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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
finches
• 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.
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
characteristics
• His ideas were rejected by scientists
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)
1.
2.
3.
4.
Overproduction
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
reasoning
– 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
traits
• 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
population
#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
adaptation
#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
Chapter 15
Natural Selection
“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
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
weight
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
1.
2.
3.
4.
Fossil Record
Biogeography
Anatomy and Embryology
Biological Molecules
Fossils
• Remains or traces of an organism that died
long ago
• Show that different types of organisms
appeared at different times and places on
Earth
• 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
Evidence
of Whale
Evolution
Ancestor –
land dwelling
mammal
Transitional
(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
swimming
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
young);
– Possible explanation is that these animals evolved in isolation on the
Australian continent
Evidence #3 – Anatomy and
Embryology
• Anatomy – the study of the body structure of
organisms
• Embryology – study of how organisms
develop
• 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
tasks.
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
structure
– 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
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
development
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