• 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
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:
o Differences 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.
o Not 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
o Not 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:
o Observational studies
o Comparative method
• 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:
o Defining and testing effective control groups is critical
o All of the treatments (controls and experimental) must be handled exactly alike
o Randomization is a key technique for equalizing other, miscellaneous effects among control
and experimental groups. In essence, it is another way to avoid bias.
o Repeating the test on many individuals is essential. Larger sample sizes are better.
• Replicated experiments or observations do two things:
o They reduce the amount of distortion in the estimate caused by unusual individuals or
o Replicated 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. Observational Studies
• When an experiment is impractical, a careful observational study may be the next best method for
evaluating a hypothesis.
• Vast majority of organisms are ectothermic, means that their body temperatures are determined
by the temperatures of their environments
• Body temperature has a profound effect on an ectotherm’s physiological performance
• Desert iguanas can survive short exposures to body temperatures as low as 0ºC and as high as
o They can only function between about 15ºC - 45ºC
Within this range, cold iguanas run and digest slowly, tire quickly, and hear poorly
As they get warmer, they run and digest more quickly, tire more slowly, and hear
• The relationship between physiological performance and temperature is called a th