Download Week 09 Lecture Notes

Learning Objectives
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Charles Darwin and his contribution to the theory of evolution
Define natural selection and how it relates to evolution
The relationship between natural selection and evolution
What is adaptive radiation?
What is the difference between homologous and analogous?
How do fossils provide a historical record of evolution?
How is evolution observed at the molecular level?
Hardy-Weinberg equilibrium
What are the agents of evolution?
Three types of selection: stabilizing, disruptive, and directional
Learning Objectives
• Human impact on natural selection
 Industrial melanism
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Guppies as an example of natural selection
What is the biological species concept?
What are the two categories of barriers to reproduction?
What are the six isolating mechanisms that fall into the
category of pre-zygotic barriers to reproduction?
• Post-zygotic barriers to reproduction
Charles Darwin, Father of Evolution
• Charles Darwin
(1809 -1882)
• English naturalist who
first suggested an
explanation for why
evolution occurred
• Published in 1859, On the
Origin of Species by
Means of Natural
Selection
This brings us back to the Theory of
Evolution from Chapter 1
Figure 14.1 The theory of
evolution by natural selection
was proposed by Charles Darwin
Natural Selection
• Charles Darwin’s work on evolution challenged
established worldviews
 Darwin proposed a mechanism for evolutionary
change called natural selection
 Darwin’s hypothesis for evolutionary change, after
much testing, eventually became accepted as theory
Darwin’s Voyage on HMS Beagle
• Darwin voyaged from 1831 - 1836 on the HMS Beagle, a ship
mapping the world’s coastlines
• Darwin observed firsthand different plants and animals in
various locales
• These observations played an important role in the
development of his thoughts about the nature of life on earth
Figure 14.3 The five-year voyage of HMS Beagle
Darwin’s Evidence
Darwin made several observations that helped lead him to
believe that species evolve rather than remain fixed:
1) fossils of extinct
organisms resembled
those of living organisms
2) geographical patterns
suggested that
organismal lineages
change gradually as
individuals move into new
habitats
3) islands have diverse
animals and plants that
are related to yet different
from their mainland
sources
Figure 14.5 Four Galápagos finches and what they eat
Darwin observed that, although all the finches
shared a common ancestor, their beak sizes had
evolved to suit their food. Darwin termed this
“descent with modification.”
Malthus’ Influence
• Thomas Malthus’ Essay on the Principle of
Population (1798) provided Darwin with a
key insight
 while human populations tend to increase
geometrically, the capacity for humans to
feed this population only grows
arithmetically
Figure 14.6 Geometric and arithmetic progressions
Malthus contended that the human population
growth curve was geometric, while the human
food production growth curve was only
arithmetic
Darwin’s Malthusian Influence
• Darwin expanded Malthus’ view to include every
organism
 all organisms have the capacity to over-reproduce
 only a limited number of these offspring survive and produce the
next generation
• Darwin associated survivors with having certain physical,
behavioral, or other attributes that help them to live in
their environment
 by surviving, they can pass their favorable characteristics on to
their own offspring
Figure 14.7 Sea turtle hatchlings
Many organisms, such as sea turtles, produce more offspring than will actually
survive
The Theory of Natural Selection
Darwin envisioned the frequency of favorable
characteristics increasing in a population through a process
called natural selection
 favorable characteristics are specific to an environment; they
may be favored in one but not in another
 organisms whose characteristics are best suited to their
particular environment survive more often and leave more
offspring
Survival and Fitness
• Darwin’s selection concept is often
referred to as the “survival of the fittest”
 Darwinian fitness does not refer always to the
biggest or the strongest
 fitness, in evolutionary theory, refers to
organisms who, due to their characteristics,
survive more often and leave more offspring
On the Origin of Species
• Darwin drafted his ideas in 1842 but hesitated to publish them for 16
years
• Another researcher, Alfred Russel Wallace, sent an essay to Darwin
outlining a theory of evolution by natural selection
 Wallace had independently arrived at the same mechanism for evolution
that Darwin had
 Darwin arranged for a joint presentation of their ideas in London
• Darwin finally published On the Origin of Species in 1859
The Beaks of Darwin’s Finches
• Darwin’s finches are a closely related group of distinct
species
 all the birds are similar to each other except for the shape of their
beaks
 genetic differences account for the physical differences in the
beaks – How do these differences arise?
