Sexual selection (Robert Quinlan, ANTH 468
If you look at this skull in
figure 1, what would you conclude about this animal’s diet?
Figure 1. http://www.boneclones.com/BC-036.htm
You might guess that it was a
meat eater, right? It does have huge
canine teeth that would appear to be well adapted to catching and eating
game. Well, if you guessed it’s a meat
eater you would be wrong. This animal subsists almost entirely on shoots and
leaves.
Now look at the skull in
figure 2 below. If I told you that it was about in proportion to the skull in
figure 1 (that is it’s about 80% smaller) and that it’s a full grown animal,
would you think that it’s the same species as figure 1? I bet many of you would not, but you’d be
wrong. It is the same species: They’re both gorillas. The big one in figure 1
is an adult male, and figure 2 is an adult female. Such sex differences are
called sexual
dimorphism.
Figure 2. http://www.boneclones.com/BC-035.htm
Our question is then, how is
that male and female gorillas are so different? They’re so different that you
might not even guess, just by looking at them, that they’re the same
species. How could evolution do that to
a species? Well, we have to back to the
basics of sexual reproduction to understand the selective pressure that result
in often big differences between males and females.
Sex differences in cell division: Anisogamy
If you remember back to high
school biology, you may recall that there are two types of cell division. One
type, called mitosis, is
involved in the growth and maintenance of tissues. In mitosis the cells in your
body replicate their DNA and organelles, and from one cell you end up with two
identical daughter cells. The daughter
cells have the full compliment of your DNA in 23 pairs of chromosomes. Your
body is undergoing mitosis right now as your skin cells replicate themselves,
die and slough off. For our purpose it’s no bid deal and there aren’t sex
differences in mitosis that we need to be concerned about.
There is another form of cell
division called meiosis
involved in the production of sex cells.
Meiosis is considerably different that mitosis because you end up with
daughter cells that only contain half the DNA of the parent cell. It works like
this: First, you start with a diploid
cell (with chromosome pairs) either in the testis if you’re male or ovaries if
you’re female (see figure 3, which only has one chromosome pair for ease of
presentation).
Figure 3. http://genetics.gsk.com/graphics/meiosis-big.gif
That cell duplicates its DNA;
then the chromosome pairs get pulled apart in the first division. In the second
division the duplicated half of the chromosome pairs are pulled apart and you
end up with four haploid
daughter cells (they do not have chromosome pairs). Basically you could draw a tail on the four
daughter cells in figure 3 and call them sperm.
But the process is a bit different females (figure 4).
Figure 4. http://faculty.southwest.tn.edu/rburkett/A&P2_reproductive_system_lab.htm
In females there is a big
difference: At each cell division in
meiosis all of the cytoplasm
and organelles (junk and
functional stuff) in the gonad
cell go into one relatively large daughter cell. The leftover DNA in the other daughter cell
becomes a thing called a polar
body, which is ultimately absorbed.
The result is that females tend to produce relatively few, huge and
expensive gametes called
eggs, while males produce many millions of tiny, cheap gametes called
sperm. Figure 5 gives you an idea of the
relative size of egg and sperm, where the giant egg is surrounded by many tiny
sperm.
Furthermore, once a male hits
spermarche (sexual maturity coinciding with the first nocturnal emission
or “wet dream” in humans) he produces millions and millions of sperm throughout
his life. Sperm are so plentiful and
cheap to produce that many males cast them away with glee and reckless
abandon! In contrast eggs are relatively
costly: A human female is born with all the eggs she will ever produce (about
30,000 of them) which are suspended half way through the meiotic process,
waiting to finish their division into gametes, and make their way down the
fallopian tube once per month after menarche. In mammals especially, this basic difference
in the “cost” of producing gametes echoes throughout much of the reproductive
process.
