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Evolution and the
Diversity of Life
What is Evolution?
In the simplest biological terms
evolution is defined as change
over time. However, it is much
more than that.
Who was Charles Darwin?
• Charles Darwin first published an
explanation of how species changed
over time, or evolved.
• Darwin argued that contemporary
species arose from ancestors
•Through a process of “descent
with modification,” with natural
selection as the mechanism.
How did he explain evolution?
• Natural Selection: Occurs in natural
populations when organisms with
certain traits pass those traits on to
the next generation.
• The following are required for natural
selection to take place:`
1. Organisms can change over generations.
2. Individuals with certain heritable traits
produce more surviving offspring than
others.
3. The result of natural selection is
evolutionary adaptation.
Natural Selection
• Natural selection is the
mechanism for evolution.
• It only acts on populations.
• It changes the frequency of
genes in a population –
not in an individual.
What traits are usually
selected?
• Traits that allow the organism to
survive and reproduce.
http://evolution.berkeley.edu/evolibrary/article//bergstrom_02
Peppered
Moth Example
• Originally White
• Now more black than white
• Causes
– Industrial Pollution
• Soot covered trees
How does natural
selection happen?
Evolution only occurs when there is
a change in gene frequency within
a population over time.
Change Over Time
Evidence for evolution
Fossils
evidence of an organism that lived some
time in the past.
The Fossil record
The ordered sequence
of fossils as they
appear in rock layers
which reveals the
appearance of
organisms in a
historical sequence.
Fossils
Fossils show a succession from
simple forms early in the fossil
record to complex forms that
appear much later.
Fossils and Geological Time
• Scientists will date the rocks in an
attempt to determine the age of the
fossil.
• Two ways to do this:
– Relative dating: When you compare the
depth of the layer of rock the fossil was
formed in relative to other layers of rock.
– Radiometric dating: Uses
isotopes.
Relative Dating
• Relative dating uses layers to determine
age by order of appearance.
• Does not give you a specific age in
years, just relative to the layers above
and below the fossil.
• Compare rock layers to layers of
clothing in your laundry hamper…Oldest
on the bottom (unless disturbed).
Radiometric Dating
• Technique involves using radioactive
isotopes, which are atoms with unstable
nuclei that break down (decay) over
time, giving off radiation.
Biogeography
Is the study of the geographic
distribution of species.
Convergent and Divergent
Evolution
• Divergent
– Organisms with the same ancestral genetic heritage
migrate to different habitats and evolve into species
with different external forms and structures, but
continue to use the same type of habitats
• Ex) Ostrich
• Convergent
– The process by which species evolve in different
places of times and, although they have
different genetic heritages, develop similar
external forms and structures as a result of
adaptation to similar environments
• Ex) shapes of sharks
Divergent Evolution
Convergent Evolution
• occurs when similar
environmental
pressures and natural
selection
– Produce similar
(analogous)
adaptations in
organisms from
different
evolutionary
Figure 25.5
lineages
Homologous Structures –
structural features with a
common evolutionary origin –
shared by related species.
Although used for such different functions as throwing,
swimming, and flying, the same basic structural plan is evident in
them all. In each case, the bone shown in color is the radius.
Analogous StructuresStructural features which serve the
same function in different
species, but they evolved
independently.
Ex:
Butterfly wings, Bat wings
Bird wings
• FIGURE 16.2 The
Bones Are
Homologous, the
Wings Are Not The
supporting bone
structures of both bat
wings and bird wings
are derived from a
common tetrapod
(four-limbed) ancestor
and are thus
homologous. However,
the wings
themselves—an
adaptation for flight—
evolved independently
in the two groups.
Vestigial Structurescharacteristics of organisms that have
lost all or most of their original
functions. Humans have a vestigial
tailbone. Vestigial toes in the horse.
Vestigial limbs in whales and snakes.
Structural Adaptations
A. Mimicry: A structural adaptation
that protects an organism by copying
the appearance of another species.
–
Example: Gopher snake
B. Camouflage – A structural
adaptation that allows an organism
to blend in with its environment.
