Transcript Charles Darwin 1809-1882

Charles Darwin
1809-1882
Descent with Modification
And the Origin of Species
Born 12 February 1809
Charles Darwin at age 31
Full-scale replica of the Beagle sailing
off the coast of South America.
General plan of the Beagle
based on a drawing by a
shipmate during the voyage
of the Beagle. Darwin
wrote that, “I have just
room to turn around and
that is all.”
Voyage of HMS Beagle –1831-1836
A five year, circumnavigation of the globe, with the principal objective to map
the coast line of South America.
Darwin’s
Finches
Galapagos Islands
.
Voyage of HMS Beagle –1831-1836
A five year, circumnavigation of the globe, with the principal objective to map
the coast line of South America.
The Darwinian Thesis
Fact 1. Organisms have enormous reproductive potential
(Malthus)
Exponential growth
Time
Numbers
Fact 2. Populations are at equilibrium (Observation)
Logistic Growth
Time
Fact 3. Resources are limited (0bservation).
If organisms have enormous reproductive potential, yet do not
realize that potential owing to the fact that resources are limited,
then there must be ….
Inference 1. … a struggle for existence.
Fact 4. Individuals are unique; there is individual variation.
(Observation)
Variation in shell color and banding
pattern within a single species of
Caribbean snail.
Darwin was impressed with the fact that
no two individuals are exactly alike.
In contrast to the Platonic idea that the
eternal idealized type was what
mattered, Darwin made individual
variation an integral part of his theory
of evolutionary change.
Inference 2. If there is a struggle for existence and there is
individual variation, then some individuals, owing to their unique
set of traits, will be better equipped to prevail in the struggle for
existence. In other words,
NATURE SELECTION will occur.
Trait in Offspring
Fact 5. Some individual variation can be transmitted from one
generation to the next (personal observation, experience with animal
breeding).
Offspring resemble their
parents, but not precisely.
Thus, differences among
individuals in the parental
generation tend to be reflected
in individuals of the
descendent generation.
Trait in Parents
Within species, each individual is unique. That is, for most
biological traits there is individual variation. But what are the
sources of this variation?
Imagine that you take a sample of Mockingbirds from the
population that exists on the University of Miami campus.
Northern Mockingbird
You measure and record bill
length on several hundred
specimens, and then cast
your data into a frequency
distribution, just as
instructors cast test scores
into a frequency distribution
(the “curve”) for purposes of
assigning grades.
Why aren’t all of the
mockingbirds identical with
respect to bill length? That is,
what are the sources of
variation in bill length in this
population?
Bill Length
Sources of Variation
• Sexual Dimorphism -- differences due to sex
• Ontogenetic Variation -- differences due to age
• Environmental Effect -- differences due to environment
• Artifactual Variation -- Imprecision in measurement
• Genetic Variation -- Variation due to genetic differences among individuals
Parental
Generation
Offspring
Only birds with large bills
are allowed to reproduce
and thus contribute to the
next generation
By selecting birds with
large bills to produce the
next generation, we get a
statistical response to this
artificial selection: on
average bill length in the
descendent birds exceeds
that of the parental
generation. In addition,
some individuals in the
offspring generation
exceed any in the parental
generation in bill length.
Inference 3. Natural selection, operating over the immensity of
Geologic time, will produce evolutionary change, or DESCENT
WITH MODIFICATION, to use Darwin’s expression.
IN SUMMARY: According to the Darwinian
Thesis, evolution proceeds by means of
agents of natural selection, operating on
heritable variation within populations, to
bring about ancestor-descendant change.
That substantial changes can be induced in
plants and animals by artificial selection is
obvious. Consider the many very different
breeds of dogs, all descended from a single
ancestral species (the wolf).
But can we find examples of natural
selection operating in nature?
Natural Selection in the House Sparrow, Passer domesticus.
• Native to Europe
• Presently ranges from northern
• Introduced into North America Canada to Oaxaca, Mexico
at New York ca. 1850
• A human commensal, hence the
• Spread Across North America, specific name, domesticus.
arriving in California by 1910
On 1 February 1898, in Providence, Rhode Island, an unusually
severe winter storm killed or incapacitated a large number of
House Sparrows.
• One hundred-thirty-four dead or moribund house sparrows
were brought to the laboratory of Professor Herman Bumpus.
• He divided the birds into those that were dead (nonsurvivors) and those that were alive when he received them
(survivors).
• He then weighed each bird, measured total length and wingspan.