• birds with larger beaks make more of a protein called BMP4
Figure 14.9 A diversity of finches
on a single island
The Grants and the Finches
• Darwin supposed that the birds evolved from a single
ancestor to become individual species who specialized
in particular foods
• Peter and Rosemary Grant studied the medium ground
finch on the island of Daphne Major in the Galápagos
• Measured beak shape
over many years,
recorded feeding
preferences
• Finches preferred to feed on
small, tender seeds
• Finches switched to larger,
harder-to-crack seeds when
the small seeds became
hard to find
• Beak depth increased when
only large, tough seeds were
available
Figure 14.11 Evidence that natural selection alters beak
size in Geospiza fortis
Finches: Evolution in Action
• The Grants’ work with the
medium ground finch is an
example of evolution in action
 average beak depth increased
after a drought
• only large-beaked birds were
able to crush the bigger seeds
and survive to make the next
generation
 when wet periods returned, smaller
beaks prevailed at handling the
more plentiful small seeds
Punctuated equilibrium vs. Phyletic gradualism?
Adaptive Radiation
• Darwin’s finches on the
Galápagos are an example of
adaptive radiation
 in adaptive radiation, a cluster of
species changes to occupy a
series of different habitats within a
region
 each habitat offers different
niches to occupy
• a niche represents how a species
interacts both biologically and
physically with its environment in
order to survive
 each species evolves to become
adapted to that niche
The Evidence for Evolution
• There are many lines of evidence supporting Darwin’s
theory of evolution
 fossil record comprises the most direct evidence of
macroevolution
 fossils are the preserved remains, tracks, or traces of
once-living organisms
• they are created when organisms become buried in sediment
• by dating the rocks in which the fossils occur, one can get an
accurate idea of how old the fossils are
History of Evolution
• Fossils in rock represent a history of
evolutionary change
 fossils are treated as samples of data and are
dated independently of what the samples are
like
 successive changes through time are
observed in the data
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35
Millions of years ago
Figure 14.14
Testing the
theory of
evolution with
fossil titanotheres
40
45
50
Anatomical Record
• The anatomical record also reflects evolutionary
history
 for example, all vertebrate embryos share a basic set
of developmental instructions
Figure 14.15 Embryos show our early evolutionary history
Homologous vs. Analogous
• Homologous structures are derived from the
same body part present in an ancestor
 for example, the same bones might be put to different
uses in related species
• Analagous structures are similar-looking
structures in unrelated lineages
 these are the result of parallel evolutionary
adaptations to similar environments
• this form of evolutionary change is referred to as convergent
evolution
Figure 14.16 Homology among
vertebrate limbs
Molecular Evolution
• Traces of our evolutionary past are also
evident at the molecular level
 organisms that are more distantly related
should have accumulated a greater number of
evolutionary differences than two species that
are more closely related
 the pattern of divergence can be seen at the
DNA and protein levels
Figure 14.17 Molecules reflect
evolutionary divergence
Molecular Clock
• Evolutionary changes accumulate at a
constant rate in some proteins
 this constancy is referred to as a molecular
clock
 different proteins can evolve at different rates
Figure 14.18 The molecular clock
of cytochrome c
Vertebrate Phylogeny
Evolution of Systems
• The irreducible-complexity
fallacy refers to claims that
the molecular machinery of
the cell is irreducibly complex
• Yet natural selection acts on
the systems and not the
parts so that, at every stage
of evolution, the system is
functioning
 for example, the mammalian
blood clotting system has
evolved in stages from much
simpler systems
Genetic Change in Populations
• Variation within populations puzzled many scientists
 why don’t dominant alleles drive recessive alleles out of
populations?
