Evidence for
Evolution
Today’s Objective
(learning goal)
To identify how fossils are used as
evidence in changes within a species.
Clues to the Past
• About 95 percent of the species that have existed are
extinct—they no longer live on Earth.
• The oldest rocks that have been found on Earth formed
about 3.9 billion years ago.
• Among other techniques, scientists study fossils to learn
about ancient species.
• Fossils are evidence of organisms that lived long ago that
are preserved in Earth’s rocks.
TYPES OF FOSSILS
Fossils Types
Formation
A trace fossil is any indirect evidence
Trace fossils
Casts
left by an animal and may include a
footprint, a trail, or a burrow.
When minerals in rocks fill a space
left by a decayed organism, they make
a replica, or cast, of the organism.
A mold forms when an organism is
Molds
Petrified
fossils
Amber-Preserved
or
frozen fossils
buried in sediment and then decays,
leaving an empty space.
Petrified-minerals sometimes penetrate
and replace the hard parts of an organism
At times, an entire organism was
quickly trapped in ice or tree sap that
hardened into amber.
• Paleontologists, scientists who study ancient life, are like
detectives who use fossils to understand events that
happened long ago.
• They use fossils to determine the kinds of organisms that
lived during the past and sometimes to learn about their
behavior.
• Paleontologists also study fossils to gain knowledge about
ancient climate and geography.
• By studying the condition, position, and location of rocks
and fossils, geologists and paleontologists can make
deductions about the geography of past environments.
• For example, if they find a fossil of a plant that resembles a
present day plant that can only survive in mild weather,
they can infer that the conditions were mild when that
plant was living as well.
•
For fossils to form, organisms usually have to be
buried in mud, sand, or clay soon after they die.
• Fossils are not usually found in other types of rock because of the ways those rocks
• Most
These rocks form at relatively low
form.fossils are found in sedimentary rocks.
temperatures and pressures that may prevent damage to the organism.
• For example, the conditions under which metamorphic rocks form often destroy
any fossils that were in the original sedimentary rock.
• Few organisms become fossilized because, without burial,
bacteria and fungi immediately decompose their dead
bodies.
• Occasionally, however, organisms do become fossils in a
process that usually takes many years.
• Sediments from
upstream rapidly
cover the body,
slowing its
decomposition.
Minerals from the
sediments seep into
the body.
• Over time, additional
layers of sediment compress
the sediments around the
body, forming rock. Minerals
eventually replace all the
body’s bone material.
• A Protoceratops drinking at
a river falls into the water and
drowns
• Earth movements
or erosion may
expose the fossil
millions of years after
it formed.
• Scientists use a variety of methods to determine the age of
fossils.
• One method is a technique called relative dating.
• If the rock layers have
not been disturbed, the
layers at the surface
must be younger than
the deeper layers.
• Thus, the fossils in the top layer must also be
younger than those in deeper layers.
• Using this principle, scientists can determine relative age
and the order of appearance of the species that are
preserved as fossils in the layers.
• To find the specific ages of rocks, scientists use radiometric
dating techniques utilizing the radioactive isotopes in rocks.
• Radioactive isotopes are atoms that are unstable and break
down, or decay, over time, giving off radiation.
• Because every radioactive isotope has a characteristic
decay rate, scientists use the rate of decay as a type of
clock.
• The half-life of an isotope is the time it takes for half of the
isotope in a sample to decay
• A radioactive isotope forms a new isotope after it decays.
• If you can know the amount of an unstable isotope
that was in a sample
• And you know the rate at which that isotope decays
• And you can measure the amount of that isotope
presently in the sample
• You can figure out how old the sample is
•
14C
is used to date organic samples like wood, hair,
shells, and other plant and animal products
• Atmospheric 14C is incorporated into organic
molecules by plants during photosynthesis
• Animals that eat the plants get 14C from the plants
they eat
Black dots represent carbon, grey dots carbon14
• Scientists use carbon-14 to date fossils less than 70 000
years old.
