Natural Selection and Evolution
Earth’s History
 Earth Origins:
4.6 billion years old – originally a ball of hot molten rock – bombarded by meteorites and extreme
volcanic activity – the atmosphere was inhospitable but volcanic gases paved the way for our current
atmosphere
 Earth’s Oceans:
Formed about 3.9 billion years ago after the Earth began to cool and allow the water vapor to
condense into water forming the oceans – its is in these oceans that the first living organisms appeared 3.5
billion years ago
History in the Rocks
 Fossils: Any evidence of an organism that lived long ago – classified by how the fossil is formed
 Types of fossils:
 Casts - A mold of an organism filled by minerals in the surrounding rock – produces a replica
 Trace Fossils - Markings or evidence of animal activities – footprints, trails, and burrows
 Types of fossils:
 Imprints - Fossils that form before sediment hardens into rock – leaves or feathers that fall
into mud and leave imprints
 Molds - When an organism is buried, it decays leaving an empty space that has the exact
shape of the organism
 Petrified Fossils - The hard parts of an organism are sometimes penetrated and replaced by minerals
atom for atom, when minerals harden an exact stone copy of the original organism is produced
 Amber-Preserved & Frozen Fossils - Entire, intact organism is caught in ice or tree sap (amber) –
very rare – preserves the internal parts of organisms important to scientific study of extinct species
What is Paleontology?
 Paleontology- study of geological periods and fossil remains
Why are sedimentary rocks important for Earth’s history?
 Sedimentary rocks contain important information about the history of the Earth.
 They contain fossils, the preserved remains of ancient plants and animals.
 Differences between successive layers indicate changes to the environment which have occurred over
time.
 Sedimentary rocks can contain fossils because, unlike most igneous and metamorphic rocks, they
form at temperatures and pressures that do not destroy fossil remnants.
Formation of Fossils in Sedimentary Rock
a. organism dies and falls into a sandy or muddy bottom of a body of water or is carried there by floods
b. overtime sediment is deposited on top of the dead organism until it is completely covered
c. mud and sand begin to compress around the organism – forming a sequence of sedimentary rocks – usually
only the hardest parts of the organism remain
d. fossil becomes imbedded in the rock, then geological events, such as Earth movements or erosion, the
fossil can come to the surface
e. scientists discover the fossil and extract it from the surrounding rock – then the fossil is studied
How do you tell the age of a fossil?
Relative Dating- the layers of sedimentary rock are layered with its newest layer on top and the oldest on
the bottom – fossils are determined the same way - deeper fossils are older then shallower fossils
Radiometric Dating- radioactive isotopes are used – the fossil emits a decay rate and scientists using
radioactive isotopes whose decay rate is known and if the known decay rate matches the fossil the age can
be determined – radioactive decay
Earth’s Geologic Time Table
 Precambrian Era-
Life begins – 3.5 billion years ago – spherical and filamentous organisms that resemble
photosynthetic bacteria cyanobacteria and dome-shaped stromatolites – Precambrian
accounts for 87% of Earth’s history – prokaryotes dominated the early Precambrian era
 Eukaryotes appeared about 1.5 billion years ago
 By the time the Precambrian era ended about 544 million years ago the oceans were filled
with unicellular and multi-cellular organisms such as algae, sponges, and jellyfish
 Paleozoic Era From 544 t 245 million years ago – characterized by the appearance of many plants and
animals –
 Scientists call this era the explosion of life Seas were filled with worms, echinoderms, and primitive arthropods (Examples: insects,
spiders, lobsters, and crabs)
 1st half – fishes and earliest vertebrates appeared, some evidence of plant life on land
 Middle – amphibians appeared
 2nd half – reptiles appeared
 Mesozoic Era 245 to 66 million years ago – many geological and life changes during this era
 Divided into periods:
 Triassic – mammals 1st appeared – the 1st were small mouse-like and 1st dinosaurs appeared
 Jurassic – began 208 million years ago – Age of Dinosaurs – figure 17.9 on page 407
 Cretaceous – began 144 million years ago – the spread of the mammals and the evolution of
flowering plants such as oak, fig, and elm trees
 Geological changes – the hypothesis of continental drift – how the continents move or plate tectonics
 Cenozoic Era “The Age of Mammals”
 66 million years ago – we live in this era
 mammals flourish, evolution of mammals into modern groupings, primates spread across the
planet about 60 million years ago– humans 1st appeared 200,000 years ago
The Origins of Life
 Spontaneous Generation- The idea that life was produced from nonliving matter
