Download 2. Evolution under Artificial Selection Oil Content in

2. Evolution under Artificial Selection
Oil Content in Corn
Artificial selection has been carried out on a variety of
traits in a number of organisms.
Although some examples only a biologist could love, other
examples have had an important impact on agriculture,
including selection on
• birth weight, growth rate, and milk production in cows
• egg production in chickens
• back-fat in pigs
• grain yield in wheat
One of the longest running studies
where evolutionary change has
been documented began in corn in
1896 at the University of Illinois.
Two lines of corn were artificially selected. In one line,
those plants with a high oil content were used as seed in
the next generation. In the second line, plants with a low
oil content were used as seed.
[Dudley and Lampert (1992)]
(Why has the lower line tapered off?)
Body Weight in Mice
More complicated traits also respond to artificial selection.
6-week weight (g)
For example, the weight of mice at six weeks of age was
selected, again in two separate lines (for heavier and for
lighter mice).
Generation
[Roberts (1966)]
(The dashed lines refer to a subset of the two lines in
which artificial selection was reversed.)
(The dotted lines refer to a subset of the two lines in which
artificial selection was stopped.)
Thorax Length in Drosophila
Thorax length in adult Drosophila melanogaster shows a
similar response to selection.
Thorax length (mm)
Here two separate lines were selected: one for longer flies
and one for shorter flies.
Generation
[Robertson 1955]
(The dashed lines refer to a subset of the two lines in
which artificial selection was reversed
(The dotted lines refer to a subset of the two lines in which
artificial selection was stopped.)
Abdominal Bristle Numbers
The number of bristles is a trait that is fairly easy to score
in Drosophila melanogaster.
Yoo (1980) selected for an increased number of bristles in
six replicate lines.
Abdominal Bristle Number
*
*
*
Selection relaxed
Generation
The number of bristles changed from around ten to around
forty in 90 generations!
After about 90 generations, selection was then stopped (*).
(Why would the number of bristles decrease after
artificial selection was stopped?)
As long as the initial population is genetically
variable, artificial selection is almost always successful
and the trait under selection changes over time.
Even if you start with a genetically homogeneous
population, artificial selection will still work, but it takes
longer since selection can only act on the new mutations
that occur.
For example, Mackay et al (1994) selected on abdominal
bristle number in a highly inbred line of Drosophila
(=extremely low in genetic variability).
Nevertheless, over 120 generations the high and low lines
differed by 12 bristles on average!
OK - So a trait changes over time under selection, but
could that ever lead to two different species?
Habitat Selection in Drosophila
Rice and Salt (1988, 1990) designed an experiment to test
the hypothesis that selection could drive speciation among
sympatric populations.
Definitions: from Bush and Howard (1986)
Species: "A group of populations whose evolutionary
pathway is distinct and independent from that of
other groups...achieved by the group’s reproductive
isolation from other groups."
Sympatry: "Two populations are sympatric if
individuals of each are physically capable of
encountering one another...with moderately high
frequency."
Allopatry: "Allopatric populations are separated by
uninhabitated space...across which migration
(movement) occurs at a very low frequency."
Rice and Salt constructed an ingenious maze within which
larval flies were placed.
Within the maze, flies could choose
• lightness or darkness by moving left or right [selection
for phototaxis]
• up or down [selection for geotaxis]
• acetaldehyde (white vial) or ethanol (black vial)
[selection for chemotaxis]
In addition, flies were collected from the eight "habitats"
during three time periods: early (E), middle (M), and late
(L) [selection for development time].
Flies were allowed to mate within the maze (females tend
to mate only when they have located food).
For the control lines, all flies within the habitats were
mixed and 120 females chosen.
For the selected lines, 60 females flies were drawn from
habitat 5E (dark, up, acetaldehde) and 60 from habitat 4L
(light, down, ethanol).
Larvae from the experimental females 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.)
Control
Selection
Selection +
(Offspring that
switched habitats
destroyed)
Over the 35 generations of the experiment, habitat
specialization evolved in the selected lines:
• the offspring of females collected from habitat 5E
(solid squares) were more likely to return to 5E
• the offspring of females collected from 4L (empty
squares) were more likely to return to 4L.
No habitat specialization evolved in the control flies.
Since females tend to mate near the food vials, gene flow
between 5E and 4L flies had virtually ceased by the end of
the experiment.
The first step of speciation had occurred!
SOURCES:
• Rice and Salt (1988) Speciation via disruptive
selection on habitat preference: Experimental
evidence. American Naturalist 131: 911-917.
• Rice and Salt (1990) The evolution of reproductive
isolation as a correlated character under sympatric
conditions: Experimental evidence. Evolution 44:
1140-1152.