• Carbon-14 (14C) is an isotope of carbon, that has 6
protons and 8 neutrons
•
14C
decays to 14N at a constant rate
• Every 5,730 years half the 14C in a sample will emit a
beta particle (electron) and decay to 14N
• Thus 5,730 years is called the half life of 14C
• For example, if the half-life of 14C (Carbon 14) is
5,730 years
and a sample today has 1,000 14C atoms
after 5,730 years 500 14C atoms will remain
256 14C atoms
After 5730
years
or 1 half-life
128 14C and
128 14N atoms
After 11,460 yrs
or 2 half-lives
64 14C and
192 14N atoms
After 17,190 yrs
or 3 half-lives
32 14C and
224 14N atoms
After 22,920 yrs
or 4 half-lives
16 14C and
240 14N atoms
After 28,650 yrs
or 5 half-lives
8 14C and
248 14N atoms
After 34,380 yrs
or 6 half-lives
4 14C and
252 14N atoms
After 40,110 yrs
or 7 half-lives
2 14C and
254 14N atoms
• Scientists use potassium-40, a radioactive isotope that
decays to argon-40, to date rocks containing potassium
bearing minerals.
• Based on chemical analysis, chemists have determined that
potassium-40 decays to half its original amount in 1.3 million
years.
• Scientists always analyze many samples of a rock using as
many methods as possible to obtain consistent values for
the rock’s age.
• Errors can occur if the rock has been heated,
causing some of the radioactive isotopes to be lost
or gained.
• The fossil record indicates that there were several episodes of
mass extinction that fall between time divisions.
• A mass extinction is an event that occurs when many
organisms disappear from the fossil record almost at once.
Phylogenetics
The study of evolutionary relationships among
groups of organisms (species, populations), which
are discovered through:
1.Molecular sequencing data – DNA sequencing
and protein synthesis
2.Morphological data matrices – homologous
structures, analogous structures, and embryonic
development
Molecular Sequencing
Homologous Structures
• Scientists Noticed Animals With Backbones
Had Similar Bone Structure
– Same Structures Different Function
• Arms, Wings, Legs, Flippers
– Limb Bones Develop In Similar Patterns
– Help Scientist Group Animals
Homologous Structures
Analogous Structure
Same function with different structures
Homologous Body Structures
• Not All Serve Important Functions
– Vestigial Organs
• Appendix In Man
• Legs On Skinks
Similarities In Early Development
• Embryonic Structures Of Different Species
Show Significant Similarities
CLADOGRAMS
Divergent evolution is the
accumulation of differences
between groups which can
lead to the formation of new
species, usually a result of
diffusion of the same species
to different and isolated
environments that blocks the
gene flow among the distinct
populations. This allows
differentiation of
characteristics through
genetic drift and natural
selection.
Convergent
evolution is the
process by which
unrelated or
distantly related
organisms evolve
similar body forms,
coloration, organs,
and adaptations.
Adaptive radiation is the evolution of an
animal or plant group into a wide variety
of types adapted to specialized modes of
life.
Puncuated Equilibrium
A hypothesis in evolutionary biology which proposes that
most species will exhibit little net evolutionary change for
most of their geological history, remaining in an extended
state called stasis.
When significant evolutionary change occurs, the hypothesis
proposes that it is generally restricted to rare and
geologically rapid events of branching speciation (the
evolutionary process by which new biological species arise)
called cladogenesis.
Cladogenesis is the process by which a species (organisms
that breed and produce offspring that are also fertile) splits
into two distinct species, rather than one species gradually
transforming into another.
Morphological Change
Evolutionary Time Scales
Microevolution: Short time scale events (generation-togeneration) that change the genotypes and phenotypes of
populations.
The term coevolution is used to describe
cases where two (or more) species
reciprocally affect each other’s evolution.
Coevolution is likely to happen when different
species have close ecological interactions with
one another. These ecological relationships
include:
1. Predator/prey and parasite/host
2. Competitive species
3. Mutualistic species
Genetic drift -random fluctuations in the numbers of allele differences in a
population
*Takes place when the occurrence of alleles, increases and decreases by chance over time.
*Typically occurs in small populations, where infrequently occurring alleles face a greater
chance of being lost
*Will continue until the involved allele is either lost by a population or until it is the only
allele present in a population at a particular location
*Can cause a new population to be genetically distinct from its original population, which
has led to the hypothesis that genetic drift plays a role in the evolution of new species.
Gene flow (also known as gene migration) is the transfer of alleles or genes from one
population to another.
Migration into or out of a population may be responsible for a marked change in allele
frequencies (the proportion of members carrying a particular variant of a gene).
Immigration may also result in the addition of new genetic variants to the established
gene pool of a particular species or population.
Mutations - when a DNA gene is damaged or changed
in such a way as to alter the genetic message carried
by that gene. Mutation causes a high rate of natural
selection in a changing environment.
