Evidence for Evolution

Today’s Objective (learning goal)

To identify how fossils are used as evidence in changes within a species.

• 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.

Clues to the Past

• Fossils are evidence of organisms that lived long ago that are preserved in Earth’s rocks.

TYPES OF FOSSILSFossils Types Formation

Trace fossils

Casts

Molds

Petrified fossils

Amber-Preserved

orfrozen fossils

A trace fossil is any indirect evidence

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

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.

• Most fossils are found in sedimentary rocks. These rocks form at relatively low temperatures and pressures that may prevent damage to the organism.

• Fossils are not usually found in other types of rock because of the ways those rocks form.

• 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.

• A Protoceratops drinking at a river falls into the water and

drowns• 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.

• 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

Phylogenetics

The study of evolutionary relationships among groups of organisms (species, populations), which are discovered through:1.Molecular sequencing data – DNA sequencing

and protein synthesis2.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 EquilibriumA 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-to-generation) 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 FittestAKA Natural Selection

Survival of the Fittest

Key ConceptOver 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 EvolutionKey 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