Evolution
What Darwin Never Knew
Charles Darwin
 Born in England – 1809
 Sailed on the HMS Beagle’s five-year voyage mapping
the coastline of South America.
 Observed and recorded characteristics of species on the
trip
 Developed a scientific theory of biological evolution explains how modern organisms evolved over long
periods of time through descent from common
ancestors.
 Wrote On the Origin of Species
 Darwin's Finches
Darwin’s 3 patterns of biodiversity
 Species vary globally
 Species vary locally
 different, yet related, animal species often occupied
different habitats within a local area
 Species vary over time
Species vary globally
 Different, yet ecologically similar, animal species
inhabited separated, but ecologically similar, habitats
around the globe
 found flightless, ground-dwelling birds that were similar on
different continents.
 South America = rheas
 Africa = ostriches
 Australia = emu.
Species vary locally
 Different, yet related, animal species often occupied
different habitats within a local area
 Galapagos Islands – the shape of the tortoises’ shells corresponds to
different habitats.
 Isabela Island has high peaks, is rainy, and has abundant vegetation
that is close to the ground. A tortoise from Isabela Island has a domeshaped shell and short neck.
 Hood Island, in contrast, is flat, dry, and has sparse vegetation. A long
neck and a shell that is curved and open around the neck and legs
allow the Hood Island tortoise to reach sparse, high vegetation.
Species vary over time
 Darwin collected fossils = the preserved remains or
traces of ancient organisms.
 Darwin noticed that some fossils of extinct animals
were similar to living species.
 He discovered fossils of Glyptodont
where armadillos currently live.


Why did Glyptodont disappear
Why did they resemble armadillos?
Scientists who shaped Darwin’s
thinking
 Hutton and Lyell - geologists
 The earth is extremely old
 Processes that changed the past are the same that
operate in the present
Scientists who shaped Darwin’s
thinking etc.
 Lamarck – Although his ideas were
FLAWED they shaped Darwin’s thinking
 Organisms could change during their
lifetimes by selectively using or not using
various parts of their bodies
 Individuals could pass these acquired traits
on to their offspring, enabling species to
change over time.
 Organisms have an inborn urge to become
more complex and perfect, and to change
and acquire features that help them live
more successfully in their environments.
Scientists who shaped Darwin’s
thinking etc.
 Malthus = if the human population grew unchecked,
there wouldn’t be enough living space and food for
everyone
 War, famine and disease would work against population growth
 Darwin realized that most organisms don’t survive and
reproduce, he wondered which individuals survive…and
why?
Artificial selection
 Artificial selection = process in which nature provides
the variations, and humans select those they find
useful.
 Darwin put artificial selection to the test by raising and
breeding plants and fancy pigeon varieties.
16.3 Natural selection
 Natural selection = the process by which organisms
with variations most suited to their environment
survive and leave more offspring
 Natural selection occurs when there is
 1) A struggle for existence – more offspring are produced
than can survive
 2) Variation and adaptation
 3) Survival of the fittest
1) Struggle for existence
 Grasshoppers can lay more than 200 eggs at a time,
but only a small fraction of these offspring survive to
reproduce.
2) Variation and Adaptation
 Adaptation = heritable characteristic that increases an
organism’s ability to survive and reproduce in its
environment
 EX. Green color is an adaptation: The green grasshoppers
blend into their environment and so are less visible to
predators.
3) Survival of the Fittest
 Fitness = the ability of an organism to survive and
reproduce
 Because their color serves as a camouflage adaptation, green
grasshoppers have higher fitness and so survive and
reproduce more often than yellow grasshoppers do.
Natural Selection
 Green grasshoppers become more common than
yellow grasshoppers in this population over time.
 More grasshoppers are born than can survive
 Individuals vary in color and color is a heritable trait
 Green grasshoppers have higher fitness in this
particular environment
Natural Selection
 Natural selection does not make organisms “better”.
 It is a process that enables organisms to survive and
reproduce in a local environment.
 If local environmental conditions change, some traits
that were once adaptive may no longer be useful
 Salamander Evolution
Principle of common descent
 According to the principle of common
descent, all species—living and
extinct—are descended from ancient
common ancestors.
 Darwin proposed that, over many
generations, adaptation could cause
species to evolve into new species.
 He also proposed that living species
are descended, with modification,
from common ancestors—an idea
called descent with modification.
Evidence for Evolution
Age of the Earth and Fossils
 Radioactive dating of rocks
indicate that Earth is about
4.5 billion years old—plenty
of time for evolution by
natural selection to take
place.
 Since Darwin,
paleontologists have
discovered hundreds of
fossils that document
intermediate stages in the
evolution of many different
groups of modern species
Evidence for Evolution
Homologous Structures
 Homologous Structures – structures that are shared by
related species and are inherited from a common
ancestor
 For example, the front limbs of amphibians, reptiles,
birds, and mammals contain the same basic bones.
Evidence for Evolution
 Vestigial structures are inherited from ancestors,
but have lost much or all of their original function due
to different selection pressures acting on the
descendant.
 Ex. hipbones of bottlenose dolphins
 In ancestors, hipbones played a role in terrestrial
locomotion. As the dolphin lineage adapted to life at sea,
this function was lost
 Why would an organism possess structures with little
or no function?