• G.H. Hardy and W. Weinberg, in 1908, studied allele
frequencies in a gene pool
 in a large population in which there is random mating, and in the
absence of forces that change allele frequencies, the original
genotype proportions remain constant from generation to
generation
 because the proportions do not change, the genotypes are said
to be in Hardy-Weinberg equilibrium
 if the allele frequencies are not changing, the population is not
evolving
Allele Frequencies
• Hardy and Weinberg arrived at their
conclusion by analyzing the frequencies of
alleles in successive generations
 frequency is the proportion of individuals with
a certain characteristic compared to an entire
population
 knowing the frequency of the phenotype, one
can calculate the frequency of the genotypes
and alleles in the population
p+q=1
• By convention, the frequency of the
dominant allele is designated by the letter
p and that of the recessive allele by the
letter q
• Because there are only two alleles, the
sum of p and q must always equal 1
Hardy-Weinberg Equilibrium
• In algebraic terms, the Hardy-Weinberg
equilibrium is written as an equation
p2
+
2pq
+
q2
=
1
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Phenotypes
Figure 14.22
HardyWeinberg
equilibrium
Genotypes
BB
Frequency of genotype in
the population (number in
a population of 1,000 cats)
Bb
360 cats
360/1,000 = 0.36
Number of alleles in the
population (2 per cat)
720 B
Frequency of alleles in the
population (total of 2,000)
480 cats
480/1,000 = 0.48
160 cats
160/1,000 = 0.16
480 B + 480 b
320 b
720 B + 480 B = 1,200 B
1,200/2,000 = 0.6 B
Sperm
p = 0.6
q = 0.4
B
B
p2 = 0.36
b
Eggs
p = 0.6
BB
b
Bb
Bb
pq = 0.24
pq = 0.24
bb
q2 = 0.16
bb
q = 0.4
480 b + 320 b = 800 b
800/2,000 = 0.4 b
Hardy-Weinberg Assumptions
•
The Hardy-Weinberg equilibrium only works if the
following five assumptions are met
1.
The size of the population is very large or effectively infinite.
2.
Individuals mate with one another at random.
3.
There is no mutation.
4.
There is no immigration or emigration.
5.
All alleles are replaced equally from generation to generation
(natural selection is not occurring).
Hardy-Weinberg and Humans
• Most human populations are large and
randomly mating with respect to most
traits and thus are similar to an ideal
population envisioned by Hardy and
Weinberg
 for example, the frequency of heterozygote
carriers for recessive genetic disorders can be
estimated using the Hardy-Weinberg
equilibrium
Agents of Evolution
• Five factors can alter the proportions of
homozygotes and heterozygotes enough
to produce significant deviations from
Hardy-Weinberg predictions
1.
2.
3.
4.
5.
mutation
migration
genetic drift
nonrandom mating
selection
Table 14.1 Agents of Evolution
Agents of Evolution: Mutation
• Mutation is a change in a nucleotide
sequence in DNA
 mutation rates are generally too low to
significantly alter Hardy-Weinberg proportions
 mutations must affect the DNA of the germ
cells or the mutation will not be passed on to
offspring
 however, no matter how rare, mutation is the
ultimate source of genetic variation in a
population
Agents of Evolution: Migration
• Migration is the movement of individuals
between populations
 the movement of individuals can be a
powerful force upsetting the genetic stability
of natural populations
• the magnitude of the effects of migration is based
on two factors
– the proportion of migrants in the population
– the difference in allele frequencies between the migrants
and the original population
Agents of Evolution: Genetic Drift
• Genetic drift describes random changes
in allele frequencies
 in small populations, the frequencies of
particular alleles may be changed drastically
by chance alone
 in extreme cases, individual alleles of a given
gene may be
• all represented in few individuals
• accidentally lost if individuals fail to reproduce or
die
Animation: Simulation of
Genetic Drift
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Agents of Evolution: Founder Effect
•
A series of small populations that are isolated
from one another may come to differ strongly as
the result of genetic drift
 founder effect occurs when one of a few individuals
migrate and become the founders of a new, isolated
population at some distance from their place of origin
• the alleles that they carry will become a significant fraction of
the new population’s genetic endowment
• the founder effect is important in the evolution of organisms
on oceanic islands
Agents of Evolution: Nonrandom
mating
• Nonrandom mating occurs when
individuals with certain genotypes mate
with one another either more or less
commonly than would be expected by
chance
 sexual selection is choosing a mate often
based on physical characteristics
 nonrandom mating alters genotype
frequencies but not allele frequencies
Agents of Evolution: Selection
• Selection, according to Darwin, occurs if
some individuals, due to their inherited
characteristics, leave behind more
progeny than others
• in artificial selection, a breeder selects for the
desired characteristics
• in natural selection, conditions in nature
determine which kinds of individuals in a
population are the most fit
• there are three types of natural selection
Figure 14.24 Three kinds of natural
selection
Stabilizing Selection
• Stabilizing selection is a form of
selection in which both extremes from an
array of phenotypes are eliminated
 the result is an increase in the frequency of
the already common intermediate phenotype
 for example, human birth weight is under
stabilizing selection
Figure 14.25 (a) Examples of
selection
Disruptive Selection
• Disruptive selection is a form of selection
in which the two extremes in an array of
phenotypes become more common in the
population
 selection acts to eliminate the intermediate
phenotypes
 for example, beak size in African black-bellied
seedcracker finches is under disruptive
selection because the available seeds are
only large or small
Figure 14.25 (b) Examples of
selection
Directional Selection
Directional selection is a form of selection
that occurs when selection acts to eliminate
one extreme from an array of phenotypes
 In Drosophila, flies that fly toward light can be
selected against and those that avoid light selectively
bred, producing a population of flies with a greater
tendency to avoid light
 Fossil records show that European black bears
increased in size during glacial periods, but
decreased in size between ice ages
Figure 14.25 (c) Examples of
selection
Sickle-Cell Disease
Sickle-cell disease is a hereditary disease
affecting hemoglobin molecules in the blood
 The disorder results from a single nucleotide change
in the gene encoding beta-hemoglobin
• this causes the sixth amino acid in the chain to change from
glutamic acid (very polar) to valine (nonpolar)
• as a result, the hemoglobin molecules clump together and
deform the red blood cell into “sickle-shape”
Figure 14.27 Why the sickle-cell
mutation causes hemoglobin to clump
Sickle Cell: Non-Mendelian?
