5 Steps to a 5 AP Biology, 2014-2015 Edition

140 ❯ STEP 4. Review the Knowledge You Need to Score High

Adaptations

Anadaptationis a trait that if altered, affects the fitness of the organism. Adaptations are

the result of natural selection and can include not only physical traits such as eyes, finger-

nails, and livers but also the intangible traits of organisms. For example, lifespan length is

an adaptation, albeit a variable one. Mating behavior is also an adaptation—it has been

selected by natural selection because it is an effective strategy. An individual with a differ-

ent form of mating behavior may do better or worse than the average, but a change is likely

to have some effect on reproductive success. For example, individuals whose mating strat-

egy is to attempt to court women by running at them, arms flailing while screaming wildly,

and salivating heavily, do worse than the average male.

Let’s take a look at how such a behavioral adaptation can evolve. Reproductive matu-

rity is a good example. Female chimpanzees become reproductively mature at around the

age of 13. Females that mature at age 12 spend less time growing and may therefore be

more susceptible to problems with pregnancy. Females that mature at 14 have lost valu-

able time—their earlier-maturing peers have gained a year on them. You can imagine that

from generation to generation, females that matured at age 13 became better represented

in the population compared to faster and slower maturers. Although there will always be

individuals that differ from the mode, we can view age at reproductive maturation as an

adaptation.

Types of Selection

Natural selection can change the frequencies of alleles in populations through various

processes. The most commonly described are the following three:

1. Directional selection.This occurs when members of a population at one end of a spec-
   trum are selected against, while those at the other end are selected for. For example,
   imagine a population of elephants with various-sized trunks. In this particular environ-
   ment, much more food is available in the very tall trees than in the shorter trees.
   Elephants with what length trunk will survive and reproduce the most successfully?
   Those with the longest trunks. Those with shorter trunks will be strongly selected
   against (and those in the middle will also be in the middle in terms of success). Over
   time we expect to see an increasing percentage of elephants with long trunks (how
   quickly this change occurs depends on the strength of selection—if all the short-trunked
   elephants die, we can imagine that the allele frequencies will change very quickly).
   (See Figure 12.1.)


2. Stabilizing selection.This describes selection for the mean of a population for a given
   allele. A real example of this is human infant birth weight—it is a disadvantage to be
   really small orreally big, and it is best to be somewhere in between. Stabilizing selec-
   tion has the effect of reducing variation in a population (see Figure 12.1).


3. Disruptive selection.Also known as diversifying selection,this process can be regarded as
   being the opposite of stabilizing selection. We say that selection is disruptive when
   individuals at the two extremes of a spectrum of variation do better than the more
   common forms in the middle. Snail shell color is an example of disruptive selection.
   Imagine an environment in which snails with very dark shells and those with very light
   shells are best able to hide from predators. Those with an in-between shell color are
   gulped up like escargot at a cocktail party, creating the double-hump curve seen in
   Figure 12.1.

KEY IDEA

Mike (freshman
in college):
“Learn these
selection types...
they make good
multiple-choice
questions.”