Figure 5. http://www.geocities.com/CapeCanaveral/Hall/2955/imagens/fertilization.jpg
Think about it for a moment:
From the point of conception a woman is tied to her fetus for about 10
months. After birth women are tied to
infants by lactation (breastfeeding). During
exclusive breastfeeding, when a child does not eat much supplemental food, a
woman puts about 670 calories per day into milk production. That’s more than a
Big Mac with some fries. In the
Now consider a man’s minimum
parental investment: If you
include dinner and a movie, then it’s about one day. Most males reach spermarche at about 15 (13
in the
Table 1. Sex
differences in reproductive potential for men and women
|
|
Women |
Men |
|
minimum parental investment |
1200 days (40 months) |
1 day |
|
reproductive career |
32.5 years |
53 years |
|
maximum reproductive potential |
10 |
30-1000 |
|
|
|
|
Female choice and male-male competition
This big difference has
several implications: Because reproduction
is so expensive for females, they should tend to be very choosy. Remember that
we define fitness as “one’s genetic representation in future generations.”
During each pregnancy a female human (or other great ape, which tend to have
one offspring at time) is only nurturing half of her genes in her fetus. The
other half come from some male. Hence,
she should be very careful about the genes she takes “on board” to mix with her
own. After all, who wants to take care of some loser’s genes that might even
reduce the chance that one’s own genes make it into future generations? Females then should look for males with good
genes. But how do you tell which males
have good genes?
Females should prefer signs
of good genes that are difficult to fake.
One idea is that males with good genes are better able to resist
pathogens (like skin infections etc.).
Another idea is that honest signals of genetic quality should be
energetically expensive to make. So for example, peacocks grow huge, flashy
tails that can make life hard for them. This is known has handicap theory. The
healthiest peacocks can make the best tails, which are hard to make. Peahens,
then, choose their mates according to the quality of their tail. Some sexually dimorphic
traits like the peacock’s tail evolved through one mechanism of sexual
selection known as female choice. [needs a bit more here: Zahavi & Hamilton-Zuk links].
Can you imagine something
analogous to the peacock’s tail in humans?
Some evolutionists suggest that artistic and musical ability might be
like the peacock’s tail. Others think that beards are a possible honest signal
of male genetic quality. Maybe it’s
true. How might you test those hypotheses?
There is another dimension of
female choice. Because pregnancy and lactation are energetically expensive
females should want help: It takes lots of calories to make and nurse a baby,
and it’s difficult to do other things, like look for food, when you’re
breastfeeding. In species (or
environments) where males can help, then females should prefer males that show
signs of willingness and ability to provision her and her offspring. One study by psychologist David Buss showed
that women in 37 different cultures were, in fact, attracted to men with
resources that they could invest in offspring. [needs more: links?]
Female choice is only half
the story. If you imagine that female
reproduction is very expensive and male reproduction is very cheap, then that
creates a problem for males too. That
is, access to willing females is limited and males must compete for the
attention and sexual access to females.
Males with genetically heritable traits that make them better able to
compete for mates are more likely to pass on their genes and those traits that
make them better competitors evolve through the second mechanism of sexual
selection known as male-male competition.
In many species male-male competition gets played out in combat among
males. In that case larger males tend to
gain access to females, and male-male competition leads to the evolution of sex
differences in body size. For example, gorilla males are 80% larger than
gorilla females, which is most likely due to competition for access to
mates. Male gorillas also have large
canine teeth for fighting. Both traits – large body size and canine teeth – are
said to be sexually selected (fig. 1).
In theory there can be male
choice and female-female competition. This reversal is particularly the case
when males provide extensive parental care.
More accurately we should refer to inter-sexual choice and intra-sexual
competition as the two mechanisms of sexual selection or mechanisms by
which sex differences evolve.
What about humans?
Among humans we might expect
some sexually selected differences.
Human females face many of the same problems that other animals face in
terms of mate choice. Women should be
choosy about the genes that they bring “onboard” to mix with their own because
pregnancy, lactation and later childcare are especially expensive for
humans. Mistakes in mate choice can be
very costly for women.
