A history of life and natural selection

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Early predictionsImagine you live in the 1600s- so you have no modern technology and no way to see microscopic organisms.

You see maggots growing on raw meat and mold growing on old bread. You know these organisms are alive, so where did they come from?

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Where did life come from? Before the 1700s, people believed in spontaneous generation The idea that life can come from nonliving things

This is mainly due to people not being able to observe how things like maggots were created

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Redi’s experiment

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Around 1650, Francesco Redi performed an experiment to see how maggots were created

He used covered and uncovered meat in jars and found maggots came from flies NOT MEAT!!!

Pasteur’s Experiment In the mid 1800’s, Louis Pasteur performed

another experiment looking at where bacteria came from, but instead he used a flask with a curved neck- so air could get in but bacteria could not

He found bacteria did not spontaneously generate

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Biogenesis Soon after Pasteur, spontaneous generation was rejected and people biogenesis (the principle that states all living things come from other living things) was accepted

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The origin of organic compounds

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First Organic Compounds The estimated age of the Earth is over 4

billion years old This was found by radiometric dating (which

we will learn about another day) All elements found in organic compounds

today are thought to have existed on earth and in the rest of the solar system when the Earth formed

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First Organic Compounds We know Earth’s early atmosphere contained

ammonia, hydrogen gas, water, and compounds made of hydrogen and carbon (like methane)

In the 1920s, two scientists, Alexander Oparin and John Haldane proposed that with heat, these compounds would form simple organic compounds (like amino acids)

Oparin also thought over time these compounds may react to form more complex compounds (like proteins)

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First Organic Compounds In 1953, Stanley Miller

and Harold Urey experimented with and confirmed Oparin and Haldane ideas

Organic compounds were produced using the same gases that were found in Earth’s early atmosphere and heat

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First Organic Compounds

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Many similar experiments have been done, and the following have been produced with similar conditions to Early Earth:

Amino acids ATP nucleotides

First Organic Compounds We also know the Earth’s

atmosphere contained a lot of CO2, which interferes with the formation of organic compounds

So, scientists believed these compounds were created in places not exposed to CO2 , like undersea hot springs

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First Organic Compounds Another explanation is that organic

compounds could have been carried to Earth by debris from space (like meteorites)

We have recently found meteorites covered in organic material

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The First Life Forms How might the first cells have been created? Microspheres and Coacervates have been

produced from mixtures of organic chemicals (similar to the one’s produced in the previous experiments)

How are they like cells? Both can take in materials from there

surroundings, grow, and reproduce

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The First Life Forms RNA came before DNA!!! In the 1980s, Thomas Cech found that

there is a type of RNA that acts as a chemical catalyst (like an enzyme)

He called it a ribozyme Other studies based on his work have

concluded that ribozymes also direct their own replication

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Hammerhead Ribozyme

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The First Life Forms So…the ingredients – RNA, Microspheres,

and Coacervates – formed the first cells RNA was the genetic material and probably

acted like an enzyme Microspheres and coacervates were the

membrane and organelles.

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The First Life FormsWhat did the first cells look like?We know there was little to no oxygen in the

early atmosphereThe oldest fossils we have resemble

prokaryotes (bacteria)The only food available would have been

organic moleculesFrom this evidence, we conclude the first

organisms would have been anaerobic, heterotrophic prokaryotes

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The First Life FormsEventually, the heterotrophs used up most

or all of the organic molecules and would die unless…

Autotrophs evolved and multipliedThese organisms would have been

chemosynthetic, not photosyntheticChemosynthesis is much simpler

(remember photosynthesis is very complicated and uses many enzymes-which were not around at this time)

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The First Life FormsChemosynthesisCO2 serves as the carbon source to

create organic moleculesEnergy is obtained from the oxidation

of various inorganic substances-like sulfur

Archaea are modern day bacteria that most likely resemble the first life forms

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The First Life FormsFrom fossils, we know that

photosynthetic organisms developed about 3 bya and were unicellular

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The First Life FormsPhotosynthesis produces oxygen, which

was deadly to some of these unicellular organisms.

