Chapter 8. Adaptation

Chapter 8. Adaptation

Adaptation = a trait that increases fitness of its possessor. Also, the process that results in this condition. The only evolutionary force that can create adaptation is natural selection.
Some traits are obviously adaptive:

If we claim that a trait is adaptive we need to show that it increases fitness in an environment and also must be able to construct a plausible retrospective Darwinian scenario. This is harder then it sounds, as the following examples show:

Polar bear's white fur as an adaptation to provide camouflage during seal hunting.

Polar bears do not seem to hunt so that camouflage is beneficial (sit-and-wait ambush, crushing seal lairs)
There is an alternative explanation - hairs could function to increase solar heating.

Giraffes do not seem to exploit their long necks, preferring to feed at shoulder height and on low foliage (Fig. 8.2).
There is an alternative explanation - sexual selection on males (males with long/stout necks and massive skulls) prevail in male combats over females, females are more receptive to this class of male (Table 8.1).

Not all differences among populations reflect adaptations to different selective presures. Some may be the result of drift (e.g. the founder effect).
It is not always easy to decide what a trait is "for". Although long necks may occasionally help giraffes feed high in the canopy, the trait may have evolved for a completely different reason.
Not every adaptation is perfect: there are tradeoffs and historical constraints.
Natural selection always lags behind environmental change. The traits observed in a population today are the result of natural selection acting in the past.

Adaptations can be studied by means of experiments, field observations and by comparisons among species when their phylogenetic relations are known. Following is an example of an observational study.

Behavioral thermoregulation in ectotherms (organisms whose body temperature depends on environmental temperatures)

Desert iguanas in the laboratory prefer sites with environmental temperatures which provide them with body temperatures that maximize several physiological functions. Field-captured individuals also have temperatures in the optimal range (Fig. 8.8).
Garter snakes choose rocks to burrow under in such a fashion that their body temperatures are maintained close to the optimum (Fig. 8.9, 8.10, Table 8.2)

Phenotypic plasticity = same genotype produces different phenotype in different environments. Examples abound, but are they adaptive?
Phototactic behavior in Daphnia (Fig, 8.17): there is genetic variance for the trait, so it is vulnerable to natural selection. The most plastic clones are those with a history of exposure to fish (selective agent).

Tradeoffs - if a trait increases fitness in component of an organism's biology, it may decrease fitness elsewhere. Neither component will be optimized. Examples:

Developmental constraints may prevent optimization of a trait: Fuchsia excorticata (Fig. 8.24, Table 8.3) retains its flowers after they have been fertilized, but advertises this fact by having flowers turn from green to red. Why not just drop the flowers? Answer - flowers must be retained long enough for pollen tube to reach ovary.
Lack of genetic variation may prevent optimization of a trait or evolution of new traits. If a lake dries out, fish don't evolve into amphibians -- they die!