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15
Mechanisms of Evolution
Chapter 15 Mechanisms of Evolution
Key Concepts
• 15.1 Evolution Is Both Factual and the Basis of Broader Theory
• 15.2 Mutation, Selection, Gene Flow, Genetic Drift, and
Nonrandom Mating Result in Evolution
• 15.3 Evolution Can Be Measured by Changes in Allele
Frequencies
• 15.4 Selection Can Be Stabilizing, Directional, or Disruptive
• 15.5 Genomes Reveal Both Neutral and Selective Processes of
Evolution
• 15.6 Recombination, Lateral Gene Transfer, and Gene Duplication
Can Result in New Features
• 15.7 Evolutionary Theory Has Practical Applications
Chapter 15 Opening Question
What evidence do biologists use to
confirm the theory of evolution?
Concept 15.1 Evolution Is Both Factual and the Basis of Broader
Theory
Theory—In everyday speech, an untested hypothesis or a guess.
Evolutionary theory is not a single hypothesis, but refers to our
understanding of the mechanisms that result in genetic changes in
populations over time.
Use that understanding to interpret changes in and interactions
among living organisms.
Evolution is based on factual evidence-genetics, fossils, phylogeny,
morphology, embryology.
Figure 15.2 Milestones of Evidence in the Development of Evolutionary Theory
Chapter 15 Example of Micro Speciation video on finches
Concept 15.1 Evolution Is Both Factual and the Basis of
Broader Theory
Biological populations
change over time, or evolve.
Evolutionary change is
observed in laboratory
experiments, in natural
populations, and in the fossil
record.
Even before Darwin, biologists had suggested that species had changed over
time, but no one had proposed a convincing mechanism for evolution.
Charles Darwin was interested in
geology and natural history.
Figure 15.1 The Voyage of the Beagle
Concept 15.1 Evolution Is Both Factual and the Basis of Broader
Theory
From the observations and insights made on the voyage, and new
ideas from geologists on the age of the Earth, Darwin developed
an explanatory theory for evolutionary change:
• Species change over time.
• Divergent species share a common ancestor.
• The mechanism that produces change is natural selection.
CHARLES RETURNS HOME
BECAME VERY POPULAR DUE TO ALL THE
SPECIMENS, SAMPLES HE SENT BACK
HE BECAME FRIENDS WITH Charles Lyell, a
GEOLOGIST
Lyell stated “The Earth must be very old, since
geological formations are slow & gradual.”
Charles
Lyell
. INFLUENCES ON DARWIN
A. CHARLES LYELL & FARMERS
DARWIN thought
GEOLOGICAL changes
could change PLANT &
ANIMAL forms CHARLES
LYELL
He observed farmers
breeding animals for
SELECTIVE TRAITS ARTIFICIAL SELECTION
His Mother bred fancy
pigeons.
Darwin suspected Selection
also OCCURRED in
NATURE -Natural Selection
B. THOMAS MALTHUS 1766-1834
CLERGYMAN who wrote about ECONOMICS ESSAY
“PRINCIPLE OF POPULATION”
He said “THE HUMAN POPULATION WAS GROWING SO
FAST THAT RESOURCES WOULD SOON RUN OUT,
PEOPLE WOULD DIE DUE TO DISEASE, WAR & OTHER
DISASTERS”
THOMAS MALTHUS CONT.
• “PLANTS & ANIMALS PRODUCE FAR MORE
OFFSPRING THAN CAN SURVIVE”
• He even suggested that lower class family
size be regulated so they could not
produce more young than they could
support economically.
Alfred Russell Wallace
Had some of the same ideas as Darwin
• He actually contacted Darwin for advice on publishing
his theory on Evolution!
• He also had a theory on Natural Selection, although he
did not use this term.
1859 “THE ORIGIN OF SPECIES” IS PUBLISHED 30
YRS. AFTER THE BEAGLE
Fossil film clip
• 5 KEY CONCEPTS of Darwins Theory on Evolution:
• 1.MODERN ORGANISMS ARISE THROUGH
EVOLUTION
• 2.EACH SPECIES comes from a PRECEDING
ONE, they have a COMMON ANCESTOR.