–
Example: Snowshoe hare
Gopher Snakes?
The one on the left is a rattlesnake,
the one on the right is a gopher snake.
Snowshoe Hare
Summer
Winter
Embryology – Early embryos of
very different organisms closely
resemble each other.
Morphology:
the study of animal form. How
animal parts function together to
make them run, fly, swim, eat,
survive.
Functional morphology: how a
structure works.
Ecological morphology: how a
structure is used in nature.
Molecular Data:
Causes of genetic variation in
populations at the molecular level
(DNA, RNA and proteins).
• Can be used for comparison with
all organisms.
• Provides tons of information,
including tracking mutations.
Molecular clocks help track evolutionary time
Molecular Clocks
• The molecular clock
– Is a yardstick for measuring the
absolute time of evolutionary change
based on the observation that some
genes and other regions of genomes
appear to evolve at constant rates
Neutral Theory
• Neutral theory states that
– Much evolutionary change in genes and
proteins has no effect on fitness and
therefore is not influenced by Darwinian
selection
– And that the rate of molecular change in
these genes and proteins should be
regular like a clock
Difficulties with Molecular
Clocks
• The molecular clock
– Does not run as smoothly as neutral
theory predicts
Applying a Molecular
Clock: The Origin of HIV
• Phylogenetic analysis shows that HIV
– Is descended from viruses that infect
chimpanzees and other primates
• A comparison of HIV samples from
throughout the epidemic
– Has shown that the virus has evolved in a
remarkably clocklike fashion
Molecular Clock of Disease
• Research a zoonotic disease
– Determine when, where, and how a
disease first entered a human
population.
– Phylogenetic Analysis
– Present and Global diversity
– How would the evolution of your
disease help epidemiologist to
identify how disease is spread and
how to prevent future outbreaks?
Evolutionary Tree/
Cladogram:
Adaptations
and
Natural Selection
Vocabulary
Gene Pool: The total number of
genes present in a population.
Allelic Frequency: the frequency of
an allele for a specific trait.
Genetic equilibrium: when the
allelic frequency of alleles remains
the same over time.
Vocabulary
Darwinian Fitness: the contribution
an individual makes to the gene pool
of the next generation relative to the
contributions of other individuals.
Changes in Populations
Gene variations:
Mutations-changes in DNA
Gene flow-movement of genes
from one population to another
Reproduction-introduces new gene
combinations.
• Genetic variations in populations
– Contribute to evolution
Inherited Variation
• Individuals possessing alleles
suited for their environment will
reproduce and population will
continue because of their success.
• Population genetics provides a
foundation for studying evolution
• Microevolution
– Is change in the genetic makeup of a
population from generation to
generation
The Modern Synthesis
• Population genetics
– Is the study of how populations
change genetically over time
– Reconciled Darwin’s and Mendel’s
ideas
• The modern synthesis
– Integrates Mendelian genetics with
the Darwinian theory of evolution by
natural selection
– Focuses on populations as units of
evolution
Gene Pools and Allele Frequencies
• A population
– Is a localized group of individuals that are
capable of interbreeding and producing
fertile offspring
MAP
AREA
•
Fairbanks
Fortymile
herd range
•
Whitehorse
• The gene pool
– Is the total aggregate of genes in a
population at any one time
– Consists of all gene loci in all
individuals of the population
Gene Flow
– Causes a population to gain or lose alleles
– Results from the movement of fertile
individuals or gametes
– Tends to reduce differences between
populations over time
A finite supply of
environmental resources
• As population increases, each
individual has a harder time
getting resources needed to
survive.
• Birth rate decreases, mortality
increases, overall growth rate
slows.
