The Theory of Evolution
Evolution-The process where organisms
change over a period of time (a new species
from previous species).
Theories of evolution (process of change through
time) attempt to explain the similarities and
differences among species.
A species is a group of organisms that look alike
and are capable of producing fertile offspring.
• Darwin’s Theory: Natural Selection
– Charles Darwin, an English scientist,
took a voyage on a ship named the
Beagle, which lasted five years.
– He studied the local animals and plants
on various islands and continents.
Darwin noticed that each species
seemed well adapted to its
– By studying fossils, he realized that
some forms of life no longer existed.
– After his voyage, Darwin wrote a book
called On the Origin of Species by
Natural Selection. This book presented
a theory of evolution that totally
changed biology.
– Darwin used the data he gathered from
the natural world to form his well-known
idea of evolution by natural selection.
Best adapted to
living in the water
Best adapted
to living in the
•Darwin’s Theory was based on the presence of variation
among members of a species and the process that he called
“natural selection.”
• In Darwin’s theory, environmental
pressures act as a force for the
natural selection of the best
adapted individuals—those with
helpful adaptations that enable
them to survive and reproduce
successfully. According to
Darwin, evolution occurs because
of natural selection. The
environment doesn’t dictate the
changes, rather the environment
dictates who will survive. He
wanted to explain how species
adapt to their environment over
many generations.
Evidence of Evolution:
Scientists have identified about 1.5 million
different species.
They expect there to be between 20 billion and
100 billion species that exist.
Scientists estimate that more than 99% of all
species that have existed are now extinct.
Scientists can have an idea about ways species
have changed in five ways: fossils,
embryological development, structural
similarities, physiological make-up (anatomy),
& DNA comparisons.
Most scientists today consider similarities in
DNA to be the best indicator of how closely 2
species are related. Why do you think this is the
case? DNA similarities are the most direct
indicator that species inherited their genes from a
common ancestor.
Fossil Record
Fossils can show structural
evolutionary changes over time.
Fossils provide an actual record of
Earth’s past life-forms. The change
over time (evolution) can be seen in
the fossil record.
Usually found in Sedimentary rock.
Fossil formation:
– Shortly after an animal dies, they have to be
completely covered in mud or clay.
– These mud particles get compressed and
layered until sedimentary rock forms.
– Minerals replace wood, shells, and bone, and
the organism becomes petrified or fossilized.
Radiometric dating and relative dating
determine the time period in which an
organism lived.
– Relative Dating: tells which fossils are older than other fossils
based on where they are found in sedimentary rock layers.
– Radiometric Dating: Process that determines the approximate age
of a fossil. This technique measures the amount of radioactive
isotopes found in the fossil. The approximate age can then be
calculated based on the half-lives of the radioactive isotopes. A
half-life is the amount of time that it takes for half of a radioactive
material to decay into a different material.
• Scientists then compare modern species that lived millions of years
• Fossil comparisons can
– what changes have occurred
– why some species no longer
– when new species split off
from older ones along an
evolutionary time line
• Scientists can now extract
DNA from fossils. They
can compare the fossilized
organisms’ amino acid and
nucleotide sequences
(from the DNA) to the living
species. This shows:
– how closely organisms are
– whether a gene mutation
played a role in the
formation of a new species
Punctuated Equilibrium:
• suggests that long intervals may pass at times in which
little or no change may occur, and then suddenly those
periods may be interrupted by short bursts of quick,
radical transitions. Most scientists now believe that a
combination of gradual evolution & punctuated
equilibrium have occurred over time.
– Comparative Anatomy
• Scientists look at the structural similarities among living organism.
• Homologous Structures
– Parts of different organisms that are similar in structure and usually
varying in function. Homologous structures are structures with a
common evolutionary origin.
– Develop from the same tissues as embryos and have similar internal
– Look different on the outside
– Have different functions
» Ex: Forelimb of a bat, human, crocodile, and bird all have a
humerus, ulna, carpals, and radius, but they have different functions
• Analogous Structures
– Structures that have similar functions but are not believed to have
evolved from a common ancestor.
– Ex: A bird’s wing & a butterfly’s wing are analogous structure because
both have the same function, to enable flight, but both types of wings
are structurally different.
• Vestigial Structures
– structures that seem to have no useful purpose now although they
resemble structures that are useful in other species.
– Ex: Flightless wings of ostriches, sightless eyes to the cave
salamanders, pelvis bone/hind-limb bone in whales, appendix in
humans, etc.
• Embryology
– The evolutionary history of organisms is also seen
in the development of embryos.
