Evolution__3rd_Edition

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An adaptation may benefit one
level of organization but not
another
Lion hunts benefit lions but not
mammals as a whole
CHAPTER 11 / The Units of Selection 293
11.1 What entities benefit from the adaptations produced
by selection?
It is a familiar idea that life can be divided into a series of levels of organization, from
nucleotide to gene, through cell, organ, and organism, to social group, species, and
higher levels. Which, if any, of these levels does natural selection act on and produce
adaptations for the benefit of? In a fairly superficial analysis, the answer does not matter.
If an adaptation benefits an individual organism, it will often also benefit its species at a
higher level and, at a lower level, all the parts that make up the individual. But there can
be conflicts between these levels. In some cases, what benefits an organism may not also
benefit its species, and in these cases the evolutionary biologist needs to know which
level natural selection most directly benefits. The question therefore matters when we
are studying particular adaptations. If we are seeking to understand why an adaptation
evolved, we need to know what entities adaptations in general evolve for the benefit
of. The question also has a more general, almost philosophical, interest: the theory of
evolution should include a precise, and accurate, account of why adaptations evolve.
The issue can be made clearer in an example. Let us consider the adaptations that can
be seen when lions go on a hunt. Lions often hunt alone, but they can improve their
chance of success by hunting in a group. Here is part of a description of a group of
hunting lions by Bertram (1978).
When prey have been detected, a wildebeest herd perhaps, the lions start to stalk towards
them. As they get close, they take different routes, some going on straight ahead, and some
to the sides, so the prey herd is approached by lions stalking them from different directions.
... Eventually one lion gets close enough to make a rush at a wildebeest, or else a
lion is detected by the prey.
Then the trap is sprung. Panicked wildebeest run in all directions, some of them
into the reach of other lions. The cooperative behavior of the hunting party is here an
adaptation for catching food, but it is not the lion’s only feeding adaptation. The lions’
muscular jaws and limbs, their teeth and five senses, all contribute to the success of the
hunt. Lions are well adapted for feeding: although some hunts are unsuccessful, and
individual lions may starve to death, the lions in the Serengeti Plains of Kenya spend
about 20 hours a day in rest or sleep, and only 1 hour a day on average in hunting.
Visitors tend to think lions are lazy.
When a lion hunt is successful, there are benefits for all but the highest biological
levels of organization. The individual lions obviously benefit, as does the pride. Each
time a hunt is successful, there will be a small incremental increase in the species’ chance
of survival, or avoiding extinction. The survival probability will also be increased, if by
a smaller amount, for the genus Felis and the cat family Felidae. The hunt’s effect at
higher levels will depend on exactly what prey the lions caught. Almost all the Serengeti
lions’ food is made up of other mammals, so when we reach the class Mammalia, the
effect of the hunt has probably become neutral. The lion’s gain is the wildebeest or the
zebra’s loss and the chance of survival of the class Mammalia is more or less unaffected.
The beneficial effect of the hunt spreads downwards as well as up from the individual
lion. As the survival of the lion is increased, so is the survival of its constituents: the