• Persons homozygous for the sickle-cell genetic
mutation frequently have a reduced lifespan
 red blood cells that are sickled do not flow smoothly
through capillaries
 oxygen transport is affected
• Heterozygous individuals make enough
functional hemoglobin to keep their red blood
cells healthy
Sickle-Cell Disease: Frequency
• The frequency of the sickle-cell allele is about
0.12 in Central Africa – How would we show
this in the Hardy-Weinberg equation?
 one in 100 people is homozygous for the defective
allele and develops the fatal disorder
 sickle-cell anemia strikes roughly two African
Americans out of every thousand
• If natural selection drives evolution, why has
natural selection not acted against the defective
allele in Africa and eliminated it from the
population there?
Heterozygote Advantage
The defective allele has not been eliminated
from Central Africa because people who are
heterozygous are much less susceptible to
malaria
 the payoff in survival of heterozygotes makes
up for the price in death of homozygotes
• this is called heterozygote advantage
• stabilizing selection occurs because malarial
resistance counterbalances lethal anemia – what
would a graph of this selection look like?
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Figure 14.28 How
stabilizing selection
maintains sickle-cell
disease
Sickle-cell
allele in Africa
1–5%
5–10%
10–20%
Falciparum
malaria in Africa
Malaria
Peppered Moths and Industrial
Melanism
• The frequency of dark individuals relative to light
individuals of the peppered moth (Biston
betularia) has increased in industrialized areas
 darker forms may be less susceptible to predation
 industrial melanism describes the evolutionary
process in which darker individuals become more
common than light individuals in a population as a
result of natural selection
 this phenomenon has been observed in other species
of moths in industrialized areas in Eurasia and North
America
Figure 14.29 Tutt’s hypothesis explaining
industrial melanism
Industrial Melanism: Reversible?
• Due to a reduction in air
pollution, a reversal of
industrial melanism seems
to be occurring in England
and America
Figure 14.30 Selection against
melanism
Natural Selection: Guppies
Guppies are colorful fish that are popular for
aquariums
 on the island of Trinidad, guppies are found in two
very different stream environments
• in pools above waterfalls, the guppies are found along with
the killifish, a seldom predator of guppies
• in pools below waterfalls, the guppies are found in pools
along with the pike cichlid, a voracious predator of guppies
• guppies can move between pools by swimming upstream
during floods
Selection on Color in Guppies
• Guppy populations above and below
waterfalls exhibit many differences
 guppies in high-predation pools are not as
colorful as guppies in low-predation pools
 guppies in high-predation pools tend to
reproduce at an earlier age and attain
relatively smaller adult body sizes
 these differences suggest the function of
natural selection
Figure 14.31 The evolution of
protective coloration in guppies
Guppies: Experiments in Selection
• John Endler conducted experiments on guppies to
determine whether predation risk was really the driving
selective force in this system
 in a controlled laboratory setting, he created artificial pool
environments in which he placed guppies in one of three
conditions:
• with no predator present
• with killifish present (low predation risk)
• with cichlid present (high predation risk)
 after 10 guppy generations, he found that the guppies from noor low-predation risk pools were both larger and more colorful
than the guppies from the high predation risk pool
 he later found the same results in field experiments
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
No
predation
14
Figure 14.32
Evolutionary
change in
spot number
Spots per fish
13
Low
predation
12
11
10
High
predation
9
8
0
4
8
Months
(a)
(b)
b: Courtesy H. Rodd
12
Speciation
• Speciation is the macroevolutionary
process of forming new species from preexisting species
 it involves successive change
• first, local populations become increasingly
specialized
• then, if they become different enough, natural
selection may act to keep them that way
The Biological Species Concept
• Ernst Mayr coined the biological species
concept, which defines species as
“groups of actually or potentially interbreeding
natural populations which are reproductively
isolated from other such groups”
• Populations whose members do not mate with
each other and cannot produce fertile offspring
are said to be reproductively isolated and,
thus, members of different species
Barriers to Reproduction
• Barriers called reproductive isolating
mechanisms cause reproductive isolation
by preventing genetic exchange between
species
 prezygotic isolating mechanisms
• prevent the formation of zygotes
 postzygotic isolating mechanisms
• prevent the proper functioning of zygotes once
they have formed
Table 14.2 Isolating Mechanisms
Hint: This would be a
good table to review
for the exam!
