Natural and Artificial Selection
Biology H
Lamarck’s Theory of Evolution
Lamarck hypothesized that species evolve through use and disuse and the inheritance of acquired traits
The mechanisms he proposed are unsupported by evidence
The Voyage of the Beagle
During his travels on the Beagle, Darwin collected specimens of South American plants and animals
He observed adaptations of plants and animals that inhabited many diverse environments
His interest in geographic distribution of species was kindled by a stop at the Galápagos Islands near the
equator west of South America
Darwin made two major points in his book:
Many current species are descendants of ancestral species
Natural selection is a mechanism for this evolutionary process
Darwin developed two main ideas:
Evolution explains life’s unity and diversity
Natural selection is a cause of adaptive evolution
Descent with Modification
The phrase descent with modification summarized Darwin’s perception of the unity of life
The phrase refers to the view that all organisms are related through descent from an ancestor that lived
in the remote past
In the Darwinian view, the history of life is like a tree with branches representing life’s diversity
Observation #1: For any species, population sizes would increase exponentially if all individuals that are
born reproduced successfully
Observation #2: Populations tend to be stable in size, except for seasonal fluctuations
Observation #3: Resources are limited
Inference #1: Production of more individuals than the environment can support leads to a struggle for
existence among individuals of a population, or competition, with only a fraction of their offspring
surviving
Observation #4: Members of a population vary extensively in their characteristics; no two individuals are
exactly alike
Observation #5: Much of this variation is inherited
Inference #2: Survival depends in part on inherited traits; individuals whose inherited traits give them a
high probability of surviving and reproducing are likely to leave more offspring than other individuals
“Fitness” in evolutionary terms is the individual who produces the most offspring that live to reproduce
Inference #3: This unequal ability of individuals to survive and reproduce will lead to a gradual change in
a population, with favorable characteristics accumulating over generations
Natural selection
The major microevolutionary process that results in differential survival and reproduction (works on
phenotypes)
There is variation among individuals
More are born than can survive
There is competition for resources
Those individuals that are most fit for their environment survive, reproduce and pass on their alleles.
Natural selection produces an increase over time in adaptation of organisms to their environment
If an environment changes over time, natural selection may result in adaptation to these new conditions
Organisms cannot “choose” to adapt to changes in their environment; the variation must already be in
the gene pool. Those with the variation that helps them, survive and reproduce. Those that don’t have
the variation, die.
Selection of gene frequencies
Directional selection: shifts allele frequencies in a certain direction in response to environmental
pressures; moths
Stabilizing selection: favors the most common forms of a trait in a population; human babies
Disruptive selection: favors forms at both ends of the range of variation and selects against the
intermediate forms; finches
Peppered moths
Artificial Selection
In artificial selection, humans have modified other species over many generations by selecting and
breeding individuals with desired traits
Dogs
Farm animals
Corn