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Chapter 6
Organic
Organic
Evolution
Evolution
Origins of Darwinian
Evolutionary Theory
Pre-Darwinian Evolutionary Ideas
Before 18th century species origins based on
mythology and superstition, not science
Most creation myths did not include the idea of
change in species after creation
Some Greek philosophers developed early ideas
of evolutionary change
No major changes in ideas of creation until 18th
century
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Lamarckism: The First Scientific
Explanation of Evolution
Lamarck was the first
to provide a complete
explanation of
evolution (1809)
Proposed Inheritance
of Acquired
Characteristics as the
mechanism for
evolutionary change.
Darwin’s Great Voyage of Discovery
5 year voyage that began
in 1831, Darwin was
almost 23 years old.
Darwin was hired as the
ships naturalist.
Observations made during
the trip provided the
underlying evidence for
his theory of evolution by
natural selection
Larmarck’s Concept
Termed “transformational” because in claims that
individuals transform their characteristics to
produce evolution
Lamarck’s theory has been rejected due to genetic
evidence of chromosomal inheritance through
gametes rather than acquired characteristics.
Darwin’s theory is termed “variational” based on
the distribution of genetic variation in populations
Evolutionary change is caused by differential survival
and reproduction among organisms that differ in
hereditary traits.
Travels on the Beagle
Most famous stop on
the voyage was a 5
week visit to the
Galapagos islands
Darwin considered his
observations taken
there as the “origin of
all my views”
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After the Beagle
Darwin returned to
England in 1836
Published the journal
of his voyage 3 years
later “The Voyage of
the Beagle” was a
great success.
Darwin did not
publish his theory of
evolution for more
than 20 years.
Influences on Darwin’s Theory
Thomas Malthus
Wrote an essay
discussing principles
of population growth
Major point was that
populations tend to
increase beyond the
capacity of the
environment to
support them.
Getting Scooped
In 1858 received a
manuscript from a
colleague Alfred
Russel Wallace
Wallace had
summarized the major
points of the theory
Darwin had been
working on for 20
years
Influences on Darwin’s Theory
Charles Lyell
Suggested the
principle of
Uniformitarianism
Laws of physics and
chemistry remain the
same throughout
earth’s history
Past geological events
occurred by natural
processes similar to
those seen today
The Push to Publish
Developed idea over time and presented it
in 1844 as an unpublished essay
Began to assemble data into a planned 4volume book “as perfect as I can make it”
Plans were interrupted
Finally Published
Persuaded to publish one page statement with
Wallace’s paper in the Journal of the Linnean
Society
Prepared “abstract” of his planned 4 volume set
the next year (1859) titled “On the Origin of
Species by Means of Natural Selection, or the
Preservation of Favoured Races in the Struggle
for Life”
Produced many other books on evolution and
other topics over the next 23 years
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The Evidence for Evolution
Fig. 6.9
Perpetual Change
Main premise of
Darwinian Evolution is
the idea that the living
world is always changing
Most direct evidence is
from the fossil record
• Fossil record is biased
because preservation is
selective
• Most organisms leave no
fossils; the record is always
incomplete and requires
interpretation.
Geological Time
Law of Stratigraphy
Method of relative dating with the oldest layers
at the bottom and the youngest at the top of the
sequence.
Time divided into Eons, Eras, Periods, and
Epochs (see back inside cover of your text)
Radiometric methods more precise
Use radioactive decay of naturally occurring
elements to determine absolute age in years of
rock formations
Trends in the fossil record
Horse Evolution Shows Clear
Trend (Figure 6.11)
From Eocene to Recent periods,
genera and species of horses were
replaced.
Earlier horses had smaller sized
and fewer grinding teeth, and
more toes.
Reduction in toes and increase in
size and numbers of grinding
teeth correlate with environmental
changes.
Change occurred in both features
of horses and numbers of species
Evolutionary Trends
Fossil record used to view change across
broadest scale of time.
Most animal species survive ~1-10 million
years on average-but this is highly variable
Trends in fossil diversity produced by
different rates of species formation vs
extinction through time.
