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how low fecundity or a short life span can evolve by natural selection. In think-
ing about this, we must be clear about the level at which selection acts on these
traits (see Chapter 3). Some biologists used to think that species such as codfishes
produce hundreds of thousands of eggs in order to compensate for high mortality
and ensure the survival of the species. Likewise, it has sometimes been suggested
that animals die of old age to make room for a vigorous new generation. But we
have noted that the future persistence or extinction of a species cannot affect, and
is irrelevant to, the course of natural selection among individuals (see Chapter 3).
So how can individual selection result in low reproductive rates or short life spans?
The life history traits with which we are concerned are the ages at which repro-
duction begins and ends, fecundity at each age, and the average survival to each
possible age [11, 49, 55]. These traits affect the growth rates of populations and are
major components of a genotype’s fitness. The age to which individuals survive in
nature is often shorter than their potential life span, which would be attained only
if extrinsic mortality factors, such as predation, disease, and food shortage, were
not operating. The maximum life spans cited at the start of this chapter are closer
to potential life spans than average realized life spans.
An organism acquires from its environment a certain amount of energy and
nutrients, which are allocated among several functions, especially self-mainte-
nance (hence, survival), growth, and reproduction. (The “growth” portion is ulti-
mately allocated to the other two functions.) We expect that there will be trade-
offs among functions: a fitness benefit of one function that is correlated with a
fitness cost of another function. (This is another way of saying “there is no such
thing as a free lunch.”) The fraction of energy and nutrients allocated to reproduc-
tion is sometimes referred to as reproductive effort. The trade-off between repro-
duction and all other functions is often called the cost of reproduction.FIGURE 11.2 Variation in fecundity (number of offspring). (A) Spawning oysters release
clouds of minuscule eggs and sperm. (B) A coconut is a single enormous seed, and the
coconut palm (Cocos nucifera) can produce only a few at a time. (C) Poplars (Populus)
produce millions of tiny seeds, with fluffy hairs that enable dispersal by wind. (D) This
X-ray of a kiwi (Apteryx) shows the bird’s enormous egg. (D, photo courtesy of Otoro-
hanga Zoological Society.)Futuyma Kirkpatrick Evolution, 4e
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