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EVOLUTION The History of Life
8.1 Fossil Evidence of Change
O Learning Goal: Explain and provide
examples for various types of evidence that
supports evolution – fossils
Earth’s Early History
O Earth = 4.6 billion years old
O Land – molten body
O Volcanoes
O Very very hot!
O Atmosphere
O Made of volcanic gasses
O H2O, CO2, SO2, CO, H2S, HCN, N2, H2
Clues in Rocks
O 3.5 billion years ago life first appeared
O Fossils – preserved evidence of organisms
O Any living thing can form a fossil
O Most decompose before they become
fossilized
Dating Fossils
O Over time layers of rock form
O Younger layers of rock are deposited on top of
older layers
Relative Dating
O Determines the age of rocks by comparing
them with rocks in other layers
Radiometric Dating
O Uses the decay of radioactive isotopes to
measure the age of a rock
O Uses half-life of the isotope
O Amount of time it takes for half the original
isotope to decay
O Uranium 238 = 4510 million years
O Carbon 14 = 60,000 years
Geologic time scale
O Era = hundreds of millions of years
O Periods = tens of millions of years
O Epochs = a few million years
O Geologic time scale
Important events
O Precambiran time = 90% of Earth’s history
O 1st life in oceans
O Cambrian explosion = rapid evolution of
most major animal groups
O 1st life on land
Cenozoic era
O Most recent era
O Mammals evolved!
O 1st resembled shrews
Mass extinctions
O Many species become extinct in a short
amount of time
O 65-70% of all species alive
O Occur every 25-30 million years
O Generally result in a new era starting
K-T boundary
O About 65 million years ago
O Extinction of the dinosaurs
O Most likely caused by asteroid hitting earth
O Why would this cause extinction?
Plate tectonics
O Earth is made of several large plates that
can move!
O Movements of plates help species diversify
O Why?
8.2 The Origin of Life
O Evidence indicates that a sequence of
chemical events preceded the origin of life
on Earth and that life has evolved
continuously since that time.
Early theories
O Spontaneous generation – life arises from
nonlife
O Mice could be created by putting hay in a
corner
O Flies come from rotting meat
Francesco Redi - 1668
O Hypothesized that flies (not meat) produced
other flies
Theory of Biogenesis
O Only living organisms can produce other
living organisms
O Not accepted until mid 1800s
Louis Pasteur
Modern Theories
O If life can only come from pre-existing life
how did the first life appear?
O Series of chemical events early in Earth’s
history
O Complex organic molecules made from
simpler ones
O Metabolic pathways developed
Primordial Soup Hypothesis
O If Earth’s early atmosphere had a mix of
certain gases, organic molecules could have
been made from simple reactions
O UV light from sun & electricity from lighting =
energy source
O Organic molecules made = precursors to life
Miller & Urey - 1953
O Experimented with primordial soup
hypothesis
O Built glass apparatus that simulated early
Earth’s conditions
O Filled with water and gases thought to make
up atmosphere
O Water boiled & electricity added
O Made organic compounds!!!
Later Discoveries
O HCN can be made even easier
O HCN can react with itself to make Adenine
Making Proteins
O Next step in forming life
O Amino acids may have bound to clay to form
first proteins
Genetic Code & Cells
O RNA = 1st coding system
O Formation of membranes = crucial to
making cells
O Not sure how cells first evolved
Cellular Evolution
O 1st cells left no fossils
O Earliest fossils = 3.5 billion years old
O Earliest rocks = 3.8 billion years old
O Chemicals in these rocks suggest life present
O Early life linked to volcanic environment
The first cells
O Prokaryotes
O Autotrophs
O Why???
O Lived in extrememe environments
O Hot springs
O Volcanic vents
O Archaea = closest relative
Photosynthesizing prokaryotes
O Oxygen absent from Earth’s atmosphere
until 1.8 billion years ago
O 1st photosynthesizing prokaryotes =
cyanobacteria
O Eventually produced enough oxygen to form
ozone.
O Once ozone established, conditions were
right for eukaryotic cells
Endosymbiont theory
O Eukaryotic cells appeared 1.8 billion years
ago
O 1966 – theory proposed
O Prokaryotes lived inside eukaryotes
O Eventually became organelles
Support for Endosymbiont Theory
O Mitochondria & Chloroplasts
O Contain their own DNA
O Arranged in circular pattern like prokaryotes
O Have ribosomes
O More like prokaryote than eukaryote
O Reproduce by fission
O Divide independently from rest of cell
8.3 Darwin’s Theory
O Learning Goal: Explain how natural selection
(variation, inheritance, competition, survival)
is one form of evolution.
Darwin on the HMS Beagle
O On the ship as a naturalist
O Collect biological and geological samples
O 5 year voyage
O Read Charles Lyell’s Principles of Geology
O Proposed that Earth was millions of years old
The Galapagos Islands
O Off the coast of South America
O Noticed islands had slightly different
varieties of animals
O Also noticed animals on islands were similar
to animals in South America
O Had very different environments
O So why were they similar?