Human males are able to
contribute substantial parental care, either directly or by provisioning the
mother and her offspring, which can free women from work that conflicts with
childcare. This fact sets up a kind of
two-way street for human sexual selection: (1) Women should be choosy not only
about the quality of her mate’s genes, but also about his willingness and
ability to provide resources for childrearing.
(2) Men should also be choosy about their mates’ genetic quality because
men may invest a lot of time and resources into a mating relationship and
offspring, and men often forgo other mating opportunities to invest in one
family. Hence, men should be choosy
about another quality of their mates: Their potential fidelity. After all, if
man invests substantial resources in a relationship with a woman and her
offspring, then he should want to be as sure as he can be that those offspring
share his genes. That means men should
look for signals of fidelity along with signals of good genes.
Human males are about 15 to
20% larger than are human females depending on the population. This sexual dimorphism reflects at least
moderate male-male physical competition for access to mates or access to
resources for mating. There may be even
greater sexually selected psychological differences between men and women.
In western industrial
populations, like the
Even with aggression and
sexual “permissiveness” there is substantial overlap between men and
women. Janet Shibley Hyde conducted a “meta-analysis” of many studies
of psychological sex differences, which shows that the average difference
between men and woman was only about .8 to 1 standard deviation
for traits that are likely sexually selected.
Figure 6 shows that there is substantial overlap between men and women
for attitudes toward casual sex, for example.
In the figure the vertical lines show the average for men and women, and
the bell-shaped curves show the variation around the average. What the figure shows is that there are many
women who are more accepting of casual sex than are many men, but the general
tendency is for women to be less accepting of casual sexual relationships.
David Schmidt conducted a
study of college students around the world, which shows that the variation in
gender differences in attitudes toward casual sex is influenced by
culture. In societies where men and
women are more equal in economic power and opportunities, there is less sex
difference in attitudes toward casual sex.
But there is still some sex difference: Women still tend to be less
accepting of casual affairs than are men.
Culture also influences what
we believe to be appropriate sexual behavior for men and women. A series of fascinating psychological studies
uses as technique called the “bogus pipeline” to examine our culturally shaped
notions of sexual propriety. In the
bogus pipeline method one group of subjects is given a questionnaire and is
hooked up to a fake lie detector (the bogus pipeline). The other group of
subjects simply fills out the questionnaire anonymously. Terri Fisher did a bogus pipeline study of
sexual experience (number of sex partners and number of one-night-stands), and
the results are shown in figure 7: Women
college students tend to under-report their sexual experience, while men
exaggerate their experience.
Figure 6. Difference between men and women in
attitudes toward casual sex. Dotted lines indicate men, solid lines indicate
women
The bogus pipeline shows
something else very important. Notice
that in the pipeline condition there is no difference in the average level of
sexual experience of men and women (fig. 7 “Pipeline”). However, there is a big difference in the
standard deviation: Men have much more
variation in the level of sexual experience than do women, meaning that some
men have much more sexual experience than do others. Such variation among males is precisely what
we expect to see under male-male competition and female choice – some males are
being relatively excluded from the mate pool, while others are highly
successful. In contrast, most women have about the same level of sexual
experience.
Figure 7. Expectations for appropriate gender behavior
influences our responses to questionnaires.
Bars represent the mean and “whiskers” represent the standard deviation. “Sexual Experience” includes number of partners
and number of one-night-stands.
Summary & Conclusion
Basic sex differences in the
costs of reproduction create different pressures on males and females. Females should be choosy about the quality of
genes they bring on board and the willingness and ability of their mate to help
with childrearing. Males should be
competitive for access to choosy mates, and when men provide parental care they
should be concerned about their mate’s fidelity. Among humans these differences in costs of
reproduction may account for moderate sexual dimorphism in body size, and
dimorphism in aggression and acceptance of casual sex. Cultural differences do appear to play a role
in either narrowing or widening the gap between men and women. In no society, however, have we ever observed
a full reversal in sex differences: No
where are women on average larger and more aggressive than men, and no where do
women appear to be more motivated on
average to have casual sex than are men.
