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Ch. 15: Mechanisms of Evolution
15.1 – Evolution is both Factual and the Basis of Broader Theory
15.2 – Mutation, Selection, Gene flow, Genetic Drift, and
Nonrandom Mating result in Evolution
15.3 – Evolution can be measured by changes in allele frequencies
15.4 – Selection can be stabilizing, Directional, and Disruptive
15.5 – 15.6 Skip
AP Biology C 2013
Charles Darwin
What do we know
about him?
Charles Darwin 1809-1882
• 1830’s – took a 5 year trip around the world on a
ship called the HMS Beagle
• He went to the Galapago’s Islands, and was intrigued
by unique organisms such as giant tortoises and
• He noticed similarities and differences among the
many organisms he saw as he traveled
– He noticed striking differences between the species he
saw in South America and those of Europe
– He noted that most of the animals on the Galapago’s
Islands were endemic – unique to that location and
nowhere else
He concluded that
the finches must
have adapted to
their habitat and
food available on
that continent.
George Culvier 1769-1832
• Spent years reconstructing the appearance of
unique organisms from fossil bones
• Gave evidence that some organisms in the
past differ greatly from any living species and
that some organisms had become extinct.
• Promoted the idea of catastrophism – sudden
geological catastrophies caused extinction of
large groups of organisms at certain points of
the past
Charles Lyell (1797-1875)
• Thought that the geologic processes that have
changed the shape of Earth’s surface in the
past continue to work in the same ways –
concept is called uniformitarianism
• Darwin used this idea in his writings
Jean Baptiste Lamarck (1744-1829)
• Thought that simple organisms could arise
from nonliving matter
• Also thought that simple forms of life develop
into more complex forms
• Proposed that individuals could acquire traits
during their lifetime as a result of experience
or behavior – inheritance of acquired
• His ideas were rejected by scientists
Chapter 15
Section 1 History of Evolutionary
Lamarck’s Theory of Evolution
Click below to watch the Visual Concept.
Visual Concept
3 Major Propositions of Darwin
1. Species change over time (evolve)
2. Divergent species share a common ancestor
3. Natural Selection
– Mechanism that produces change in species
– The differential survival and reproduction of
individuals in a population based on variation in
their traits
Darwin published his ideas in a book called, Origin of Species, published
in 1859. It stirred considerable interest (and controversy) among
scientists and the public.
Theory of Natural Selection (4 parts)
Genetic Variation
Struggle to survive
Differential Reproduction
#1 - Overproduction
• More offspring can be produced that can
survive to maturity
• Darwin used Malthus’s book as part of his
– Human populations can increase more quicklythat food supplies and that populations are often
limited by conditions such as war, disease, or lack
of food
– Darwin realized that the environment limits the
population of ALL organisms by causing deaths or
by limiting births
#2 – Genetic Variation
• Within a population, individuals have different
• Ex – some deer have thicker fur or longer legs
than others. The deer with thicker fur will
have offspring with thicker fur
• Occasionally, new traits may appear in a
#3 – Struggle to Survive
• Individuals must compete with each other in
what Darwin called a “struggle for existence”
• Some variations improve an individual’s chance to
survive and reproduce, but some variations
reduce this chance
– Ex – thicker furred deer may survive better in the cold
• A trait that makes an individual successful in its
environment, such as thick fur, is called an
#4 – Differential Reproduction
• Organisms with the best adaptations are most
likely to survive and reproduce
• And through inheritance, the adaptations will
become more frequent in the population, so
populations may begin to differ
Survival of the fittest
• Term Darwin sometimes used
• Describes natural selection
• Fitness – is a measure of an individual’s
hereditary contribution to the next generation
– A fit individual is one that has offspring that also
live long enough to reproduce
Chapter 15
Natural Selection
3 Patterns of Natural Selection
1. Stabilizing Selection
– Individuals with the average form of a trait have
the highest fitness
– Reduces variation in populations
– Example – Predators can easily catch larger visible
lizards and the slower smaller lizards. The average
ones best survive!
– Example – human birth
3 Patterns of Natural Selection
Disruptive Selection
– Individuals with either extreme variation of a trait
have greater fitness than individuals with the
average form of the trait
– Increases variation in population
– Example – bill sizes in black-bellied finches
• Food source is two different kinds of seeds from marsh plants (soft
and hard)
• The birds with large bills can easily crack the hard seeds
• Birds with the small bills can crack the soft seeds
• Birds with medium bills do not survive well because they are less
efficient that the other birds at cracking either seed.