• Mutation and sexual recombination
produce the variation that makes
evolution possible
• Two processes, mutation and sexual
recombination
– Produce the variation in gene pools that
contributes to differences among
individuals
Mutation
– Are changes in the nucleotide
sequence of DNA
– Cause new genes and alleles to
arise
Point Mutations
• A point mutation
– Is a change in one base in a gene
– Can have a significant impact on
phenotype
– Is usually harmless, but may have
an adaptive impact
Mutations That Alter Gene
Number or Sequence
• Chromosomal mutations that
affect many loci
– Are almost certain to be harmful
– May be neutral and even beneficial
• Gene duplication
– Duplicates chromosome segments
Mutation Rates
• Mutation rates
– Tend to be low in animals and plants
– Average about one mutation in every
100,000 genes per generation
– Are more rapid in microorganisms
Sexual Recombination
• In sexually reproducing
populations, sexual recombination
– Is far more important than mutation
in producing the genetic differences
that make adaptation possible
Natural Selection
• Differential success in reproduction
– Results in certain alleles being passed
to the next generation in greater
proportions
Genetic Drift
• Statistically, the smaller a sample
– The greater the chance of deviation
from a predicted result
• Genetic drift
– Describes how allele frequencies can
fluctuate unpredictably from one
generation to the next
– Tends to reduce genetic variation
CWCW
CRCR
CRCR
Only 5 of
10 plants
leave
offspring
CRCW
CWCW
CRCR
CRCR
CRCW
CWCW
CRCR
CRCW
CRCW
CRCR
CWCW
CRCW
CRCR
CRCR
CRCW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3
Only 2 of
10 plants
leave
offspring
CRCR
CRCR
CRCR
CRCR
CRCR
CRCR
CRCR
CRCR
CRCW
CRCW
Generation 2
p = 0.5
q = 0.5
Figure 23.7
CRCR
CRCR
Generation 3
p = 1.0
q = 0.0
The Bottleneck Effect
– A sudden change in the environment
may drastically reduce the size of a
population
– The gene pool may no longer be
reflective of the original population’s
gene pool
(a) Shaking just a few marbles through the
narrow neck of a bottle is analogous to a
drastic reduction in the size of a population
after some environmental disaster. By chance,
blue marbles are over-represented in the new
population and gold marbles are absent.
Original
population
Bottlenecking
event
Surviving
population
• Understanding the bottleneck
effect
– Can increase understanding of how
human activity affects other species
(b) Similarly, bottlenecking a population
of organisms tends to reduce genetic
variation, as in these northern
elephant seals in California that were
once hunted nearly to extinction.
The Founder Effect
• The founder effect
– Occurs when a few individuals
become isolated from a larger
population
– Can affect allele frequencies in a
population
• Natural selection
– primary mechanism of adaptive
evolution
• Natural selection
– Accumulates and maintains favorable
genotypes in a population
• Polymorphism
• Phenotypic polymorphism
– Describes a population in which two or
more distinct morphs for a character are
each represented in high enough
frequencies to be readily noticeable
• Genetic polymorphisms
– Are the heritable components of
characters that occur along a continuum
in a population
• Genetic variation
– Occurs in individuals in populations of
all species
– Is not always heritable
(a) Map butterflies that
emerge in spring:
orange and brown
(b) Map butterflies that
emerge in late summer:
black and white
• Measuring Genetic Variation
• Population geneticists
– Measure the number of polymorphisms in
a population by determining the amount
of heterozygosity at the gene level and
the molecular level
• Average heterozygosity
– Measures the average percent of loci that
are heterozygous in a population
Variation Between
Populations
• Most species exhibit
geographic variation
– Differences
between gene pools
of separate
populations or
population
subgroups
1
2.4
3.14
5.18
8.11
9.12
10.16
13.17
1
2.19
3.8
4.16
9.10
11.12
13.17
15.18
6
7.15
19
XX
5.14
6.7
XX
• Some examples of geographic
variation occur as a cline, which is
a graded change in a trait along a
geographic axis
Heights of yarrow plants grown in common garden
EXPERIMENT
Researchers observed that the average size
Mean height (cm)
of yarrow plants (Achillea) growing on the slopes of the Sierra
Nevada mountains gradually decreases with increasing
elevation. To eliminate the effect of environmental differences
at different elevations, researchers collected seeds
from various altitudes and planted them in a common
garden. They then measured the heights of the
resulting plants.
