Download Speciation - Mission Hills High School

Types of
Natural Selection
Chapter 13
Standard Gene Pool
This is a standard bell curve graph for a gene
pool
 What does this graph show us?
Frequency of Phenotype

Phenotype (height)
Directional Selection

Natural selection can alter a
trait in a gene pool by
pushing it in one direction
Directional Selection Example
Stabilizing Selection

Natural Selection chooses the average, squeezing the
bell curve into the middle
Stabilizing Selection
Key
Low mortality,
high fitness
High mortality,
low fitness
Birth Weight
Selection
against both
extremes keep
curve narrow
and in same
place.
Stabilizing Selection Example
Disruptive Selection

The bell curve is split pushing the
population in 2 separate directions
Disruptive Selection
Low mortality,
high fitness
High mortality,
low fitness
Population splits
into two subgroups
specializing in
different seeds.
Beak Size
Number of Birds
in Population
Key
Number of Birds
in Population
Largest and smallest seeds become more common.
Beak Size
Genotype vs. Phenotype

In all 3 types of selection, nature chose
based on phenotype; NOT genotype