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Chapter 10, 11, 12
In science, theories are statements or models that have been tested and confirmed many times. Theories have
some important properties:
1. They explain a wide variety of data and observations
2. They can be used to make predictions
3. They are not absolute. They serve as a model of understanding the world and can be changed as the
world view changes. In Biology, theories can explain many questions and observations.
Some questions that can be answered by evolution:
- Why do so many different animals have the same structures, the arm bones in humans are the same
bones as a flipper in a whale?
- Why do organisms have structures (vestigial) that they no longer use, like the appendix in a human?
Wings that have no functions in penguins?
- Why are there bones and fossil evidence of creatures that no longer exist?
- Why is the sequence of DNA very similar in some groups of organisms but not in others?
- Why do the embryos of animals look very similar at an early stage?
The Theory of Evolution is considered a Unifying Theory of Biology because it answers many of
these questions and offers explanations for the data.
10-1 Early Ideas About Evolution
Origins of Life: Early ideas revolved around spontaneous generation(life arises from nonlife). Experiments
refuted this idea.
o Francesco Redi – tested the idea that flies arose spontaneously from rotting meat.
o John Needham – tried to disprove Red’s findings by using his own experiment.
o Lazzaro Spallanzani – showed that microorganisms will not grow in boiled and sealed gravy but
will grow in boiled gravy that is left open to the air.
o Louis Pasteur – showed that sterile broth remained free of microorganisms until exposed to air.
He proved that that biogenesis (only living organisms can produced living organisms) was true.
Many scientists proposed ideas about the origins of life, which eventually led to the theory of
evolution.
o Evolution is the biological change process by which descendants come to differ from their
ancestors.
o A species is a group of organisms that can reproduce and have fertile offspring.
o Thomas Malthus – wrote an essay suggesting that the human population, if unchecked,
eventually would outgrow it food supply, leading to competitive struggle for existence.
o Jean-Baptiste Lamarck – theory of acquired characteristics. Some thought you could gain or
lose features if you overused or did not use them, and you could pass these traits on to offspring.
Ex. A lizard that didn’t use its legs would eventually not have legs and his offspring wouldn’t.
Ex. A giraffe stretched its neck to reach higher leaves, and the stretched neck would be inherited
by offspring.
Lamarck’s theory was proven to be wrong! Why? Logically it doesn’t work. If you had a leg
amputated, your children would not be born with no leg. Features gained in life are not passed on.
o Charles Lyell: processed occurring now have shaped Earth’s geological features over long periods
of time.
o Alfred Wallace – wrote to Darwin speculating on evolution by natural selection based on his
studies of the distribution of plants and animals. Darwin presents Wallace’s essay, and realizes that
this applies to all populations of organisms.
o
Theories of geologic change set the stage for Darwin’s theory
Ecologists recognize that the earth is many millions of years old.
This helped Darwin accept and explain how it may take many, many years for life to change as well.
10-2 Darwin’s Observations
Charles Darwin - Proposed theory of evolution (organic, or biological, evolution). Darwin set sail around
the world in the h.m.s. Beagle. He made many observations on the Galapagos Islands. He realized that a huge
number of species inhabit the earth. He was impressed by how well suited they were for whatever environment
that they inhabited.
Galapagos Islands – small group of islands off the coast of South America that influenced Darwin the most.
• Variation is a difference in a physical trait.
– Galápagos tortoises that live in areas with tall plants have long necks and legs.
– Galápagos finches that live in areas with hard-shelled nuts have strong beaks.
• An adaptations is a feature that allow an organism to better survive in its environment.
– Species are able to adapt to their environment.
– Adaptations can lead to genetic change in a population. (ex. Porcupine and its quills)
– Survival of the fittest = natural selection
 Darwin observed fossil and geologic evidence supporting an ancient Earth.
 Darwin found fossils of extinct animals that resemble modern animals.
 Darwin found fossil shells high up in the Andes mountains.
10-3 Theory of Natural Selection
Darwin published On the Origin of the Species in which he proposed a mechanism for evolution that he called
natural selection. Organisms are modified during their descent, generation by generation, from common
ancestors.
Several key insights led to Darwin’s idea for natural selection.
• Darwin noticed a lot of variation in domesticated plants and animals.
• Artificial selection is the process by which humans select traits through breeding.
Natural selection is a mechanism by which individuals that have inherited beneficial adaptations
produce more offspring on average than do other individuals.
• Natural selection explains how evolution can occur.
• Fitness is the measure of survival ability and ability to produce more offspring.
• Organisms struggle for existence by competing for resources (food and space).
• Some competition would lead to death of some individuals while others would survive.
• Natural selection acts on existing variation among individuals of a species.
10-4 Evidence of Evolution
Evidence for evolution in Darwin’s time came from several sources.
• Fossils provide evidence of evolution. Paleontology is the study of fossils or extinct organisms.
•The study of geography provides evidence of evolution. Geographic distribution indicates common
ancestral species.
•Embryology provides evidence of evolution. Similarities in early development implies similar genes.
•The study of anatomy provides evidence of evolution.
 Homologous structures are similar in structure but different in function.
 Homologous structures are evidence of a common ancestor.
 Analogous structures are not evidence of a common ancestor.
Molecular and genetic evidence support fossil and anatomical evidence. Two closely-related
organisms will have similar DNA sequences. Evolution unites all fields of biology.
Population – includes number of same species in same area that can reproduce.
Over time, those differences that help organisms survive and reproduce in their environment become more
common. Meanwhile, differences that are not beneficial become less common.
