3.Disruptive selection- when individuals with extreme phenotypes at either
end of the population distribution have higher fitness than individuals with
average phenotypes. This tends to increase genetic and phenotypic variation.
(though it is uncommon)
•Example in textbook: industrial melanism peppered moth demonstrates genetic
change in response to darkened branches to avoid predation (camoflauge) illustrates
natural selection and evolutionary response over a short period of time.
•Population genetics: method for predicting changes in gene frequency in response
to selection (the potential for evolution exists in all populations).
•Example: cactus wren has no source of water, so must avoid gaining too much
heat from its environment. only live in Arizona desert. The orientation of the cactus
wren changes seasonally, so the nest is facing away form the cool winds in winter, and in
the summer, is facing toward the cool wind.
•Reaction norm: observed relationship b/w the phenotype and the environement.
Example: heat energy heat accelerates most life processes. The butterfly grow faster
at high temperatures. However, different environments differ in this relationship.
•Examples of acclimization: growing thicker fur, producing smaller leaves,
increasing number of red blood cells, and producing enzymes that remain fluid at diff
•Process of modifying the body’s structure and metabolic machinery. (is reversible!)
•However, increased tolerance to one extreme brings reduced tolerance to another.
•Textbook example: creoste bush change temperature optima with seasons. Their
acclimization results from changes in the viscosity of the membranes directly involved in
•A species capacity for acclimization may reflect the range of conditions in its
•Developmental responses: when the attribute of the indivudal is modified for a
long time, even its whole lifetime. Example: African grasshopper change colours with
response to seasons to hide from predators this change is irreversible!
•Genotype-environment interaction – when a genotype performs better in one
environment than the other. these relationships can be revealed by reciprocal
•An example of the experiment: the growth rate of fence lizards reveal both genetic
determination and phenotypic plasticity. When there are fewer resources, lizards cannot