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Chapter 10- Studying Adaptation: Evolutionary Analysis of Form and
Explanation of organismal design is among the triumphs of the theory of evolution by natural
Individuals in previous generations varied in their design, and the ones with the best designs
passed on their genes in greater numbers
A trait, or integrated suite of traits, that increases the fitness of its possessor is called an
adaptation and is said to be adaptive
In order to prove that a trait is an adaptation, we need first to determine what a trait is for and
then show that individuals possessing the trait contribute more genes to future generations than
individuals lacking it
10.1 – All Hypotheses Must Be Tested: Oxpeckers Reconsidered
Read Example on Oxpeckers Pg. 364-366.
Oxpeckers are vampires and eaters of earwax
Even when they do eat ticks, Oxpeckers prefer adult females that have already engorged
themselves with blood- that is, ticks that have already done their damage to the host
When studying adaptations, there are some key things to keep in mind:
oDifferences among populations or species are not always adaptive. There are two species of
oxpecker; one has red bills, the other yellow. It is possible that each color is adaptive for the
species that wears it. But it is also possible that the difference is not adaptive at all.
Mutations causing different colors may have become fixed in the two oxpeckers by genetic
drift. At the molecular level, much of the variation among individuals, populations, and
species may be selectively neutral.
oNot every trait of an organism, or every use of a trait by an organism, is an adaptation. While
feeding on large mammals, oxpeckers may sometimes meet a potential mate. This does not
necessarily mean that feeding on large mammals evolved because it creates mating
oNot every adaptation is perfect. Feeding on the blood and earwax of large mammals may
provide oxpeckers with high-quality meals. But because many large mammals migrate long
distances, it may also expose oxpeckers to the risk of an unpredictable food supply.
Three methods used to test hypotheses about the adaptive significance of traits:
oObservational studies
oComparative method
10.2 Experiments
Experiments are the most powerful method for testing hypotheses. A good experiment restricts the
difference between study groups to a single variable.
Read Experiment on jumping spiders and flies Pg. 367- 371
Jumping spiders tended to retreat from flies that gave the wing-waving display with marked wings,
but attacked flies that lacked either wing markings, wing waving, or both.
Important points about experimental design:
oDefining and testing effective control groups is critical
oAll of the treatments (controls and experimental) must be handled exactly alike
oRandomization is a key technique for equalizing other, miscellaneous effects among control
and experimental groups. In essence, it is another way to avoid bias.
oRepeating the test on many individuals is essential. Larger sample sizes are better.
Replicated experiments or observations do two things:
oThey reduce the amount of distortion in the estimate caused by unusual individuals or
oReplicated experiments allow researchers to understand how precise their estimate is by
measuring the amount of variation in the data. Knowing how precise the data are allows the
use of statistical tests. Statistical tests, in turn, allow us to quantify the probability that the
result we observed was simply due to chance.
Large sample sizes are better but researchers have to trade off the costs and benefits of collecting
ever more data.
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