From
Psychology: An Evolutionary Approach,
Gaulin & McBurney, Prentence Hall, 2001.
EXERPTS FROM CH.2 – book is on reserve at the library
-This version was produced from a scan of the book, so some errors might be present; if you run into a
confusing sentence, assume there is a small error in the transcription and read through it-
Evolutionary Psychology, Natural Selection, and Sexual Selection
Darwin's Argument
1.
Natural populations (of giraffes, daisies, or people) could grow exponentially.
Consider an asexual organism that reproduces once per day. If we begin with one
of these organisms, we will have two tomorrow, four the next day, eight the next,
and so on. After a month we will have 2
29
(about 540 million), and after two and a
half months we will have more of these creatures than there are atoms in the universe! This
conclusion is not limited to organisms that reproduce rapidly. To illustrate this point, Darwin
selected the most leisurely reproducer he could think of- elephants. He assumed that a
female elephant would begin reproducing by the time she was 30 years old and that she
would then produce one offspring every 10 years, up to a total of six offspring, when she
would die of old age. Given these assumptions, he calculated that after a mere 500 years such a
female would have 15 million living descendants. Darwin ran out of computational power at
that point- but modern computers can show that after a few thousand years she would have so
many descendants—each of whom would be in the process of producing yet more
descendants—that the earth would be a ball of elephants expanding outward at
the speed of light! This is exponential growth.
2.
Despite this potential for exponential growth, natural populations normally are rel
atively stable.
As you have no doubt noticed, this sector of the universe is not a ball
of elephants expanding outward at the speed of light. There are no mistakes in the
calculations, so what is wrong? It must be that some elephants do not produce six
offspring over their lifetimes. The calculations tell us what would happen if the^
did. Since the conclusion is false, the assumptions are wrong, which leads to the
third logical statement.
3.
Many individuals do not leave as many offspring as they might.
This failure is
often due to limited resources. What would a ball of elephants expanding outward
at the speed of light eat? What would they breathe? The resources needed to make
more elephants are finite. Every time elephant
A
reproduces, her offspring use food,
water, and other resources that would have otherwise been available for the offspring of
elephant
B.
When a population colonizes a new, unexploited area, it maw
grow exponentially for a while, but its very success means that it will soon grow to the
point where, again, resources are inadequate to support the reproduction of all.
4.