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More on Natural Selection
Bio Standard 8a, 7a, b, and c
• Orgs differ by DNA they
possess.
• DNA: code that forms your
traits.
• DNA makes up genes- set of
instructions for one trait.
• Chromosomes carry the
genes.
• Some traits are dominant
(shows up in offspring) or
recessive (doesn’t show).
DNA
Chromosomes
Genotype and Phenotype
• Genotype: genes/DNA
that an org contains for
a trait.
• Phenotype:
observable/physical
traits of an individual.
• Natural selection works
on phenotypes, what
org looks like.
• States that individs whose
characteristics are well-suited
to the environment will
survive.
•  Survival of the Fittest.
To Work…
Populations
have:
1) Variation
among
individuals
2) Pop. size had to increase
3) Traits Are Passed on to
Offspring
To Work..4) Struggle for
Survival
Peppered Moth Story:
Natural Selection Example
• England: 2 types of moths
(same species): black and
white.
• White: abundant, black: rare.
Peppered Moth Story:
Natural Selection Example
• Trees the moths
lived on were
whitish colored.
• Predatory birds
could only see
dark moths to eat.
 Huge white
moth pops.
Then, the Industrial
Revolution happened…
Coal was burned…created
soot…
 Trees
lined
with
soot…
SO…Now
who is best
fit for this
environment
??
• Black moths now camouflaged w/
trees.
• Black moths  abundant, passed
on genes to offspring.
•Survival of the fittest!
•Natural selection works on an
individual’s phenotype (physical
features), not its genotype (DNA)
Types of Natural Selection
1) Stabilizing
Selection:
Selective
pressures select
against the 2
extremes of a trait.
• Ex: it is better to be
average!
Types of Natural Selection
2) Directional
selection: one
extreme of the
trait experiences
selection against
it.
• Ex: giraffes w/
short necks
Types of Natural Selection
3) Disruptive selection:
Selection pressures
act against individuals
in the middle of the 2
extremes of the trait.
• Ex: it’s not good to be
average
Peppered Moth Virtual Lab