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Evolution Unit
Earth’s History
Evolution Viewpoint
• Big Bang Theory
• Earth is 4.6 billion
years old
• Life has changed
over time.
• Many species failed
to survive and
became extinct.
Life’s History
Life’s existed for at least
3.6 billion years, most of
Earth’s history.
For most of its history, life
was exclusively
unicellular.
The first half of life’s
history saw only
prokaryotic organisms.
Multicellular eukaryotes
arose about 1 billion years
ago.
Life’s History
Animals diversified in the
seas about 600 million
years ago.
Plants colonized land
about 440 million years
ago and were followed
shortly by land animals.
Humans of any sort are a
very recent evolutionary
innovation (~ 7 million
years before present).
Phylogeny
• Phylon = tribe,
geny = genesis
or origin
• The evolutionary history of a species or a
group of related species.
• Traces life backward to common
ancestors.
• How did we know about past life on Earth?
Phylogeny
• Found in fossils and the fossil record.
Fossils
• Any preserved remnant or impression of
a past organism.
Fossils
• Relics or impressions of organisms from
the past.
• Earliest life forms- 3.6 billion years old.
• Earth - 4.6 billion years old.
• Problem:
– Fossil record is incomplete.
– Show changes over time from simple to
complex.
– Many fossils don't have descendants.
Evidence for Evolution – The Fossil Record
Types of Fossils
1. Mineralized
2. Organic Matter
3. Trace
4. Amber
Mineralized Fossils
• Found in sedimentary rock.
• Minerals (rock) replace cell contents.
• Ex: bone, teeth, shells
Organic Matter Fossils
• Retain the original organic matter.
• Ex: plant leaves trapped in shale.
• Comment – can sometimes extract DNA
from these fossils.
Trace Fossils
• Footprints and other impressions. No
organic matter present.
Amber
• Fossil tree resin.
• Preserve whole specimen.
• Usually small insects etc.
Fossils - Limitations
•
•
•
•
Rare event.
Hard to find .
Fragmentary.
Dating.
Fossil Dating Methods
1. Relative Dating - by a fossil's position in the
strata relative to index fossils.
• Law of Superposition
2. Absolute Dating - approximate age on a scale of
absolute time.
• called Radiometric dating
• Estimated from half-life products in the fossil.
• Ex: Carbon - 14
Potassium - 40
What do fossils tell us?
• That the geographical distribution of
organisms has changed over time.
• Reason? – The land formations of the
earth have changed.
Continental Drift
• The movement of the earth's crustal plates
over time.
• Drift is correlated with events of mass
extinctions and adaptive radiations of life.
Pangaea
• 250 million years ago.
• One super continent.
• Many life forms
brought into contact
with each other.
Mesozoic era
• Pangaea began to break
up.
• 180 million years ago.
Results:
• Geographical Isolation.
• New environments
formed.
• Old environments lost.
• As the environments
changed, so did Life.
For example:
• Australian fauna and flora
are unique.
• Separated early and
remained isolated for 50
million years.
Mass Extinctions
• At the end of each Era, the Earth
experienced a mass extinction.
• The sudden loss of many species in
geologic time.
• May be caused by asteroid hits or other
disasters.
Eamples of Mass Extinction:
Permian Extinction
Cretaceous Extinction
Permian Extinction
• 250 million years ago.
• 90% of species lost.
Cretaceous Extinction
• 65 million years ago.
• Loss of the dinosaurs.
• Good evidence that this event was caused
by an asteroid that hit in the Yucatan,
causing a “nuclear winter”.
• Earth has a layer of iridium in the crust.
Iridium is only found in outer space.
Result of Mass Extinctions
• Areas (niches) are open for the surviving
species to exploit.
• Rapid period of speciation (adaptive radiation).
• Many new
species are
formed in
a very
short period
of time.
Origins of Life
Prokaryotes (1st Life forms)
Fossil
Modern
Bacterial Mats/ Cyanobacteria
Early Thoughts on Origins of Life
Spontaneous Generation = the idea that nonliving
material can produce life
Based on observations of environments:
Idea that decaying meat produced maggots
Idea that mud produced fish
Idea that grain produced mice
Was life produced by spontaneous generation?
NO, spontaneous generation disproved by
experiments of Redi and Pasteur
Francesco Redi’s Experiment
• 1668 Italian physician
• Disproved the common belief that
decaying meat produced maggots with
his controlled experiment.
