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!
What is the Sexy Son Hypothesis?
Females choose attractive mates because it means that their own male offspring will be better able to attract mates
What is the Handicap Hypothesis? (think peacock)
- Intense secondary sexual characteristics indicate to the opposite sex that the individual can afford to waste resources on developing such an elaborate secondary characteristic
- The individual can “handicap” itself and still survive = very fit
What is coevolution?
Where two (or more) species impact each other’s evolution
What’s the difference between natural selection and coevolution?
- Natural selection: broad changes in phenotypes in response to environmental conditions
- Coevolution: Small number of traits/genes in one species are directly impacting the evolution of a second suite of traits/genes in another species
What is specific (pairwise) evolution?
Two species reciprocally impacting the other’s traits or genes
What is an evolutionary arms race?
- Symmetrical arms race
- Selection pressures are identical on two species
- Two species whose interactions result in them co-evolving the same trait
- Asymmetrical arms race
- Contrasting selection pressures
- Both species are co-evolving, however the selection pressure acting on the two species is different
Example of a symmetrical arms race?
- Only the tallest trees receive full sunlight in a mature forest
- Different trees evolve to have the same trait: height
- A tree is only tall because the next tree threatens to overshadow it
Example of an asymmetrical arms race?
- African cheetah can only run for a short distance at its top speed (120km/hr)
- Thomson’s gazelle’s (top speed = 97km/hr) is the preferred prey of African cheetahs
- Cheetah evolved traits for hunting: speed & stealth
- Gazelle evolved different traits for evading capture: endurance & evasion
Define symbiosis
- A close and prolonged interaction between organisms of different species
- Can be beneficial to both partners, neutral to one partner, harmful to one partner
Define Mutualism
- Interactions between two different species
mutually benefit each other
Define obligate mutualism
- Two species must co-operate in order for both to survive
- The survival of one species depends on the survival of the other
- EX: inside out flowers are ONLY pollinated by fig wasps
Define facultative mutualism
- Both species derive benefit from each other, but one would survive if the other went extinct
- EX: Tarantula protects frog from other predators and the frog protects tarantula eggs (as the frog eats ants, the primary predator of tarantula eggs)
- But they can survive alone
Define parasitism?
- Interaction between two different species where one benefits at the expense of the other
- Parasitism relationships are generally very finely tuned to preserve the life of the host
- Parasite needs the host to survive
What are brood parasites?
- Cuckoo finch = obligate brood parasite
- Does not raise its own offspring
- Relies on birds of other species to raise its offspring by laying its own eggs in other birds’ nests
- When the cuckoo eggs hatch, the babies will push the other eggs out of the nest
- Ensures that only the cuckoo offspring is raised
- The host can sometimes identify the cuckoo egg before it hatches
- Will push the cuckoo egg out of the nest
- The host evolves a diversity of egg patterns to try to distinguish its eggs from the cuckoo