Isolating Mechanisms
• There are six broad categories of
prezygotic isolating mechanisms






geographical isolation
ecological isolation
temporal isolation
behavioral isolation
mechanical isolation
prevention of gamete fusion
Geographical
• Geographical isolation occurs in cases when
species exist in different areas and are not able
to interbreed
 Kaibab squirrel found only in a 20 x 40 mile habitat on North Rim
of Grand Canyon
 Abert’s squirrel is found on South Rim
• Widely distributed, several subspecies
Kaibab squirrel
Abert’s squirrel
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Ecological
Ecological isolation results
from two species who occur
in the same area but utilize
different portions of the
environment and are unlikely
to hybridize
(a)
 Lions & tigers once occupied
same geographical area but
different niches
•
•
Lions: savannah
Tigers: jungle
(b)
Figure 14.33 Lions and tigers
are ecologically isolated
(c)
a-b: © Corbis RF; c: © Porterfield/Chickering/Photo Researchers
Temporal
Temporal isolation results from two species having different
reproductive periods, or breeding seasons, that preclude hybridization
•
American toad and Fowler’s toad closely related but do not breed

•
American toad breeds in early summer, Fowler’s toad later in the season
Western spotted skunk breeds in the fall, while Eastern spotted skunk
breeds in late winter
Eastern spotted
skunk
American toad
Fowler’s toad
Western spotted
skunk
Behavioral
• Behavioral isolation refers to the often elaborate courtship and
mating rituals of some groups of animals, which tend to keep these
species distinct in nature even if they inhabit the same places
Mechanical
Mechanical isolation results from structural differences
that prevent mating between related species of animals
and plants
• Example: Snails whose shells spiral in opposite
directions cannot mate
Gametic
Prevention of gamete fusion blocks the union of gametes
even following successful mating
• Sea urchins spew
gametes into ocean,
sperm & eggs form
zygotes, develop
into larvae
• Giant Red Urchin
and Purple Urchin
occupy same
habitat along
western U.S.
• Gametes are
chemically/
genetically
incompatible
Post-Zygotic Isolation
• If hybrid mating does occur and zygotes are
produced, many postzygotic factors may prevent
those zygotes from developing into normal
individuals
 in hybrids, the genetic complements of two species
may be so different that they cannot function together
normally in embryonic development
 even if hybrids survive the embryo stage, they may
not develop normally
 finally, many hybrids are sterile
Figure 14.34 Postzygotic isolation
in leopard frogs
• Similar in appearance,
overlapping ranges
• Fertilized eggs fail to
develop properly
Does Humanity Have a Hybrid Origin?
• One prominent geneticist proposes
that humans arose through a cross
between a male pig and female
chimpanzee
• Explains a number of anomalous
human characteristics:
 Low fertility and non-uniformity of human sperm
 Dermal features such as naked skin, subcutaneous fat, thermoregulatory
sweating, etc.
 Facial features such as protruding cartilaginous mucous nose, eyebrows,
earlobes, philtrum
 Many skeletal features, including short digits, thick tooth enamel, helical chewing
 Behavioral traits such as nocturnal activity, swimming, tears
• Repeated backcrossing would account for the negligible differences
between human and chimp genomes
http://www.macroevolution.net/human-origins.html#.U4y9GPldWSp
Evolution in Action: The Grants
and Darwin’s Finches
• https://www.youtube.com/watch?v=1XbY5
8rCuKY