So, these organisms must have chemically bound oxygen to other molecules to make it harmless

This is thought to be involved in the development of aerobic respiration, because the first step is binding oxygen

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The First Life FormsAs the photosynthetic organisms

developed, oxygen in the atmosphere increased

This oxygen rose into the upper atmosphere, where it was hit with sunlight

Sunlight splits O2atoms into 2 O atoms

These single O atoms reacted with O2to produce O3-which is ozone

Soon an ozone layer formed, which shielded the Earth from UV rays (which are deadly to life on land)

This allowed life on land to further develop.

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The First Life FormsHow did eukaryotes develop?Endosymbiosis- states

mitochondria and plastids were once free living prokaryotic cells that were ingested by bigger prokaryotes

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EndosymbiosisIt is possible that the eukaryotic cell

is the accumulation of several endosymbiotic events

Also, the eukaryotic genome may have increased its complexity with the transfer between different bacterial and archaeabacterial genes, genetic annealing

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EndosymbiosisObservations that support endosymbiosis:

Enzymes in mitochondria and plastids have enzymes and transport systems very similar to prokaryotes

Mitochondria and plastids replicate in a fashion that is similar to binary fission

The organelles contain a single, circular, DNA . . . just like many prokaryotes

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Continental DriftAnother way that organisms could have evolved

is through geographic isolationWhen separated from other organisms in different

environments, organisms will adapt different traitsContinental drift, the movement of the land

masses on tectonic plates, allow for separation and isolation of different organismsAt one time all the land masses formed a giant

continent, PangeaThey then separated allowing a great

diversification of life

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Radiometric DatingThis is how we know how old things areRemember back to the Periodic Table The atomic number of an element is the

number of protons (p) in an element. This number is unique for each element-

change the # and change the elementAtoms of the same element CAN have a

different number of neutrons (n).These are called isotopesMost elements have several isotopes

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Radiometric DatingRemember also, that the mass number of an

isotope is equal to p + n. Example:Carbon-12(isotopes are listed with their mass next

to the element name-12 in this case)Atomic # of carbon is 6This isotope has 6 neutrons6 + 6 = 12 (mass)

Carbon-14 would have how many neutrons?

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Radiometric DatingSome isotopes are not stable and may undergo

radioactive decay, in which their nuclei will release particles and/or energy until it becomes stable

Isotopes that go through radioactive decay are called radioactive isotopes

The rate of the decay of many radioactive isotopes has been determined

The rate of time it takes for ½ of the sample of isotope to decay enough to become stable is called the half life of that substance

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Radiometric DatingHow does half life relate to dating?

Organic matter contains a known amount of carbon-14

Carbon-14 is radioactive and decays into nitrogen-14 (β-radiation), which is stable

When an organism dies, carbon-14 continues to degrade, so that over time, there is less carbon-14

By measuring the change in carbon-14, we can deduce the age of dead organisms

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Radiometric DatingThe half-life of carbon-14 is 5,730 years If you find an organism that originally had 10

grams of carbon-14 and it has 5 grams when you find it…how old would it be? About 5,730 years old (1 half life of carbon-14

means 50% of the sample will be left) Carbon-14 is only good for dating organisms

less than 60,000 years old After that time, the carbon-14 left would be

too small to measure (or none at all)

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Radiometric Dating Radioactive Istotope Product (decays to…) Half-life (years)

Potassium-40 Argon-40 1.25 billion

Uranium-235 Lead-207 7.04 million

Uranium-238 Lead-206 4.5 billion Rubidium-87 Strontium-87 48.8 billion

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Radiometric Dating If 1/8 of the original amount of

Potassium- 40 is left in a sample, how old is it?

1/8 means 3 half lives ( ½ x ½ x ½ = 1/8 )

Half life of potassium-40 is 1.25 billion 3 x 1.25 billion years = 3.75 billion

years

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TRY THIS OUTIodine-131 is used to destroy thyroid tissue in the treatment of an overactive thyroid. The half-life of iodine-131 is 8 days. If a hospital receives a shipment of 200 g of iodine-131, how much I-131 would remain after 32 days?

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ANSWERTotal days that have passed: 32 daysTime for a half life: 8 daysNumber of half-lives: (32/8) = 4Starting sample size: 200g

200g/2 = 100g/2 = 50g/2 = 25g/2 = 12.5g

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half-life half-life half-life half-life

Who is involved and what did they think?

