L13 - Sexual Selection and Coevolution Flashcards
Why would a seemingly maladapted trait persist in a population (e.g., peacock tail)?
- Sexual selection! While the maladaptive traits are not beneficial to an individual’s survival, they are attractive to potential mates, essential for the reproduction of a species
Define primary sexual characteristics
Differences in the biological sexes directly related to reproduction (e.g. genitalia)
Define secondary sexual characteristics
- Traits which are used in sexual selection, but are
not required for sexual reproduction- Horns, pretty feathers, birdsongs…
- Direct competition or attractiveness
Define sexual dimorphism
Difference in appearance between males and females of the same species
What are the two main types of sexual selection?
- Intrasexual: within the same sex
- Epigamic (intersexual): between sexes
What is intrasexual selection?
- Direct competition between two members of the same sex for the opposite sex
- Most often seen as male-male competition for a female
- Thus, female has limited to no choice in mate selection
- This type of selection is most common among species where the males generally do not help care for offspring
What are common sexual dimorphism patterns among species that undergo intrasexual selection?
- Males are larger than females
- Males have sexual ornaments (antlers) to fight competing males
- Males tend to be similar colours to females
What is epigamic (intersexual) selection?
- Selective pressure from the member of one sex having COMPLETE CHOICE over a mate of the opposite sex
- Courtship rituals used to attract members of the opposite sex
- This type of selection is most common in species where individuals help raise the offspring together, also among individuals that mate for life
What are common sexual dimorphism patterns among species that undergo epigamic selection?
- Where males do not stay to help raise the offspring:
- Males are often much more colourful than females (extreme sexual ornamentation)
- Females are often larger than males, or of a similar size
- Where males help raise offspring:
- Both sexes are often the same colour and size
Who typically chooses in epigamic selection?
- Females !
- Male choice does exists but it is much rarer
How do males invest into propagating their genes?
- Sperm: low input & high output
- Reproductive success comes from mating with as many females as possible
- Many mates: male can afford to select a suboptimal mate
How do females invest into propagating their genes?
- Eggs: high energy investment, fixed number
- Pregnancy: energy intensive
- Reproductive success comes from breeding with the BEST male - to ensure that offspring are as fit as possible
- Can be very costly to choose a poor mate
- THUS: females get to choose
- Can be very costly to choose a poor mate
What is Bateman’s principle?
- The sex which invests the most in producing offspring will become a limiting resource over
which the other sex competes - **Males can be the sex that invest the most
Do we ever see male competition in epigamic selection?
Yes ! However, any system where mate choice exists is considered primarily epigamic
What is the Fischerian Runaway Selection Hypothesis?
- Female prefer a secondary sexual characteristic
- Female selects that mate, then passes that secondary character to her male offspring
- She also passes on a genetic trait in her daughters to desire that secondary sexual characteristic
- Creates a positive feedback, increases the desirability and extreme nature of the character over generations!