Common Descent
Darwin proposed that all plants and animals
descended from a common ancestor.
A history of life forms a branching tree called a
phylogeny.
This theory allows us to trace backward to
determine converging lineages.
All forms of life, including extinct branches,
connect to this tree somewhere.
Phylogenetic research is successful at
reconstructing this history of life.
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Homology and Phylogenetic Reconstruction
Darwin saw homology as major
evidence for common descent.
Richard Owen described
homology as “the same organ in
different organisms under every
variety of form and function.”
Vertebrate limbs show the same
basic structures modified for
different functions. (Figure
6.14)
Darwin’s central idea that apes
and humans have a common
ancestor was explained by
anatomical homologies
Ontogeny, Phylogeny and Recapitulation
Ontogeny is the history of development of an organism
throughout its lifetime.
Evolutionary alteration of developmental timing generates
new traits allowing divergence among lineages.
German zoologist Ernst Haeckel stated stages of
development represented adult forms from evolutionary
history.
“Ontogeny recapitulates phylogeny” also became known
as recapitulation or the biogenetic law.
Haeckel, Darwin’s contemporary, thought that this change
was caused by adding new features onto the end of
ancestral ontogeny; but this idea is Lamarckian.
Other factors
Ontogeny can be shortened in
evolution; terminal stages may
be deleted causing adults of
descendants to resemble
youthful ancestors.
Paedomorphosis is the
retention of ancestral juvenile
characteristics in descendent
adults. (Figure 6.17)
Organisms are a mosaic of
both; ontogeny rarely
completely recapitulates
phylogeny.
Ground-dwelling birds illustrate homologies
A new skeletal homology arises on each
lineage shown.
Different groups located at tips of branches
contain homologies that reflect ancestry.
Branches of the tree combine species into
nested hierarchies of groups within
groups.
Analysis of the living species alone can
reconstruct the branching pattern.
The pattern of nested hierarchies forms the
basis for classification of all forms of life.
Structural, molecular, and chromosomal
homologies are all combined to reconstruct
evolutionary trees.
Older theories that life arose many times
forming unbranched lineages fails to predict
the nested hierarchies of lineages;
creationism fails to provide testable
predictions; all fail as scientific hypotheses.
Embryonic similarities
Embryologist K.E. von Baer
showed early developmental
features were simply more
widely shared among different
animals groups. (Figure 6.16)
However, early development can
undergo divergence among
lineages too.
Evolutionary change in timing of
development is called
heterochrony.
Characteristics can be added late
in development and features are
then moved to an earlier stage.
Fig. 6.16
Multiplication of Species
Evolution as a Branching Process
A branch point occurs where an ancestral
species splits into two different species.
Darwin’s theory is based on genetic variation.
Total number of species increases in time; most
species eventually become extinct.
Much evolutionary research centers on
mechanisms causing branching.
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Speciation
Definition of species varies and may include
several criteria.
Members descend from a common ancestral
population.
Interbreeding occurs within a species but not
among different species.
Genotype and phenotype within a species is
similar; abrupt differences occur between species.
Allopatric Speciation
Allopatric populations occupy separate
geographical areas.
They cannot interbreed because they are
separated, but could do so if barriers were
removed.
Separated populations evolve independently and
adapt to different environments.
Eventually they are distinct enough they cannot
interbreed when reunited.
Hybridization
Hybridization is mating between divergent
populations; offspring are hybrids. (Figure 6.19)
Reproductive Barriers
Reproductive barriers are central to forming new
species.
If diverging populations reunite, before they are
isolated, interbreeding maintains one species.
Evolution of diverging populations requires they
be kept physically separate a long time.
Geographical isolation with gradual divergence
provides chance for reproductive barriers to form.
Allopatric speciation occurs in two ways:
1) Vicariant speciation occurs when climate or
geology causes populations to fragment; this may
affect many populations at one time but does itself
not induce genetic change.
2) Founder effect occurs when a small number
of individuals disperse to a distant place; this has
occurred with fruit flies in Hawaii
Reproductive Barriers
Premating barriers impair fertilization
Members may not recognize each other.
