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Evolution and Natural Selection
I have called this principle, by which each slight
variation, if useful, is preserved, by the term
Natural Selection
- Charles Darwin, The Origin of Species
Evolution and Natural Selection
We wish to ask:
• What are the main points of Darwin’s theory of
evolution?
• How did observations of nature lead to the
formulation of the theory of evolution?
• How does the process of natural selection work?
• What evidence do we have for local adaptation?
• How can natural selection affect the frequency of
traits over successive generations?
The Intellectual Climate in
Darwin’s Time
• The age of the earth was uncertain,
but studies of strata suggested a
time sequence.
• Uniformitarianism -- the concept
that present conditions and
processes are the key to the past -was becoming influential.
• Fossil discoveries revealed
unknown life forms, suggesting
extinctions had occurred.
• Similarities among organisms were
beginning to be seen as evidence of
relatedness.
Jean Baptiste Lamarck
• A predecessor of Darwin, Lamarck argued that
animals adapted to their environment
• His advocacy of gradual evolution change is
remembered mainly because he got the
mechanism wrong.
Darwin’s Theory of Evolution
• Organisms have changed over time. Those living today are
different from those that lived in the past.
• All organisms are derived from common ancestors by a
process of branching. Similarities of traits are evidence of
a recent, common ancestor.
• Change is gradual and slow, requiring a very long time.
• Natural selection is the mechanism of evolutionary change.
The Process of Natural Selection
• All populations have the capacity for geometric increase,
yet do not.
• There must be a “struggle” to survive and reproduce, in
which only a few are successful.
• Organisms vary in traits that influence their likelihood of
success in this “struggle”.
• Organisms whose traits enable them to survive and
reproduce will contribute a greater number of offspring to
the next generation.
• Offspring resemble their parents, including in these traits
that influence survival.
• Therefore the population in the next generation will
include a greater fraction of individuals with whatever
traits enabled their parents to survive and reproduce.
Natural selection requires…..
For natural selection to occur,
two requirements are
essential:
1. There must be inherited variation
for some trait. Examples: beak
size, color pattern, fleetness.
2. There must be a greater or lesser
probability of survival and
reproduction due to possession of
that trait. Examples: peppered
moth, Galapagos finches.
The peppered moth, Biston betularia. Light and dark
morphs on (a) a light background, (b) a dark background
Distribution of the
two moth
morphotypes
during the
industrial era
The Link between Evolution and
Genetics
“Evolution is the change in the genetic composition
of a population over time”
• Where does genetic variation come from?
- Mutation (raw material, ultimate source)
- sex, meiosis, and exchange of chromosome segments
- other chromosomal re-arrangements
• What are other causes of changes in the genetic
composition of a population over time?
– Non random mating
- migration
– Mutation
- natural selection
– “Sampling error or genetic drift”
The Hardy-Weinberg Equilibrium
“In a large population, with random mating, and in
the absence of forces that would change their
proportions, the proportions of alleles at a given
locust will remain constant”
Allele:
Frequency:
A
p
a
q
The H-W Equilibrium is significant because:
1. It tells us that as long as mating is random and nothing
else happens, gene frequencies will stay unchanged
indefinitely
2. It gives us a tool to calculate allele frequencies if we
know genotype frequencies, and vice-versa
Three Types of Selection
Envision a frequency distribution for a trait such as
beak size, much like a grade distribution.
• Under stabilizing selection, extreme varieties from
both “tails” of the frequency distribution are
selected against; the mean stays constant.
• Under directional selection, individuals at one tail
of the frequency distribution are favored
(“selected for”), the other tail is selected against.
The mean value shifts in the direction favored
• Under diversifying (disruptive) selection, both
extremes are favored at the expense of
intermediate varieties. The result is a bimodal
distribution.
•
Stabilizing
directional
diversifying
•
Birth weight appears to be under stabilizing
selection. Babies that are too large or too
small have higher mortality
•
Directional selection in
horse evolution. Over
time, the horse grew taller
and its hoof evolved as the
nail of the middle digit. Its
molars grew larger as the
horse became a bigger
animal, and its diet became
primarily grasses. A drier
climate resulted in changes
in habitat (open savannahs
instead of forests) and food
(grass instead of leafy
plants).
Darwin’s finches
evolved from a
common ancestor,
similar to a species
now found on the
mainland of
Ecuador.
We’ll look at this
adaptive radiation
again when we
discuss how
species compete
for food.
Sub-species of the rat snake Elaphe obsoleta, referred to
as geographical “races”. They interbreed where their
ranges meet, so are not distinct species.
A phylogenetic tree
constructed from
similarities between
cytochrome c
molecules in various
organisms.
Summary
• The core of Darwin’s theory of evolution is its
mechanism -- the theory of natural selection.
• Natural selection requires inherited variation in a
trait, and differential survival and reproduction
associated with possession of that trait.
• Examples of natural selection are well
documented, both by observation and from the
fossil record.
• Selection acts on the frequency distribution of
traits, and can take the form of stabilizing,
directional, or diversifying selection.