Chapter 13
Evolution and Natural Selection
Evolution
• Evolution is a change in
the frequency of
genetically determined
characteristics within a
population over time
• Microevolution occurs
when there are minor
differences in allele
frequency between
populations of the same
species
Evolution
• Macroevolution occurs
when there are major
differences that have
occurred over long
periods that have
resulted in so much
genetic change that
new kinds of species are
produced
Development of Evolutionary Thought
• For centuries, people believed that the various
species of plants and animals were unchanged
from the time of their creation
– Today we know this is not true, but they knew
nothing of DNA, meiosis, and genetics
Development of Evolutionary Thought
• In the mid-1700’s,Georges-Louis Buffon, a
French naturalist wondered if animals
underwent change (evolved) over time.
• Buffon had a student, Jean-Baptiste de Lamark
who suggested how these changes might
occurred. His ideas were known as acquired
characteristics.
Development of Evolutionary Thought
• Lamark’s theory of
acquired characteristics
states that traits gained
during an organism’s life
and transmitted to the
offspring.
• For example, giraffes had
short necks, but in order
to reach the leaves, their
necks stretching. When
these giraffes reproduced
their offspring acquired
their parent’s longer
necks.
Theory of Natural Selection
• Charles Darwin (1809-1882)
was the son of a physician
and member of a family
with a history scientific
curiosity.
• After briefly studying
medicine, he enrolled at
Cambridge University,
receiving his B.A. degree in
1831.
• After graduating, Darwin
left on a multi-year
expedition (1831-1836) on
the HMS Beagle, primarily
as a social companion to the
captain.
Natural Selection
• The purpose of the trip was
to chart poorly known
coastal waters off South
America.
• While the crew were
surveying, Darwin went on
many expeditions to collect
plants and animals on
mainland South America
and the islands which the
ship visited.
• Darwin noted that the
temperate species in South
America looked much more
like tropical South American
species than they did
temperate species from
Europe.
Natural Selection and Charles Darwin
• When the Beagle arrived at
the Galapagos Islands,
900km west of South
America, he observed many
new species, yet similar to
those on the mainland.
• This included 13 species of
finches, with dramatically
different beak
morphologies.
• Some were very
“unfinchlike” with bills like
insect-eating warblers.
• Some Galapagos finch
species were found only on
one island, others had
more than one.
Natural Selection
• After his return to England, Darwin
developed his ideas on evolution and
natural selection by 1844, but
delayed publishing.
• He spent 20 more years collecting
more observations to support his
theory.
• In 1858, Alfred Wallace, a naturalist
working in Indonesia, sent Darwin a
manuscript with essentially the same
theory as Darwin
• This prompted Darwin to finish his
book, On the Origin of Species by
Means of Natural Selection in 1859.
Origin of Species
• The first point of the
Origin of Species is that
all organisms are related
through descent from
some unknown ancestor
that lived in the past.
• Over time, species
changed via adaptations
which fit them to specific
ways of life.
• These ideas, he called
descent with
modification.
Origin of Species
• The second point in the Origin of
Species is that the dominant
mechanism for evolution of species
is natural selection.
• Darwin argued that evolution of
species in the natural world was
analogous to artificial selection of
domesticated animals and plants.
• Individuals with desired
characteristics are breed with other
individuals with desired
characteristics.
• This can produce tremendous
changes in the phenotype of a
species in a relatively short period
of time.
Origin of Species
• The theory of natural selection is based on the
following assumptions about the nature of living
things:
1. All organisms produce more offspring than survive.
2. No two organisms are exactly alike.
3. Among organisms, there is a constant struggle for
survival.
4. Individuals that possess favorable characteristics for
their environment have a higher rate of survival and
produce more offspring.
5. Favorable characteristics become more common in
the species, and unfavorable characteristics are lost.
Natural Selection - Giraffes
• Using Darwin-Wallace theory with
giraffes
1. In each generation more giraffes would
be born than the food supply could
support.
2. In each generation, some giraffes
would inherit longer necks, and some
would inherit shorter necks.
3. All giraffes would compete for the
same food source.
4. Giraffes with longer necks would
obtain more food, have a higher
survival rate, and produce some
offspring.
5. As a result, succeeding generations
would show an increase in the
number of individuals with longer
necks.
Natural Selection and Evolution
• The gene pool of a species is modified by:
– chance events in small populations (founder effect, genetic
drift, genetic bottlenecks)
– immigration and emigration
– Mutations
– Natural selection
• As a result, the gene pool in the next generation differs from
that of the previous generation - microevolution.
Natural Selection – Darwin’s Finches
• Darwin hypothesized that differences in bill-size lead to
differences in the success of Galapagos finches when
feeding.
• Detailed research over the last 30+ years have shown
that bill size varies among finches and is inheritable
(genetically controlled).
• During drought years in the Galapagos, the local plants
produce fewer seeds than during normal, wet years.
• All the finches prefer to eat the smaller, easier to
handle seeds first but soon they are all gone.
• All that remain are the larger seeds, which are harder
to open.
• Larger billed birds can open these larger seeds, but
smaller billed birds cannot.
Natural Selection – Darwin’s Finches
• Larger billed birds survive drought better and
leave more offspring - more fit.
• The alleles that favor large bills become more
common among the next generation.
• The average size of birds and their bills increases
in the population.
• Larger billed birds are less fit during wet years
when small seeds are abundant.
• Larger billed birds are less
efficient when feeding on
small seeds.
Hardy-Weinberg Equilibrium
• The Hardy-Weinberg equilibrium allows us to predict
the frequencies of alleles in the offspring if we know
the frequency of alleles in the parental population.
• If a population demonstrates genotype frequencies
from generation to generation that are consistent
with Hardy-Weinberg, then it is NOT evolving.
Hardy-Weinberg Equation
• The Hardy-Weinberg Equation is p2 + 2pq +q2 = 1
• p2 stands for the frequency of homozygous dominant
alleles
• 2pq stands for the frequency of heterozygous alleles
• q2 stands for the frequency of homozygous recessive
alleles
Hardy-Weinberg Equilibrium
• Conditions necessary for gene frequencies to
remain constant are the following:
1. Mating must be completely random
2. Mutations must not occur
3. The migration of individual organisms into and
out of the population must not occur.
4. The population must be very large
5. All genes must have an equal chance of being
passed onto the next generation
Effects of Natural Selection
• In natural selection,
particular phenotypes
match (or don’t) the local
environmental
conditions.
• Some phenotypes do
well, live long, and
produce abundant
offspring (and other
don’t).
• If these phenotypic
characters have a genetic
basis, individuals that
have favored alleles will
contribute more alleles
to the next generation.
Effects of Natural Selection
• In stabilizing selection,
the environment favors
individuals at the
middle of the range of
phenotypes, reducing
phenotypic variation.
• With the range of color
possibilities, the ones
that do not blend into
the environment, will
be more likely to be
killed by predators.
Effects of Natural Selection
• In directional selection, the
environment favors
individuals at the one end of
the range of phenotypes,
shifting phenotypic variation.
• For example, periods of
drought favor larger
individuals among Darwin’s
finches in the Galapagos.
• Bigger birds compete better
for scarce food, survive
better, and leave more
offspring.
• During wetter periods, the
opposite is true.
Effects of Natural Selection
• In diversifying selection,
the environment selects
against average
individuals and favors the
extremes, increasing
phenotypic variation.
• Melanistic moths are
either light colored or
dark colored to match
light or dark tree bark.
• Intermediate colors
match neither
background and increase
predation by birds.