define SEXUAL SELECTION
the aspect of selection favoring features that INCREASE MATING SUCCESS
relevance of Sexual Selection
- Historical development
- Mendelian Natural Selection depends on sexual reproduction
- Fisher: one mother and one father
- Darwin: Elaborate characters that DON’T contribute to survival are limited to one sex and associated w/ breeding
- designs usually come about in 2 ways:
- female choice: generally mate choice
- male success in combat: generally competition among members of same sex for mates
- Bateman’s drosophila experiments 1948 -> anisogamy
- Trivers 1972 => Parental investment
Bateman
Drosophila experiments
- equal numbers of each sex (so avg RS =)
- Greater variance in male RS: 21% of males no offspring v 4% females
- DK&W fig 7.3 (p183), # offspring v # of mates
- female RS linearly related to food
- frequency of copulations NO effect on female RS but directly predicts male RS graphic
- Differences seen to be consequence of ANISOGAMY
- Combination of two gametes that are very different
relevance of PI
Parental Investment and Sexual Selection (Trivers 1972)
- if PI higher in one sex, then the members of the other sex will compete for it
a. In general: female PI is high/male low
i. E.g. in mammals
1. Females -> eggs/gestation/lactation
2. Males -> one sperm
ii. Elephant seals – extreme difference in PI
1. Female: 650kg, 50kg baby, nurse 5 weeks it gains 100kg while mom loses 200
2. Male 2700kg, sperm 1/10,000,000,000 of a gram (ejaculate made in hours)
define parental investment
“any investment by a parent in an individual offspring which increases the offspring’s chance of survival (and RS) at the cost of the parent’s ability to invest in other offspring”
- defined by Trivers
- a fundamental tradeoff
- goes back to Lack’s clutch size
- parent/sibling conflict (somatic/reproductive investment)
- sex role reversal
relevance of coy females and flashy, ardent males
- Darwin observed anisogamy asymmetry and expected to see coy females and ardent males => so this is often assumed
- observations of primates offers empirical counter
- libidinous females, advertised estrous
mating effort/parenting effort
i. finite amount of reproductive effort can be spent either on parenting or on mating, then, from anisogamy argument
1. Expect that what generally pays MOST (so selection favors most)
a. Females => parenting effort
b. Males => mating effort
c. All genes in us (except on Y) spend half their time in bodies of the other sex.
i. Implication: Most sex differences may be norms of reaction
- links to
- ANISOGAMY model
- females parental invest because they risk a larger fraction of total investment in each offspring
- males invest in being “choosable” (ornaments/armaments)
- father effect
- ANISOGAMY model
male care significance
-original assumption: higher certainty of paternity would lead to more paternal care
- S&G (1992), paternity probability should vary with grouping pattern, found weak correlation, but better correlation w/ male-mother friendship
- male friends have higher chance at future mating
- females
- H&H and BJ
- FU/male better predictor of pair bond stability in which male care is given
- male care is actually mating investment, not parental
- link to Graded Signal Hypothesis
- tie in with father effect
graded signal definition
exaggerated swellings in primates represent the probability of ovulation
male care definition
Any increase in pre-reproductive survivorship because of male effort
graded signal hypothesis significance
- given risk of infanticide, females exert parental effort by copulating w/ many males
- swellings at peak when ovulate => dominant male most likely to mate at that time
- other males have lower, but non zero, paternity possibility
significance of “libidinous” females
As follows from Bateman’s experiments, only males appear at first to increase RS with more copulations. Counter to the often predicted “coy” behavior of females, primate females often exhibit “libidinous” behavior. Numerous ideas follow:
a. Increasing paternity chances, decrease infanticide by males
b. Parenting effort, no MI
c. Sperm competition => mating comp for males
d. DK&W sex selection chapter =>
- costs of resistance exceed cost of acquiescence
- material/direct benefits (higher likelihood of insemination, resources from males)
- genetic/indirect benefits (birds who have multi partners have more fit offspring, also the antechinus litter experiment)
significance of testes size
- Monogamous have small, no sperm comp
- polygynous sp have larger, sperm comp
sperm competition define
ejaculate from different males compete for fertilizations inside the female tract
significance of sperm competition
Possibility for 2 processes:
- competition between sperm of rival males
- cryptic female choice
cryptic female choice
- morphological, physiological, behavioral counter strategies of females to problem set by male responses to sperm competition
