Download Evolution and Natural Selection Jean-Baptiste Lamarck

Evolution and Natural Selection
An analogy, inspired by Rettie (1950), may
develop a proper feeling of perspective.
Let us compress the Earth's history into the
scope of a normal calendar year of 365 days.
To do this, imagine a picture of our planet
taken once each year, and these pictures run
as frames in a motion picture projector at the
rate of 144 frames per second, six times the
usual speed. Each second that our imaginary
movie is shown, 144 years of the Earth's
history flashes by. To show the entire 4.7
billion-year history, we must keep the
projector running continuously, 24 hours a day,
from midnight on New Year's Eve until the next
New Year's Eve.
Throughout January, February, March,
and much of April, the history of Earth is
unrecorded, and our film is blank. On April
12, for a few seconds only, one corner of
the screen is visible, and shows us a
picture of the oldest known rocks.
On April 20, life first appears, but only for a
few seconds and only in a small corner of the
screen. May, June, July, and August go by, and
the film is nearly as blank as before, with only
fleeting glimpses here and there of bleak and
desolate scenery, still containing no multicellular
organisms.
The picture improves during September and
October, but only after November 16, representing
the beginning of the Cambrian period some 600 million
years ago, are we watching anything even approaching
a continuously visible motion picture.
The Devonian period, or "Age of Fishes”,
begins on December 1. The "Age of Reptiles," or
Mesozoic era begins on the morning of December
14th and for 12 1/2 days the dinosaurs dominate
the scene. The extinction of the dinosaurs
ushers in the Age of Mammals on December 26
at about 11 P.M.
The 3.75 million-year old genus Homo, to
which we belong, first appears at 4:45 on New
Year's Eve, and the species Homo sapiens at
about 23 minutes to midnight.
The entire history of human civilization,
since the start of agriculture, is shown on
the screen in less than a minute!
Jean-Baptiste Lamarck
• 1744-1829
• French biologist known for
his idea that acquired traits
are inheritable.
Alfred Russell Wallace
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a contemporary of Darwin
born 1823 in England
died 1913
1848-53 Rio Amazonas & Rio
Negro
1854-62 in Malay Archipelago
documenting >125,000 species
(>1,000 new sp.)
independently discovered a
theory of natural selection
read Thomas Malthus
toured US & Canada 1886-87
Charles Darwin
• Charles Darwin 1809-1882
• H.M.S. Beagle 1831-1836
• convinced two colleagues to
present his and Wallace's
work at Linnaean Society
meeting.
• “On the Origin of Species
by Means of Natural
Selection , or the
Preservation of Favored
Races in the Struggle For
Life” published in 1859
Voyage of HMS Beagle
Robert Fitzroy
• Captain of the HMS
Beagle
• surveyor
The voyage of the Beagle
Major Influences
• geology
• experiences in the Galapagos Islands
• Thomas Malthus (English Economist)- wrote
an essay on principles of population
Darwin’s Observations and Inferences
Observation 1: Left unchecked, the number of
organisms of each species will increase
exponentially, generation to generation.
Observation 2: In nature, populations tend to
remain stable in size.
Observation 3: Environmental resources are limited.
Inference I
Production of more individuals than can
be supported by the environment
leads to a struggle for existence
among individuals, with only a fraction
of offspring surviving in each
generation.
more observations
• Observation 4: Individuals of a
population vary extensively in their
characteristics with no two
individuals being exactly alike.
• Observation 5: Much of this variation
between individuals is heritable.
Inference II
Survival in the struggle for existence is
not random, but depends in part on
the heritable characteristics of
individuals. Individuals who inherit
characteristics most fit for their
environment are likely to leave more
offspring than less fit individuals.
Inference III
The unequal ability of individuals to
survive and reproduce leads to a
gradual change in a population, with
favorable characteristics
accumulating over generations
(natural selection).
Linking Evolution and Genetics
A major problem in Darwin’s theory was the lack
of a mechanism to explain natural selection.
How could favorable variations be transmitted to
later generations?
With the rediscovery of Mendel’s work and its
vast extension in the first half of the 20th
century, the missing link in evolutionary theory
was forged.
Evolution
"descent with modification"; change
in genetically determined
characteristics of populations.
Natural Selection
The sustained differential contribution of
offspring by different genotypes.
“sustained differential”
eliminates the random element;
natural selection is deterministic.
“contribution of offspring”
emphasis on differential reproduction
rather than survival.
“survival of the fittest” -vs- “fitness”
“different genotypes”
the true unit that is selected for or
against.
Natural Selection Sequence
1. Variation occurs within populations.
2. Sexual reproduction is the fundamental
source of variation.
3. Variation must be heritable or
genetically based.
4. Organisms produce more offspring
than can possibly survive and resources
are limiting, therefore competition
among individuals within the population
occurs.
5. Individuals show differential
mortality and differential reproductive
success.
6. Superior survivors/reproducers
increase their genotypic frequency
within the population.
7. Populations become adapted to the
environment.
8. The environment is constantly
changing.
Stabilizing Selection
• extreme phenotypes are selected
against
Directional Selection
one extreme phenotype or the other
is selected against.
Disruptive Selection
average phenotype is selected
against.
example: two colored morphs (brown
& green) in a single population of
insects.
Biological Species Concept
• A species is a population of organisms
that actively or potentially
interbreed, producing viable offspring
and which remain reproductively
isolated from other such populations.
Isolating Mechanisms
geographic isolation
temporal isolation
behavioral isolation
mechanical isolation
Geographic Isolation
Galapagos Islands
Evidences for Natural Selection
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fossil record
biogeography
comparative anatomy
comparative embryology
comparative biochemistry
comparative behavior
comparative physiology
Fossil Record
Homologous Structures
Comparative Embryology
Convergent
Evolution