Atitude (m)
RESULTS The average plant sizes in the common
garden were inversely correlated with the altitudes at
which the seeds were collected, although the height
differences were less than in the plants’ natural
environments.
CONCLUSION The lesser but still measurable clinal variation
in yarrow plants grown at a common elevation demonstrates the
role of genetic as well as environmental differences.
Sierra Nevada
Range
Great Basin
Plateau
Seed collection sites
A Closer Look at Natural
Selection
• From the range of variations
available in a population
– Natural selection increases the
frequencies of certain genotypes,
fitting organisms to their environment
over generations
Changes in Populations
Reproductive success/fitness:
Individuals best suited for their
environment leave more offspring,
changing the genetic makeup of a
population.
Evolutionary Fitness
• The phrases “struggle for existence”
and “survival of the fittest”
– Are commonly used to describe natural
selection
– Can be misleading
• Reproductive success
– Is generally more subtle and depends on
many factors
• Fitness
– Is the contribution an individual makes to
the gene pool of the next generation,
relative to the contributions of other
individuals
• Relative fitness
– Is the contribution of a genotype to the
next generation as compared to the
contributions of alternative genotypes for
the same locus
What Is a Species?
– A population or group of populations
whose members have the potential to
interbreed and produce fertile offspring.
How does speciation occur?
• Speciation occurs only when
there are reproductive
barriers between the isolated
population and its parent
population.
Geographic Isolation
• A population becomes divided into
smaller populations due to
unfavorable habitats which keeps
them from mating.
• Also called Allopatric Speciation
Behavioral Isolation
• Two populations acquire different
mating rituals and behaviors.
• Also called Sympatric Speciation
Punctuated Equilibrium
Species most often diverge in spurts of
relatively sudden change.
Changes in Populations
Selection advantage:
The environment determines which
genetic variations are favorable,
those are passed on to the
population as a whole.
Directional, Disruptive,
and Stabilizing Selection
• Selection
– Favors certain genotypes by acting on the
phenotypes of certain organisms
• Three modes of selection are
– Directional
– Disruptive
– Stabilizing
• The three modes of selection
Original population
Original
population
Evolved
population
(a) Directional selection shifts the overall
makeup of the population by favoring
variants at one extreme of the
distribution. In this case, darker mice are
favored because they live among dark
rocks and a darker fur color conceals them
from predators.
Phenotypes (fur color)
(b) Disruptive selection favors variants
at both ends of the distribution. These
mice have colonized a patchy habitat
made up of light and dark rocks, with the
result that mice of an intermediate color are
at a disadvantage.
(c) Stabilizing selection removes
extreme variants from the population
and preserves intermediate types. If
the environment consists of rocks of
an intermediate color, both light and
dark mice will be selected against.
The Preservation of
Genetic Variation
• Various mechanisms help to
preserve genetic variation in a
population
Diploidy
• Diploidy
– Maintains genetic variation in the
form of hidden recessive alleles
Balancing Selection
• Balancing selection
– Occurs when natural selection
maintains stable frequencies of two
or more phenotypic forms in a
population
– Leads to a state called balanced
polymorphism
Heterozygote Advantage
• Some individuals who are
heterozygous at a particular locus
– Have greater fitness than homozygotes
• Natural selection
– Will tend to maintain two or more alleles
at that locus
• The sickle-cell allele
– Causes mutations in hemoglobin but
also confers malaria resistance
– Exemplifies the heterozygote
advantage
Frequencies of the
sickle-cell allele
0–2.5%
2.5–5.0%
Distribution of
malaria caused by
Plasmodium falciparum
(a protozoan)
5.0–7.5%
7.5–10.0%
10.0–12.5%
>12.5%
• Frequency-Dependent Selection
• In frequency-dependent selection
– The fitness of any morph declines if
it becomes too common in the
population
Neutral Variation
• Neutral variation
– Is genetic variation that appears to
confer no selective advantage
Sexual Selection
• Sexual selection
– Is natural selection for mating
success
• Intrasexual selection
– Is a direct competition among
individuals of one sex for mates of
the opposite sex
• Intersexual selection
– Occurs when individuals of one sex
(usually females) are choosy in
selecting their mates from individuals of
the other sex
– May depend on the showiness of the
male’s appearance
The Evolutionary Enigma
of Sexual Reproduction
– Produces fewer reproductive offspring
than asexual reproduction, a so-called
reproductive
handicap
Asexual reproduction
Female
Generation 1
Sexual reproduction
Female
Generation 2
Male
Generation 3
Generation 4
• If sexual reproduction is a
handicap, why has it persisted?
