Evolution of Biological
Communities
Outline
I. Evolution by Natural Selection
A. Background
B. Elements of Theory
C. Examples
II. Population Genetics
Background
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Charles Darwin was not the first scientist to think
about life’s origins or about how species might
change over time.
Lamarck
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Study of fossils suggested life evolves
Inheritance of acquired characteristics
Charles Darwin (1809-1882)
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Even as a boy, interested in nature
Accumulated observations leading to theory
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Lyell’s geology
Galapagos Islands
Artificial breeding programs
Malthus’ theories of population growth
Through these observations, Darwin began to
see adaptation and the origin of new species as
closely related processes
Descent with modification
Evolution by Natural Selection
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Observation: In any species, more offspring are
born into a population than actually survive to
reproduce
Observation: Resources in any environment
are limited
Inference: production of more individuals than
the environment can support leads to a struggle
for existence between individuals of a
population, with only a fraction of their offspring
surviving each generation
Evolution by Natural Selection
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Observation: Members of a population vary in their
characteristics
Observation: Much of this variation is heritable
Inference: Survival depends in part on inherited traits.
Individuals whose inherited traits give them a high
probability of surviving and reproducing will leave more
offspring than those without these inherited traits
Inference: leads to gradual accumulation of these
favorable characteristics in the population over
generations
Natural Selection
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Adaptations: traits which give a selective
advantage to some individuals in a population ie.
traits which increase an organisms chances of
survival and reproduction
Selective Agent/Pressure: environmental
factors that influence which individuals leave
more offspring (favor certain adaptations within
the population)
Natural Selection is the name given to the
process of differential reproduction-the
mechanism by which evolution takes place
Important Points
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Being “fit” is relative, if the environment changes,
new traits may be favored and individuals suited
to one environment may not be suited anymore
It is not “survival of the fittest” but survival and
reproduction of the fittest!
Natural selection does not have foresight-no
direction toward the “perfect” organism-blind
process whereby existing variation is sorted out
by fitness.
Population Genetics
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Variation
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Heredity an important and necessary part of the
theory, not well understood in Darwin’s time
As study of heredity and genetics developed, the
underlying control of an organisms traits has become
better understood
2 types of variation
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Non-heritable: no genetic basis, can’t be passed down,
often of environmental origin
Heritable: actual change in the DNA of an organism via
mutation or mutagen, passed down via egg or sperm
Non-heritable Variation
Heritable Variation
Population Genetics
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Gene: specific sequence of nucleotides in DNA which
code for a particular protein
Allele: alternate forms of a gene
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Examples
More precise definition of evolution:
Evolution is a change in the allele frequencies of a
population of organisms over time
Population: a group of interbreeding individuals, live in
the same area at the same time