Tortoises & Finches
O Each island had
slightly different
species
O Didn’t resemble
European animals,
but resembled South
American animals
with slight variations
Darwin’s continued studies
O Hypothesized new species could appear
gradually through small changes in
ancestral species
O Artificial selection – directed breeding to
produce offspring with desired traits
O Darwin inferred 2 things from artificial
selection:
O If humans can change species by artificial
selection then the same process could work
in nature
O Given enough time this process could
produce new species
Natural Selection
O While pondering artificial selection Darwin
read an essay by Thomas Malthus
O Said human population would eventually
outgrow food supply leading to a competitive
struggle for existence
O Applied essay to nature: some competitors
are better equipped for survival than others
O Natural Selection – animals equipped for
survival would survive, animals not
equipped for survival would die
O Survival of the fittest
4 Principles of Natural Selection
1. Individuals in a population show variations
2. Variations can be inherited
3. Organisms have more offspring than can
survive
4. Variations that increase reproductive
success will have a greater chance of
being passed on than those that do not
increase reproductive success
The Origin of Species
O 1858 – both Alfred Wallace and Darwin proposed basically the same theory
O Darwin published On the Origin of Species by Means of Natural Selection
O On the last page used the term evolution
O Now used to define cumulative changes in groups of organisms through time
O Natural selection not synonymous with
evolution
O ONE mechanism by which evolution occurs
8.4 Evidence of Evolution
O Learning Goal: Explain and provide
examples for various types of evidence that
supports evolution (fossil, molecular, bio-
geographic, embryological, observational)
Fossil Evidence of Evolution
O Some of the most significant evidence of
evolutionary change
O Can show ancient species that are similar to
current ones
O Show some species have remained
unchanged for millions of years
O Transitional fossils – intermediate in form
between species
O Contain features shared by different species
O Archaeopteryx – between a bird & a
dinosaur
O 2 major classes of traits in transitional
fossils
O Derived traits – newly evolved features (not in
fossils of common ancestors)
O Ancestral traits – more primitive features that
do appear in ancestral forms
Comparative Anatomy
O Homologous structures – anatomically
similar structures inherited from a common
ancestor
O Vestigial structures – reduced forms of
functional structures in other organisms
O Snake pelvis
O Ostrich wings
O Human appendix
O Analogous structures – can be used for the
same purpose and look similar but are not
inherited from a common ancestor
O Bird wings & fly wings
Embryological Evidence of Evolution
O Embryo – early pre-birth stage of an
organism’s development.
O Vertebrate embryos look similar during
certain phases of development but then
become totally different
Molecular Evidence of Evolution
O Many different organisms have the same
complex metabolic molecules
O Proteins
O DNA
O RNA
O Suggests these molecules evolved early on
and were passed through ancient species to
modern ones
Biogeographic Evidence
O Biogeography – study of distribution of
plants and animals on earth
O Animals in Galapagos islands were more
similar to South American animals than
European animals living in similar
environments
Adaptation
O Trait shaped by natural selection that
increases an organism’s reproductive
success
O Fitness: measure of the relative contribution
a trait makes to the next generation
O Number of good offspring produced
Camouflage
O Adaptation that allows organisms to blend in
with their environment.
O How does this contribute to fitness?
Mimicry
O One species evolves to resemble another,
more dangerous species
O How does this contribute to fitness?
8.5 Shaping Evolutionary Theory
O Learning Goal: Explain how other
evolutionary mechanisms (non-random
mating, coevolution, mutations) can account
for evolutionary changes in life forms
Genetic Drift
O Any change in allele frequencies in a
population that is due to chance
O Founder effect – small sample of a
population settle in a location separated
from the rest of the population
O Uncommon alleles can become common
O Bottleneck effect – population declines to a
very low number then rebounds
O Population now has less genetic diversity
Nonrandom mating
O Mating is not completely random in a
population
O Organisms mate with individuals close to
them
O Close to them = more like them
O Increase of homozygous individuals
Mutation
O Random change in genetic material
O If mutation provides an advantage it will
then be selected for and become more
common
O Raw material upon which natural selection
occurs
Types of Natural Selection
O Stabilizing selection
O Most common
O Gets rid of extreme expressions of a trait
O Directional selection
O An extreme version of a trait becomes more
common
O Peppered moths
O Disruptive selection
O Removes organisms with average traits
O Splits populations in two groups
Reproductive Isolation
O Prezygotic isolation – prevent reproduction
by making fertilization unlikely
O Geographic
O Ecological
O Behavioral
O Postzygotic isolation – fertilization has
occurred but hybrid offspring cannot
reproduce
O Horse + Donkey = Mule
Speciation
O Evolutionary process by which new species
arise
O Animals from two different species can’t
reproduce
O For this to occur a population must separate
and then become reproductively isolated
Allopatric Speciation
O Physical barrier divides one population into
two or more populations
O Separate populations will eventually develop
into new species
Sympatric Speciation
O Species evolves into a new species without a
physical barrier
O Ancestor species and new species live side
by side during this process
O Happens in insects and plants
Adaptive Radiation
O One species gives rise to many species in
response to the creation of new habitat
O Often follows large-scale extinctions
O Mammals after the extinction of dinosaurs
Coevolution
O Evolution of one species affects evolution of
another because they have a very close
relationship
Convergent evolution
O Unrelated species evolve similar traits
O Occurs in environments that are far apart
but have similar ecology and climate
Rate of Speciation
O Evolution is always happening!!!
O May take hundreds or millions of years
O Gradualism – evolution happens in small
gradual steps
O Punctuated equilibrium – rapid spurts of
genetic change causes species to diverge
quickly