 Francesco Redi’s Experiments- disproves spontaneous generation  Redi’s hypothesis: Only flies can produce more flies
 Redi’s steps:
 Rotten meat is placed in 2 experimental jars and 1 control jar
 Cloth was placed over 1 experimental jar
 A cork was placed over 1 experimental jar
 The control jar, with no covering, filled with fly maggots because flies landed on the meat
and laid their eggs
 The cloth covered jar had maggots on the meat as the eggs were laid on the cloth and fell
through
 The corked experimental jar had no fly maggots
 Louis Pasteur and Biogenesis Pasteur setup an experiment in which only air was exposed to a nutrient broth – no
microorganisms were allowed to get to the broth – his experiment showed that spontaneous
generation does not happen because no organisms grew in the nutrient broth
 Pasteur came up with the concept of:
 Biogenesis: living organisms come only from other living organisms – the cornerstone of
Biology
Origins: The Modern Ideas
 Simple organic molecules formed from the primitive atmosphere: prokaryotes
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The primitive atmosphere was composed of water vapor, hydrogen, methane, and ammonia and a
1930’s Russian scientist Alexander Oparin proposed that life began in the early oceans when the suns
energy and light energy (lightening) hit the atmospheres chemical components reactions occurred
producing simple organic compounds
 Oparin imagined the reactions occurred in the atmosphere and then rain poured down bringing the
organisms with it – forming a primordial soup
 In 1953 American scientists Stanley Miller and Harold Urey tested Oparins experiment by simulating
the primordial scenario  Water vapor circulated with ammonia, hydrogen, and methane that was subjected to electrical sparks
of “lightening” the mixture was repeatedly heated and cooled, simulating night and day
 After 1 week they found that the mixture develop amino acids, sugars, and other organic compounds
 The formation of complex organic compounds and pre-cells: eukaryotes Miller and Urey’s experiments showed that heat and amino acids will link together to form
small proteins – leading scientists to speculate that life began in small pools of water where
amino acids were concentrated
 Scientist Sydney Fox showed how heating amino acids could produce cells – a protocell – a large
ordered structure that carries out the activities associated with life: growth, division, and
metabolism
The Evolution of Cells
 Heterotrophic prokaryotes Were the 1st true cells
 Scientists speculate 1st forms of life were prokaryotes that were anaerobic organisms that
required no oxygen for existence, especially since the atmosphere had little to no oxygen
 These prokaryotes were heterotrophs because they obtained their food from their
surroundings
The Evolution of Cells
Heterotrophs evolved into autotrophs – autotrophs can make their own food – these organisms were similar
to archaebacteria – they live with little sunlight and oxygen – found in sulfur springs and deep sea events
 Photosynthesizing prokaryotes Autotrophs evolved into photosynthesizing prokaryotes (who produce oxygen as a by-product
of respiration)
 These new photosynthetic prokaryotes began putting large amounts of oxygen into the
atmosphere
 The event began the Oxygen revolution about 2.8 billion years ago which is seen in the fossil
record
 Ancient storms with lightening caused the oxygen to be converted into ozone – resulting in the
protective ozone layer
– preventing harmful UV rays from destroying the newly formed life
– thus paving the way for more complex organisms to evolve
Charles Darwin and Natural Selection
 Fossils interested scientists in evolution
 The fossil record has helped form the basis of early evolutionary concepts
 These fossils convinced scientists that life slowly changed over time or evolved
 18th century scientists proposed many ideas but only the theory proposed by Charles Darwin has
become accepted
 Charles Darwin is considered to be the founder of modern evolutionary theory
 Darwin studied the natural world during the voyage of the Beagle
 In 1831, at 21 years old, Darwin began his 5-year journey as a naturalist – his job was to collect,
study, and store biological specimens discovered on the journey
 Darwin noted in his observations of Galapagos animal inhabitants that they were unique to the islands
yet similar to species seen in other parts of the world
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By the end of his trip Darwin was convinced evolution occurs – that species change over time
Darwin was also interested in an essay written stating that the human population was growing faster
than the food supply
 Applying this to his studies – he knew many organisms reproduced many offspring but large numbers
of species did not cover the Earth, so Darwin concluded that there must be a struggle for existence
among individuals
 Competition for food and space, escape from predators, and the need to find shelter
 Only some individuals survived long enough to reproduce – but which ones?