– It produces genetic variation that
may aid in disease resistance
Why Natural Selection
Cannot Fashion Perfect
Organisms
• Evolution is limited by historical
constraints
• Adaptations are often
compromises
• Mendelian inheritance
– Preserves genetic variation in a
population
Generation
1
CW CW
genotype
CRCR
genotype
Plants mate
Generation
2
All CRCW
(all pink flowers)
50% CR
gametes
50% CW
gametes
Come together at random
Generation
3
25% CRCR
50% CRCW
50% CR
gametes
25% CWCW
50% CW
gametes
Come together at random
Generation
4
25% CRCR
Figure 23.4
50% CRCW
25% CWCW
Alleles segregate, and subsequent
generations also have three types
of flowers in the same proportions
Preservation of Allele
Frequencies
• In a given population where
gametes contribute to the next
generation randomly, allele
frequencies will not change
Hardy-Weinberg
Equilibrium
• Hardy-Weinberg equilibrium
– Describes a population in which
random mating occurs
– Describes a population where allele
frequencies do not change
• A population in Hardy-Weinberg
equilibrium
Gametes for each generation are drawn at random from
the gene pool of the previous generation:
80% CR (p = 0.8)
20% CW (q = 0.2)
Sperm
CR
(80%)
CW
(20%)
CR
(80%)
pq
p2
64%
CRCR
CW
(20%)
Eggs
p2
16%
CRCW
16%
CRCW
qp
4%
CW CW
q2
If the gametes come together at random, the genotype
frequencies of this generation are in Hardy-Weinberg equilibrium:
64% CRCR, 32% CRCW, and 4% CWCW
Gametes of the next generation:
16% CR from
64% CR from
+
CRCW homozygotes
CRCR homozygotes
4% CW from
CWCW homozygotes
+
16% CW from
CRCW heterozygotes
=
80% CR = 0.8 = p
=
20% CW = 0.2 = q
With random mating, these gametes will result in the same
mix of plants in the next generation:
Figure 23.5
64% CRCR, 32% CRCW and 4% CWCW plants
• If p and q represent the relative
frequencies of the only two possible
alleles in a population at a particular
locus, then
– p2 + 2pq + q2 = 1
– And p2 and q2 represent the frequencies
of the homozygous genotypes and 2pq
represents the frequency of the
heterozygous genotype
Conditions for HardyWeinberg Equilibrium
• The Hardy-Weinberg theorem
– Describes a hypothetical population
• In real populations
– Allele and genotype frequencies do
change over time
Organizing Life’s Diversity
Taxonomy- the identification, naming,
and classification of species.
Classification – grouping of organisms
based on similarities.
Aristotle:developed first
biological classification system
• Divided all organisms into
two groups: plants or
animals
• Each animal according to
where it lived
– Land
– Water
– Air
• Each plant according to
– Size
– Structure
Carolus von Linneaus
mid-1700’s
• Determined levels of
classification
– He used similarities in structure
to determine relationships among
organisms.
• Binomial Nomenclature:
Two-word naming system
(LATIN)
Taxonomic Hierarchy
– From largest to smallest:
• Kingdom-broad
• Phylum
• Class
• Order
• Family
• Genus
• Species
King Phillip Cut Open Five Green Snakes
Naming organisms
First word: genus, Second word: species
Ex: scientific name for leopard is
Panthera pardus
Dichotomous keys
• Dichotomous key: a tool that helps find
the identity of organisms/items.
