Natural Selection
Natural selection is a major
mechanism of evolution.
Evolution—change in a population’s
genetic makeup over time.
According to Darwin’s theory of natural
selection, competition for limited
resources results in differential survival.
Individuals with more favorable
phenotypes are more likely to survive and
produce more offspring, thus passing on
their traits to future generations.
On the Origin of Species by Means of
Natural Selection by Charles Darwin
Darwin
hypothesized
that all life
descended from
a common
ancestor.
Natural Selection: Darwin’s proposed
mechanism for evolution
— a population can change over time if individuals with
more fit traits leave more offspring than less fit individuals
Darwin’s background
• Loved nature, studied to be a
clergyman
• Went on HMS Beagle for voyage
around the world
Darwin’s Voyage
Darwin observed adaptations of plants and animals
and saw fossils in South America
• The fossils resembled modern animals.
• Darwin began to consider that fossils belonged to
ancestors of modern species.
In the Galapagos Islands, Darwin saw animals that
were similar to the mainland but slightly different
on each island.
Darwin inferred that adaptation to environment and
origin of new species are related.
Descent with modification – Darwin’s way
of referring to evolution
Mayr’s summary of Darwin's theory:
OBSERVATION 1: If all individuals born reproduce
successfully, a population will increase exponentially.
OBSERVATION 2: But populations
remain stable.
OBSERVATION 3: Resources are
limited
OBSERVATION 4: Variation is heritable
INFERENCE 1: production of more
individuals than can be supported by the
environment leads to a struggle for
existence, with only a fraction of offspring
surviving each generation
INFERENCE 2: Fitness: Individuals whose
inherited traits confer an advantage have a
better chance of surviving in a given
environment and will leave more offspring
INFERENCE 3:
Unequal fitness will
lead to gradual
change in a
population, with
favorable traits
accumulating over
generation
Over time, a population might eventually
accumulate enough change to become a new
species
Evolutionary fitness is measured by
reproductive success.
Genetic variation and mutation play roles
in natural selection. A diverse gene pool is
important for the survival of a species in a
changing environment.
Environments can be more or less
stable or fluctuating, and this affects
evolutionary rate and direction.
Different genetic variations can be selected
in each generation.
An adaptation is a genetic variation that is
favored by selection and is manifested as a
trait that provides an advantage to an
organism in a particular environment.
In addition to natural selection, chance
and random events can influence the
evolutionary process, especially for
small populations.
Measuring
change in a population:
The Hardy-Weinberg Equilibrium Model
Mathematical approaches such as Hardy
Weinberg are used to calculate changes in
allele frequency, providing evidence for the
occurrence of evolution in a population.
Conditions for a population or an allele
to be in Hardy-Weinberg equilibrium:
(1) large population
(2) no migration
(3) no mutations
(4) random mating
(5) no natural selection
*These conditions are seldom met
.
2
p
2
q
+ 2pq + = 1
p+q=1
Example: Graphical analysis of allele
frequencies in a population
Applications of the Hardy-Weinberg
equilibrium equation
Evolution involves changes in the gene pool.
A population in Hardy-Weinberg
equilibrium shows no change. The law tells us
that populations maintain a reservoir of
variability so that if future conditions require
it, the gene pool can change.
If recessive alleles were continually tending to
disappear, the population would soon become
homozygous. Under Hardy-Weinberg
conditions, genes that have no present
selective value will nonetheless be retained.
https://www.boundless.com/biology/genetic-variation/evolution-testing-with-hardy-weinberg/hardy-weinberg-principle-and-application/