference for a trait and males with that trait. This leads to a positive genetic correlation between the display of a trait and the preference for that trait. 

when sexual selection on females is likely to occur

when males invest heavily in offspring- through parental care or resources making them a limiting reproductive resource, in such cases females compete for access to high-quality males or their investments

across the globe, male humans are typically 10% taller than female humans. based on the set of studies reported in your readings, does the weight of the evidence support the hypothesis that this dimorphism is the result of male-male competition or the result of female choice?

The weight of the evidence supports the hypothesis that this height dimorphism is primarily the result of male-male competition rather than female choice. Studies show that taller men historically had advantages in physical contests and social dominance, which increased reproductive success, whereas evidence for a consistent global female preference for taller males is weaker and more variable across cultures.