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… evolution by natural
Today’s lecture is about
3. Selection
Summary of Darwin’s evidence and main
ideas on the Origin of Species by Means of
Natural Selection
Convergent evolution
Three types of natural selection and
experimental demonstration of two of them
Charles Darwin 1809-1882
Charles Darwin 1809-1882
On the Origin of Species by Means of Natural Selection
Darwin produced a synthesis based upon a
number of different types of evidence
Biogeography: Voyage of the Beagle
Variation and similarity between organisms in different regions suggested
organisms were related
The Fossil Record
Comparative Anatomy
Comparative Anatomy
Homologous Structures – evolved from a
common original structure
Analogous Structures - evolved from
different original structures but function
Vestigial Structures - functioned in an
ancestral organisms, but that are reduced
(in structure and function) in the
Examples in humans:
caudal vertebrae of coccyx
appendix of intestines
Homologous structures suggested common ancestors and
variation in evolution since those ancestors
Comparative embryology
Comparing the general appearance of embryos
from different species at various stages in
development shows them to be quite similar
Certain embryonic structures are shared by all
chordates, but show interesting structural and
functional changes during development, e.g. gill
Artificial Selection
… mutation?
Mutation is the source
on which natural
selection is going to act
Darwin’s main ideas:
Natural selection is differential success in reproduction
Natural selection occurs through an
interaction between the environment and the
variability inherent among the individual
organisms making up a population
The product of natural selection is the adaptation of
populations of organisms to their environment
Convergent evolution
Species from different evolutionary branches
may come to resemble one another if they live
in similar environments
Astrophytum asterias
Cactaceae Mexico and Texas
Euphorbia valida
Euphorbiaceae South Africa
Three types of Natural Selection
Fig 13.20
Is stabilizing selection the process
that has enabled organisms with
long geological histories, or at least
ancient ancestors, to survive?
What does it require?
Escherichia coli
A group of individuals
Colonies on agar in a Petri dish
Escherichia coli is a typical gram negative rod bacterium.
It is cylindrical, about 1.0-2.0 micrometers long, with
radius about 0.5 micrometers.
SEM picture
Kingdom: Eubacterium
Scientific Name: Escherichia coli
Image Courtesy of: Shirley Owens, Center for Electron Optics, MSU
Image Width: 9.5 microns
Image Technology: SEM (Scanning Electron Microscope)
Inducing evolutionary change in E. coli
Start with a wild-type population that can be stored
and grown for comparative purposes
1. Take some of the wild-type, grow it on a agar with
glucose for a day (about 6.6 generations).
2. Next day plate out onto a fresh petri dish with
new glucose.
3. At intervals take the ‘glucose fed’ population
and compare it’s growth rate on glucose with
the original wild-type population
Trajectory of comparative fitness relative to ancestor during
10,000 generations of experimental evolution in E. coli
Each point is the mean of 12 replicates.
The error bars are 95% confidence limits
Stabilizing, directional or diversifying selection?
Is this an example of stabilizing, directional or diversifying selection?
An experiment: place E. coli in a solution in a chemostat – a
vessel that has a constant input of glucose
Introduce the T4 virus!
Introduce the T4 virus!
E coli
Stabilizing, directional or diversifying selection?
T4 virus
Emergence of an E. coli
resistant mutant
How did the virus survive after
emergence of a resistant mutant?
The mutant was less efficient at glucose
metabolism – so non-mutants out competed it
Is this an example of stabilizing, directional or diversifying selection?
Sections you need to have read
13.1 13.2 13.3 13.4 13.5 13.6 13.7 13.20
the Introduction to Chapter 10
Courses that deal with this topic
Biology 102 General Biology
Biology 454 Evolutionary Mechanisms