Male and female genitalia may not be compatible.
Behavior may be inappropriate to elicit reproduction.
Sibling species are indistinguishable in appearance but
cannot mate.
Postmating barriers impair growth and
development or survival.
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Nonallopatric Speciation
Fig. 6.20
When there is no evidence of physical barriers, it is difficult to explain
diversity of close species by allopatric speciation.
The huge variety of cichlid fishes in African lakes are found nowhere
else; yet lakes are evolutionarily young and without barriers.
Sympatric speciation is the term for the hypothesis that individuals can
speciate while living in different components of the environment.
African cichlid fishes are very different in feeding specialization.
Parasites may evolve with their host species.
It is difficult to observe formation of distinct evolutionary lineages in
allopatric speciation.
From one-third to one-half of plant species show sympatric evolution
using polyploidy; in animals polyploidy is rare.
Adaptive Radiation
Adaptive radiation produces diverse
species from common ancestral stock.
New lakes and islands provide new
opportunities for organisms to evolve.
Founders who were under heavy
competition are now free to colonize
the new habitat.
The Galápagos Islands provided
excellent isolation from mainland and
each other.
Darwin’s finches are example of
adaptive radiation from ancestral finch;
finches varied to assume characteristics
of missing warblers, woodpeckers, etc.
Gradualism
Darwin’s theory of gradualism is based on
accumulation of small changes over time.
He agreed with Lyell; past changes do not depend
on catastrophes not seen today.
We observe small, continuous changes; major
differences therefore require thousands of years.
4Accumulation of quantitative changes leads to
qualitative change.
Ernst Mayr distinguishes between populational
gradualism and phenotypic gradualism.
Fig. 6.22
Darwin’s Concept
Populational gradualism occurs when a new trait
becomes more common; this is well established.
Phenotypic Gradualism This theory states that strikingly
different traits are produced in a series of small steps.
It remains controversial ever since Darwin proposed it.
Mutations that cause substantial phenotypic change are called
“sports.”
Opponents of phenotypic gradualism contend such mutations
would be selected against.
Recent work in evolutionary developmental genetics illustrates the
continuing controversy surrounding phenotypical gradualism.
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Punctuated Equilibrium
Phyletic gradualism predicts that
fossils would show a long series of
intermediate forms.
Fossil record does not show the
predicted continuous series of fossils.
Some Darwinists contend that
fossilization is haphazard and slow
compared to speciation.
Niles Eldridge and Stephen Jay Gould
proposed punctuated equilibrium.
This theory contends phenotypic
evolution is concentrated in brief
events of speciation followed by long
intervals of evolutionary stasis.
Natural Selection
Natural selection gives a natural explanation
for origins of adaptation.
It applies to developmental, behavioral,
anatomical and physiological traits.
Darwin’s theory of natural selection
consists of five observations and three
inferences.
Observation 2
Natural populations normally remain
constant in size with minor fluctuations.
1) Natural populations fluctuate in size across
generations, sometimes going extinct.
2) No natural populations can sustain
exponential growth.
• Source: Darwin and many others
Evidence for Punctuated Equilibrium
Speciation is episodic with a duration of 10,000 to 100,000
years.
Species survive for 5-10 million years; speciation may be
less than 1% of species life span.
Small fraction of evolutionary history contributes most
morphological evolutionary change.
Allopatric speciation provides a possible explanation.
1)A small founder population has little chance of leaving fossils
that will every be found.
2)After a new genetic equilibrium forms and stabilizes, the larger
but different population is more likely to be preserved.
3)However, punctuated equilibrium occurs in groups where
founder events are unlikely.
Observation 1
Organisms have great potential fertility.
1) If all individuals produced would survive,
populations would explode exponentially
(Malthus).
2) Darwin calculated that a single pair of
elephants could produce 19 million offspring in
750 years.
Observation 3
Natural resources are limited (Malthus).
These three observations lead to Inference 1:
Struggle for food, shelter, and space becomes
increasingly severe with overpopulation.
Survivors represent only a small part of those
produced each generation.
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Observations 4 and 5
All organisms show variation.