– Ex: trees, flowers, mammals, fish and
rocks.
• Keys consist of a series of choices that
lead the user to the correct name of a
given item.
• Dichotomous means "divided into two
parts". Dichotomous keys always give
two choices in each step.
Examples:
Where do living
things come from?
• One theory held in the past is referred
to as Spontaneous generation. This
theory held that nonliving material can
produce life.
• Fancesco Redi disproved this theory
with an experiment now known as the
Redi Experiment. This experiment
helped to disprove the spontaneous
generation of large organisms,
but not microorganisms.
Redi’s (1626-1697) Experiments
Evidence against spontaneous generation:
1. Unsealed – maggots on meat
2. Sealed – no maggots on meat
3. Gauze – few maggots on gauze, none on meat
The Origin of Life
• Likewise, Louis Pasteur designed
an experiment to disprove the
spontaneous generation of
microorganisms.
Louis Pasteur (1822-1895)
Pasteur's Problem
• Hypothesis: Microbes come from cells
of organisms on dust particles in the
air; not the air itself.
• Pasteur put broth into several special
S-shaped flasks
• Each flask was boiled and
placed at various locations
Pasteur's Experiment Step 1
• S-shaped Flask
• Filled with broth
• The special shaped was
intended to trap any
dust particles
containing bacteria
Pasteur's Experiment Step 2
• Flasks boiled
• Microbes Killed
Pasteur's Experiment Step 3
• Flask left at various
locations
• Did not turn cloudy
• Microbes not found
• Notice the dust that
collected in the neck of the
flask
Pasteur's Experimental Results
Question:
So what do Pasteur's experimental results
mean?
So what now?
The Theory of Biogenesis
• Pasteur’s S-shaped flask kept microbes
out but let air in.
• Proved microbes only come from other
microbes (life from life) - biogenesis
Figure 1.3
The Origin of Life
• Biogenesis – All living organisms
must come from other living
organisms.
• Simple Prokaryotic cells were
probably the first cells and over
time the would have evolved
into Eukaryotic cells.
Evidence for this is seen in
mitochondrial DNA.
Table 14.1
Mass Extinctions and
Explosive
Diversifications of Life
– The fossil record reveals an episodic
history,
• With long, relatively stable periods
punctuated by briefer intervals when the
turnover in species composition was much
more extensive.
Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin Cummings
– Extinction is inevitable in a changing world
and occurs all the time.
• However, extinction rates have not been
steady.
– Extinctions typically eliminate various
species of organisms
• And are followed by explosive
diversifications of organisms.
How many extinctions have happened?
Are we in an extinction now?
Classification and
Phylogeny
– The goal of classification is to reflect
phylogeny, the evolutionary history of
a species.
Sorting Homology from
Analogy
– Homologous structures
• What are they?
• Are one of the best sources of information
about phylogenetic relationships.
– Convergent evolution
• Involves superficially similar structures in
unrelated organisms based on natural
selection.
– Analogy
• Is similarity due to convergence.
Molecular Biology as a
Tool in Systematics
– Molecular systematics
• Compares DNA and amino acid sequences
between organisms.
• Can reveal evolutionary relationships.
The Cladistic Revolution
– Cladistics
• Is the scientific search for clades,
• Clades are distinctive branches in the
history of life.
– Cladistics
• Has ______________ traditional
classification of some organisms.
Arranging Life into
Kingdoms:
A Work in Progress
– Linnaeus designed a two-kingdom system
of classification,
• Which was replaced by a five-kingdom system
in the mid-20th century.
• Now we use a six-kingdom system,
which may change again!
Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin Cummings
– In the late 20th century,
• Molecular studies and cladistics led to the
development of a three-domain system.
Classification Schemes
Tools!
• Dichotomous Key: Allows people
to identify organisms in the real
world based on a series of two
choices at each step.
Identify This Tree
http://www.uwsp.edu/cnr/leaf/Treekey/tkframe.htm
Dichotomous Key Activity!
• Are you ready to try this in more
detail?