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Sexual Selection
Natural selection is not the only driver of phenotypic change. A huge driver of genotype and phenotype changes is sexual selection, where sexually reproducing species have preferences for the phenotypes of their mates.
Types of Sexual Selection
- Intra-sexual selection
- Competition between members of the same sex competing for access (usually) to females
- Example: evolution of horns, weapons and large body size
- Inter-sexual selection
- Female choice selection, males compete indirectly by impressing females
- Example: birds of paradise evolving extravagant mating rituals
- Sexual selection results in sexual dimorphism
Example: Mandarin ducks (CC BY-SA 3.0)
Wize Tip
Think of Intra-sexual selection like Intra-mural sports, the teams in intra-murals are from the same school or town!
Why Females are Picky
- Direct benefits
- Offspring care
- Procuring food
- Protection from predators
- Indirect benefits
- Extravagant males must have better genes, thus increases genetic quality of offspring
- Causes of pickiness:
- Anisomagy = unequal gamete size
- The egg is energetically expensive to produce, sperm is not
- Parental investment differences
- The sex with more parental investment becomes the limiting resource in the fitness of the less investing sex
When Females are NOT Picky
- Sex-role reversed species
- Females compete for male access as males invest more heavily in offspring
- Equal parental investment causes both sexes to be subject to sexual selection
- Usually in monogamous species
- Example: humans
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Example: Advantages & Disadvantages of Sexual Reproduction
Summarize the advantages and disadvantages of sexual reproduction.
Advantages:
i. Increases the likelihood of a multiple beneficial alleles being combined in
a single individual
ii. Increases variation and decreases the accumulation of deleterious alleles
iii. Increasing variation increases the likelihood your offspring will survive in
a changing environment
Disadvantages:
i. Mating is energetically expensive and time consuming
ii. Meiosis is an extra cell division, gametes (eggs in particular) are costly to
make
iii. Males in a population are costly; they do not produce their own offspring.
A single asexual female would outcompete a sexually reproducing female
Example: Fitness
In a sex-role reversed species the absolute fitness would be higher in:
Practice: Sexual Selection
Below is an image of a herd of Red deer, where the central male has large antlers and the other individuals in the image, without antlers, are females. Which of the following describe the sexual selection taking place in the image.
(CC BY-SA 2.0)
Practice: Sexual Reproduction
Why would reproducing sexually potentially be a bad thing for a species?