• He skeletonized all birds (survivors + non-survivors), so
in truth there were no “survivors.” He measured various
skeletal elements including those from the head, body, and
appendages.
• He searched for differences in size and shape between
survivors versus non-survivors.
• Bumpus published all of his original measurements.
Re-analysis of Herman Bumpus’ House Sparrow Data at
the University of Kansas
Dr. Richard F. Johnston et al.
• Eliminated mass, wing-span and total length from analysis for
being too imprecise in terms of measurement.
• Analyzed on the basis of skeletal measurements on the body,
skull, and appendages
• Used a statistical analysis that combined the information
contained in all of the measurements into a single over-all
measure of size.
• Analyzed the sexes separately, as House Sparrows are known to
exhibit sexual dimorphism, with males slightly larger, on
average, than females.
Results of the Re-analysis of the Bumpus Data
Males
Large
Small
Females
Large
Small
Males
Likelihood of Surviving
Low
Medium
High
Very High
Smaller
Larger
Body Size
Directional Selection favoring large body size in male House
Sparrows.
Selection in which individuals in one tail of the curve of
variation have higher likelihood of survival (are “selected for”) is
termed directional selection because over many generations, the
average value of the trait (in this case body size), will shift in the
same direction. Thus, in the Bumpus example, directional selection
was operating on male House Sparrows to favor those with large
body size.
Selection which operates more or less equally on both tails
of the curve of variation is called stabilizing, or normalizing
selection because under this mode of selection, the average value
of the trait does not change. Variation in the trait will be reduced,
however. In the Bumpus example, stabilizing selection was
operating on females, favoring those of intermediate body size.
A third mode of selection, disruptive selection, occurs
when individuals of intermediate value have a lower likelihood of
surviving relative to those at either end of the curve of variation.
This is termed disruptive selection.
Results of the Re-analysis of the Bumpus Data
Males
Large
Small
Females
Large
Small
Females
Likelihood of Surviving
Very High
High
Medium
Low
Body Size
Larger
Stabilizing Selection favoring intermediate
body size in House Sparrows.
Selection in which individuals in one tail of the curve of
variation have higher likelihood of survival (are “selected for”) is
termed directional selection because over many generations, the
average value of the trait (in this case body size), will shift in the
same direction. Thus, in the Bumpus example, directional selection
was operating on male House Sparrows to favor those with large
body size.
Selection which operates more or less equally on both tails
of the curve of variation is called stabilizing, or normalizing
selection because under this mode of selection, the average value
of the trait does not change. Variation in the trait will be reduced,
however. In the Bumpus example, stabilizing selection was
operating on females, favoring those of intermediate body size.
A third mode of selection, disruptive selection, occurs
when individuals of intermediate value have a lower likelihood of
surviving relative to those at either end of the curve of variation.
This is termed disruptive selection.
Selection in which individuals in one tail of the curve of
variation have higher likelihood of survival (are “selected for”) is
termed directional selection because over many generations, the
average value of the trait (in this case body size), will shift in the
same direction. Thus, in the Bumpus example, directional selection
was operating on male House Sparrows to favor those with large
body size.
Selection which operates more or less equally on both tails
of the curve of variation is called stabilizing, or normalizing
selection because under this mode of selection, the average value
of the trait does not change. Variation in the trait will be reduced,
however. In the Bumpus example, stabilizing selection was
operating on females, favoring those of intermediate body size.
A third mode of selection, disruptive selection, occurs
when individuals of intermediate value have a lower likelihood of
surviving relative to those at either end of the curve of variation.
This is termed disruptive selection.
Likelihood of Surviving
Very High
High
Medium
Low
Trait Value
Modes of Natural Selection
• Directional selection: mean changes, variation is reduced (e.g.,
male House Sparrows in the Bumpus example).
• Stabilizing selection: mean does not change, variation reduced
(e.g., female House Sparrows in the Bumpus example).
• Disruptive selection: mean does not change, variation increased.
Historical Narrative Tracing the Development of
Darwin’s Ideas Concerning the Mechanism of
Evolutionary Change (cont.’d.)
• 1842 -- Brief unpublished abstract
• 1844 -- Unpublished essay of about 250 pages
• 1858 -- Received manuscript by Alfred Russel Wallace
entitled “On the Tendency of Varieties to Depart
Indefinitely from the Ancestral Type.”
• 1858 -- Joint presentation of the theory of evolution by natural
selection before the Royal Linnaean Society of London.
• 1859 -- Publication of “On the Origin of Species by Means of
Natural Selection, or The Preservation of Favored Races in
the Struggle for Life.”