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What is evolution? Evolution is development of new types of

organisms from preexisting types of organisms over time

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Timeline of Theory of Evolution

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In the 1800s Before the 1800s most people

thought all species were permanent and did not change

They also thought the Earth was only thousands of years old

The following slides list scientists that helped to change these ideas

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James Hutton (1726-1797) Geologist that came up

with the concept of Gradualism:

a) Processes today are the same as in the past

b) Large changes are the accumulation of slow, continuous processes.

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Thomas Malthus (1766-1834) Published essay on human

population where he said limited resources control populations

It influenced Darwin on his ideas about organism’s struggle for existence

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Charles Lyell (1800’s)GeologistProposed Uniformitarianism=

processes that occur today have always occurred

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Jean BaptisteLamarck (1744-1829)NaturalistInheritance of acquired

characteristicsOrganisms change in

response to the environment (use and disuse).

Structures that are used become stronger, and structures that are not used become weaker.

Pass new trait to kidsNOT TRUE!!!

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In the 1800sScientists began to

study rock layers (strata) in the 1800s

They found that different strata formed at different times and generally the oldest layers were found at the bottom

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In the 1800sGeorges Cuvier was able to find and piece

together some fossils. He is credited with the developing the field of

paleontology, the study of fossilsHe found many extinct (no longer exist)

speciesHe also found that some strata had very

different organisms than the strata above or below them

This showed that species on Earth had changed and become extinct over time

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CatastrophismCuvier did not believe that organisms

gradually changed over time Instead he believed that a series of

catastrophic events wiped out existing populations and allowed new populations to evolve (catastrophism)He believed that these events were localized

and not globalThese catastrophes explained why there were

such different fossils from one strata to the next.

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Charles DarwinIn the 1830s, Charles Darwin took a trip around the world in the HMS Beagle, his observations on this trip led him to write theories about how organisms change over time

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Darwin’s Voyage

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Galapagos islands are located off the coast of South America

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Charles DarwinScientists before Darwin had

hypothesized about evolution, but no one was able to explain HOW it happened

Alfred Russel Wallace went on a sea voyage and had very similar ideas around the same time as Darwin, but since Darwin published his findings first, he is credited with the discovery of natural selection

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Charles DarwinOn the Galapagos Islands, Darwin saw

many animals, such as finches, that seemed to have a lot in common, but were also different in many ways

Darwin found 13 species of finches on the islands, all of which looked similar to a finch he observed in South America

This led him to believe the island finches had an ancestor in South America

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Darwin’s Finches

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Charles DarwinDarwin began to ponder how so many species of

finches could have “descended with modifications” from one ancestral species (Origin of Species)

He came up with the theory of natural selection to explain decent with modification

The result of natural selection is anaccumulation of inheritedcharacteristics that help an organism survive (evolutionaryadaptation)

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Charles DarwinDarwin was able to find additional

support for his theory of natural selection by observing selective breeding of plants, livestock or pets

This artificial selection showed how these organisms could be altered through time so that they no longer resembled their wild ancestors

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Modern definition of evolutionWith the discovery of genetics as being

the mode of trait expression, the modern definition of evolution is as follows:

Evolution: the change over time in the genetic composition of a populationEventually a population may acquire enough “changes” to become a new species

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Natural SelectionThe theory had 4 main parts:

OverproductionGenetic variationStruggle to surviveDifferential reproduction

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OverproductionOrganisms tend to produce more offspring than

can possibly survive in any given environmentThis idea actually came from Thomas Malthus-

who pointed out that the human population is growing much faster than the environment can withstand. Malthus pointed out that populations are often limited by things like disease, or lack of food.

Darwin realized that the environment limits all organisms-there is not an unlimited supply of resources in any environment

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OverproductionExample-

Clown fish lay hundreds of eggs (remember Nemo)-much more than can possibly survive

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Genetic variationDarwin noticed that within a

population, individuals had different traits (caused by genetic variation)

Also, these variations are passed on to offspring

Continuing the Nemo example, let’s say some of the babies are fast swimmers and some are slow swimmers

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How does genetic variation persist?Since natural selection culls unfavorable

genotypes, how is genetic variation maintained?Most eukaryotes are diploid (2n). They can

maintain alleles even if the phenotype is not expressed.

