Topic 4 - Fundamentals of Evolution: Sexual Selection Flashcards
Sexual selection was notice by and because
Darwin
he noticed animals have traits that would seem to inhibit their reproductive success
but the still increase FITNESS because they allow for more reproduction
Intrasexual selection
Within a sex
eg defeat/scare same-sex rivals
territory defense
fighting or other direct competition
Intersexual competition
Between sex
Make themselves attractive for the other sex
Can act on
behavior as well as morphology
eg fiddler crabs which both have one giant claw (morphology) and use it to signal for mates (behavior)
Mechanism
Females find a trait attractive
This trait leads to increased reproductive success and therefore more individuals in the next generation with the trait
However, eventually, this trait may lead to a lack of survival which is not compensated for by an increase in reproductive success
So Natural selection acts as a buffer to stop traits being sexually selected for becoming too large or common
Variance and skew
Variance is the width of the distribution (of a trait)
Skew tells us about the shape of the distribution
The greater the variance the further apart the extremes are. The greater the skew, the more asymmetric the distribution.
Skew and distribution for flashy sex
Whichever sex has the biggest variance or skew in reproductive success will evolve more elaborate traits
Which sex is choosy
Whichever sex has more investment will become choosy
If the AVERAGE reproductive of the sexes is equal (as all young have a mother and father) then
The species with the most variation around that mean will be the flashy one.
Investment
Because of size and resources in eggs vs sperm it is often said females are inherently more invested. Prof finds this oversimplified.
Sees this in terms of maternal and paternal caare
fish - often paternal
mammals - often female
brids - often biparental
Flashy in mammals
may not be flashy, might be size, aggression and territorial natures
Batemen’s gradient
Bateman hypothesized that there was a relationship between mating success and reproductive success and that this could differ between males and females
In species where investment could LIMIT the reproductive output of one sex more than the other, that sex will be choosier as to increase reproductive sex does not just mean to have more kids but more grandkids too.
Hence a better mate will lead to better offspring and eventually more grandkids
Preference for mates: Direct benefits
Females gain something directly from male: more or healthier offspring, more food, protection, better nesting
Eg make cardinals that are brighter red are better parents
It does not have to be a heritable trait, just has to benefit female
Preference for mates: Good genes/immunocompetence hypothesis
Hamilton & Zuk, 1982
Some males more genetically resistent to parasites or infection
the survive longer and have more offspring + less likely to infect females + offspring wpuld survive longer and reproduce more
Males produce a trait that signals their health, females prefer mate that displays that
Hoops to jump through for, good genes/immunocompetence hypothesis
1) that the parasites are costly
2) that there is heritable variation in immunity
3) that the males are prevented from ornamenting themselves by the parasite
4) females choose males with fewer parasites
has not been shown much
eg junglefowl
Preference for mates: pre-existing sensory bias and sensory exploitation
Females have a bias for a sensory stimulus like a color or sound.
The biases result from the way their brains are set up.
This evolved for reasons other than sexual selection (eg foraging)
For example, animals that evolved to eat crickets might be attracted to the noises they make. If a male could imitate that would attract females.
Tungera frog
Makes whines and chuck sounds
Chucks attract females and also bats
So number of chucks is determined by the balance sexual and natural selection
Why do tungera frogs like chucks
First determined that making chucks is a DERIVED trait. One that has only recently evolved
eg not present in the ancestor but shows up after the speciation event
but then found out that ANOTHER frog likes this too even though the males of that frog never made chucks (so know the preference for chucks, if not the making of chucks comes from a common ancestor).
Most likely from the common ancestor of all 4 species.
Turns out its because the cohlea of all frogs is sensitive to low frequency sounds. This is likely an evolutionary adaptation that an ancestor made to their environment and now features in all frogs.
Only two branches make chucks. Somewhere before this speciation event, there was an ancestor who found they made chucks and female frogs were attracted. This has quickly preferred by the females (who cannot ignore it due to their cochlea and the trait spread throughout all the species.
Evolutionary, male Tungara frogs have exploited an ancestral sensory bias for low frequency sounds that is present in the female auditory system.
Phylogeny
A history of organismal lineages as they change through time
Speciation event
Where two species start from a common ancestor
Parsimony
Get tot he pattern of trait expression in the descendants with the fewest changes.
