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Agents for Evolutionary
Change
There are 5 major agents for
evolutionary change.
1. Mutation
2. Genetic Drift
3. Gene Flow
4. Non-random Mating
5. Natural Selection
A.Mutation
• Mutation is a change in DNA, the hereditary material of life. An organism’s
DNA affects how it looks, how it behaves, and its physiology—all aspects
of its life. So a change in an organism’s DNA can cause changes in all
aspects of its life.
• CROSSING OVER during meiosis produces variety within a species but
MUTATION is the ultimate source of NEW genetic varieties in a species.
• A process like MUTATION might SEEM TOO SMALL-scale to influence a
pattern as amazing as the number of different beetles, or as large as the
difference between dogs and pine trees, BUT IT’S NOT.
• Life on Earth has been accumulating mutations and passing them through
the filter of natural selection for 3.8 billion years — more than enough
time for evolutionary processes to produce its biodiversity history.
B.Genetic Drift
• Genetic Drift - Population Genetics
• KEY IDEAS
1. The study of genetic traits and changes in populations is called
POPULATION GENETICS.
2. The combined genetic material of all the members of a population
is its GENE POOL.
3. Genes often have two or more different forms called ALLELES.
4. ALLELE FREQUENCY is a fraction that represents the frequency of
a particular
THE HARDY-WEINBERG PRINCIPLE
1.
• If a population is not evolving, the allele frequencies in
the population remain stable. This is called GENETIC
EQUILIBRIUM.
• a. Under specific conditions, allele frequencies in a
population REMAIN CONSTANT from generation to
generation.
• b. If specific conditions are not met, the genetic
equilibrium is disrupted and the population MAY EVOLVE.
2.
• CONDITIONS required to maintain GENETIC EQUILIBRIUM are:
• a. No natural selection
• b. Random mating
• c. No migration
• d. No mutation
• e. Very large population size
• 3. Even when all the Hardy-Weinberg conditions are met, genetic
equilibrium can be affected by RANDOM CHANGES in allele
frequencies.
• 4. The random change in allele frequencies in a population due to
chance events is called GENETIC DRIFT.
C. Gene Flow
• Gene flow—also called migration—is any movement of
genes from one population to another.
• Gene flow includes lots of different kinds of events, such
as pollen being blown to a new destination or people
moving to new cities or countries.
• If genes are carried to a population where those genes
previously did not exist, gene flow can be a very
important source of genetic variation.
• In the graphic below, the gene for brown coloration
moves from one population to another.
• The amount of gene flow that goes on between
populations varies a lot depending on the type of
organism. As you would expect, populations of
relatively sedentary organisms are more isolated from
one another than populations of very mobile
organisms.
•
• By moving genes around, it can make distant
populations genetically similar to one another, hence
reducing the chance of speciation. The less
gene flow between two populations, the more
likely that two populations will evolve into two species.
D. Non-random Mating
• The result of nonrandom mating is that some individuals have more
opportunity to mate than others and thus produce more offspring
(and more copies of their genes) than others.
E. Natural Selection
• Darwin’s Explanation Model of Evolution by Natural Selection (Mayer 1991).
• Developed as a series of 5 observations and 3 assumptions based on the observations.
Darwin’s Explanation Model of Evolution by Natural Selection (Mayer 1991).

Developed as a series of 5 observations and 3 assumptions based on the observations.
Observation 1
Organisms have great
potential fertility, which
allows for exponential
growth of populations.
Observation 2
Natural population
normally do not increase
exponentially but remain
fairly constant in size.
Observation 3
Natural resources are
limited.
Assumption 1
A struggle for existence
occurs among organisms in
a population.
Observation 4
Variation (differences)
occurs among organisms
within populations.
Observation 5
Variation (differences) is
heritable.
Assumption 2
Varying organisms
show different
survival and
reproduction,
favouring
advantageous
(desirable) traits
(= natural
selection).
Assumption 3
Natural selection,
acting over many
generations,
gradually
produces new
adaptations and
new species.
• In other words
• “Natural Selection is the process in nature that results in the most
fit organisms producing offspring.” (Miller & Levine 1995).
• Individuals whose characteristics are well-suited to their
environment survive. Individuals whose characteristics are not wellsuited to their environment either die or leave fewer offspring. This
is called Survival of the Fittest! (Miller & Levine 1995).
Role of the Environment in Natural
Selection
• crucial role in Natural Selection
• acts to select favourable variations (NOTE: it DOES NOT CAUSE VARIATION, it
only filters out unfavourable traits)
• i.e. snowshoe hare's white coat
• Natural Selection does not respond to a need to change. The genetic potential
must already exist in the population.
• i.e. Giraffes: those born, just by chance, with longer necks had a survival advantage
(reach the higher branches of trees to eat the leaves) over shorter-necked giraffes,
and would therefore live longer and produce more offspring. The next generation,
would then have more long-necked giraffes in the population. This process would
continue till nearly all the population had long necks.
• i.e. Peppered Moths
Evolution Today
• Darwin's Theory has been expanded upon since its publication.
• i.e. evolutionary processes can happen more quickly than once thought (e.g. pepper
moth and industrial melanism).
• Evolutionary research is today focussing more on genetics.
• Today we define fitness, adaptation, species and the process of evolutionary
change in genetic terms.