Download Natural Selection

Charles Darwin and the theory of
natural selection
Biology I
I. Before Darwin
• Scientists knew that
organisms had
evolved
• Fossil evidence
• Age of the Earth
I. Before Darwin
• Lamarck- aquired
characteristics
• Evolve from simple to
complex
• Organisms want to
adapt
I. Before Darwin
• Later disproved
II. Darwin
• At 21, took a job as
a naturalist on the
HMS Beagle
• Collected
specimens, took
notes of different
organisms
His voyage:
II. Darwin
• Sailed to Galapagos
Islands
• All new species, but
similar to those found
elsewhere
• Finches most famous
• Had slightly different beaks from island to island
• Differed by their diet
II. Darwin
• Darwin 22 years
developing theory of
natural selection
• Pressured into
publishing by
Wallace
• Published On the
Origin of Species by
Natural Selection
III. Natural Selection
4 Requirements:Natural Selection
1. Variation
2. Differential
Reproduction
3. Heredity
4.Lots of Time
III. Natural Selection
• 1. Individuals have
variations.
– Color, size, speed, etc.
• The variations that natural
selection acts on are
genetically linked
III. Natural Selection
• Four steps
• 2. Organisms produce
more offspring than can
survive
– competition
Only the best are going to
survive
III. Natural Selection
• 3. Beneficial
variations survive,
pass traits to offspring
• Passed through DNA
III. Natural Selection
12
10
8
• 4. Over time variations
become more frequent
in population
A
B
6
C
D
4
2
0
A
B
C
D
12
10
8
A
B
6
C
D
4
2
0
A
B
C
D
• Results in population
being different than
ancestors
III. Natural Selection
• Natural selection
usually takes a long
time
• Selection for a
particular variation
depends on
environment
Example of Natural Selection
Evolution
Evolution is the change in a population over time
IV Evidence for Evolution
•
•
•
•
•
Adaptations
Anatomy
Fossil Record
Embryology
Biochemistry
Types of Adaptations
• An Adaptive Trait- variation that is helpful
– Thick fur in cold places
• Maladaptive Trait- variation that is
harmful
– Poor eyesight
Structural Adaptations
• Physical traits
Mimicry-An adaptation
• Enables one species to resemble another
species.
• Ex.-harmless species looks like a harmful
one, good tasting vs. bad tasting.
Camouflage-An adaptation
• A species blends in with its environment to
avoid predators.
• Ex.- peppered moth,
leaf frog
Fossils
• Determine relationships among
organisms.
• Ancestors of whales were probably
land-dwelling, doglike animals.
Anatomy- Homologous
Structures
• Common evolutionary origin.
Similar in arrangement or function
or both.
• Example: forelimbs of a whale, a
crocodile and a bird wing.
Anatomy- Analogous Structures
• No common evolutionary origin,
but similar in function.
• Wings of a bat and wings of a
butterfly.
Anatomy- Vestigial Structures
• No function now but was probably useful to
an ancestor.
• Ex. A whale has leg bones, a snake has leg
bones, we have a tail bone and wisdom
teeth.
Embryology
• Earliest stage of growth and development
• Embryos of a fish, a chicken, a pig, a cow, a
rabbit, and a human are almost identical.
• They all have gill slits and a tail.
Biochemistry
• Comparisons of DNA and RNA
• Now monera is divided into two separate
kingdoms based on their biochemistry.
V. Mechanisms of Evolution
• Genetic Drift
• Gene Flow
• Artificial Selection
• Natural Selection
Sexual Selection
Genetic Drift
• Change in allele
frequency by
CHANCE
Gene Flow
• Change in allele frequency as a
result of MIGRATION
Artificial Selection
Humans choose desired traits and
manipulate breeding
Sexual Selection
• Special case of
Natural Selection
• Variations selected
for their ability to
attract a mate
Geographic isolation
• Physical barrier
divides a
population
• New selective
pressures
• Two different
species
Reproductive isolation
• No longer able to
mate
• Different genetic
information
• Mating behavior
differences
Selective Pressures
• Environmental
• Examples: drought, lack of
food, extreme temperatures,
predation
Sources of Variation
• GENE FLOW moves genes among
populations
• SEXUAL REPRODUCTION
introduces new gene combinations
• Random MUTATIONS in DNA
lead to the formation of new alleles
(sound familiar?)
Mutations
• Can be beneficial, neutral, or
harmful for the organism
• Only mutations in gametes are
inherited by offspring
VI. Natural selection affecting
populations
• Three major types of
natural selection are:
–Stabilizing Selection
–Directional Selection
–Disruptive Selection
Stabilizing selection
• Average individual is selected for
Directional selection
• One extreme trait is selected for
Disruptive Selection
• Both extremes
are selected for
• Average selected
against.
How do we determine a new
species?
• A species is
considered a
group of
organisms that
can:
1. Interbreed
2. Produce Fertile
offspring
Two species of meadowlark