- Refers to things happening within the female reproductive tract
-goes back to conflict of interest between sexes in mating
“sex role reversed” species
- species with sex roles reversed are consistent with Trivers’ PI
a. where male PI greater, evidence of sexual selection on females
i. but PI difficult to measure
ii. what of benefits that are not exclusive to an individual offspring
1. if they are sharable/jointly consumable/like public goods or toll goods
- e.g. pipefish- females compete for males, who choose the more ornamental females who produce larger clutches
- sex roles can become reversed due to seasonal resource availability (katydids/guppies)
- link to PI
- females compete for males, who choose the more ornamental females who produce larger clutches
implications of Maynard Smith’s parental care game
Implications:
i. Can get no paternal care even if [paternity] confidence high AND can get paternal care (male care? since maybe not father) even if paternity confidence low
ii. Can get all four ESS’s from males and females facing different limiting factors
- link to H&H and BJ
a. FU/male is the same as p value
i. Imp bc of criticism that females can’t appear out of nowhere
ii. FU/male addresses criticism
iii. Ache example
1. p > 60% for male to seek additional mating opp
assumptions of JMS’s paternal care model
i. Effect of number of parents providing care on offspring survival
1. P0 < P1 < P2 (effect of no parent < one parent < two parents)
ii. # of offspring Mom can produce if she also provides care vs if she doesn’t
1. w < W (# if care < # if no care)
iii. dad’s probability of additional matings if he doesn’t provide care
1. p
iv. “dad’s” probability of paternity
1. c (can be high or low)
results of JMS’ paternal care game
FOUR different ESS’s depending only on the values of P, W, and p:
i. No care
1. If WP0 > wP1 or female will care
2. And P0(1+p) > P1 or male will care
ii. Male only
1. If WP1 > wP2 or female will care
2. And P1 > P0(1+p) or male will desert
iii. Female only
1. If wP1 > WP0 or female will desert
2. And P1(1+p) > P2 or male will care
iv. Both care
1. If wP2 > WP1 or female will desert
2. And P2 > P1(1+p) or male will desert
tradeoffs inherent in JMSs paternal care game
An Asymmetrical Game
i. Note the tradeoffs for males
1. If mom cares
a. His choice is between wP2c if he stays and wP1c+pwP1c if he doesn’t
b. Opportunity cost of staying is mating elsewhere foregone
c. Opportunity cost of deserting is parenting care foregone
d. So will do better to care IF
i. wP2c > wP1c+pwP1c
1. boils down to P2/P1 > p+1
e. Note that c devalues payoffs for BOTH alternatives to the same extent. The c’s cancel.
f. Stay if wP2c > wP1c(1+p) [i.e. P2/P1 > p+1]
g. Whether c approaches 1 or approaches 0 MAKES NO DIFFERENCE in this game
2. If mom doesn’t care
a. His choice between WP1c if he cares and WP0c+pWP0c if he leaves
b. Again opportunity cost of staying is mating elsewhere, opportunity cost of deserting is paternal care foregone, so will do better to care IF:
i. WP1c > WP0c+pWP0c
ii. Again, c devalues payoffs for both alternatives to the same extent. The c’s cancel.
iii. AS LONG AS C DOESN’T VARY SYSTEMATICALLY, whether it approaches 1 or 0 makes no difference in this game (confidence of paternity doesn’t matter)
advertised estrus
go straight to graded signaling hypothesis
coefficient of relatedness
r in Hamilton’s rule (rb>c)
- the chance that a gene is shared between relatives
- so any gene in baby (50/50 chance it’s from dad/mom - because of meiosis)
e. what of a gene that affects a parent’s treatment of her offspring?
i. Cost to parent – gene for it IS IN THE PARENT
1. Gene will certainly (probability=1) suffer that cost
ii. Benefit to kid – will the gene ITSELF reap the benefit?
1. This depends on whether that gene is in the child, so what are the chances?
a. 50% (probability=.5) and that means .5b>c
2. specific probabilities of genes being shared among all relatives can be calculated
a. r = coefficient of relatedness
iii. expectation from Mendelian natural selection
1. social behavior adjusted to vary with Hamilton’s rule (setting of kin altruism is specific)
a. must be reliable cues to recognize relatives
b. THREE variables (r, B, C and they all matter)
-link to kinship, altruism
maternal condition
lots of effects (early origins of disease, etc), e.g. great tits and clutch size
Assumptions:
i.Population sex ratio generally Fisherian
ii.Differential benefit of condition by sex (i.e. bodily condition correlated to RS – you have more babies if you’re in better shape)
iii.Mom can predict condition of offspring
iv.THEN advantage to bias own sex ratio
-important when considering how extreme sex ratios might come about (but still be consistent with Fisher)
- link with Trivers-Willard Effect
- link with potential rate of reproduction
