Download Types of Natural Selection

TYPES OF NATURAL SELECTION
Pgs 324-325
EVOLUTION
•Evolution is the change in a population’s genetic material (alleles)
over generations
•Genotype frequencies do not stay the same
•A population that is not evolving is in genetic equilibrium (HardyWeinberg equilibrium)
•Genotype frequencies stay the same
THREE TYPES OF NATURAL SELECTION
•Organisms best suited to their environment live to reproduce and
pass on their genes
•Acts on a phenotype
•Varying types of selection
1. DIRECTIONAL SELECTION
•Individuals with a more extreme form of trait have higher fitness
•Shifts normal distribution to the right or left
Graph Key
Red: before selection
event
Blue: after selection
event
Suppose termites in an
area begin to build
deeper nests. Anteaters
with long tongues could
more effectively prey on
termites than those with
short or average tongue
length
2. STABILIZING SELECTION
•Having average form of trait has highest fitness
•Normal distribution becomes narrower, see less individuals at the
extremes
Blue curve shows variation after a new
predator is introduced. Predator can
easily capture the large, visible lizards
and the small slower lizards. Thus,
selection against these extremes body
types reduces the size range in lizards.
3. DISRUPTIVE SELECTION
•Individuals with any extreme trait has a higher fitness and most
likely results in two new species
•Now have 2 curves in distribution
•We also call this divergent evolution
The white limpets blend
in with barnacles on
rocks. On bare rocks,
dark-shelled limpets
blend in. However, the
tan colored limpets are
easy to spot by birds.
PRACTICE- ON YOUR WHITE BOARDS
Oyster shells come in a variety of colors ranging from light brown
to dark brown, with most being medium brown. The rocks where
they live are medium brown in color. Some birds fly into the area
and start eating oysters. How will the distribution curve of oyster
shell color change?
PRACTICE ON YOUR YOUR WHITE BOARDS
Light-colored oysters in blend into the lighter colored rocks by the
shore. The dark-colored oyster blend into the shadows cast by
larger rocks on sunny days. A group of birds flies in and settles in
this area. How will the distribution curve change?
PRACTICE- ON YOUR WHITE BOARDS
Oysters have shells of different strengths. Most oysters have
medium strength shells, but some have thin, weak shells, and a few
have thick strong shells. A population of crabs moves into the area
and they are able to open weak and medium strength shells. How
will the distribution of shell strength change?