COMMON DESCENT
• 3.FITNESS comes from ADAPTATION
• 4. SUCCESSFUL ADAPTATION allows
ORGANISMS to SURVIVE & REPRODUCE
• 5.ADAPTATION IS ANY CHARACTERISTIC
THAT INCREASES AN ORGANISM’S FITNESS
Concept 15.1 Evolution Is Both Factual and the Basis of Broader
Theory
In 1858, Darwin received a paper from Alfred Russel Wallace with
an explanation of natural selection nearly identical to Darwin’s.
Both men are credited for the idea of natural selection.
Darwin’s book, The Origin of Species, was published in 1859.
5 PRINCIPLES OF DARWIN’S THEORY OF
EVOLUTION
• 1844 DARWIN wrote his theory on
EVOLUTION-called it NATURAL SELECTION
• THERE ARE 5 POINTS TO HIS THEORY
• 1. There is VARIATION in POPULATIONSVARIATIONS ARE PASSED ON FROM
PARENT TO OFFSPRING
– His trip to the Galapagos
• 2. SOME VARIATIONS ARE FAVORABLE-IF
FAVORABLE IT IMPROVES THE ORGANISMS
ABILITY TO LIVE & REPRODUCE
– INFLUENCE BY FARMERS/BREEDERS
DARWIN’S INFLUENCES
MALTHUS & LYELL
• 3. MORE YOUNG are PRODUCED than CAN
SURVIVE -Only a few live long enough to reproduce.
– Malthus’ Essay
• 4. THOSE THAT SURVIVE & REPRODUCE HAVE
FAVORABLE ____? A larger & larger portion of the next
generations will inherit these favorable variations.
– ? Lamarck
• 5. GRADUALISM-Over a large amount of time, small
changes accumulate & populations change
– Lyell
Concept 15.1 Evolution Is Both Factual and the Basis of Broader
Theory
By 1900, the fact of evolution was established, but the genetic basis
of evolution was not yet understood.
Then the work of Gregor Mendel was rediscovered, and during the
20th century, work continued on the genetic basis of evolution.
A “modern synthesis” of genetics and evolution took place 1936–
1947.
Concept 15.2 Mutation, Selection, Gene Flow,
Genetic Drift, and Nonrandom Mating Result in Evolution
Mutations can be deleterious, beneficial, or have no effect (neutral).
Mutation both creates and helps maintain genetic variation in
populations.
Mutation rates vary, but even low rates create considerable variation.
Chapter 15 Micro v. Macroevolution
Micro
Macro
Two causes are drift and
natural selection
Above species levels
Generation-to-generation
change in frequencies of
alleles.
Includes:
•Genetic drift
•Natural selection
•Gene flow
•Mutation
Refers to the bigger
picture (e.g. evolution of
mammals, flowering
plants, large scale
history of life)
Concept 15.2 Mutation, Selection, Gene Flow,
Genetic Drift, and Nonrandom Mating Result in Evolution
Because of mutation, different forms of a gene, or alleles, may exist
at a locus.
Gene pool—sum of all copies of all alleles at all loci in a population.
Allele frequency—proportion of each allele in the gene pool.
Genotype frequency—proportion of each genotype among
individuals in the population.
Figure 15.3 A Gene Pool
Figure 15.4 Many Vegetables from One Species
Many of Darwin’s
observations came from
artificial selection of
domesticated plants and
animals.
Selection on different
characters in a single
species of wild mustard
produced many crop plants
Concept 15.2 Mutation, Selection, Gene Flow,
Genetic Drift, and Nonrandom Mating Result in Evolution
Laboratory experiments also show genetic variation in populations.
Selection for certain traits in the fruit fly Drosophila melanogaster
resulted in new combinations of genes that were not present in the
original population.
Figure 15.6 Artificial Selection Reveals Genetic Variation
Concept 15.2 Mutation, Selection, Gene Flow,
Genetic Drift, and Nonrandom Mating Result in Evolution
Adaptation—a favored trait that evolves through natural selection.
Adaptation also describes the process that produces the trait.
Individuals with deleterious mutations are less likely to survive and
reproduce and to pass their alleles on to the next generation
Migration of individuals between populations results in gene flow,
which can change allele frequencies.
Genetic drift—random changes in allele frequencies from one
generation to the next.
In small populations, it can change allele frequencies. Harmful
alleles may increase in frequency, or rare advantageous alleles
may be lost.
Concept 15.2 Mutation, Selection, Gene Flow,
Genetic Drift, and Nonrandom Mating Result in Evolution
A population bottleneck—
an environmental event
results in survival of only a
few individuals.
Genetic drift can change
allele frequencies.