• Redi disproved the spontaneous
generation of large organisms.
Louis Pasteur’s Experiment
In the mid-1800s, Pasteur
designed an experiment that
disproved the spontaneous
generation of
microorganisms.
Biogenesis= the idea that living
organisms come from other living
organisms
Experiments disproving spontaneous generation lead to
the theory of biogenesis.
Modern Theories of How Life
Developed on Earth
Theory of Chemical Evolution
The evolution of life by abiogenesis under
primitive Earth conditions.
• Reducing atmosphere present.
• Simple molecules
– Ex: H2O, CH4, H2, NH3
Steps of Chemical Evolution
1. Monomer Formation (amino acids)
2. Polymer Formation
(proteins)
3. Protobiont Formation
4. Origin of Heredity
(DNA & RNA)
Complex Molecule Formation
• Requires energy
sources:
– UV radiation
– Radioactivity
– Heat
– Lightning
– These factors were
present on primitive
Earth before life
started.
Alexander Oparin 1930s
• Hypothesized steps of chemical evolution from primitive
earth conditions.
• Earth’s early atmosphere contained no free oxygen.
Instead, it was composed of water vapor, carbon dioxide,
nitrogen, methane, and ammonia.
• Oparin suggested that energy from the sun, lightning,
and Earth’s heat triggered chemical reactions to produce
small organic molecules from the substances present in
the atmosphere.
• Then, rain washed the molecules into the oceans to form
a primordial soup that gave rise to life.
Miller and Urey, 1953
• Tested Oparin’s
hypothesis by simulating
primitive earth conditions
in the lab.
• Organic monomers
formed, including amino
acids, sugars, lipids,
nucleotides, and ATP.
• Early earth conditions
could have formed
monomers for life's
origins.
Alternate View of Origin of Life
Life developed in
Volcanic Vents.
• Could easily supply
the energy and
chemical precursors
for chemical
evolution.
• Most primitive life
forms are the
prokaryotes found in
or near these vents.
Meteorite Hypothesis
• Space seeds
• Molecules necessary for life arrived here
on meteorites, rocks from space that
collide with Earth’s surface.
• Many meteorites contain some organic
matter.
• These molecules, which are necessary for
the formation of cells, might have arrived
on Earth and entered its oceans.
Hypothesis
• Life is a natural outcome of chemical
evolution.
• Life could be possible on many planets in
the universe.
Modern Earth
•
•
•
•
Oxidizing atmosphere (contains oxygen).
Prevents new abiotic formation of life.
Life is present.
Biogenesis (life comes from life)
Descent with Modification:
A Darwinian View of Life
Question?
How did the diversity of life originate?
Through the process of Evolution.
What is Evolution?
The kind we’re talking about is sometimes called organic evolution to distinguish it
from non-biological changes over time.
Definition: Evolution is the progressive change in organisms (populations, not
individuals) over time.
These processes have transformed life on earth from its beginnings
to today's diversity.
Evolution is the most pervasive
principle in biology.
Charles Darwin
• Father of the modern
theory of evolution.
• Theory of Evolution
by Natural Selection
• Descent with
Modification.
Darwin's Background
• Trained as a Naturalist (after trying religion
and medicine).
Darwin’s Voyage of Discovery
A reconstruction of the HMS Beagle sailing off Patagonia.
Voyage of the Beagle
Result
• Darwin's training and travel opportunities
allowed him to formulate and support his
ideas on Natural Selection.
Galapagos Finches
Evolution
Nothing in biology makes sense except in
the light of evolution. – Theodosius
Dobzhansky
Charles Darwin in later years
Darwin’s Ideas Did Not Develop in a Vacuum
Contributor’s to Darwin’s thinking included:
Charles Lyell – uniformatarianism.
1797-1875
Georges Cuvier – species extinction.
1769-1832
Darwin’s Ideas Did Not Develop in a Vacuum
Contributor’s to Darwin’s thinking included:
Thomas Malthus – struggle for existence.
1766-1834
Jean Baptiste de Lamarck – evolution by
acquired characteristics.
1744-1829
Alfred Russel Wallace Independently Drew the Same
Conclusions as Darwin
Papers from Wallace and Darwin were jointly presented (with little
impact) to the Linnaean Society in 1858.
Darwin’s Observations and Inferences
Observation 1: Left unchecked, the number of
organisms of each species will increase exponentially,
generation to generation.
