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How Organisms Evolve
The Theory of Evolution
Chapters 14-15
• Evolution is the process of change in the
inherited traits of a population of
organisms from one generation to the
next.
– The inherited traits that are the basis of
evolution come from the genes that are
passed on to offspring during reproduction.
Evolution can be traced through
the fossil record.
The Theory of Evolution
• There are two major mechanisms driving
evolution:
– Natural selection is a process causing inheritable
traits that are helpful for survival and reproduction
become more common in a population, and
harmful traits to become more rare.
A comparison of
skeletal structures
shows how the
whale has evolved
from smaller land
animals.
large
ground
finch
small tree
finch
woodpecker
finch
Evolution by Natural Selection
leaves behind evidence of ancestors.
Evolution by Natural Selection
leaves behind evidence of ancestors.
Human tail
Ostrich
wings
Snake
legs
Cave salamander eyes
Vestigial structures are
remnants from ancestors
that no longer serve a purpose.
Natural Selection on short time scales
Dog thumbs
Natural
Selection
leads to
Convergent
Evolution
Anole lizard adapts
shorter legs over 14
years when introduced
to islands with shorter
vegetation than their
natural habitat.
Human ear muscles
Isolated animals in
Australia evolved
similar adaptations to
mainland animals.
Independent evolution
of traits for a common
purpose or niche is
convergent evolution.
The Theory of Evolution
• An individual does not evolve.
• A population, however, can change its
phenotype over time.
• Evolution is a change in the genetic
make-up of a population over
generations.
– A population is a group that includes all the
members of a species living in a given area.
The Hardy-Weinberg Principle
• There must be no mutation.
• There must be no gene flow between populations.
– No movement of alleles into or out of the population.
• The population must be very large.
• All mating must be random.
• There must be no natural selection.
– All genotypes must reproduce with equal success.
Violation of any of these conditions
will cause a population to evolve.
Understanding Evolution
• To understand evolution, we must first
understand under what circumstances a
population will not evolve.
– The Hardy-Weinberg Principle describes the
conditions that cause an equilibrium population.
• A population that is not evolving, in which allele
frequencies remain the same.
Mutations cause Evolution
• Mutations are changes in the DNA
sequence of a gene.
– Most mutations arise during DNA replication.
• Inherited mutations are rare but important.
– Mutations occur in roughly 1 out of every
100,000 gametes produced.
• ~1 gene out of 60,000 alleles in a human.
– Mutations are the source of new alleles.
Mutations cause Evolution
• Mutations are not goal directed.
– Mutations arise randomly, not because of an
environmental need.
Antibiotic resistance in
bacteria occurs spontaneously
in only a few individuals, not
because the bacteria were
exposed to the antibiotic.
Gene Flow accelerates Evolution
• Flowering plants use pollen and seeds to
exchange genes.
Pollen contains sperm cells
which may be carried long
distances by wind or animal
pollinators.
When the pollen reaches
flowers of a new population,
its alleles are integrated into
that new population.
Gene Flow accelerates Evolution
• Gene flow is the movement of alleles between
populations.
• New alleles enter populations when
emigrating individuals interbreed with the
existing population.
In baboon troops, juvenile
males migrate away from
their troop to find a new
troop to join, bringing their
gene pool with them.
Population Size affects Evolution
• The size of a population can both hinder
and accelerate evolution.
• Events often occur that allow only a
small subset of the population reproduce,
limiting the number of alleles available.
– These chance events that change allele
frequencies cause genetic drift.
– Genetic drift describes a change in allele
frequencies that occurs entirely from
chance.
Population Size affects Evolution
• Genetic drift has a much larger affect on
small populations than larger ones.
Population Size affects Evolution
• Population bottlenecks are an example
of genetic drift.
– In a population bottleneck, a population
size is drastically reduced such that only a
few individuals.
Only the alleles of the
survivors contribute to
the genetic diversity of
the next generation.
Population Size affects Evolution
Population Size affects Evolution
• Population bottlenecks are an example
of genetic drift.
• Population bottlenecks are an example of
genetic drift.
• The Northern Elephant Seal was hunted to
almost extinction in the 1800s.
• The cheetah was once found on 5 continents.
It is now an endangered species.