Some variation is heritable.
1) Darwin only noted the resemblance of
parents and offspring.
2) Gregor Mendel’s mechanisms of heredity
were applied to evolution many years later.
• Sources: Animal breeding and systematics
Natural Selection can be viewed as a two-part
process: random and non-random
Production of variation among organisms is
random; mutation does not generate traits
preferentially.
The nonrandom component is the survival
of different traits.
1) Differential survival and reproduction is
called sorting; random processes may sort.
2) Natural selection is sorting that occurs
because certain traits give their possessors
advantages relative to others.
Revisions of Darwin’s Theory
Neo-Darwinism
Darwin did not know the mechanism of inheritance.
Darwin saw inheritance as a blending of parental traits.
He also considered an organism could alter its heredity through use
and disuse of parts.
August Weismann’s experiments showed an organism could
not modify its heredity.
Neo-Darwinism is Darwin’s theory as revised by Weismann.
Mendel’s work provided linkage through inheritance that
Darwin’s theory required.
Ironically, early geneticists thought mutations could cause
speciation in a single large step; selection was merely an
eliminator.
Inferences 2 and 3
Inference: There is differential survival and
reproduction among varying organisms in a
population.
Inference: Over many generations,
differential survival and reproduction
generates new adaptations and new species.
Source: Darwin
Criticisms of Natural Selection
Some critics contend natural selection cannot
generate new structures, only modify old ones.
(Irreducible complexity)
1) Many structures could not perform their function in
early evolutionary stages.
2) However, many structures evolved initially for
purposes different from the present.
3) Early feathers functioned in thermoregulation; they
later became useful for flight.
Emergence of Modern Darwinism:
The Synthetic Theory
In 1930s, a synthesis occurred that tied together
population genetics, paleontology, biogeography,
embryology, systematics and animal behavior.
Population genetics studies evolution as change in
gene frequencies in populations.
Microevolution is change of gene frequency over
a short time.
Macroevolution is evolution on a grand scale,
originating new structures and designs, trends,
mass extinctions, etc.
The synthesis combines micro- and
macroevolution and expands Darwinian theory.
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Microevolution: Genetic Variation and
Change Within Species: The Gene Pool
Fig. 6.27: Frequency of
type B allele in Europe
Different allelic forms of a gene constitute
polymorphism.
All alleles of all genes that exist in a
population are collectively the gene pool.
Allelic frequency is the frequency of a
particular allelic form in a population.
Genetic Equilibrium
Whether a gene is dominant or recessive does not
affect its frequency; dominant genes do not
supplant recessive genes.
In large two-parent populations, genotypic ratios
remain in balance unless disturbed.
This is called the Hardy-Weinberg equilibrium.
It accounts for the persistence of rare traits such as
albinism and cystic fibrosis caused by recessive
alleles.
Calculating H-W equilibrium
Genotype frequency can be calculated by expanding the
binomial (p - q)2 where p and q are allele frequencies.
For example, an albino is homozygous recessive and the
trait is represented by q2 in the formula: p2 + 2pq + q2 = 1.
Albinos (homozygous recessive) occur in one in 20,000;
therefore q2 = 1/20,000 and q = 1/141.
Non-albino (p) is 1 - q = 140/141.
Carriers would be 2pq or 2 x 140/141 x 1/141 = 1/70; one
person in 70 is a carrier.
Eliminating a “disadvantageous” recessive allele is nearly
impossible.
Selection can only act when it is expressed; it will continue
through heterozygous carriers
See the box on page 122
How Genetic Equilibrium is Upset
How Genetic Equilibrium is Upset
In natural populations, Hardy-Weinberg equilibrium is
disturbed by one or more of five factors.
1. Genetic Drift (Figure 6.28)
2. Nonrandom Mating
a. A small population does not contain much genetic variation.
b. Each individual contains at most two alleles at a single locus; a mating
pair has a maximum of four alleles to contribute for a trait.
c. By chance alone, one or two of the alleles may not be passed on.
d. Chance fluctuation from generation to generation, including loss of
alleles, is genetic drift.
e. There is no force causing perfect constancy in allelic frequencies.
f.The smaller the population, the greater the effect of drift.
g. If a population is small for a long time, alleles are lost and response to
change is restricted.
a. If two alleles are equally frequent, one half of the population will be
heterozygous and one quarter will be homozygous for each allele.
b. In positive assortative mating, individuals mate with others of the
same genotype.
