Evolution
Unit
AP Biology
What is Evolution?
• Change in the genetic makeup of a population
over time.
• Fitness – those with favorable variations for
survival and reproduction.
– Populations can evolve, not individuals.
• Diverse gene pool good for long-term survival
of a species. Genetic variations are important!
• How do genetic variations occur?
Where does Variation come from?
 Mutation

random changes to DNA
 errors in mitosis & meiosis
 environmental damage
 Sexual reproduction

mixing of alleles
 genetic recombination
 new arrangements of alleles in every offspring
 new combinations = new phenotypes
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Genetic variation in a population
Essence of Darwin’s ideas
 Natural selection
heritable variation exists in populations
 over-production of offspring

 more offspring than the environment can support

competition
 for food, mates, nesting sites, escape predators

differential survival
 successful traits = adaptations

differential reproduction
 adaptations become more
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common in population
Lamarckian vs. Darwinian view
 LaMarck

in reaching higher
vegetation giraffes
stretch their necks &
transmits the acquired
longer neck to offspring

 Darwin

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giraffes born with longer
necks survive better &
leave more offspring who
inherit their long necks
Natural Selection
•
•
•
•
Major mechanism of evolution
Environment is always changing
Acts upon the phenotype of the population
Based on Darwin’s idea that resources are limited
and that there is competition for those resources.
• Adaptation = a genetic variation favored by
natural selection.
• When allele frequencies shift, speciation occurs
– Thus, the frequency change is NOT RANDOM
Effects of Selection
 Changes in the average trait of a population
DIRECTIONAL
SELECTION
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giraffe neck
horse size
STABILIZING
SELECTION
DISRUPTIVE
SELECTION
human birth weight
rock pocket mice
Natural selection
in action
Resistance…
NOT immunity!
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MRSA
Heterozygote Advantage
 Keeps the recessive
allele in the population
 Ex: Sickle Cell Anemia



aa – dies of sickle cell
anemia
Aa – some side affects
BUT resistant to malaria!
AA – no disease present
BUT prone to malaria
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Hidden variations can be exposed through selection!
Terminal
bud
Lateral
buds
Cabbage
Artificial selection
Brussels
sprouts
Leaves
Flower cluster
Kale
Cauliflower
Stem
Flower
and
stems
Broccoli
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Wild mustard
Kohlrabi
In addition to natural
selection, evolutionary
change is also driven
by random processes…
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Genetic Drift
 Chance events changing frequency of
traits in a population

not adaptation to environmental conditions
 not selection

founder effect
 small group splinters off & starts a new colony
 it’s random who joins the group

bottleneck
 a disaster reduces population to
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small number & then population
recovers & expands again but
from a limited gene pool
 who survives disaster may be random
Ex: Cheetahs
 All cheetahs share a small number of alleles

less than 1% diversity
 2 bottlenecks

10,000 years ago
 Ice Age

last 100 years
 poaching & loss of habitat
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Conservation issues
 Bottlenecking is an important
Peregrine Falcon
concept in conservation
biology of endangered
species
loss of alleles from gene pool
 reduces variation
 reduces adaptability

Breeding programs must
consciously
outcross
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Golden Lion
Tamarin
Human Impact on variation
 How do we affect variation in other
populations?

Artificial selection/Inbreeding
 Animal breeds

Loss of genetic diversity
 Insecticide usage

Overuse of antibiotics
 resistant bacterial strains
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