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Biol 202: Lecture 4
Ecology & Evolution II
History of Evolution
• On November 24, 1859, Charles Darwin
published On the Origin of Species by Means of
Natural Selection.
• Darwin made two points in The Origin of
Species:
– (1) Today’s organisms descended from ancestral
species.
– (2)Natural selection provided a mechanism for
evolutionary change in populations.
History of Evolution
• Idea of evolution DID NOT originate with
Charles Darwin
• Ancient Greeks: Empedocles (495-435 BC),
Aristotle (384-322 BC) described “concepts” of
change in living organisms over time
• Georges-Louis Buffon (1707-1788) spent
years studying comparative anatomy. “Change
must have occurred during the history of life on
earth”
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History of Evolution
• Erasmus Darwin (1731-1802) - physician and
grandfather of Charles Darwin was interested in
questions of origin and change and believed in
the common ancestry of all organisms
• Jean Baptiste Lamarck (1744 - 1829) -French
zoologist who studied animal classification
believed that species are not constant and that
existing species were derived from preexisting
species.
Lamarck and Evolution
• In 1809, Jean Baptiste Lamarck published a
theory of evolution based on his observations
of fossil invertebrates
• Central to Lamarck’s mechanism of evolution
were the concepts of use and disuse of parts
and of inheritance of acquired characteristics.
– Species could not go extinct
• Lamarck’s Mechanism of Evolution
– Body parts used extensively to cope with the
environment became larger and stronger,
while those not used deteriorated.
– Modifications acquired during the life of an
organism could be passed to offspring.
– A classic example of these is the long neck
of the giraffe in which individuals could
acquire longer necks by reaching for leaves
on higher branches and would pass this
characteristic to their offspring.
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Darwin
• Charles Darwin (1809-1882)
• HMS Beagle (1831-1836)
5 weeks on Galapagos Islands
• On voyage Darwin began
formulating his ideas about
evolution of species
• By the early 1840’s Darwin had developed the
major features of his theory of natural selection
as the mechanism for evolution.
• In 1844, he wrote a long essay on the origin of
species and natural selection, but he was
reluctant to publish his theory and continued to
compile evidence to support his theory.
• In June 1858, Alfred Wallace sent Darwin a
manuscript containing a theory of natural
selection essentially identical to Darwin’s.
• Later that year, both Wallace’s paper and
extracts of Darwin’s essay were published.
• Darwin quickly finished The Origin of Species
and published it the next year (1859).
• While both Darwin and Wallace developed
similar ideas independently, the essence of
evolution by natural selection is attributed to
Darwin because he developed and supported the
theory of natural selection so much more
extensively and earlier.
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• Central to Darwin’s view of the
evolution of life is descent with
modification.
• All present day organisms are
related through descent from
unknown ancestors.
• Descendents of these ancestors
accumulated diverse
modifications/adaptations that fit
them to specific ways of life and
habitats.
Darwin’s Observations & Inferences
• Observation #1: All species have
such great potential fertility that
their population size would increase
exponentially if all individuals that
are born reproduced successfully.
• Observation #2: Populations tend
to remain stable in size, except for
seasonal fluctuations.
• Observation #3: Environmental resources are limited.
• INFERENCE #1: Production of more individuals than the
environment can support leads to a struggle for existence
among the individuals of a population, with only a fraction of
the offspring surviving each generation.
• Observation #4: Individuals of a population
vary extensively in their characteristics; no two
individuals are exactly alike.
• Observation #5: Much of this variation is
heritable.
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• INFERENCE #2: Survival in the struggle for
existence is not random.
– Those individuals whose inherited characteristics
best fit them to their environment are likely to leave
more offspring than less fit individuals.
• 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 the
generations.
Darwin’s Main Ideas: A Summary
1. Natural selection is differential success in
reproduction (unequal ability of individuals to
survive and reproduce).
2. Natural selection occurs through an interaction
between the environment and the variability
inherent among the individual organisms making
up a population.
3. The product of natural selection is the adaptation of
populations of organisms to their environment.
Some Definitions
• Evolution - change of population over time
• Microevolution - change in the gene pool
of a population across generations (species
level)
• Macroevolution - formation of new species
and new designs; evolutionary change
above the level of species.
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Microevolution
Battling malaria: natural
selection is fast!
• DDT developed in
1942, used in India in
late 1940’s
• 95% effective initially
but 16 months later
only 20% effective
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Industrial Melanism
• Classic example of natural selection
• Kettlewell (1959) conducted experiments to
measure the survival of different genotypes
• Calculated change in frequency of dark and
light individuals
Industrial
Melanism
Increase of melanic
forms from 1848 to
1948
A population’s gene pool is defined
by its allele frequencies
• A population is a localized group of individuals that
belong to the same species.
– One definition of a species (among others) is a group of
populations whose individuals have the potential to
interbreed and produce fertile offspring in a nature.
• The total aggregate of genes in a population at any one
time is called the population’s gene pool.
– It consists of all alleles at all gene loci in all individuals of a
population.
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The Hardy-Weinberg Theorem describes a
non-evolving population
• This theorem states that the frequencies of
alleles and genotypes in a population’s gene
pool will remain constant over generations in
the absence of certain “forces” or “events”
• Populations at Hardy-Weinberg equilibrium
must satisfy five conditions.
(1) Very large population size. In small populations,
chance fluctuations in the gene pool can cause
genotype frequencies to change over time.
(2) No migrations. Gene flow, the transfer of alleles
due to the movement of individuals or gametes into
or out of our target population can change the
proportions of alleles.
(3) No new mutations. If one allele can mutate into
another, the gene pool will be altered.
(4) Random mating. If individuals pick mates with
certain genotypes, then the mixing of gametes will
not be random and the Hardy-Weinberg equilibrium
does not occur.
(5) No natural selection. If there is differential
survival or mating success among genotypes, then
the frequencies of alleles in the next variation will
deviate from the frequencies predicted by the HardyWeinberg equation.
• Evolution usually results when any of these five
conditions are not met.
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Hardy-Weinberg Calculations
• If frequency of the dominant allele (A) in
the population is p, and the frequency of the
recessive allele (a) is q, then p + q = 1
• Frequency of heterozygotes (Aa), dominant
homozygotes (AA) and recessive
homozygotes (aa) is:
•
p 2 + 2pq + q2 = 1
• AA Aa
aa
The effect of selection on a variable
characteristic/trait
• Natural selection can affect the frequency of a
heritable trait in a population, leading to
(1) directional selection
(2) diversifying selection
(3) stabilizing selection
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Speciation: the link between
microevolution and
macroevolution
• The fossil record
chronicles two patterns of
speciation: anagenesis
and cladogenesis.
• Anagenesis is the
accumulation of changes
associated with the
transformation of one
species into another.
• Cladogenesis,
branching evolution,
is the budding of one
or more new species
from a parent species.
– Cladogenesis
promotes biological
diversity by increasing
the number of species.
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The biological species concept emphasizes
reproductive isolation
• In 1942 Ernst Mayr enunciated the biological
species concept to divide biological diversity.
– A species is a population or group of populations
whose members have the potential to interbreed
with each other in nature to produce viable, fertile
offspring, but who cannot produce viable, fertile
offspring with members of other species.
• Species are based on ability to reproduce, not
physical similarity.
• Meadowlarks vs humans.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Reproductive Isolation:
Mechanisms
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Modes of Speciation
• Two general modes of speciation are
distinguished by the mechanism by which gene
flow among populations becomes interrupted.
• allopatric speciation vs sympatric speciation
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
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Speed of Evolution
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