Observation 2: In nature, populations tend to remain
stable in size.
Observation 3: Environmental resources are limited.
Inference 1: Production of more individuals than can be supported by the
environment leads to a struggle for existence among individuals, with only a
fraction of offspring surviving in each generation.
Darwin’s Observations and Inferences
Inference 2: Survival in the struggle
for existence is not random, but
depends in part on the heritable
characteristics of individuals.
Individuals who inherit
characteristics most fit for their
environment are likely to leave more
offspring than less fit individuals.
Darwin’s Observations and Inferences
Inference 3: The unequal ability of individuals to survive and reproduce leads to a
gradual change in a population, with favorable characteristics accumulating over
generations (natural selection).
Taken together, these three inferences are a statement of
Darwin’s Theory of Evolution.
Darwinian View
• History of life is like a tree with branches
over time from a common source.
• Current diversity of life is caused by the
forks (or splits) from common ancestors.
Example
“The Origin of Species”
• 1859 published The
Origin of Species
• Documented the
occurrence of
evolution.
• Suggested that the
mechanism for
evolution was Natural
Selection.
Nature
• Determines which characteristics are
favorable.
• Determines who survives.
• Result - “Natural Selection”
Artificial Selection
• When man determines the characteristics
that survive and reproduce.
• Result - the various breeds of animals and
plants we’ve developed.
Ex - Mustard Plant
Original
Cultivars
Evolution Success Measured By
•
•
•
•
Survival
Reproduction
Variation within a population
Whoever lives long enough and has kids is
the “winner” in evolution.
Comment
• Acquired characteristics may allow a
species to evolve "outside" of Natural
Selection.
• Ex: culture, learning
Proof for Evolution
1. Biogeography
2. Fossil Record
3. Taxonomy
4. Comparative Anatomy

Homologous features

Analogous features

Vestigial organs
5. Comparative Embryology
6. Comparative Biochemistry and Molecular
Biology (DNA sequences)
7. Artificial Selection
Biogeography
• The geographical distribution of species.
• Problem:
– Species mixtures on islands
– Marsupials in Australia
Taxonomy
• Science of Classification.
Main Categories
•
•
•
•
•
•
•
Kingdom
Phylum or Division
Class
Order
Family
Genus
Species
Evidence for Evolution - Comparative Morphology
Why use the same skeletal plan for these very different appendages?
Comparative Anatomy
1. Homologous
Structures Common "building
plan” with divergent
functions.
Mammal forelimbs
Homologous structures
It’s Critical (and often difficult) To Distinguish Homology
from Analogy
Homologous structures, like the bat wing and gorilla arm, are similar because
they are derived by modification of a shared ancestral structure.
Homology is the key to establishing phylogenies.
Distinguishing Homology from Analogy
Analogy is similarity due to convergent evolution.
Analogy mistaken for homology confuses phylogenies.
Another Set of Analogies Created by Convergent Evolution
Ocotillo of the
US southwest
Allauidia of
Madagascar
Comparative Anatomy
2. Vestigial Organs Rudimentary
structures of
marginal, if any, use.
Whale Legs
Human Example of Vestigial Organ
Comparative Embryology
• Closely related organisms go through
similar stages in their embryonic
development.
• Ex: Gill pouches in vertebrates
Evidence for Evolution - Comparative Embryology
Why do embryos of different animals pass through a similar developmental stage?
Recent discoveries of the conservation of molecular mechanisms of development are
even more compelling.
Comparative Embryology
Molecular Biology
• Study of Evolution at the DNA or protein
levels.
• Related species have similar DNA
sequences.
• Related species share a common
ancestral DNA. The closer the
relationship, the more similar the DNA
sequences should be.
Comparative Biochemistry and
Molecular Biology
Evidence of
Evolution –
Conservation and
Diversification at
the Molecular
Level
Why should
different
organism possess
related genes?
Why does the
degree of
relationship of
genes match their
degree of
relationship
established by
other methods?
What Drives Evolution?
There are 5 sources of genetic variation.
Only natural selection
makes a population better
adapted (more fit) to its
environment.
Mutations Provide Raw Material For Evolution
One type of mutation at the level of
the gene.
One type of mutation at the
level of the chromosome.
Mutations are usually neutral or harmful in their effects; only rarely are they beneficial.
