Download 16.3 – Darwin Presents His Case Natural Selection

16.3 – Darwin Presents His Case
Chapter 16: Darwin’s Theory of
Evolution
16.3 – Darwin Presents His Case
THINK ABOUT IT
 Darwin wanted to gather as much evidence as he could to
support his ideas before he made them public.
 His work contradicted the accepted beliefs of the time.
 In 1859 Darwin published his first complete work on
evolution: On the Origin of Species
16.3 – Darwin Presents His Case
Evolution by Natural Selection
 The Mechanism of Evolution = Natural Selection
 Natural Selection – Process by which organisms that
are most suited to their environment survive and
reproduce successfully.
 Well-adapted individuals survive and reproduce.
 Acts only on inherited traits because they are passed to offspring
16.3 – Darwin Presents His Case
Evolution by Natural Selection
 Driving Forces for Natural Selection
 The Struggle for Existence
 Variation and Adaptation
 Survival of the Fittest
16.3 – Darwin Presents His Case
The Struggle for Existence
 When more individuals are produced than can survive –
all of the members of the population must compete for
resources.
 Darwin called this the struggle for existence.
 Example:
 Classroom full of 12 students. The room has 12 desk and enough food
for 12 people. Suddenly 2 individuals are added to the classroom. With
only the same resources available all of the members must compete for a
desk “living space” and food.
16.3 – Darwin Presents His Case
Variation and Adaptation
 Variation among individuals of a population occurs
naturally. Differences between characteristics
provide some individuals with a greater chance of
survival.
Adaptation - Any heritable characteristic that
increases an organism’s ability to survive and
reproduce in its environment.
16.3 – Darwin Presents His Case
Variation and Adaptation
 Adaptations: include body parts or structures, colors for
camouflage or mimicry, physiological functions, and behaviors.
 Example: The scarlet king snake (harmless) exhibits mimicry—
It copies the coloration of the eastern coral snake (poisonous),
so predators avoid it, too.
16.3 – Darwin Presents His Case
Survival of the Fittest
 Fitness describes how well an organism can survive and
reproduce in its environment.
 Individuals with characteristics that are well-suited to their
environment – a have high fitness.
 Individuals with characteristics that are not well-suited to
their environment – a have low fitness.
 Survival of the fittest!!!!!
 In evolutionary terms, survival means reproducing and
passing adaptations on to the next generation.
Briefly discuss
the diagram
below:
16.3 – Darwin
Presents
His Case
• What is occurring in this diagram?
• Natural Selection
16.3 –Darwin’s
Darwin Presents
His Case
Finches:
(All 13 different species of finches have different beaks)
16.3 – Darwin Presents His Case
Natural Selection
 This hypothetical population of
grasshoppers changes over time as a
result of natural selection.
 Grasshoppers can lay more than 200
eggs at a time, but only a small fraction
of these offspring survive to reproduce.
16.3 – Darwin Presents His Case
Natural Selection
 Certain variations, called
adaptations, increase an
individual’s chances of surviving
and reproducing.
 In this population of grasshoppers,
heritable variation includes yellow
and green body color.
 Green color is an adaptation: The
green grasshoppers blend into their
environment and so are less visible
to predators.
16.3 – Darwin Presents His Case
Natural Selection
 Because their color serves as a
camouflage adaptation, green
grasshoppers have higher fitness and
so survive and reproduce more often
than yellow grasshoppers do.
16.3 – Darwin Presents His Case
Natural Selection
 Green grasshoppers become more
common than yellow grasshoppers in
this population over time because
more grasshoppers are born than
can survive, individuals vary in color
and color is a heritable trait, and
green grasshoppers have higher
fitness in this particular environment
16.3 – Darwin Presents His Case
Natural Selection
 Natural selection:
 Does not make organisms “better.”
 Allows an organism to pass its genes to the next
generation.
 Doesn’t move in a fixed direction.
 If the environment changes, traits that were once
useful may no longer be.
 If the environment changes too quickly the population
may go extinct.
16.3 – Darwin Presents His Case
Natural Selection
 Peppered Moth Evolution
 Majority of the population
200 years ago was light
colored
 The industrial evolution in
England caused wide
spread pollution and killed
the lichens the moths were
camouflaged for and left
the tree bark covered in
dark soot.
 Birds easily found the lighter
individuals and preyed upon
them.
 Only the darker individuals survived to
reproduce and the population became
dominated by darker moths
16.3 – Darwin Presents His Case
Descent with Modification
 Descent with modification - Descendants of the earliest
organisms spread into different habitats. Over time the
descendants adapted to their environment. (With Modification)
 Example: Snowshoe Hare and Jackrabbit
16.3 – Darwin Presents His Case
Common Descent
16.3 – Darwin Presents His Case
Common Descent
 Picture from Darwin’s Notebook
shows one of the first evolutionary
trees
 A single “tree of life” links all living
things.
16.3 – Darwin Presents His Case