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Evolution for
Beginners
What is evolution?
A basic definition of evolution…
“…evolution can be precisely defined as any
change in the frequency of alleles within a
gene pool from one generation to the next."
- Helena Curtis and N. Sue Barnes, Biology, 5th ed. 1989 Worth Publishers, p.974
So what does the definition mean?
• Evolution is a change in the number of
times specific genes that code for specific
characteristics occur within an interbreeding
population
• Individuals don’t evolve, populations do
• There is no implied “improvement” in
evolution
Genetic Variation and
Evolution
• Evolution: changes through time
1. Species accumulate difference
2. Descendants differ from their ancestors
3. New species arise from existing ones
A brief history of evolution
Charles Darwin was born on
February 12, 1809 in Shrewsbury,
England.
From 1831 to 1836 Darwin served as
naturalist aboard the H.M.S. Beagle on
a British science expedition around the
world.
He observed much variation in related
or similar species of plants and animals
that were geographically isolated from
each other.
These observations were the basis for
his ideas.
A brief history of evolution
Darwin presumed that populations of individuals
changed over time, and, in 1844, he developed
the concept of the driving force for evolution. It
wasn’t until many years later that he published
his idea.
“I have called this principle, by which
each slight variation, if useful, is
preserved, by the term Natural Selection.”
—Charles Darwin from "The Origin of Species“,
1859
Natural selection: mechanism of
evolutionary change
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Natural selection: proposed by Darwin as the
mechanism of evolution
individuals have specific inherited characteristics
they produce more surviving offspring
the population includes more individuals with
these specific characteristics
the population evolves and is better adapted to its
present environment
Natural Selection
Darwin knew nothing of
genes, but what he did have
were two observations and a
little inference that provided
the motive force for
evolution.
•Darwin: Evolution is descent with modification
Natural Selection
Observation 1: Organisms generally
have more offspring than can survive to
adulthood.
Observation 2: Offspring are not
identical. There is variation in their
appearance, size, and other
characteristics.
Natural Selection
Inference: Those organisms that
are better adapted to their
environment have a greater
likelihood of surviving to adulthood
and passing these characteristics
on to their offspring.
Survival of the “fittest.”
Survival of the “fittest.”
Darwin’s
theory for
how long
necks
evolved in
giraffes
Evolution of species
• Based on 3 mechanisms
– 1.
Sources of variation
– 2.
Method of selection for those
characteristics that would be passed on
– 3.
A mechanism for retaining changes
1. Sources of variation
• Genetic diversity thru mutations that are not
lethal
– Physical or behavioral traits
• Sexual reproduction between genetically
different individuals
1. Sources of variation
1. Sources of variation
Gene Flow
• Many species are made up of local populations whose members tend to
breed within the group.
• Each local population can develop a gene pool distinct from that of other
local populations.
• Members of one population may breed with occasional immigrants from
an adjacent population of the same species.
-introduce new genes or alter existing gene frequencies in the
residents.
This is called hybridization.
If the hybrids later breed with one of the parental types, new genes are
passed into the gene pool of that parent population.
This process, is called introgression. It is simply gene flow between
species rather than within them.
•In either case, gene flow increases the variability of the gene pool.
Purebred naturally evolved region specific wild species can be threatened
with extinction in a big way through the process of genetic pollution i.e.
uncontrolled hybridization, introgression and genetic swamping which leads
to homogenization or replacement of local genotypes as a result of either a
numerical and/or fitness advantage of introduced plant or animal.
2. Method of selection
• Reproductive fitness is the method of
selection.
– Competition, escaping from and eluding
predators
– Finding a good mate
Selection
• Artificial selection: a breeder selects for
desired characteristics
Selection
• Natural selection: environmental conditions
determine which individuals in a population
produce the most offspring
• 3 conditions for natural selection to occur
– Variation must exist among individuals in a population
– Variation among individuals must result in differences
in the number of offspring surviving
– Variation must be genetically inherited
3. Maintenance of Variation
• Frequency-dependent selection: depends
on how frequently or infrequently a
phenotype occurs in a population
– Negative frequency-dependent selection: rare
phenotypes are favored by selection
– Positive frequency-dependent selection:
common phenotypes are favored; variation is
eliminated from the population
• Strength of selection changes through time
Maintenance of Variation
• Fitness of a phenotype does not
depend on its frequency
• Environmental changes lead to
oscillation in selection
Fitness and Its Measurement
• Fitness: A phenotype with greater
fitness usually increases in frequency
– Most fit is given a value of 1
• Fitness is a combination of:
– Survival: how long does an organism live
– Mating success: how often it mates
– Number of offspring per mating that survive
The Limits of Selection
• Genes have multiple effects
– Pleiotropy: sets limits on how much a
phenotype can be altered
• Evolution requires genetic variation
– Thoroughbred horse speed
Selection for
increased speed in
racehorses is no
longer effective
Maintenance of Variation
Directional selection: acts to eliminate one
extreme from an array of phenotypes
Genetic equilibrium is a
basic principle of
population genetics.
Hardy–Weinberg principle states that the genotype frequencies in a
population remain constant or are in equilibrium from generation to
generation unless specific disturbing influences are introduced.
Those disturbing influences include
1. non-random mating,
2. new mutations, selection,
3. random genetic drift
4. and gene flow.
Five agents of evolutionary change
Genetic Drift
• Interbreeding is limited to the members of local populations.
• If the population is small, Hardy-Weinberg may be violated.
• Chance alone may eliminate certain members out of proportion to their
numbers in the population. In such cases, the frequency of an allele
may begin to drift toward higher or lower values.
• Ultimately, the allele may represent 100% of the gene pool or, just as
likely, disappear from it.
•Drift produces evolutionary change, but there is no
guarantee that the new population will be more fit
than the original one.
•Evolution by drift is aimless, not adaptive.
Change in the allele frequencies (or gene frequencies) of a population from one generation to the next
due to the phenomena of probability in which purely chance events determine which alleles (variants
of a gene) within a reproductive population will be carried forward while others disappear.