Chapter 10- Studying Adaptation: Evolutionary Analysis of Form and Function
Explanation of organismal design is among the triumphs of the theory of evolution by natural selection
Individuals in previous generations varied in their design, and the ones with the best designs passed on their genes in
A trait, or integrated suite of traits, that increases the fitness of its possessor is called an adaptation and is said to be
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:
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 opportunities.
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 circumstances
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.
10.3- Observational Studies
When an experiment is impractical, a careful observational study may be the next best method for evaluating a hypothesis.
Behavioral Thermoregulation Vast majority of organisms are ectothermic, means that their body temperatures are determine by the temperatures of
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 about 47ºC
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 more keenly
The relationship between physiological performance and temperature is called a thermal performance curve
Given the s