 Darwin began experiments where he would select certain variations in his pigeons and breed the
pigeon’s for the desired trait
 These breeding experiments are called – artificial selection
 Darwin wanted to know if there was a force in nature similar to artificial selection
 Darwin examined his data and began to form his idea of evolution by natural selection
 Natural selection is a mechanism for change in populations that occurs when organisms with favorable
variations for a particular environment survive, reproduce, and pass these variations or traits on to
the next generation
 Organisms with less favorable variations are less likely to survive to pass on their traits
 Each new generation will be made up of organisms that have the favorable trait
 Darwin published his findings in 1859 in his book “On the Origin of Species by Natural Selection”
How natural selection works
 Over population of a species. Example: 1000’s of fish eggs laid at once
 Within any population individuals will show slight variations. Examples: in fishes: color, fin and tail
size, and speed
 Individuals with favorable traits in their environment are most likely to survive than individuals with
less favorable variations. Example: fishes whose skin color blends in with its surroundings will less
likely fall prey to predators
 Surviving individuals will breed and produce offspring with the variations that allowed them to
survive predator attacks
Natural Selection and Adaptations
 Structural adaptations arise over many generations
 The mole rat adapting larger teeth and claws to be able to dig deeper holes and avoid predators
 Structural adaptations that change the structure of body parts:
 Example: can develop quickly in a geological perspective of a minimum of 100 years
 Mimicry: structural adaptation that provides protection for an organism by enabling it to copy the
appearance of another species
 Example: the coral snake and milk snake
 Camouflage: a structural adaptation that enables an organism to blend in with its surroundings
 Example: the insect – the walking stick – looks like a branch or stick on a tree
 Physiological adaptations can develop rapidly
 These are changes in organism metabolic processes:
 Example: bacteria becoming resistant to penicillin
Evidence for
Evolution
 Fossils show changes overtime
 Anatomical studies indicate evolutionary relationships
 The organisms look different from the outside and vary in function, yet the details of their skeletons
are similar
 Scientists use these similarities as evidence of evolution from a common ancestor – as the ancestors
moved to new environments they adapted to the new surroundings
 This accounts for the differences that are seen
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Homologous structures: a modified structure that is seen among different groups of descendents –
having a common evolutionary origin
 Example: forelimbs of bats, crocodile, and humans
 Analogous structures: structure without a common evolutionary origin that is similar in function but
not in structure
 Example: wings of birds and insects
 Functionless structures indicate evolutionary pathways
 Structures that once had a function but are no longer used but continue to be passed from
generation to generation:
 Example: the human appendixes, the eyes of the mole rat – these structures are called vestigial
structures
 Embryological development shows evolution from a common ancestor
 In early embryological studies it is found that fish, reptiles, birds, and mammals are all similar in
structure – each have a tail and gill slits – some of which do not develop the tail and gills, while others
do
 Genetic comparisons may reveal hidden relationships
 Noting similarities in nucleotide sequences of DNA can help determine relationships between
organisms
 Example: Human DNA is 20% identical to mouse DNA and 98% identical to chimpanzee DNA
Mechanisms of Evolution
 Population Genetics and Evolution
 An organism cannot adapt during its lifetime
 Natural selection operates only on populations over many generations
 Populations evolve – individuals DO NOT
 Genes change overtime when there is high allelic frequency in the gene pool – an entire collection of
genes among a population
 Allelic frequency – the percent of a particular trait in the gene pool
 When the frequency of alleles does not change from generation to generation there is no genetic
frequency
 Changes in genetic equilibrium lead to evolution
 Populations at genetic equilibrium are not evolving
 Evolution can only occur when genetic equilibrium is disrupted
What factors cause changes to genetic equilibrium?