3 Patterns of Natural Selection
Directional Selection
– Individuals that display a more extreme form of a
trait have greater fitness than individuals with an
average form of the trait
– Average shifts in one direction
– Example – Hunters hunt for the biggest animals,
therefore the smaller ones survive!
Evidence of Evolution
Fossil Record
Anatomy and Embryology
Biological Molecules
• Remains or traces of an organism that died
long ago
• Show that different types of organisms
appeared at different times and places on
• Some fossils are of organisms that have
become extinct
• Most powerful evidence of evolution
Transitional Species
• Fossil record shows that species have differed
in a gradual sequence of forms over time
• Transitional species – have features that are
intermediate between those of hypothesized
ancestors and later descendent species
Section 2 Evidence of Evolution
Chapter 15
of Whale
Ancestor –
land dwelling
(mammal) –
lived in coastal
waters; could
swim by kicking
its legs and
using tail for
balance. It
could waddle
on land with its
short legs
Transitional (mammal) – lived in
ocean; resembled a dolphin,
propelled itself with massive tail;
had forelimbs that were flippers
and tiny hind limbs that could not
have been used for walking or
Modern whales – have forelimbs
that are flippers; They have tiny
non-functioning hip bones at the
rear of their bodies
Evidence #2 - Biogeography
• Biogeography – study of the locations of organisms around
the world
• Darwin and another scientist, Wallace, observed animals that
seemed closely related yet were adapted to different
environments in nearby regions
• They also observed animals that seemed unrelated but that
had similar adaptations to similar environments in regions
that were far apart
• Mammals of Australia provide the evidence of biogeography
– There are native Australian animals that resemble wolves, cats, mice,
moles or anteaters
– However, most Australian mammals are marsupials (pouches for
– Possible explanation is that these animals evolved in isolation on the
Australian continent
Evidence #3 – Anatomy and
• Anatomy – the study of the body structure of
• Embryology – study of how organisms
• The forelimbs are used in different
ways in each animal; yet each limb
has a similar bone structure
• One explanation is that an early
ancestor shared by all of these
vertebrates had a forelimb with a
similar bone structure
• As generations passed, different
populations of descendents
adapted to different environments
• Bones inherited from ancestors may
have become modified for different
Body Structures
• Homologous Structures – anatomical structures that
occur in different species and that originated by
heredity from a structure in the most recent
common ancestor of the species
– Might have different function
– Example – forelimbs on birds, dolphins, humans
• Analagous structures – have closely related
functions but do not derive from the same ancestral
– Example – wings on bats, birds and moths – all have
different underlying structure
Vestigial Structures
• Structures that serve no function but that
resemble structures with functional roles in
related organisms
• Example – human tail bone, coccyx is made up
of 4 fused vertebrae that resemble the bones
in an animal’s tail
• Another Example – human appendix and
pelvic bones of modern whales
Chapter 15
Vestigial Features
Click below to watch the Visual Concept.
Visual Concept
Development of Animal Embryos
• Stages of vertebrate embryo development are
very alike
• One possible explanation for these similarities
is that vertebrates share a common ancestor
and have inherited similar stages of
Chapter 15
Section 2 Evidence of Evolution
Similarities in Embryology
Click below to watch the Visual Concept.
Visual Concept
Evidence #3 – Biological Molecules
• In all species, DNA
and RNA are the
molecular basis for
inheritance of traits
• Biologists can
compare the DNA,
RNA, proteins, and
other biological
molecules from many
different organisms
• Phylogeny – the
relationships by
ancestry among
groups of organisms
• Can be modeled
using different
Artificial Selection
• Process occurs when a human breeder
chooses individuals that will parent the next
• Example – humans may choose to breed oat
plants that yield more grain per stalk or
greyhounds that run faster
• Evolution is ongoing – many species may be
evolving at once
• Coevolution – when 2 or more species have
evolved adaptations to each other’s influence
• Examples
– Animals evolved strategies to avoid being eaten
– Microbes evolved to live with certain animals
– Bacteria become resistant to various antibiotics
Other Terms to Know
• Gene Flow – migration of individuals and
movements of gametes between populations
• Genetic Drift – random changes in allele
frequencies from one generation to the next
• Population Bottleneck – Large populations
pass through environmental events that only a
small number of individuals survive; thus a
loss of genetic variation occurs
Other terms to Know continued…
• Founder Effect – A colonizing population is
unlikely to possess all of the alleles found in
the gene pool of its source population
• Sexual Selection – Specific type of nonrandom
mating in which an organism’s phenotype
influences its ability to attract males