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Chapter 15: Evolution Notes Darwin’s Theory of Evolution:
page 368
Evolution: change over time; the process by which modern
organisms have descended from ancient organisms
Scientific Theory: a well supported testable explanation of
phenomena that have occurred in the natural world
Charles Darwin: born in England 1809, on the same day as
Abraham Lincoln
 traveled around the world on H.M.S. Beagle

 Made numerous observations and collected evidence that led
him to propose a hypothesis about the way life changes over
time, the theory of evolution
Darwin’s Observations:
 Patterns of diversity: he observes such a large variation of
species in similar environments
 EX:
in England
but not in
Australia even though the environment was similar
 Living Organisms and Fossils: He observes that fossils of
ancient organisms resembled organism that were still alive.
Others looked unlike anything he has ever seen

 Galapagos islands: many islands close together with very
different climates. Darwin observes that the characteristics
of many animals and plants varied noticeably among
different islands

These differences are related to the environments in which the respective
tortoises live, and the types of food they eat. The domed tortoises tend to
live in the moist high-lands and take their food from grasses and low-lying
shrubs. The saddle-backed tortoises, on the other hand, live in arid regions
and feed on plants that are mostly above their head, most notably the treelike Opuntia (prickly pear) cactuses. The arched shell permits them to stretch
their heads high, giving them a longer vertical reach.
On the origin of species: Darwin’s book published in 1859 that
summarized all his findings from his trip around the world.
Artificial Selection: nature provides the variation and humans
select those variations that they find useful. ** Used to improve
crops and livestock
Selective breeding transformed teosinte's few fruitcases (left) into modern
corn's rows of exposed kernels (right).
 Natural Selection: Over time, results in changes in the
inherited characteristics of a population and increase fitness
(ability of an individual to survive and reproduce in its
specific environment) of a species.

 Darwin's illustrations of beak variation in the finches of the
Galápagos Islands, which hold 13 closely related species that
differ most markedly in the shape of their beaks. The beak of
each species is suited to its preferred food, suggesting that
beak shapes evolved by natural selection.
 Adaptation: an inherited characteristic that increases and
organism’s chance of survival.

 Struggle for Existence: members of each species compete
regularly to obtain food, living space and other necessities of
life.
 Organisms produce more offspring than - given the limited
amounts of resources - can ever survive, and organisms
therefore compete for survival. The Atlantic cod for instance
lays around five million eggs a year while Darwin calculated
that even the low reproducing elephant produces more young
than will ever live to maturity. Only the successful
competitors will reproduce themselves. Darwin referred to
this competition as the "struggle for existence".
 Descent with modification: natural selection produces
organisms that have different structures, establish different
niches or occupy different habitats. This causes today’s
species to look different from their ancestors.

 Common Descent: all living organisms are related to one
another
Evidence of Evolution:
 Fossil record

 Geographic Distribution of Living Species
 Homologous Body Structures: structures that have different
mature forms but develop from same embryonic tissues
(Wings and legs all descended from the fish fin)

 Vestigial Organs: remnants of organs that have no function
now

 Similarities in Embryology

Evolution of Populations:
A population is a group of individuals of the same species that
interbreed and share a common group of genes
Gene pool- all the genes, including all the different alleles, that are
present in a population
Relative frequency of an allele- the number of times that the
allele occurs in a gene pool, compared with the number of times
other alleles for the same gene occur. Expressed as a percentage
Evolution- any change in the relative frequency of alleles in a
population
Single-gene traits – any traits controlled by one gene. (EX:
widow’s peak in humans)
 natural selection on single-gene traits can lead to changes in
allele frequencies and thus to evolution

Polygenic Traits- any traits controlled by 2 or more genes (EX:
height in humans)
 Natural selection can affect the distributions of phenotypes
in any of three ways: directional selection, stabilizing
selection or disruptive selection.
 Directional selection: when individuals at one end of the
curve have higher fitness than individuals in the middle or at
the other end. (EX: seed size and bird beak size)
 Stabilizing selection: when individuals near the center of the
curve have higher fitness than individuals at wither end of the
curve. (EX: weight of human infants at birth)
 Disruptive selection: when individuals at the upper and
lower ends of the curve have higher fitness than individuals
near the middle (EX: seed size and bird beak size)

Genetic Drift: in a small population, individuals that carry a
particular allele may leave more descendents than other
individuals, just by chance. Over time, a series of chance
occurrences of this type can cause an allele to become common in
a population
Founder effect: genetic drift due to the migration of a small
subgroup of a population. (EX: Fruit Flies migrating from
mainland to different Hawaiian Islands)
Hardy-Weinberg Equilibrium: allele frequencies in a population
will remain constant unless one or more factors cause those
frequencies to change. Also called genetic equilibrium
 Five condition are required to maintain genetic equilibrium
from generation to generation
 Random mating
 Large population
 No movement into or out of the population
 No mutation
 No natural selection
Speciation: formation of new species
 Reproductive isolation: when the members of 2 population
cannot interbreed and produce fertile offspring
 Behavioral isolation: when two populations are capable of
interbreeding but have differences in courtship rituals or
other reproductive strategies that involve behavior.
 Geographic isolation: when 2 populations are separated by
geographic barriers such as rivers, mountains, or bodies of
water.
 Temporal isolation- when 2 or more species reproduce at
different times