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Natural Selection
And other mechanisms for Evolution
Darwin’s Observations and Inferences
Genetic Variation
• Individuals in a species vary from one to
another. These variations can be passed
from parent to offspring.
Over Production of Offspring
• Individuals in populations tend to
produce more offspring than can survive.
Struggle for existence
• Over population creates competition.
• Environmental resources are limited.
• Those that are more efficient will have an
advantage for survival.
Differential survival and reproduction
Darwin’s Observations and Inferences
Inference 1:
Production of more individuals than can
be supported by the environment leads to
a struggle for existence among individuals,
with only a fraction of offspring surviving
in each generation.
Inference 2:
Survival in the struggle for existence is not
random
• depends in part on the heritable
characteristics
• Those who inherit characteristics
most fit for their environment are
likely to leave more offspring
Darwin’s Observations and Inferences
Inference 3:
• The unequal ability of individuals to survive and reproduce
leads to a gradual change in a population, with favorable
characteristics accumulating over generations (natural
selection).
This is Evolution by Natural Selection
Darwin’s Ideas Updated
• A major problem in Darwin’s theory was
the lack of a mechanism to explain natural
selection
– How could favorable variations be transmitted
to later generations
• With the rediscovery of Mendel’s work and
its vast extension in the first half of the
20th century, the missing link in
evolutionary theory was forged.
• Darwinian theory supported by genetics is
known as the modern synthesis.
Darwin in his
early years.
Genetics of Evolutionary
Theory
Darwin did not
understand how traits
were passed on to
each generation.
Today we define
fitness, adaptation,
species, and process of
evolutionary change
in genetic terms.
Terms to Know
What is the purpose of the
adaptation in this animal?
• FITNESS
– Ability of an individual to survive and reproduce
• ADAPTATION
– Fitness is the result of adaptations
– Adaptation is any individual characteristic that
increases an organism’s chance of survival
Support for Darwin’s Theory
1.
Inherited variation
exists within the
genes of every
population or
species
– Law of
segregation
– Law of
independent
assortment
Support for Darwin’s Theory
2. In a particular
environment,
some individuals
of a population
or species are
better suited to
survive and have
more offspring
Support for Darwin’s Theory
3. Overtime, the traits
that make certain
individuals of a
population able to
survive and reproduce
tend to spread in that
population
Divergence – Accumulation of
differences between groups
Support for Darwin’s Theory
4. There is
overwhelming
evidence that
living species
evolve from
organisms that
are extinct.
Mechanisms for Evolution
• Geographic isolation leads to reproductive isolation
– Two populations are separated by geographic barrier
• Example; river, mountain, ocean
– Two gene pools formed
Kaibab squirrel of north rim of Grand
Canyon
Albert squirrel of south rim of
Grand Canyon
Mechanisms for Evolution
• Temporal isolation leads to reproductive isolation
– Two or more species reproduce at different times
– Example: brown trout and rainbow trout live in same
stream but cannot interbreed because they breed in
different months of the year
Rainbow Trout
Brown Trout
Patterns for Natural Selection
• Divergent evolution – pattern in evolution in which
species that were once similar to an ancestral species
diverge and become increasingly distinct.
• divergent evolution eventually results in a new species
Polar Bear
Brown Bear
Patterns for Natural Selection
• Adaptive radiation – when an ancestral species evolves into
an array of species to fit the number of diverse habitats.
– Shown when new species evolve rather quickly
– Happens when a colony moves to a new environment that lacks
any competing organisms
– adaptive radiation is a form of divergent evolution.
•Examples: Hawaiian honeycreepers and finches of Galapagos.
Patterns for Natural Selection
Convergent Evolution
Can lead to development of analogous structures
A) organ pipe cactus
(N. & S. America)
B) Euphorpia
(African deserts)
PATTERNS OF EVOLUTION
• Parallel Evolution
– Two related
species making
similar
evolutionary
changes after
their divergence.
– Marsupial and
Placental
mammals
Patterns for Natural Selection
• Convergent Evolution – when two species
look alike but do not have a common
ancestor
• Similar in appearance
• Similar in function
• Different in development, origin, &
anatomy
Opossum
Aye-Aye
SHARK
DOLPHIN
Patterns for Natural Selection
Coevolution – when two species evolve in response
to changes in one another
•Flowers & Insects
•Flowers & Birds
•Flowers & Mammals
Evidence for Evolution – Structural Adaptation
• STRUCTURAL ADAPTATIONS
– Mimicry
– Camouflage
http://science.howstuffworks.com/animal-camouflage2.htm
Natural Selection in Action
 Camouflage
When the environment changes over
time
(a) A flower mantid
in Malaysia
–Natural
selection may
result in
adaptation to
these new
conditions
(b) A stick mantid
in Africa
Figure 22.11
Natural Selection in Action
 Counter Shading
Natural Selection in Action
 Eye spots
Natural Selection in Action
 Disruptive Coloration
Natural Selection in Action
 Mimicry
 Monarch or Viceroy Butterfly
Summary
• Evolution is change in
species over time
• Heritable variations exist
within a population
• These variations can
result in differential
reproductive success
• Over generations this can
result in changes in the
genetic composition of
the population.