Sexual Selection Flashcards
Reproductive rates in Men
A man could reproduce much faster than a woman could
If a man made little commitment to rearing
If a man had multiple partners
Reproductive Rates in Women
Having multiple partners would not increase a woman’s reproductive rate
Mating Systems: Monogamy
a mating system in which males and females each have only a single reproductive partner
Mating Systems: Polyandry
a mating system in which women may have more than one reproductive partner
Polygyny
a mating system in which men may have more than one reproductive partner
Is monogamy the norm
Is monogamy the norm?
85% of human societies allow polygyny
Most of the more than 1,000 societies that permit polygyny are small tribal groups
Prohibitions on polygyny are recent
Historical records suggest that prohibitions date nowhere more than 500 years
Marriage practices are not a perfect reflection of mating practices
Marital infidelity
Divorce and remarriage
Parental investment
anything a parent does for a particular offspring that both increases that offspring’s reproductive prospects and decreases the parent’s reproductive prospects
A limited resource, like income or allowance
What you spend on one thing is not available for something else
Ex. gestation, lactation, groceries, medical fees, clothing, tuition, time spent, etc.
Biparental investment
When both sexes of parent contribute significant amounts of parental investment
The norm in human populations
If males could accelerate their reproductive rate by acquiring more mates, why do they slow themselves down by investing in offspring?
Human Parental Investment
Under certain conditions, offspring who receive investment from two parents do much better than offspring who receive it from only one
Biparental investment is more likely to evolve in species with helpless young
Extreme dependence of human infants may have favored the evolution of biparental investment in our species
Dynamics of Mating Competition
Members of the fast sex compete, while members of the slow sex choose
The more the investment, the more at stake
In a monogamous species, females and males tend to be equally competitive and equally choosy
Humans are neither strongly polygynous nor fully monogamous
Both sexes will evolve to compete for mates, with competition being more intense in males
Both sexes will evolve to be choosy about their mating partners, with women being more selective
We would predict that women will be more choosy and men more competitive
Buss and Schmidt (1993)
Asked men and women to rate how likely they would be to consent to sexual intercourse with someone they viewed as desirable, given that they had known the person for various periods of time
Participants rated their willingness on a scale from -3 (definitely not) to +3 (definitely yes)
Men consistently indicate a greater willingness to engage in sexual intercourse than women do
Choosiness / Homosexuality
For a heterosexual male, this is a problem…
The people with whom he must negotiate his sexual desires are women, whose desires differ
Homosexual men negotiate with individuals whose desires are more coincident
Thus, we would expect that homosexual men would be more successful in achieving large numbers of sexual partners than heterosexual men
Homosexual men report having hundreds or even thousands of partners.
Lesbian and straight women do not differ on average in their numbers of sexual partners
Donald Symons (1995)
“Beauty is in the adaptations of the beholder.”
Nothing in and of itself is intrinsically beautiful
We find this beautiful (and not beautiful) because it was (or was not) adaptive for us to in the EEA
Mates provide two kinds of resources
Good Genes
Good Investment
The best mates are the ones that provide both good genes and good investment.
How do we know which mates have the best genes and will provide the best investment?
Sex and Anisogamy
A comprehensive definition is that males produce small gametes (sperm) that seek out the larger, less mobile gametes (eggs) produced by the female.
Isogamy
a condition where the gametes from each partner engaged in sexual reproduction are of equal size- NO MALE / FEMALE- Same sex.
Anisogamy
A situation in which the gametes from sexually reproducing species are of different sizes.
Males produce small highly mobile gametes in large numbers
Females produce fewer and larger eggs
Size of gametes
The ancestral state of equally sized gametes quickly breaks down into two strategies
Providers and seekers
Both strategies are stable
They resist invasion from other strategies
This confers upon them an advantage
Why Sex- Costs
Time and effort is spent attracting, defending, and copulating with mates.
Such effort could have been directed into reproduction
Individuals may be vulnerable to predation during mating, especially during intercourse or courtship displays
There is a risk of damage during the physical act of mating
A risk of disease transmission from one individual to another exists
The recombination of genes that follows sex may throw up a homozygous condition for a dangerous recessive allele
Sex introduces same sex competition
The Red Queen Hypothesis
In low-fertility organisms (like humans) there is currently only one plausible explanation for sexual reproduction
To cope with the parasites and pathogens that make us sick
According to the Red Queen Hypothesis
Sex evolved as a defense against rapidly evolving pathogens
The Red Queen Hypothesis- Continued
Parasites and pathogens are small, rapidly reproducing organisms that eat us from the inside out
Can kill us or seriously reduce our fitness
Because they reproduce rapidly, they evolved and adapt quickly
Shorter generation times
Their environment is the host’s body
They get better and better at exploiting that host
Hosts evolve defenses too
Pathogens have the upper-hand because their generation time is much, much shorter
The Red Queen Hypothesis- Continued
By the time a host reaches reproductive age, he or she will have provided a stable environment for tens of thousands of generations of parasites and pathogens
If the host reproduces asexually, they pass an identical copy of their genotype to their offspring
Since their own parasites and pathogens are already adapted to this genotype, this will burden their offspring
It would be better to start off life with a new and different genotype
We have sex to increase parasite and pathogen resistance
Pathogen Resistance
we should choose mates on the basis of their pathogen resistance
We should have evolved to view signs of consistent good health as attractive.
Beauty is in the adaptations of the beholder.
Those people with good pathogen resistance will be seen as attractive.
