Download Artificial Selection

Darwin’s Theory of
Evolution by Natural
Selection
Section 15-3
Darwin Presents His Case
Artificial Selection
• Artificial Selection
– The selection, by humans, of
organisms with useful traits.
– People decide which plants or
animals they will let reproduce.
– This leads to gradual
improvements each generation.
Artificial
Selection
Kuri
The obvious success of animal
and plant breeding led
Darwin to conclude that:
•Species are not uniform - there
is variation
•Species are not fixed, they
can change over time
•Darwin realized that if
artificial selection could
make such dramatic
changes in organisms
there is no reason the
same process shouldn’t
be occurring in nature.
Variation
in nature
Reading Malthus led
Darwin to conclude that:
Differential Reproductive Success…
•In nature, there must be “winners”
and “losers”
Those with favorable traits vs those with
unfavorable traits
•The “winners” survive and leave more
offspring relative to the “losers”.
Survival of the Fittest
What are some traits that could
make a plant or an animal better
able to survive?
•
•
•
•
•
•
•
•
More offspring
Faster
Larger
Smaller
Brighter color
Longer teeth
Camouflage
Survive drought
• Spines to protect from
predators
• Shell
• Able to dig
• Deep roots
• High photosynthetic rate
• Chemical defenses
against herbivores
Evolutionary Fitness
• The reproductive success of an
individual relative to the rest of
the population.
Fitness
example
• A mutation occurs in an animal
– causes it to be 25% more likely to be
killed by a predator than similar
individuals without the mutation
– able to produce 50% more offspring
than similar individuals without the
mutation
• Should the frequency of this mutation
increase or decrease in the population?
• Does the mutation increase or decrease
the fitness of the animal?
Darwin proposes Natural
Selection as the engine that
drives evolution
• Sometimes referred to as “survival
of the fittest”.
– Note: never by Darwin
Main principles of Darwin’s theory
of evolution by natural selection
1. Variation – variation exists among
individuals of any species.
2. Heredity – Some of the variation is
heritable; that is, it is genetic in nature
and will be passed on to offspring.
3. Differential Reproduction (Fitness) – Some
of the heritable traits will confer a
reproductive advantage to individuals
with the trait.
 Individuals with the trait will have more
surviving offspring (which will also have the
advantageous trait)
Main principles of Darwin’s theory of
evolution by natural selection
• If all three necessary conditions
are met (variation, heritability,
differential reproduction), then
evolution occurs.
Implications of natural selection
Darwin recognized an implication of
his theory of evolution through the
process of natural selection was that…
 Each living species has descended,
with changes, from other species
over time.
 Darwin referred to this as descent
with modification.
Descent with
Modification
Implications of natural selection
• Common Descent
– All species, living or extinct, were
derived from common ancestors.
– A single “tree of life” links all living
things.
Implications of natural selection
Common Descent
Checkpoint
• What is artificial selection?
The selection, by humans, of organisms
with useful traits.
• What are two observations/conclusions
Darwin drew from artificial selection?
Species are not uniform (there is
variation)
Species are not fixed, they can
change over time
Checkpoint
• The driving force of evolution is…
Natural selection
• What is fitness in an evolutionary
sense?
The reproductive success of an
individual relative to the rest of the
population.
Checkpoint
• What are the three necessary
conditions for evolution by
natural selection?
Variation
Heritability
Differential reproduction
Checkpoint
• Descent with modification
means…
Each living species has descended,
with changes, from other species over
time.
There is grandeur in this view of life,
with its several powers, having been
originally breathed into a few forms
or into one; and that, whilst this
planet has gone cycling on according
to the fixed law of gravity, from so
simple a beginning endless forms
most beautiful and most wonderful
have been, and are being,
evolved…Charles Darwin
Chuck
says…
That’s
enough
notes for
today.
Nothing in biology makes sense
except in the light of evolution...
Theodosius Dobzhansky
Common
Descent or
Descent with
Modification
PTEROSAUR
CHICKEN
PENGUIN
ANCESTOR
REPTILE
PORPOISE
BAT
HUMAN