11-1 Genetic Variation within a Population
A population shares a common gene pool.
Genetic Variation in a population increases the chances that some organisms will survive
Genetic variation leads to phenotypic variation.
Phenotypic variation is necessary for natural selection.
Genetic variation is stored in a population’s gene pool.
– made up of all alleles in a population
– allele combinations form when organisms have offspring
Allele frequencies measure genetic variation.
– measures how common allele is in population
– can be calculated for each allele in gene pool
Genetic Variation comes from several sources
1. Mutation is a random change in the DNA of a gene. Caused by mistakes in DNA replication or mutagens.
 can form new allele
 can be passed on to offspring if in reproductive cells
2. Recombination forms new combinations of alleles.
 usually occurs during meiosis (crossing-over)
 parents’ alleles arranged in new ways in gametes (gene shuffling). The 23 pair of chromosomes can
produce 8.4 million different combinations of genes (independent assortment).
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•
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11-2 Populations, not individuals, evolve
1. Single-gene trait – controlled by a single gene. Only 2 possible phenotypes.
(ex. widow’s peak or no widow’s peak)
2. Polygenic trait – controlled by 2 or more genes. Many phenotypes possible.
(ex. height or skin color in humans)
* Symmetrical bell-shaped curve is typical of polygenic traits.
11-3
Other mechanisms of evolution
Are there any conditions under which evolution will not occur?
Situation in which allele frequencies remain constant is called genetic equlibrium. Five conditions are
required to maintain genetic equilibrium.
 Random mating, large population, no emigration or immigration, no mutations, no natural selection
11-5 New species can arise when populations are isolated.
Factors such as natural selection and chance events can change the relative frequencies of alleles in a
population. But how do these changes lead to the formation of new species or speciation?
Species – group of organisms that breed with one another and produce fertile offspring.
Isolating mechanisms:
Reproductive isolation – occurs when member of 2 population cannot interbreed and produce
fertile offspring. Can develop several ways:
1) Behavioral isolation – don’t reproduce because of different courtship rituals or other types of
behavior differences. Ex. Different songs in birds
2) Geographic isolation – 2 populations are separated by geographic barriers such as rivers,
mountains, or bodies of water. Ex. Rivers, mountains, bodies of water
3) Temporal isolation – 2 or more species reproduce at different times
11-6
Patterns in Evolution
Convergent evolution – evolution toward similar characteristics in unrelated species, resulting from
adaptations to similar environments.
Divergent Evolution – evolution of one or more closely related species into different species, resulting
from adaptations to different environmental conditions.
Coevolution – process in which 2 or more species evolve in response to changes in each other.
12-1 The Fossil Record
More than 99% of all species that have ever lived on Earth have become extinct.
The fossil record shows:
1) fossils occur in a particular order
2) most organisms that ever lived on earth are now extinct
3) modern organisms have unicellular ancestors
** The fossil record is an incomplete record of life because many organisms died without leaving any trace of
their existence.
Radiometric Dating of Fossils:
o Isotopes are atoms of an element that differ in their number of neutrons.
o A half-life is the length of time required for half of the radioactive atoms in a sample to decay.
o Is you are going to “date” a sample of rock that you think may be very old, use a radioactive isotope with a
long half-life.
12-2 Geologic time scale (p. 366 H - M Book)
The geologic time scale divides Earth’s history based on major past events.
Mesozoic era occurred after the Paleozoic era. Cenozoic era is the most recent era.
To be used as an index fossil, a species must be easily recognized and must have existed for a short period over
a wide geographic range.
• Index fossils can provide the relative age of a rock layer.
– existed only during specific spans of time
– occurred in large geographic areas
• Index fossils include fusulinids and trilobites.
12-3 Origin of Life
4 billion years ago, no oceans existed on earth. The earth was very hot so no liquid water existed.
There are two organic molecule hypotheses (chemical evolution).
o Miller-Urey experiment– showed that simple organic molecules could be made from inorganic
compounds. Simulated lightning on a mixture of gases created simple organic molecules.
o meteorite hypothesis – organic molecules may have come from space on a meteorite or asteroid.
Hypotheses of early cell structure (cellular evolution)
o iron-sulfide bubbles hypothesis – around thermal vents in the ocean, gases bubble up within iron
sulfide chimney structures forming the basic organic compounds within the iron sulfide walls which
acted as the first cell membranes.
o lipid membrane hypothesis – lipid molecules spontaneously form membrane-enclosed spheres which
could then form around a variety of organic molecules.
A hypothesis proposes that RNA was the first genetic material.
o Ribozymes are RNA molecules that catalyze their own replication.
o DNA needs enzymes to replicate itself.
12-4 Early Single-Celled Organisms
o The oldest known fossils (3.5 billion years ago) were prokaryotic cells similar to Archaea, which live in
extreme environments, such as hot springs or volcanic vents.
o Prokaryotic cells such as Cyanobacteria, which are photosynthetic autotrophs, evolved and added
oxygen to atmosphere. This led to the evolution of aerobic prokaryotes, and eventually, eukaryotes.
Eukaryotic cells may have evolved through endosymbiosis 1.5 to 1.8 million years ago.
o Endosymbiosis is a relationship in which one organism lives within the body of another.
o Mitochondria and chloroplasts may have developed through endosymbiosis.
Evolution of sexual reproduction increased diversity
o Genetic variation is an advantage of sexual reproduction.
o Sexual reproduction may have led to the evolution of multicellular life.