• Embryos of many vertebrates look very similar (esp. in
the early stages of development)
• EX: Embryos of fish, chicken, tortoises, and a humans
indicate that they share a common ancestry because of
the presence of gills and tails
– Physical similarities suggest organisms may have
some genetic similarities and a common ancestor.
Anatomical Similarities: the brain
• According to the plate tectonics theory, the earth’s
surface is divided into large plates that float just underneath
the earth’s surface. The plates continually move causing
earthquakes, volcanic activity, & the continental drift.
• Scientists believed that the continents were once
consolidated together in one single landmass, called
Pangaea. Over time, the continents were formed as the
plates drifted apart.
• Biogeography is the study of how plants & animals are
distributed around the world. Scientists use biogeography
to figure out how & when species may have evolved.
– Ex: Some species are isolated to specific continents, like Apes
(& all ape fossils) are found only in Africa & Asia.
– Ex: Marsupials, mammals with pouch are found only in
– Comparative Biochemistry
• Physiology: the study of processes and functions
• Scientists study the biochemical compounds of different
• Ex: A protein, cytochrome c, is required for aerobic
respiration. Cows, fish, and monkeys all contain
cytochrome c, which leads scientists to believe they
evolved from a common ancestor.
Comparisons of DNA
If species have changed over
time, then the genes that
determine their characteristics
should also have changed. As
species evolved, one change
after another should have
become part of their genetic
instructions through mutation.
Therefore, more and more
changes in a gene’s nucleotide
sequence should build up over
DNA is the BEST way to
determine if two organisms are
– Ex: After using gel electrophoresis
to determine DNA patterns, it is
now known that red pandas are
more closely related to raccoons
than they are to giant pandas.
– Ex: DNA studies have been useful
for interpreting the relationships
among apes and humans.
• We use fossil records, comparative anatomy
[structural similarities] (homologous structures),
comparative embryology (development before birth),
comparative DNA, and comparative biochemistry
[physiological similarities] to find similarities in
organisms. This is our basis for modern classification.
• Introduction to Natural Selection:
–Natural Selection: Way in which
populations change as certain organisms
reproduce and pass on their genes to
future generations.
» Natural selection is the survival and
reproduction of the organisms, which are
genetically best adapted to the environment.
Light to attract
Big mouth to catch prey
– Natural Selection depends on five things:
1. All species have genetic variation
2. Competitions for survival (food, habitat, etc.)
3. Environmental challenges
4. Survival of the fittest: Organisms that are better
adapted to live long enough to produce more
offspring than organisms that are not very well
5. Traits (Dominant or Recessive) of best adapted
organisms tend to become more frequent in the
» Ex: Peppered Moths
» Ex: Alligators live in the Everglades and make up
a population because they all live together in a
particular area and are able to reproduce.
– Organisms with less favorable variations are less
likely to survive and pass on traits to the next
– Natural selection does NOT
cause new traits; it favors traits
already present that make an
organism better able to survive.
– Organisms interact in an area;
this interaction leads to
competition, and the
organisms that are best
adapted are going to live long
enough to survive and produce
offspring. They pass these
traits to their offspring.
Eventually, the traits better
suited for survival become more
common, and fewer and fewer
individuals without the trait are
produced. This is a genetic
shift, which brings about
speciation: the development
of a new species.
Diversity in Gene Pools
• Diversity the variety of traits; this makes
organisms of the same species different from
one another.
• Traits are passed from generation to generation
through genes, which are sections of DNA. The
total number of genes that account for different
traits in a species is called its gene pool.
– Ex: Humans are very diverse. We have different hair
color, skin color, & eye color; we have different heights &
weights; we have different sizes & shapes of noses, eyes
& ears; & we even have different personalities,
intelligence, & talents. The different genes that account for
all of these different traits make up the human gene pool.
• Species that have a large gene pool have a
greater diversity because they have more
combinations of genes available to them.
• A small gene pool limits a species’ diversity.
• Gene pools of species can increase due to
mutations, or it can decrease when traits die
out. As a species becomes more specialized
to a specific environment, it loses its ability to
adapt to changes. Species can lose traits
through natural selection, genetic drift, the
threat of extinction, & selective breeding.
• Genetic drift is the change in a gene pool generated by
– Ex: If a species of insect can have white or brown eyes, &
both eye colors are equally beneficial & equally found in a
population, then just because of chance, more offspring are
born with brown eyes during one season. Because more of
the population has brown eyes, the next generation will more
than likely have brown eyes. Over time, the population drifts
toward having more brown eyes than white eyes, & pretty
soon, the gene for white eyes may be lost.