The presence of a vestigial structure does not affect an
organism’s fitness. Natural selection would not eliminate it.
Evidence for Evolution
Embryology
 Similar patterns of embryological
development provide evidence that
organisms have descended from a
common ancestor.
 Researchers noticed a long time ago
that the early developmental stages of
many animals with backbones (called
vertebrates) look very similar.
 PBS embryology clip
 Which Embryo?
Evidence for Evolution
Genetics & Molecular Biology
 At the molecular level, the
universal genetic code and
homologous molecules provide
evidence of common descent.
 DNA and RNA carry
information from generation to
generation and to direct protein
synthesis
 Similar genes and proteins are
found in many organisms

Hox genes – determine the headto-tail axis in embryonic
development
Chapter 17 Evolution of
Populations
 Genetics in Evolution
 In genetic terms, evolution is any change in the relative
frequency of alleles in the gene pool of a population over
time
 Changes in genes and chromosomes generate variation
 Natural selection acts directly on phenotype, not genotype

Some individuals have phenotypes that are better suited to their
environment than others. These individuals produce more
offspring and pass on more copies of their genes to the next
generation.
Populations and Gene Pools
 A population = a group of individuals of the same species
that mate and produce offspring.
 A gene pool = of all the genes, including all the different
alleles for each gene that are present in a population.
 relative frequency = is the number of times a particular
allele occurs in a gene pool, compared with the number
of times other alleles for the same gene occur.
Populations evolve, not individuals
 Evolution is any change in the relative frequency of alleles
in the gene pool of a population over time.
 Natural selection operates on individuals, but resulting
changes in allele frequencies show up in populations.
Populations, rather than individuals, evolve.
Sources of Genetic Variation
 Mutation
 Mutations matter in evolution only if they can be passed
from generation to generation.
 The mutation must occur cells that produce eggs or sperm.
 Genetic Recombination in Sexual Reproduction
 Most hereditable differences are from recombination
 Independent assortment & crossing over during meiosis
 Lateral Gene Transfer
 Organisms pass genes from one individual to another that is
not its offspring
 It can occur between organisms of the same species or
different species.
How Natural Selection Works
 Evolutionary fitness = the success in passing genes to the
next generation.
 Evolutionary adaptation is any genetically controlled trait
that increases an individual’s ability to pass along its
alleles.
Single and Polygenic Traits
 The number of phenotypes produced for a trait depends
on how many genes control the trait.
 A single-gene trait is a trait controlled by only one gene

Single-gene traits may have just two or three distinct phenotypes.
 Polygenic traits are traits controlled by two or more genes.

A single polygenic trait often has many possible genotypes and
even more different phenotypes.
Natural Selection of single-gene
traits
 Natural selection on single-gene traits can lead to
changes in allele frequencies and, thus, to changes in
phenotype frequencies.
 Black lizards might be able to absorb sunlight. Higher body
temperatures may allow the lizards to move faster, escape
predators, and reproduce.
Natural Selection of Polygenic Traits
 Natural selection on polygenic traits can affect
the distributions of phenotypes in three ways:
 Directional Selection
 when individuals at one end of the curve have
higher fitness than individuals in the middle or at
the other end
 Stabilizing Selection
 occurs when individuals near the center of the
curve have higher fitness than individuals at
either end.
 Disruptive Selection
 occurs when individuals at the upper and lower
ends of the curve have higher fitness than
individuals near the middle
Genetic Drift
 Genetic drift = a random change in allele frequency that
occurs in small populations when an allele becomes more
or less common simply by chance.
 bottleneck effect = a change in allele frequency following a
dramatic reduction in the size of a population.
 founder effect = when allele frequencies change as a result
of the migration of a small subgroup of a population
Genetic Equilibrium
 Genetic equilibrium = allele frequencies in the
population remain the same = the population will not
evolve.
 Hardy-Weinberg principle = allele frequencies in a
population should remain constant unless one or more
factors cause those frequencies to change.
(Frequency of AA) + (Frequency of Aa) + (Frequency of aa) = 100%
(Frequency of A) + (Frequency of a = 100%)
Conditions that cause evolution to
occur
 5 Conditions that disrupt the Hardy-Weinberg principle
and cause evolution to occur
(1) Nonrandom mating
(2) Small population size
(3) Immigration and Emigration
(4) Mutations
(5) Natural selection
The process of Speciation
 Species = a population whose members can interbreed
and produce fertile offspring
 Speciation = the formation of a new species
Reproductive Isolation
 Reproductive isolation = occurs when a population splits
into two groups and the two populations no longer
interbreed.
Isolating Mechanisms
 Behavioral Isolation = populations develop differences in
behavior or courtship rituals
 Eastern and western meadowlarks can mate, but their songs are
different and don’t
 Geographic Isolation = populations are separated by a barrier
 The Albert’s squirrel isolated by the formation of the Grand Canyon
 Temporal Isolation = reproduce at different times
 Orchids have flowers that last one day and must be pollinated on
that day
Molecular Clocks
 Molecular clock = uses mutation rates in DNA to estimate the
time that two species have been evolving independently.
 Neutral mutations tend to accumulate in the DNA of different
species at about the same rate.
 The more differences between the DNA sequences, the more
time has elapsed since the two species shared a common
ancestor.