Mutations “Just Happen”
Mutations occur at random without regard to whether they have a beneficial,
neutral or harmful effect.
For this reason, mutations are a randomly acting evolutionary force.
Mutation
Mutation is the only source of new alleles in a species.
Mutation acting alone works too slowly to drive evolution.
Loss of an allele
due to mutation
With an average mutation
rate, it takes ~ 70,000
generations, far more
than the number of
generations of modern
humans, to reduce allele
frequency by 50%.
Chromosome Sources of Genetic Variation
• Mutations.
• Recombination though sexual
reproduction.
– Crossing-over
– Random fertilization
Natural Selection
Natural selection leads to adaptation – an increase in the fitness of a population in a
particular environment.
Natural selection works because some genotypes are more successful in a given
environment than others.
Successful (adaptive)
genotypes become more
common in subsequent
generations, causing an
alteration in allele
frequency over time that
leads to a consequent
increase in fitness.
It’s not natural – but this is one
outcome of strong selection.
Modes of Natural Selection
1. Stabilizing
2. Directional
3. Disruptive
4. Sexual
Three Forms of Natural Selection
Stabilizing Selection
• Selection toward the average and against
the extremes.
• Ex: birth weight in humans
Human Birth Weight Is Under Stabilizing Selection
Modern medicine relaxes this and other forms of selection.
Stabilizing Selection for the Sickle Cell Allele
In heterozygous form, the sickle cell allele of -globin confers resistance to
malaria. Therefore, the allele is maintained, even though it’s harmful in
homozygous form.
Directional Selection
• Selection toward one extreme.
• Ex: running speeds in race animals.
• Ex. Galapagos Finch beak size and food
source.
A Galapagos Finch, the Subject of a Classic Study of
Evolution in Action
Peter and Mary Grant and their
colleagues observed how beak
depth, a significant trait for
feeding success, varied in
populations experiencing
climactic variations.
Beak Depth Changed in a Predictable Way in Response to Natural
Selection
Significantly, beak depth is a
genetically determined trait.
Directional Selection
Evolution of pesticide resistance
in response to selection.
Disruptive Selection
• Selection toward both extremes and
against the norm.
• Ex: bill size in birds
• Disruptive/Diversifying Selection - can split
a species into several new species if it
continues for a long enough period of time
and the populations don’t interbreed.
Sexual Selection
• May not be adaptive to the environment, but
increases reproduction success of the individual.
• Sexual dimorphism.
• Secondary sexual features for attracting mates.
Life’s History
and Diversity
The Big Picture
One Hypothesis of Hominan
Evolution
Hominans are the group of species (that
includes us) on the line of descent from
the last common ancestor of modern
humans and chimpanzees.
Note that many species of hominans
have existed but there is only one extant
species – Homo sapiens.
Not all details of hominan evolution (the
branching pattern of the cladogram) are
firmly established.
Africa is Birthplace of
Hominans
Sahelanthropus tchadensis – Possibly
the Earliest Hominan
This fossil skull is from a 7 million year old hominan that lies at the root of
the hominin lineage.
The specimen, named Toumaï (hope of life in the local language) is that of a
young boy.
Artist’s Conception of Two Hominan Species
Homo habilis stayed
close to home (Africa)
Homo ercetus traveled
widely through Africa
and Asia.
The Ebb and Flow of Hominan Species
Nature (2004) 248 (Oct 28):1043
Note that the roots of ancient and modern migrations (with the exception
of H. floresienses) begin in Africa
Genetic Evidence Indicates That Modern Humans Arose In Africa
About 150,000 Years Ago
Starting About 60,000 Years Ago, Modern Human
Migrated From Africa
Genetic similarities and differences among modern populations indicates that
modern humans migrating from Africa displaced existing hominads.
This is the Out-of-Africa hypothesis.
Neandertals Disappeared Soon After Modern Human
Arrived in Their Range
Neandertal and modern human skulls
Neandertals are not our ancestors.
Neandertals may have disappeared
because they were outcompeted by our
own species.
When Did The First Modern Humans Arise?
Anatomically modern humans arose
~ 150,000 years ago.
Culturally modern humans probably did not
exist until ~ 100,000 years ago.
17,000 year old paintings from the Lascaux cave in France.
http://www.nhm.ac.uk/natureonline/evolution/what-isevolution/natural-selection-game/theevolution-experience.html
http://phet.colorado.edu/en/simulation
/natural-selection
The End