• The threat of extinction or any drastic decrease in
population size often accelerates genetic drift. The
remaining organisms have a more limited gene pool. This
new gene pool may favor a different trait than what was
seen in the original population. When a small population
repopulates, it results in what is called the founder effect.
• After 80% of a population of long-tailed mice were
eliminated because of a hurricane, the remaining
20% survived & all belonged mostly to one closely
related family. In this mice family, they have a
genetic defect that causes them to have short
stumpy tails instead of long tails. Because of the
founder effect, the gene pool now contains a
larger percentage of this genetic defect, which
causes the trait to become more common in the
new population.
3. Nature uses natural selection & genetic drift for
determining genes that are passed on; however,
human can use artificial selection through
selective breeding.
a. Farmers can select traits that make their crops more profitable
& easily harvested.
b. Through selective breeding, certain “undesirable” traits have
been bred out of the gene pool.
c. Because this has been done for many years with wheat, the
crop that is now produced is almost genetically uniform with
little variation.
Because wheat has little diversity, it may not be able to grow in different
climates, & the lack of variation makes it susceptible to evolving diseases &
Mutations can add genetic variations. In stable environments, mutations may
have little or no benefit, or they are likely to be harmful. In a changing
environment, however, mutations may allow organisms to adapt.
– Speciation can occur when members of populations no
longer interbreed as a result of geographic isolation &
temporal isolation.
• Geographic Isolation
– physical barrier divides a population into two or more separate groups
– Ex: a volcano eruption separates islands. The populations have to
adapt to their new environment. Over time each population may
become a different species
• Temporal (Reproductive) Isolation
– Different reproductive cycles (seasonal). Due to the species not being
able to interbreed, they may become so different they can no longer
– Ex: Frogs live in the same areas. Some of the frogs mate during the
Spring, while others mate during the Fall. These frogs don’t interbreed
because of temporal isolation; eventually, these frogs may become so
different they can no longer interbreed and are two separate species.
• Behavioral Isolation:
– A type of isolation between populations due to differences in courtship
or other mating behaviors.
– 2 populations will not interbreed because of different ritualistic
• Ex. 2 similar species of birds may overlap in territories and may be capable
of producing fertile offspring, but because they sing a slightly different
mating song, they will not interbreed in the wild.
Evolution may take several paths that can ultimately lead to the
formation of a new species from a single ancestral species; there are 3
patterns of evolution that lead to a new species: species may evolve
apart (diverge), evolve similar structures and appearances (converge),
or evolve together (coevolve).
– Convergent Evolution – Pattern of evolution in which 2 or more unrelated
species become more & more similar in appearance as they adapt to the
same kind of environment, often resulting in analogous structures.
• Ex: Whales & fish have similar characteristics since both had to evolve methods
of moving through the same medium—water.
– Divergent Evolution – Pattern of evolution in which 2 or more related
species gradually become more dissimilar.
• Ex: kit fox & red fox
• Adaptive Radiation – ex. of divergent evolution; many related species evolved
from a single ancestral species (**if an organism invades an area where there are
few competing species & new habitats are available, new species will evolve) ex:
– Coevolution – the joint change of two or more species in close interaction
• Ex: predator & prey; parasite & host; plant-eating animal & the plants
they feed on; pollination between plants & animals (bats & light-colored
Evolution: Pattern
of evolution in
which two closely
related species
gradually become
more and more
Evolution: Pattern of
evolution in which two
unrelated species
gradually become
similar to each other
through adaptation to a
common environment,
often resulting in
analogous structures
Parallel Evolution:
Pattern of evolution in
which two species
maintain the same
degree of similarity
while each undergoes
changes along an
independent path.
• Natural selection can result in a species that is better
ADAPTED to a particular environment. The individuals
that produce the greatest number of offspring are the best
– Adaptations are considered any trait, physical or
behavioral, that helps the organism survive.
– Natural selection acts on populations NOT individual
organisms. It changes the frequencies of alleles, Tall vs.
Short, etc. (peppered moth) Populations evolve, individuals
don’t because natural selection operates only on populations
over many generations.
• EX: Albino deer will not survive long enough to
reproduce. Therefore, each new generation is made up
largely of offspring from parents with the most favorable
• Ex: The Peppered Moths in England were all gray with a few
darker and black varieties. The gray moths blended in with the
trees, so most of them survived, and the black moths were
eaten by birds. After the industrial revolution, there was a lot of
soot in the air that settled on the trees making them black.
Today, in England most of the Peppered Moths are black,
instead of gray.
• Natural selection causes changes in the frequencies of alleles.
