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Evolution in Natural Populations
Dispersal in island plant populations – Cody & Overton 1996
Evolution in Natural Populations
Dispersal in island plant populations – Cody & Overton 1996
• Studied weedy, wind-dispersed plants
located on the islands off the west
coast of Vancouver Island
• Censused the plant populations of 200
islands and a region of the mainland
over a 10-year period
Extinction and recolonization events occurred
frequently on the islands.
How might seeds that successfully colonize an island differ
from the majority of seeds on the mainland?
After a plant has colonized an island, how might seed
morphology evolve over time?
Evolution in Natural Populations
Dispersal in island plant populations – Cody & Overton 1996
Studied two species:
• Hypochaeris radicata
• Lactuca muralis.
Pappus
Both are Asteraceae (composites).
Both produce achenes consisting of a
feathery pappus and a seed.
Dispersal ability is related to the volume
of the pappus divided by the volume
of the achene (VP / VA).
Seed
Evolution in Natural Populations
Dispersal in island plant populations – Cody & Overton 1996
Old island populations of
Hypochaeris radicata have significantly
(P<0.01) lower dispersal ability than
mainland populations.
Recent migrants of Lactuca
muralis have greater dispersal
ability than mainland plants.
Dispersal ability significantly
decreases (P<0.01) as the age of
the population increases.
Evolution in Natural Populations
Dispersal in island plant populations – Cody & Overton 1996
These populations have evolved in a manner consistent with strong
natural selection for dispersal ability among colonists and against
dispersal ability among residents of these islands.
Source: Cody and Overton. 1996. Short-term evolution of reduced
dispersal in island plant populations. J. of Ecology 84:53-61.
Evolution and Artificial Selection
Artificial selection has been carried out on a variety of
traits in a number of organisms.
Many are relevant even to non-biologists,
particularly agricultural traits like:
• birth weight, growth rate, and milk production in
cows
• egg production in chickens
• back-fat in pigs
• grain yield in wheat
Evolution and Artificial Selection
Oil content in corn – Dudley and Lampert 1992
One of the longest running studies
documenting evolution
• began in 1896 at the University of Illinois
Two selection treatments:
• High oil content
• Low oil content
Why has the lower line
tapered off?
Evolution and Artificial Selection
Body Weight in Mice – Roberts 1966
More complicated traits also respond to artificial
selection.
• Dashed lines: artificial
selection reversed
• Dotted lines: artificial
selection stopped.
Evolution and Artificial Selection
Body length in Drosophila – Roberts 1955
More complicated traits also respond to artificial
selection.
• Dashed lines: artificial
selection reversed
• Dotted lines: artificial
selection stopped.
Evolution and Artificial Selection
Bristle number in Drosophila – Yoo 1980
Response to selection is similar across six replicates
• The number of bristles
quadruples in 90
generations!
Why would the number
of bristles decrease
when artificial
selection was
stopped?
Evolution and artificial selection
As long as the initial population is genetically variable,
artificial selection is almost always successful and the trait
under selection changes over time.
• Even starting with a genetically homogeneous population,
artificial selection still works, but it takes longer.
• Why?
• Mackay et al (1994) selected on abdominal bristle number in
a highly inbred line of Drosophila (=extremely low in genetic
variability).
• Over 120 generations the high and low lines differed by 12
bristles on average!
Evolution and artificial selection
OK - So a trait changes over time under selection, but could
that ever lead to two different species?
Provisional definition:
Species – An interbreeding group of organisms that is
reproductively isolated from (does not interbreed
with) other groups of organisms
(What do you think of this definition?)
Evolution and Artificial Selection
Habitat selection in Drosophila – Rice & Salt 1988, 1990
• Flies choose
• lightness or darkness (left vs.
right) – selection for phototaxis
• up or down – selection for
geotaxis
• acetaldehyde (white vial) or
ethanol (black vial) – selection
for chemotaxis
• Experimenters choose time period:
early (E), middle (M), and late (L) –
selection for development time
Evolution and Artificial Selection
Habitat selection in Drosophila – Rice & Salt 1988, 1990
• Flies mated within the maze
• Control lines: 120 females chosen
randomly
• Selected lines – 60 females each from:
• dark, up, acetaldehde, early (5E)
• light, down, ethanol, late (4L)
Evolution and Artificial Selection
Habitat selection in Drosophila – Rice & Salt 1988, 1990
• Experimental larvae were
mixed and placed together
in the maze to start the
next generation.
• Controls were run through
the maze separately.
• Offspring of mothers collected
from 5E and half of the controls
were raised on a chemical that
turned their eyes brown.
Evolution and Artificial Selection
Habitat selection in Drosophila – Rice & Salt 1988, 1990
% Brown-eyed flies
5E
4L
Control
Selection
Selection plus
(Offspring that switched
habitats destroyed)
Evolution and Artificial Selection
Habitat selection in Drosophila – Rice & Salt 1988, 1990
5E
4L
% Brown-eyed flies
Control
Selection
Selection plus
(Offspring that switched
habitats destroyed)
No habitat specialization evolved in the control flies
Evolution and Artificial Selection
Habitat selection in Drosophila – Rice & Salt 1988, 1990
% Brown-eyed flies
5E
4L
Control
Selection
Selection plus
(Offspring that switched
habitats destroyed)
Habitat specialization evolved in the selected lines.
• Offspring of females collected from 5E were more likely to
return to 5E
• Offspring of females collected from 4L were more likely to
return to 4L
Evolution and Artificial Selection
Habitat selection in Drosophila – Rice & Salt 1988, 1990
% Brown-eyed flies
5E
4L
Control
Selection
Selection plus
(Offspring that switched
habitats destroyed)
Habitat specialization evolved in the selected lines
• Since females tend to mate near the food vials, interbreeding
between 5E and 4L flies had virtually ceased by the end of
the experiment.
• The first step of speciation had occurred!
Image credits
Blackcap: www.dicktruedesign.com/PRINTS/prints.fin&feather.htm
Achene: www.bartleby.com/61/imagepages/A4achene.html