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EVOLUTION REVIEW SHEET
The first life on Earth
Geologists have dated the earth to have formed about 4.6 billion years ago. This age was determined by radioactive
dating of rocks. It is assumed that the earth is at least as old as the oldest rocks and minerals composing its crust.
Life on earth has existed for three billion years. Prior to that, simple molecules may have formed complex
molecules that may have formed complex organic molecules that eventually formed into cells with nuclei about a billion
years ago. With the development of cells with nuclei about a billion years ago, there was a great increase in the rate of
evolution of increasingly complex multi-celled organisms. The rate of evolution of new species has been uneven since then,
perhaps reflecting the varying rates of change in the physical environment.
How do we organize and classify all living things
The great diversity of life on earth has lead scientists to come up with a classification system in which organisms
are grouped according to their evolutionary relationships. The smallest group is species. Species that have recently evolved
from a common ancestor are placed together in a larger group called a genus. The complete sequence of grouping from the
smallest group to the largest and most inclusive group is: species genus family order class phylum kingdom.
At present, the most widely used system includes 5 kingdoms. These kingdoms are based on evolutionary relationships.
Kingdom Plantae Kingdom Fungi Kingdom Animalia

Kingdom Protista
(mostly single-celled with nucleus such as paramecia)

Kingdom Monera
(single-celled organisms with no nucleus such as bacteria)
Evolution accounts for the diversity of species that we see today
Evolution is a process of change through time. Evolutionary theory is a unifying principle for the biological
sciences. It provides an explanation for the differences in structure, function, and behavior among life forms.
Modern ideas of evolution provide a scientific explanation for three main sets of observable facts about life on
earth: (1) the enormous number of different life forms we see about us. (2) the systematic similarities in anatomy and
molecular chemistry we see within the diversity, and (3) the sequence of changes of fossils found in successive layers of
rock that have been formed over more than a billion years.
Evidence for Evolution
(1) Paleontological Evidence = Fossils are the direct or indirect remains of organisms preserved in media such as
sedimentary rock, amber, ice, or tar. Since the beginning of the fossil record, many new life forms have appeared and most
old forms have disappeared. The many traceable sequences of changing anatomical forms, inferred from ages of rock
layers, convince scientists that the accumulation of differences from one generation to the next has led eventually to
species as different from one another as bacteria are from elephants.
(2) Anatomical Evidence = homologous structures and/or vestigial structures provide evidence of common ancestry and
relatedness among modern species
(3) Molecular evidence (comparing DNA and similar molecules within organisms) substantiates the anatomical evidence
from fossils and provides additional detail about the sequence in which various lines of descent branched off from one
another.
(4) Embryological Evidence – comparing embryos at early stages of development allows us to conclude that many
organisms have a common ancestor
Darwin’s Natural Selection Explains How Evolution Happens
A central concept of the theory of evolution is natural selection, which arises from three well-established
observations:
(1) There is some variation in inheritable characteristics within every species of organism, (Variations refer to the
characteristics in an individual that are different from that of other individuals in the same species.)
(2) some of these characteristics will give individuals an advantage over others in surviving to maturity and reproducing and
(3) those individuals will likely to have more offspring, which will themselves, be more likely than others to survive and
reproduce. The likely result is that over successive generations, the proportion of individuals that have inherited advantagegiving characteristics will tend to increase.
Selectable characteristics can include details of biochemistry, such as the molecular structure of hormones or
digestive enzymes, and anatomical features that are ultimately produced in the development of the organism, such as bone
size or fur length. They can also include more subtle features determined by anatomy, such as acuity of vision or pumping
efficiency of the heart. By biochemical or anatomical means, selectable characteristics may also influence behavior, such as
weaving a certain shape of web, preferring certain characteristics in a mate, or being disposed to care for offspring.
Charles Darwin proposed the theory of Natural Selection. Key concepts in Darwin’s theory include:
(1) Overproduction: Within a population more offspring are born than can possible survive.
(2) Competition: Since the number of individuals in a population tends to remain constant from generation to generation, a
struggle for survival is suggested.
(3) Survival of the Fittest: The individuals who survive are the ones best adapted to exist in their environment due to the
possession of variations that maximize their fitness.
(4) Reproduction: Individuals that survive and then reproduce transmit these variations to their offspring.
(5) Evolution of new species=Speciation: As time and generations continue, adaptations are perpetuated in individuals and
new species evolve from a common ancestral species, or the accumulation of differences from one generation to the next
has led eventually to species as different from one another as bacteria are from elephants.
The Struggle for survival: Overproduction leads to competition within a species. They compete for (finite) or limited
resources needed for survival.
Variation: New traits that can lead to evolution come from genetic variation. Variations that give individuals an advantage
over other individuals as they compete for the same finite resources, are said to have adaptive value.
That is, the particular traits enable the organism to survive and reproduce under a given set of environmental conditions.
For example, an animal’s color may allow it use camouflage to hide from predators
Selection by the environment: Traits that have adaptive value in a particular environment will give the individuals that
posses that trait, a competitive advantage. The trait will give the organism’s offspring a competitive advantage as well,
making them more likely to survive and reproduce. Eventually, the proportion of individuals that possess that beneficial
trait will increase. Eventually, nearly all individuals in that population will possess that trait.