Populations that go through
bottlenecks loose much of
their genetic variation.
Concept 15.2 Mutation, Selection, Gene Flow,
Genetic Drift, and Nonrandom Mating Result in Evolution
Founder effect—genetic drift changes allele frequencies when a
few individuals colonize a new area.
Concept 15.2 Mutation, Selection, Gene Flow,
Genetic Drift, and Nonrandom Mating Result in Evolution
Nonrandom mating:
Selfing, or self-fertilization is common in plants. Homozygous
genotypes will increase in frequency and heterozygous genotypes
will decrease.
Sexual selection—mates are chosen based on phenotype, e.g.,
bright-colored feathers of male birds.
There may be a trade-off between attracting mates (more likely to
reproduce) and attracting predators (less likely to survive).
Concept 15.3 Evolution Can Be Measured by Changes in Allele
Frequencies
Evolution can be measured by change in allele frequencies.
Allele frequency =
number of copies of allele in population
total number of copies of all alleles in population
Concept 15.3 Evolution Can Be Measured by Changes in Allele
Frequencies
For each population, p + q = 1, and q = 1 – p.
Monomorphic: only one allele at a locus, frequency = 1. The allele is
fixed.
Polymorphic: more than one allele at a locus.
Genetic structure—frequency of alleles and genotypes of a
population.
Concept 15.3 Evolution Can Be Measured by Changes in Allele
Frequencies
Hardy–Weinberg equilibrium—allele frequencies do not change
across generations; genotype frequencies can be calculated from
allele frequencies.
If a population is at Hardy-Weinberg equilibrium, there must be no
mutation, no gene flow, no selection of genotypes, infinite
population size, and random mating.
Concept 15.3 Evolution Can Be Measured by Changes in Allele
Frequencies
At Hardy-Weinberg equilibrium, allele frequencies don’t change.
Genotypes frequencies:
Genotype AA Aa aa
Frequency p2 2pq q2
Figure 15.11 One Generation of Random Mating Restores Hardy–Weinberg Equilibrium
Concept 15.3 Evolution Can Be Measured by Changes in Allele
Frequencies
Probability of 2 A-gametes coming together:
p  p  p 2  (0.55) 2  0.3025
Probability of 2 a-gametes coming together:
Overall probability of obtaining
2 a heterozygote:
2
q  q  q  (0.45)  0.2025
2 pq  0.495
Concept 15.3 Evolution Can Be Measured by Changes in Allele
Frequencies
Populations in nature never meet the conditions of Hardy–
Weinberg equilibrium—all biological populations evolve.
The model is useful for predicting approximate genotype frequencies
of a population.
Specific patterns of deviation from Hardy–Weinberg equilibrium help
identify mechanisms of evolutionary change.
Concept 15.4 Selection Can Be Stabilizing, Directional, or
Disruptive
Qualitative traits—influenced by alleles at one locus; often discrete
qualities (black versus white).
Quantitative traits—influenced by alleles at more than one locus;
likely to show continuous variation (body size of individuals).
Figure 15.12 Natural Selection Can Operate in Several Ways
Natural selection can act on
quantitative traits in three ways:
• Stabilizing selection favors
average individuals.
• Directional selection favors
individuals that vary in one
direction from the mean.
• Disruptive selection favors
individuals that vary in both
directions from the mean.
Figure 15.13 Human Birth Weight Is Influenced by Stabilizing
Selection
Stabilizing selection reduces variation in populations, but does
not change the mean.
Figure 15.14 Long Horns Are the Result of Directional Selection
Directional selection—individuals at one extreme of a character
distribution contribute more offspring to the next generation.
Figure 15.15 Disruptive Selection Results in a Bimodal Character Distribution
Disruptive selection—individuals at opposite
extremes of a character distribution contribute more
offspring to the next generation.
Concept 15.5 Genomes Reveal Both Neutral
and Selective Processes of Evolution
Fitness of genotypes:
Genotypes of higher fitness increase in frequency over time; those of
lower fitness decrease over time.
Concept 15.5 Genomes Reveal Both Neutral
and Selective Processes of Evolution
Genome size and organization also evolves.
Genome size varies greatly.
If only the protein and RNA coding portions of genomes are
considered, there is much less variation in size.
Figure 15.20 Genome Size Varies Widely
Figure 15.21 A Large Proportion of DNA Is Noncoding
Concept 15.5 Genomes Reveal Both Neutral
and Selective Processes of Evolution
Much of the noncoding DNA does not appear to have a function.