• 1) This increases homozygous and decreases heterozygous genotypes.
• 2) It does not change allelic frequencies.
c. Inbreeding is preferential mating among close relatives.
• 1) Inbreeding increases homozygosity.
• 2) While positive assortative mating affects one or a few traits, inbreeding
affects all variable traits.
• 3) Inbreeding increases the chance that recessive alleles will become
homozygous and express.
• 4) Inbreeding cannot change gene frequencies; genetic drift does and both
are common in small populations.
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How Genetic Equilibrium is Upset
3. Migration
a. Migration prevents different populations
from diverging.
b. Continued migration between Russia and
France keeps the ABO allele frequencies from
becoming completely distinct
How Genetic Equilibrium is Upset
5. Interactions of Selection, Drift and Migration
a. Subdivision of a species into small populations
that exchange migrants promotes rapid evolution.
b. Genetic drift and selection allow many
combinations of many genes to be tested.
c. Migration allows favorable new combinations
to spread.
d. Interactions of all factors produce change
different from what would result from one alone.
e. Perpetual stability almost never occurs across
any significant amount of evolutionary time.
Macroevolution: Major
Evolutionary Events
How Genetic Equilibrium is Upset
4. Natural Selection
a. Natural selection changes both allelic
frequencies and genotypic frequencies.
b. An organism that possesses a superior
combination of traits has a higher
relative fitness.
c. Some traits are advantageous for
certain aspects of survival or
reproduction and disadvantageous for
others.
d. Sexual selection is selection for traits
that obtain a mate but may be harmful
for survival.
e. Changes in environment alter selective
value of traits making fitness a complex
problem.
Quantitative Variation
Quantitative traits show
continuous variation with no
Mendelian segregation pattern.
Such traits are influenced by
variation at many genes.
Such traits show a bell-shaped
frequency distribution.
Stabilizing selection favors the
average and trims the extreme.
Directional selection favors an
extreme value to one side.
Disruptive selection favors the
extremes to both sides and
disfavors the average
Fig. 6.31
Fig. 6.32
Speciation links macroevolution to microevolution.
The timescale of population genetics processes is from tens
to thousands of years.
Rates of speciation and extinction are measured in millions
of years.
Periodic mass extinctions occur in tens to hundreds of
millions of years.
Five mass extinctions have been dramatic.
Study of long-term changes in animal diversity focuses on
this longest timescale
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Speciation and Extinction
Through Geological Time
A species has two possible fates: become extinct or give
rise to new species.
Rates of speciation and extinction vary among species.
Lineages with high speciation and low extinction produce
the greatest diversity.
Lineages whose characteristics increase probability of
speciation and confer resistance to extinction should come
to dominate.
Species selection is differential survival and multiplication
of species based on variation among lineages.
Species-level properties include mating rituals, social
structuring, migration patterns, geographic distribution,
etc.
Effect macroevolution
Effect macroevolution is similar but differential
speciation and extinction is caused by variation in
organismal-level properties rather than species-level
properties.
Food specialists would therefore be more likely to be
geographically isolated.
A lineage of specialized grazers and browsers has high
speciation and extinction rates.
A lineage of generalist grazers and browsers shows neither
branching speciation nor extinction during the same time.
Interestingly, the two lineages have similar numbers of
individual animals alive today. (see figure 6-33)
Mass Extinction
Periodic events where huge numbers of taxa go
extinct simultaneously are mass extinctions.
The Permian extinction occurred 225 million
years ago; half of the families of shallow water
invertebrates and 90% of marine invertebrates
disappeared.
The Cretaceous extinction occurred 65 million
years ago and marked the end of the dinosaurs and
many other taxa.
Mass extinctions appear to occur at intervals of 26
million years.
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