Balancing selection: Natural selection selects for two or more phenotypes in a population (balanced polymorphism)Heterozygote advantage: the heterozygous

expression of a genotype has a greater fitness than either allele alone (sickle cell anemia)

Frequency-dependent selection: the fitness of one phenotype becomes less fit as it becomes more common in a population 64

How does genetic variation persist? (cont)

Neutral variation: some genetic variation in a population does not have an effect on the fitness of an organism (is not selected for or against)Pseudogenes: Have been inactivated

by mutation. They still persist, but have no effect on fitness

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Struggle to surviveIndividuals compete with each other to

get the resources necessary for survivalSome variations improve an organisms

chances for survival/reproduction and some reduce an organisms chances for survival/reproduction

A trait that makes an individual successful in its environment is called an adaptation

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Struggle to surviveContinuing the Nemo example-Let’s say the

clown fish offspring must swim away from the angler fish, which will eat themThe slow ones tend to be eatenThe fast ones tend to get awaySo being a fast swimmer is an adaptation

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Differential reproduction Darwin concluded that organisms with the

best adaptations are most likely to survive and reproduce. Then those that survive will pass on

their adaptations to their offspring So, the FAST clown fish in our example

will survive, reproduce, and pass their “fast” genes on to their offspring

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Natural Selection This theory is often called “Survival of the Fittest”

BUT fitness does not necessarily mean strongest Fitness is a measure of an individual's

hereditary contribution to the next generation So if an organism is able to produce many offspring

(in other words, pass on its hereditary information) and those offspring survive to reproduce, it is considered FIT

Having lots of babies AND grandbabies means an organism is FIT

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Lamarck vs. Natural Selection

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Another ExampleA population of

cockroaches living in a field is sprayed with pesticide

A few of the cockroaches are resistant and survive

Those survivors pass on the resistance to their offspring, so now the pesticides no longer kill the population

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Peppered MothsPeppered moths rest on bark

of oak trees. Birds eat the ones they can see During early 19th

century, most moths blended in with the oak bark (light brown/ green specks)

Post Industrial Revolution-soot and pollution stained the tree bark dark brown.

Population of dark moths grew- light colored moth population shrank.

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Types of Natural Selection Predation selection

Speed Mimicry- copying Camouflage- blending in

Physiological selection: body functions use of O2; efficiency disease resistance

Sexual Selection- mating 73

Natural Selection and Evolution Natural Selection can only work on expressed

phenotypes (the most fit is selected for)!!! Natural selection over many generations can lead

to evolution Survival AND reproduction are important for

natural selection Organisms DON’T TRY TO CHANGE…they cannot

“develop an adaptation” or “become immune” Natural Selection does NOT always lead to

organisms adapting- 99% of species that have ever lived on earth are now extinct.

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Types of Natural Selection Stabilizing selection- favors average

individuals; reduces variation in a population

Directional selection- favors one of the extreme variations of a trait and can lead to rapid evolution of a population

Disruptive selection- favors both extreme variations of a trait, resulting in no intermediate forms of the trait and leading to the evolution of 2 new species.

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What types are these?

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Types of Natural Selection Let’s say a fish can be small, medium or

large. Generation 1 has more medium fish (average) than small or large fish (extremes) Example 1: There are 2 predators, one eats only small fish and one eats only large fish. So, the frequency of medium fish would go up in generation 2 and this would be stabilizing selection

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Types of Natural SelectionLet’s say a fish can be small, medium

or large. Generation 1 has more medium fish (average) than small or large fish (extremes)Example 2: There is 1 predator, and it can only eat the small and medium fish. So, the frequency of large fish would go up in generation 2 and this would be directional selection

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Types of Natural SelectionLet’s say a fish can be small, medium or

large. Generation 1 has more medium fish (average) than small or large fish (extremes)Example 3: There is 1 predator, and it only eats the medium fish. So, the frequency of medium fish would go down and small or large fish would go up in generation 2; and this would be disruptive selection

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