Natural Selection the gradual process by which
biological traits become either more or less common
in a population as a function of the effect of
inherited traits on the reproductive success of
organisms interacting with their environment
Stabilizing Selection, extreme varieties from both
ends of the frequency distribution are eliminated.
The frequency distribution looks exactly as it did in
the generation before
Directional Selection - individuals at one end of the
distribution of beak sizes do especially well, and so
the frequency distribution of the trait in the
subsequent generation is shifted from where it was
in the parental generation
Diversifying (disruptive) Selection - both extremes are
favored at the expense of intermediate varieties. This is
uncommon, but of theoretical interest because it
suggests a mechanism for species formation without
geographic isolation
Evolution By Natural Selection
• The Struggle for Existence
• Survival of the Fittest
• Descent with Modification
The Struggle for Existence
• Malthus’ Influence
– High Birth Rates & Limited Resources Would
Force Life & Death Competition
• Each Species Struggles For:
– Food
– Living Space
– Resources
Survival of the Fittest
• Fitness
– Ability of an Individual To Survive &
Reproduce
• Adaptation
– Inherited Characteristic That Increases an
Organisms Chance for Survival
Survival of the Fittest
• Adaptations Can Be:
–Physical
• Speed, Camouflage, Claws, Quills, etc.
–Behavioral
• Solitary, Herds, Packs, Activity, etc.
Survival of the Fittest
• Fitness Is Central To The Process Of
Evolution
• Individuals With Low Fitness
– Die
– Produce Few Offspring
Survival of the Fittest
AKA Natural Selection
Survival of the Fittest
Key Concept
Over Time, Natural Selection Results In
Changes In The Inherited
Characteristics Of A Population. These
Changes Increase A Species Fitness In
Its Environment
Natural Selection
• Cannot Be Seen Directly
• It Can Only Be Observed As Changes In A
Population Over Many Successive
Generations
– Radiation
– Fossil Record
Population Growth
• Thomas Malthus, 1798
– Economist
– Observed Babies Being Born Faster Than People
Were Dying
• Population vs. Food Supply
Population Growth
• Key Concept
– Malthus Reasoned That If The
Human Population Continued To
Grow Unchecked, Sooner or Later
There Would Be Insufficient
Living Space & Food For Everyone
• Famine, Pestilence
• Political Instability, War
• Death Rate Will Increase To
Balance Population & Food
Supply
Population Growth
• Darwin Realized Malthus’s
Principles Were Visible In
Nature.
• Plants & Animals Produce
Far More Offspring Than
Can Be Supported.
– Most Die
– If They Didn’t – Earth Would
Be Overrun
Natural Variation & Artificial Selection
• Abandoned The Idea That Species Were
Perfect & Unchanging
• Observed Significant Variation in All Species
Observed
• Observed Farmers Use Variation To Improve
Crops & Livestock (Selective Breeding)
Natural Variation & Artificial Selection
• Natural Variation
– Differences Among Individuals Of A
Species
• Artificial Selection
– Selective Breeding To Enhance Desired
Traits Among Stock or Crops
Natural Variation & Artificial Selection
Key Concept
In Artificial Selection, Nature Provided
The Variation Among Different
Organisms, And Humans Selected
Those Variations That They Found
Useful
Evidence of Evolution
Key Concept
Darwin Argued That Living Things Have Been Evolving On
Earth For Millions of Years. Evidence For This Process
Could Be Found In:
– The Fossil Record
– The Geographical Distribution of Living Species
– Homologous Structures of Living Organisms
– Similarities In Early Development
Geographic Distribution of Living Species
• Different Animals On Different Continents
But Similar Adaptations To Shared
Environments
Darwin's Theory
1. Individual Organisms In Nature Differ From
One Another. Some Of This Variation Is
Inherited
2. Organisms In Nature Produce More
Offspring Than Can Survive, And Many Of
These Offspring Do Not Reproduce
Darwin's Theory
3. Because More Organisms Are Produced
Than Can Survive, Members Of Each Species
Must Compete For Limited Resources
4. Because Each Organism Is Unique, Each Has
Different Advantages & Disadvantages In
The Struggle For Existence
Darwin's Theory
5. Individuals Best Suited To Their Environment
Survive & Reproduce Successfully – Passing Their
Traits To Their Offspring.
6. Species Change Over Time. Over Long Periods,
Natural Selection Causes Changes That May
Eventually Lead To New Species
Darwin's Theory
7. Species Alive Today Have Descended With
Modifications From Species That Lived In
The Past
8. All Organisms On Earth Are United Into A
Single Tree Of Life By Common Descent