 Mutations: spontaneous, environmental causes such as radiation and chemicals
 Mutations are important – they cause genetic changes in a gene pool
 Some mutations are harmful but once in a while these mutations are favorable for evolution
 Genetic drift: alteration of allelic frequencies by chance processes – common among small populations
 Example: Amish – small population causing extra toes gene to express it self in 1 and 14 people rather
than 1 in 1000 people
 This can allow recessive genes to be expressed more readily – recessive genes that carry genetic
disorders – Tay-Sachs
 Movement of Individuals: genetic equilibrium can also be affected by immigration of individuals into
and out of a population
 The factor that causes the greatest change in gene pools is natural selection
Natural Selection acts upon the variation in populations
 3 types of natural selection
 stabilizing selection
 directional selection
 disruptive selection
 Stabilizing selection:
 Favors average individuals
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Example: butterflies
Dark colored butterflies cannot avoid predators – stand out against sky
Light colored butterflies cannot avoid predators – stand out against plants
Sky colored butterflies can avoid predators because they match the skies color
Directional selection:
When one of the extreme forms of a trait is favored by natural selection
Example: insects who burrow deep into trees will only be able to be caught by long beaked
woodpeckers who can use their beaks to reach them deep in the tree
 The long beaked woodpeckers will have more food then shorter beck woodpeckers and the short beck
woodpeckers will die out and then the long beaked woodpeckers will become the norm
 Disruptive selection:
 Individuals with either of the 2 extreme forms of a trait are at a selective advantage
 Example: marine limpets vary in color from white to dark brown – white limpets on a white rock avoid
the sight of predators – dark brown limpets on dark rock avoid the sight of predators but tan colored
limpets are easily seen on either rock and predators are able to prey on these limpets
 Disruptive selection eliminates the middle or intermediate forms of an organism
The Evolution of Species
 Physical barriers can prevent interbreeding
 Geographic isolation – occurs if a physical barrier separates a population into groups
 This is one way in which new species form
 Example: frogs separated by deforestation eventually the frogs genes will not resemble the old ones
and through adaptation and natural selection the groups of frogs become so far apart in their genes
that they are considered a new species
 Geographic isolation can lead to differences in mating behaviors
 Gene pools become closed and no new genes are being introduced – eventually reproductive isolation
occurs – when formerly interbreeding organisms are prevented from producing fertile offspring
 Example: frogs – if one group mates in the fall and another in the summer – they fail to be able to
mate together because they cannot breed at the wrong time of the year
 Speciation can occur when chromosome numbers change
 Speciation can occur quickly or slowly
 Quick: example: polyploids only take one generation
 Gradualism: that species originate through a gradual build up of new adaptations
 Supported by fossil record
 Idea proposed by Charles Darwin
 Punctuated equilibrium: speciation occurs quickly in rapid bursts with long periods of stability in
between
 Also supported by the fossil record
Patterns of Evolution
 Species diversify when introduced to new environments
 Adaptive radiation: the process of evolution of an ancestral species into an array of species that
occupy different niches or places in their environment
 Adaptive radiation is an example of divergent evolution
 Divergent evolution: the pattern of evolution in which species that once were all similar to the
ancestral species become more and more distinct
 Example: finches in Hawaii
 All finches came from the same ancestor but each has different plumage and beak size
 Distantly related species can evolve similar features
 Convergent evolution: the pattern of evolution in which distantly related organisms evolve similar
traits
 Example: dolphins and fishes having similar body shapes
 These species occupy similar environments and face similar selection pressures
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