Transcript Document

Sexual selection, a brief review
Some basic principles.
Some examples.
Difference between sexual and natural selection.
Sex role reversal.
Video -Why Sex (Evolution Series)
Hey,
look at
me!!
In general, success at mating:
For a male, is limited by the number of females
he can convince to mate with him.
For a female, is limited by the number of eggs/pregnancies
she can produce.
Leads to:
Mate Competition (male-male, for access to females)
Mate Choice (female choice, for best quality males)
Begins with difference in gamete size:
Sperm are very small (“inexpensive”)
Eggs are large and nutrient rich (“expensive”).
Anisogamy = having gametes of very different size
Small gamete is the male gamete (sperm)
.
Large gamete is the female gamete (egg).
Eggs (or pregnancies) are more “expensive” than
ejaculates.
Sperm are very small (“inexpensive”)
Eggs are large and nutrient rich (“expensive”).
So, females INVEST more in gametes, and have more
investment in each offspring compared to males;
Asymmetry in sexual reproduction established…
What effects does this have on mating behavior?
Males: fertilize as many eggs as possible (maximize
number of matings).
Females: make as many high-quality offspring as
possible (maximize quality of each offspring
through mate choice).
Mate competition
(usually male-male for access to females)
Mate Choice
(usually female choice among a selection of males)
How is sexual selection different from natural
selection?
For both sexes, reproductive success (number of
offspring produced in a lifetime) is determined
by process of natural selection (success at
survival and reproduction).
Puzzle of sexual dimorphism (differences in males and
females in appearance or behavior)
Darwin “Sight of a peacock makes me sick.”
Sexual selection is a subset of natural selection that
has to do with mating success.
If there is variation in a trait that affects ability to
mate successfully, and that trait is heritable, then
individuals with that trait will become more
common over time.
Sexual selection is a subset of natural selection that
has to do with mating success.
If there is variation in a trait that affects ability to
mate successfully, and that trait is heritable, then
individuals with that trait will become more
common over time.
What are some of these “sexually-selected traits?
In general, success at mating:
For a male, is limited by the number of females
he can convince to mate with him.
For a female, is limited by the number of eggs/pregnancies
she can produce.
Leads to:
Mate Competition (male-male, for access to females)
Mate Choice (female choice, for best quality males)
Some species show sex-role reversal, then:
Females compete for males, and
Males are the “choosy” sex.
When would you predict this would occur?
When would you predict sex-role reversal?
When males invest more than females in offspring. (Examples:
seahorses, giant water bugs, wattled jacanas)
Happens under unusual ecological conditions.
Why Sex?Video
Why do males and females look so different/
Why do male peacocks have that ridiculous tail?
Why are so many birds monogamous (but also fooling
around)?
Why is there sex-role reversal in some species (jacanas)?
Evolutionary psychology: how can we explain human
mate choice (what is beauty, do we smell our mates)?
Does sex ultimately explain art, music and humor?
What limited reproductive success for male peacocks?
What limited reproductive success for female peacocks?
Why is monogamy common among many birds, such as
the forest birds that Emlen studied in the video?
Female promiscuity is also favored in these birds. What was
the evidence for this, and how is it that a monogamous
species is also promiscuous?
Why do males care for young in the jacana?
Why do females have harems of males in the jacana?
What do you think of the studies done
by the evolutionary psychologists on
human mating preferences?