Some noncoding DNA can alter the expression of surrounding
genes.
Some noncoding DNA consists of pseudogenes.
Some consists of parasitic transposable elements.
Concept 15.5 Genomes Reveal Both Neutral
and Selective Processes of Evolution
The amount of nonconding DNA may be related to population size.
Noncoding sequences that are only slightly deleterious are likely to
be purged by selection most efficiently in species with large
population sizes.
In small populations genetic drift may overwhelm selection against
these sequences.
Concept 15.6 Recombination, Lateral Gene Transfer,
and Gene Duplication Can Result in New Features
Sexual reproduction results in new combinations of genes and
produces genetic variety that increases evolutionary potential.
But in the short term, it has disadvantages:
• Recombination can break up adaptive combinations of genes
• Reduces rate at which females pass genes to offspring
• Dividing offspring into genders reduces the overall reproductive
rate
Concept 15.6 Recombination, Lateral Gene Transfer,
and Gene Duplication Can Result in New Features
Why did sexual reproduction evolve? Possible advantages:
• It facilitates repair of damaged DNA. Damage on one chromosome
can be repaired by copying intact sequences on the other
chromosome.
• Elimination of deleterious mutations through recombination
followed by selection.
Concept 15.6 Recombination, Lateral Gene Transfer,
and Gene Duplication Can Result in New Features
• In asexually reproducing species, deleterious mutations can
accumulate; only death of the lineage can eliminate them
Muller called this the genetic ratchet—mutations accumulate or
“ratchet up” at each replication; Muller’s ratchet.
Concept 15.6 Recombination, Lateral Gene Transfer,
and Gene Duplication Can Result in New Features
• The variety of genetic combinations in each generation can be
advantageous (e.g., as defense against pathogens and parasites).
Sexual recombination does not directly influence the frequencies of
alleles. Rather, it generates new combinations of alleles on which
natural selection can act.
Concept 15.6 Recombination, Lateral Gene Transfer,
and Gene Duplication Can Result in New Features
Lateral gene transfer—individual genes, organelles, or genome
fragments move horizontally from one lineage to another.
• Species may pick up DNA fragments directly from the
environment.
• Genes may be transferred to a new host in a viral genome.
• Hybridization results in the transfer of many genes.
Concept 15.6 Recombination, Lateral Gene Transfer,
and Gene Duplication Can Result in New Features
Lateral gene transfer can be advantageous to a species that
incorporates novel genes.
Genes that confer antibiotic resistance are often transferred among
bacteria species.
Concept 15.7 Evolutionary Theory Has Practical Applications
Molecular evolutionary principles are used to understand protein
structure and function.
Puffer fish have a toxin (TTX) that blocks sodium ion channels and
prevents nerve and muscle function.
Genes for sodium channel proteins in puffer fish have substitutions
that prevent TTX from binding.
Study of these gene substitutions aid in understanding how sodium
channels function.
Concept 15.7 Evolutionary Theory Has Practical Applications
Living organisms produce many compounds useful to humans. The
search for such compounds is called bioprospecting.
These molecules result from millions of years of evolution.
But biologists can imagine molecules that have not yet evolved. In
vitro evolution—new molecules are produced in the laboratory to
perform novel and functions.
Concept 15.7 Evolutionary Theory Has Practical Applications
In agriculture, breeding programs have benefited from evolutionary
principles, including incorporation of beneficial genes from wild
species.
An understanding of how pest species evolve resistance to
pesticides has resulted in more effective pesticide application and
rotation schemes.
Concept 15.7 Evolutionary Theory Has Practical Applications
Molecular evolution is also used to study disease organisms.
All new viral diseases have been identified by evolutionary
comparison of their genomes with those of known viruses.
Answer to Opening Question
Changes in surface proteins make influenza virus strains
undetectable to the host’s immune system (positive selection for
changes in surface proteins).
By comparing ratios of synonymous to nonsynonymous
substitutions, biologists can detect which mutations are under
positive selection.
Answer to Opening Question
They then assess which current flu strains show the greatest
number of changes in these positively selected codons.
These flu strains are most likely to survive and lead to flu epidemics
of the future, so they are the best targets for new vaccines.
Figure 15.24 Evolutionary Analysis of Surface Proteins Leads to Improved Flu Vaccines