Download 15-3 Darwin Presents His Case

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Objectives:
Describe how natural variation is used in artificial
selection.
Explain how natural selection is related to species’ fitness.
Identify evidence Darwin used to present his case for
evolution.
State Darwin’s theory of evolution by natural selection.
Publication of On the Origin of
Species

 Darwin’s ideas
challenged
fundamental scientific
beliefs
 Discussed his work
with friends
 Shelved his manuscript
for years
 Told his wife to publish
it when he died
Publication of On the Origin of
Species (cont’d)

 1858
 Alfred Russel Wallace
wrote an essay
summarizing his
thoughts on
evolutionary change
and sent it to Darwin
 This caused Darwin to
want to publish his
findings
Publication of On the Origin of
Species (cont’d)

 1859
 Darwin published On
the Origin of Species
 Suggested a mechanism
for evolution (natural
selection)
 Caused a sensation
Blue Ticket Question

 What event motivated
Darwin to publish his
ideas?
Blue Ticket Answer

 Darwin received Wallace’s essay
 Play: http://www.sciencechannel.com/games-andinteractives/charles-darwin-game/
Inherited Variation and Artificial
Selection

 1 of Darwin’s most important insights was that members
of each species vary from 1 another in important ways
Inherited Variation and Artificial
Selection (cont’d)

 Variation exists both in
nature and on farms
 Heritable variation—
differences that are
passed from parents to
offspring
 Happened in the wild
 Revolutionary idea
Inherited Variation and Artificial
Selection (cont’d)

 Animal breeders use heritable variation (genetic
variation) to improve crops and livestock
 Selected only the organism that had the desirable traits to
reproduce (artificial selection)
Inherited Variation and Artificial
Selection (cont’d)

 In artificial selection, nature provided the variation, and
humans selected those variations that they found useful
 Has produced many diverse domestic animals and crop
plants
Evolution by Natural Selection

 Darwin compared
processes in nature to
artificial selection
 Developed a scientific
hypothesis to explain
how evolution occurs
 Greatest contribution
The Struggle for Existence

 Struggle for existence: members of each species compete
regularly to obtain food, living space, and other necessities of
life
 Predators—catch more prey if they are faster, etc.
 Prey—survive if they are better protected
Survival of the Fittest

 A key factor in the
struggle for existence is
fitness
 Fitness: the ability of
an individual to
survive and reproduce
in its specific
environment
Survival of the Fittest (cont’d)

 Darwin suggested that
fitness is the result of
adaptations
 Adaptations: any
inherited characteristic
that increases an
organism’s chance of
survival
 Successful adaptations
allow organisms to
become better suited to
their environment to
survive and reproduce
Survival of the Fittest (cont’d)

 Adaptations can be
 Anatomical (structure)
 Characteristics
 Physiological
processes (functions)
 Behavior
 Live
 Hunt
Survival of the Fittest (cont’d)

 The concept of fitness was
central to the process of
evolution by natural selection
 Survival of the fittest
 Organisms that are not well
suited to their environment
 Die
 Leave few offspring
 Organisms that are better
suited to their environment
 Survive
 Reproduce more
successfully
Survival of the Fittest (cont’d)

 In natural selection
 Only certain individuals
of a population produce
new individuals
 The traits being selected
contribute to an
organism’s fitness in its
environment
 Takes place without
human control or
direction
Survival of the Fittest (cont’d)

 Over time, natural
selection results in
changes in the inherited
characteristics of a
population
 These changes
increase a species’
fitness in its
environment
Survival of the Fittest (cont’d)

 Cannot be seen directly
 Observed as changes
in a population over
many successive
generations
Blue Ticket Question

 What did Darwin mean
when he described
certain organisms as
“more fit” than others?
Blue Ticket Answer

 Organisms that are better suited to survive in their
environment and pass their traits on to the next
generation
Descent with Modification

 Darwin suggested that
over long periods of
time, natural selection
produces organisms
that have different
structures or occupy
different habitats
 Species today look
different from their
ancestors
Descent with Modification (cont’d)

 Descent with modification: each living species has
descended, with changes, from other species over time
 Implies that all living organisms are related to one another
Descent with Modification (cont’d)

 Common descent: all
species—living and
extinct—were derived
from common ancestors
Evidence of Evolution

 Darwin argued that living things have been evolving on Earth for
millions of years.
 Evidence for this process

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Fossil record
Geographical distribution of living species
Homologous structures of living organisms
Embryology (similarities in early development)
The Fossil Record

 Darwin suggested that Earth was many millions of years
old
 Countless species had come into being, lived for a time, and
then vanished
The Fossil Record (cont’d)

 Scientists document the
fact that life on Earth
has changed over time
by comparing fossils
from older rock layers
with fossils from
younger layers
 Fossils point out
uncertainties in our
understanding of
exactly how some
species evolved
Geographic Distribution of Living
Species

 Darwin’s theory of
descent with
modification made
scientific sense of why
he sometimes saw
different animals that
had similar anatomies
and behaviors
Geographic Distribution of
Living Species (cont’d)
1) Species now living on
different continents had
each descended from
different ancestors
2) Some animals on each
continent were living
under similar ecological
conditions and were
exposed to similar
pressures of natural
selection
3) Different animals ended
up evolving certain
striking features in
common

Homologous Body Structure

 Homologous structures: structures that have different mature
forms but develop from the same embryonic tissues
 Provides strong evidence that all 4-limbed vertebrates have
descended, with modifications, from common ancestors
Homologous Body Structure
(cont’d)

 Similarities and
differences help
biologists group
animals according to
how recently they last
shared a common
ancestor
Homologous Body Structure
(cont’d)

 Vestigial organs: organs so
reduced in size that they are
traces of homologous organs
in other species
 The presence of a vestigial
organ may not affect an
organism’s ability to survive
and reproduce
 Natural selection would not
cause the elimination of that
organ
Similarities in Embryology

 The early stages
(embryos) of many
animals with backbones
are very similar
 The same groups of
embryonic cells develop
in the same order and
similar patterns to make
the tissues and organs of
all vertebrates
 Make homologous
structures
Blue Ticket Question

 What are homologous
structures?
Blue Ticket Answer

 Homologous structures are structures that have
different mature forms but develop from the same
embryonic tissue
Summary of Darwin’s Theory

 Organisms
 Differ, and some variation is heritable
 Make more offspring than can survive and many that do
survive do not reproduce
 Compete for limited resources
 Have different advantages and disadvantages in the
struggle for existence
 Pass their heritable traits to their offspring (natural
selection)
 Are descended with modification from ancestral species
that lived in the distant past
 Evolved from common ancestors (tree of life)
Strengths and Weaknesses of
Evolutionary Theory

 Scientific advances
have confirmed and
expanded most of
Darwin’s hypotheses
 Evolutionary theory
continues to change as
new data are gathered
and new ways of
thinking arise
15-3 Exit Ticket
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