By the 1890s, only 20
individuals survived.
Though now there are
about 30,000 individuals
in the wild, genetic
analysis shows they are
nearly identical.
Cheetahs
Loss of habitat, conflict with
humans, as well as its own loss
of genetic variation have
threatened its survival.
The genetic inbreeding in
cheetahs has led to low
survivorship, poor sperm
quality, and greater
susceptibility to disease.
Population Size affects Evolution
Population Size affects Evolution
• Population bottlenecks are an example
of genetic drift.
• Population bottlenecks are an example
of genetic drift.
• Isolated colonies founded by a small number
of organisms can lead to founder effects.
• Isolated colonies founded by a small number
of organisms can lead to founder effects.
The fig wasp carries
fertilized eggs to a particular
fig variety where its offspring
hatch, mate, and search for
a new fig to lay eggs.
Each type of fig has its own
genetically unique fig wasp
species.
Mating enhances Evolution
• Mating within a population is almost
never random.
• Non-random mating has
large effects on the
distribution of different
genotypes within a
population.
– In animals, individuals
have preference for
particular traits.
Mating display of a male frigatebird.
One couple among the
original 200 Amish to
immigrate to Lancaster, PA
had a defective allele
giving their children short
limbs and extra fingers.
Because the founding population was rather
small, this syndrome is now found in 14% of
this Amish population.
Mating enhances Evolution
• Non-random mating leads to sexual selection.
Male mandrill
baboon displaying his teeth and rear end as
a show of aggression to gain female favor.
Mating enhances Evolution
Mating enhances Evolution
• Non-random mating leads to sexual selection.
• Non-random mating leads to sexual selection.
Males fighting for attention from
picky females leads to sexual
dimorphism.
Male fringehead fish competing for female attention.
Mating enhances Evolution
• Mating within a population is almost never
random.
• Sometimes the pressure to assert dominance
for female favor has its drawbacks.
The long neck of the
giraffe allows it to
exert dominance
over other males, but
also makes it rather
difficult to drink.
Natural Selection leads to Evolution
• Not all genotypes are equally beneficial.
• Natural selection is the process by which
favorable traits become more common in
a population.
The long neck of the
giraffe is beneficial for
accessing hard-toreach leaves for food.
Natural Selection leads to Evolution
Principles of Natural Selection:
– It does not cause genetic changes in individuals.
– Though natural selection acts on individuals, it
is populations that are changed by evolution.
– Evolution leads to unequal success at
reproduction among organisms bearing different
alleles.
– Evolution is not progressive; it does not make
organisms “better”.
Sexual Selection
vs
Natural Selection
Why do females prefer
elaborate ornaments?
Runaway Sexual
Selection: males have
the trait, females have
the preference.
Natural Selection leads to Evolution
• Natural selection is often termed “survival
of the fittest”.
– Fitness is the reproductive success of an
organism.
This jewel
cichlid has high
fitness, judging
from the babies
seen.
Sexual Selection
vs
Natural Selection
Satin bowerbirds
Why do females prefer
elaborate ornaments?
Honesty Advertisement:
the trait is directly advertising
a good gene or a good parent.
Sexual Selection
vs
Natural Selection
long-tailed widowbird
• Natural selection is often termed “survival
of the fittest”.
– Not all genotypes are equally beneficial.
Why do females prefer
elaborate ornaments?
Handicap Hypothesis: With
this handicap, I’m still here!
Natural Selection leads to Evolution
– Antibiotic resistance evolves by natural
selection.
Satin bowerbirds
Before the introduction of penicillin (World War II), very
few resistant bacteria were known.
The resistance came from a mutated digestive enzyme, capable
of breaking down penicillin.
After a few generations, the frequency of the penicillindestroying allele is 100%.
Antibiotic resistance evolves by
Natural Selection.
• Methicillin-resistant Staphylococcus aureus
(MRSA) is resistant to beta-lactam antibiotics
including the penicillins and cephalosporins.
• S. aureus is a common bacteria that infects skin,
blood, and respiratory systems.
• MRSA causes the same diseases as
normal S. aureus but cannot be
retarded by common antibiotics.
• Our introduction of antibiotics into
the environment (food, water, soil)
has selected for resistant strains.