If environmental conditions change, traits that were formerly associated with a low survival value may, in a changed
environment, have greater adaptive value and increase accordingly. Examples include: Roaches, mosquitoes, and houseflies
resistant to insecticides. (Resistance is not in response to the insecticide. The insecticide acts an environmental selecting
agent.)
Please note that Darwin only observed that there were variations among organisms of the same species – HE DID
NOT PROPOSE OR SUGGEST A REASON FOR THOSE VARIATIONS.
Female Choice and Natural Selection
According to Charles Darwin and others who followed him, a trait that gives a male mating and fertilization
advantage will evolve in a population because males with such traits will produce more offspring than their competitors.
Assuming the trait is heritable, offspring experiencing the beneficial trait will, in turn, achieve greater reproductive success
than their competitors, and so on, through future generations. Some of these traits may have evolved because they attract
the attention of females. Females tend to be more selective than males in choosing a mate partly because males can produce
millions of sperm, whereas females’ eggs are few and far between. Thus, egg availability is a limiting factor in reproductive
success. Therefore, males tend to compete for female attention and not vice versa.
Female choice is a behavior exhibited in a large variety of species. Females generally prefer male traits that most
strongly stimulate their senses. For example, female green tree frogs are attracted to the male that can call the loudest and
most frequently. Female guppies are attracted to the most brightly colored male.
For more complex organisms such as bird and mammals, natural selection appears to favor females who choose
mates that provide them with some direct benefit that will increase their ability to produce offspring, their ability to survive
or to increase the survival of their offspring. Benefits include food, a safe haven or even fewer parasites.
Why is the genetic basis for variations within a population?
(1) Gene mutation is a major source of variation. Mutations can be spontaneous and provide the raw material for
evolution. Chromosomal alterations may result in new combinations of genes in organisms causing variations.
Environmental mutagens that cause gene mutations include ultraviolet radiation from sunlight, x-rays, nuclear radiation, and
cosmic rays, as well as chemicals such as nicotine, asbestos, and cyanide.
When mutations occur in sex cells, the mutations can be passed on to the offspring. If they occur in body cells, they
can be passed on to other body cells only. The experiences an organism has during its lifetime can affect its offspring only
if the genes in its sex cells are changed by the experience.
(2) Sexual reproduction involves the sorting and recombining of genes during meiosis and fertilization resulting in a
great variety of new possible gene combinations (recombination) which produce variations.
How is evolution represented in a diagram (cladogram)?
A careful examination of the fossil record reveals that the process of evolution is often not a linear trend of progression
Instead, there is no definite pattern to evolutionary change. The process is similar to tree growth, in which some branches
survive with little change, some change dramatically in many directions, and some die off. For example, the evolution of
horses is well described from abundant fossil evidence. Horse evolution is often depicted as progressing from a small, 4toed browser to a large, hoofed grazer. In actuality the evolutionary pathway of horses had many branches. Species on
some branches actually decreased in size. Branches with browsing horses existed at the same time as branches with grazing
species. (GRADUALISM VS PUNCTUATED EQUILIBRIUM – refer to textbook)
Extinction
Extinction occurs when a species no longer continues to produce more members of its own kind. Fossil evidence
has shown that during the history of life there have been a number of mass extinction events. In mass extinction, a large
percentage of the existing species go extinct, and some entire groups may be wiped out. The worst extinction event was
about 250 million years ago, when 96% of all species in the seas and on land went extinct. About 65 million years ago
another mass extinction occurred where more than half of all species on earth including the dinosaurs became extinct in a
relatively short period. Some scientists believe that the greatest mass extinction of all time is happening now due to the
effects of the human population on other species. The inability of a species to reproduce may be due to changes in the
environment to which the species cannot adapt. Environment changes include but are not limited to decreased rainfall and
decreased average temperature.
You should be able to:
Hypothesize how particular variations within a chosen population might
influence its survival rate;
Investigate the affects of mutagens on a group of individuals;
Predict the fate of snow leopards based on genetic make-up;
Read and analyze data; Construct a graph;
Research Darwin’s finches;
Compare/contrast the case study of the peppered moth to that of insect
resistance to insecticides;
Analyze how organisms are adapted to attract mates;
Draw conclusions as to how these adaptations favor reproduction;
Trace the evolutionary path of an animal
Compare/contrast human behavior to that of other organisms
Integrate the concepts of mutation and genetic recombination
to explain how variations occur within a population;
Simulate the process of natural selection;
Interpret excerpts from The Origin of Species by Charles
Darwin;
Investigate variations in bird feet relate to their ability to adapt
to their environment
Analyze survey results;
Test Charles Darwin’s theory of behaviors that have evolved
through natural selection. The behavior in question is female
choice;
Compare adaptations of males to those of females;
Demonstrate how the evolution of the horse appears to be
similar to that of the growth of a bush;
Hypothesize the conditions that have selected today’s horse;
Research the evolution of the horse