Processes of Evolution
How gene pools change across generations
The wisdom tooth question
• Remember this question?
• Some evidence exists that over several
centuries, the number of people born
with small wisdom teeth or no wisdom
teeth has increased. Using your best
understanding of Natural Selection,
explain how selection could cause this.
How selection works:
• Variation exists
• Traits are inheritable
• Some traits have more survival value
than others (differential survival).
• Some individuals have a better chance
of reproduction than others and a better
chance of passing on their traits
(differential reproduction).
Is this selection?
• “Humans in the past needed wisdom
teeth, but since our way of eating is
different, we no longer need them.
Because we no longer need them,
they’re getting smaller.”
Is this selection?
• “Our parents may have had genes for
large wisdom teeth. Because they’re too
big for the child, the child may get a
mutation that causes him to have smaller
wisdom teeth.”
Is this selection?
• “Way in the past two people who had
small wisdom teeth probably mated and
since then people have had smaller
wisdom teeth.”
Is this selection?
• “We inherited wisdom teeth from our
ancestors even though they have no
purpose. Because they have no purpose,
the genes are recessing from our
population.”
Is this selection?
• “People could be born with smaller or no
wisdom teeth because of natural
variation that exists. Sometimes the
mouth is too small to contain the wisdom
teeth without proper dentistry, so people
with large wisdom teeth may die from
impactions and infections.”
Natural Selection
• There must be variation in the original population:
• Variation in size of wisdom teeth
• A few people are born without wisdom teeth (random
mutation)
Natural Selection
• There must be
differential survival
(selection) due to the
variation:
• People with large
wisdom teeth are
more likely to get
impacted teeth, which
can get infected.
Infections can be fatal.
Natural Selection
• There must be differential reproduction:
• Young people who have impacted
wisdom teeth may die of an infection
before they reproduce. Their genes
that produced large wisdom teeth are
not passed on.
Thinking Question:
• People in Western culture often have access
to good dental care, and can have wisdom
teeth removed before they cause problems.
• How does this affect the process of natural
selection?
• Will wisdom teeth continue to get smaller in
Western nations? (The words “need” and
“purpose” should not be used in your
answer!)
Antibiotic Resistance
• We will watch a short video on the rise of
antibiotic resistance.
• At the end, you should be able to explain
how natural selection works has caused
the rise in antibiotic-resistant bacteria.
(Note it is bacteria that become resistant,
not people. Bacteria do not become
“immune” — they do not have immune
systems.)
Thinking Question:
• Use the principles of natural selection to
explain antibiotic resistance. Be sure to
include these in your answers:
• Variation in the original population.
• Differential survival.
• Differential reproduction
• “Need,” “purpose,” and “immune” should
not be in your answer!
Genes and Evolution
• Genes are the units of heredity.
• Genes code for proteins, which result in
our set of traits.
• Genes are passed from parent to
offspring through the sex cells.
Genes in Individuals
• Different “versions” of
genes are alleles.
• Dominant alleles are
expressed in the
phenotype (expressed
trait) even if only one
copy is inherited.
• Recessive alleles are
expressed only if two
copies are inherited.
“Genotype” is a description of the
alleles for a given trait in an
individual: BB, Bb, or bb
The Gene Pool Concept
• The “gene pool” of a population is the
entire collection of alleles for a given trait
throughout a given population.
• The word for all genes for all traits in an
individual or population is genome.
Hardy-Weinberg Equilibrium
• Allele ratios in a gene pool will not change
from generation to generation (that is, no
evolution) only if all these things are true:
• No mutation
• Large population
• No migration
• No selection
• Random mating
Mutations
• Small mutations appear randomly in
populations.
• The appearance of mutations changes
allele ratios by
• “breaking” functional alleles (as in
genetic disorders)
• adding new alleles
Population size
• In large populations, random events
have a very small effect.
• In small populations, because fewer
individuals have any given trait, random
events can have a larger effect. Changes
in gene ratios caused by random events
are called “genetic drift.”
A population bottleneck is genetic drift.
Founder effect is genetic drift.
Migration
• Migration into and out of a population
can change gene ratios.
• Immigrants can bring in new mutations,
or a different ratio of alleles.
• Emigrants may take away a high
proportion of a certain allele.
• Small population are more affected than
large populations.
Selection
• Selection may increase or decrease the
frequency of certain alleles:
• Directional selection: favors one end of
a range over another.
• Disruptive selection: disfavors the
midrange.
• Stabilizing selection: favors the midrange.
Mating Behavior
• Mate choice among most organisms is
selective, not random.
• Sexual selection may favor traits that are
in conflict with natural selection. For
example, bright-colored male guppies
attract more females, but are also more
visible to predators.
Evolution Happens
• Because perfect Hardy-Weinberg
equilibrium is never met with in nature,
all populations experience small shifts in
gene ratios with each generation.
• Gene ratio shifts may fluctuate with
cyclical changes in climate. Long-term
changes in habitat (such as global
climate change) can shift the gene ratios
far enough to bring about speciation.