In the moths, it was the same species of moth; however, the
dark allele was more commonly expressed than the light allele.
3 types of Natural Selection:
(in which the population is affected)
• Stabilizing Selection
– Natural selection favors average individuals
• Ex: Insects: if too large, they may be eaten by birds; if too small,
can’t find food.
• Directional Selection:
– Favors organisms with an extreme form of a trait
• Ex: Finches: During drought food was scarce, the finches with
larger beaks were able to find food as a result the average size of
beaks increased
• Disruptive Selection
– Favors organisms with both extremes and eliminates the
• Ex: If only very large seeds and very small seeds are available,
then birds with large and small beaks would be best adapted, and
the number of average size beaks would decrease.
• External Fertilization
– Animals like fish produce very large
numbers of egg and sperm and release
them into the water
– Large numbers are necessary because most
eggs will not be fertilized and even the
fertilized eggs don’t have a high survival
rate. Most will die.
• Internal Fertilization
– Sperm and egg are more likely to fuse
together and develop into an adult
– Therefore, only small numbers of egg and
sperm are needed because most will survive
– This give animals with internal fertilization an
• Behavioral Adaptations
– Elaborate mating rituals, ex: male peacock
– Ritualistic fights among males to attract females
– Hibernation:
• Many animals that do not live in large groups to survive when the
environment is not well suited for survival they hibernate.
• The animal is not exposed to predators or the environment
• Metabolism, including digestion, respiration, and heart rate, slows down
– Estivation:
• A resting state similar to hibernation that some animals use in hot weather
(instead of cold weather for hibernation). Since reptiles and amphibians are
ectothermic, they must still be able to survive even if the temperature
surrounding them gets too hot. If the environment gets too hot or too dry,
frogs & reptiles will estivate.
• Estivation allows animals to conserve energy by slowing their respiration
and heartbeat. These organisms will not require as much food or water to
• Some organisms, like reptiles, can use 90-95% less energy when they are
– Migration:
• The movement of an animal from one region or climate to
another for a specific period of time. This is usually a seasonal
travel to a more favorable environment .
• Examples of migrating animals: hummingbirds, robins, ducks,
sea turtles, elk, whales, & monarch butterflies
– Social groups among animals are useful in helping find
food and defend themselves. This is also a
disadvantage because if there is a natural or
environmental disaster, disease, etc., then the entire
group could be killed.
• Adaptations for protection against predators:
– Protective coloring:
• Camouflage is a structural adaptation that provides
protection for an organism to blend in with its
surroundings. Organisms that are well camouflaged
are more likely to escape predators and survive to
– Ex: The coloration of flounder allows it to avoid
• Warning coloration is a type of protective
adaptation in which the organism is venomous or
toxic to other organisms.
– Ex: Poison Dart Frogs in the rain forest use their bright
colors to send a signal stating that they are poisonous.
Yellow & red are colors that usually mean DANGER!
– Protective resemblance:
• Mimicry is a structural adaptation that provides protection
for an organism copying the appearance of another species
that is potentially harmful.
– Ex. The non-venomous king snake mimics the venomous coral snake.
– Ex. The non-nasty viceroy butterfly mimics the nasty-tasting monarch
• Self-mimicry is a special kind of protective adaptation,
whereby one part of an animal mimics another part of its
– Ex. Insects have coloration that appears to be large eyes to scare off
– Ex. Some insects display self-mimicry when their anterior & posterior
regions look alike.
• Plant Adaptations
– Seed dispersal in angiosperms (flowering
• Wind, like dandelions or maples
• Hooks, like hitchhikers
• fleshy fruits, like apples, so that animals can eat
them and disperse them through their feces
– Without dispersal, then the offspring would fall
close to the parent plant and would compete
for sun, water, nutrients, etc.
• If organisms are not suited for their survival, they could become
endangered or extinct giving way for the better suited
– Extinction leads to species replacement.
• Over long periods of time, events, such as climate changes and natural disasters,
result in some species becoming extinct, which means that they disappear
permanently. Species that are better suited to the new conditions may replace those
that became extinct.
• Ex: The extinction of dinosaurs was followed by the evolution of modern mammals
and birds over millions of years.
• Populations continually change due to the
– Overpopulation—There are more offspring
produced because of good conditions. Soon
enough, however, there are too many.
– Competition—Due to problems, certain
populations compete with each other.
– Survival of the fittest—The ones better suited to
the environment will survive.
– Natural Selection—A mechanism that explains
how populations evolve.
– Reproduction—Individuals that survive will
– Speciation—The process in which new species
Survival of the Fittest
Natural Selection