Transcript Lecture 2 - Organic Origins Debate

Post-Copulatory Sexual
Selection
The Coevolutionary Battle
of the Sexes (Part II)
The Myth of Monogamy
Monogamy, polygyny, polyandry
 Sexual vs. social monogamy
 Aristotle was among the first to write about
polyandry
 Darwin generally assumed that females were
monogamous
 In the 1960s-1970s, it was realized that
polyandry abounded in nature

Cooperation or Competition?

Males and females have disparate goals
 Males attempt to fertilize as many ova as
possible; they are constrained mainly by
the number of available females
 Females attempt to maintain choice over
which male fertilizes their ova
 This leads to sexual conflict and
coevolution of reproductive traits
Genitalia
Females of many species
store sperm
 Increases female choice
 Relative testis size
 Penile-vaginal coevolution
(e.g., labia)
 “Accessories”
 Male bean weevil
intromittent organ

Sperm Variation
Sperm length has been selected (direct
benefits, intermale competition, counteradaptations)
 Constrained by sperm length vs. numbers
 Sperm motility predicts successful fertilization
 Speed and duration of “swimming”
 Non-nucleate sperm enhance ejaculate “bulk”
 Sperm “train” for faster access

Kamikaze Sperm

As noted in your text, Baker & Bellis (1988)
report “kamikaze sperm”; however
 When sperm were observed killing each
other, how do we know that it was
between two different ejaculates?
 Supposedly good sperm morphs (e.g., “egg
getters”) had chromosomal errors
 Results could not be replicated
Last-Male Sperm Precedence


The last male to mate with a female tends to
secure a disproportionate number of
fertilizations
How do several matings affect paternity?
 Fair raffle
 Loaded raffle
 Second ejaculate displacing first
 Female internal “pump” displaces first
ejaculate
Female Choice


Early conceptualizations of post-copulatory
sperm competition assumed female passivity
There has recently been research on cryptic
female choice
 Female comb jelly (Beroë) nucleus selects
among several male nuclei
 Diploid ova select for compatibility
 Female feral fowl differentially eject sperm
from lower-status males
Benefits of Polyandry


Direct benefits
 Sperm shortage
 Fertility insurance
 Gifts (e.g., nutrients)
 Parental care
Indirect benefits
 Compatibility (e.g., MHC)
 Parasite resistance
The Wrap-Up
Myth of monogamy
 Competition and coevolution
 Variation in genitalia and sperm
 Problems with “kamikaze sperm” theory
 Last-male sperm precedence
 Female choice
 Benefits of polyandry

Things to Come



The evolutionary origins and impact of
human language
Memetics
Gene-culture coevolutionary models