FUN THINGS TO DO WITH BEANS: Variation,
Natural Selection, and Adaptation
To REVIEW the structure of the
simulation:
The
simulation involves populations of two organisms interacting as predator and
prey. In each population there was heritable
variation in traits which influence success at avoiding capture (prey) or
at getting food (predators). We used
variation in these demographic parameters to (survival of prey, feeding success
of predators) to drive simulated reproductive success (fitness). Changes in the
frequency of these traits over generations, then, amount to evolution due to
natural selection.
The
prey population – the beans – manifested six
phenotypes which were simply inherited; in effect, we assumed the
reproduction was clonal so that offspring beans were always of the same phenotype as the parent. We began with a population of 600 beans (100
of each type) spread over a habitat of grassy lawn.
The
predator population – expressed heritable variation in feeding apparatus (mouthparts);
assumptions about reproduction and inheritance are the same as for
beans.
(NOTE
that there could have been significant variation among individuals in other
traits as well – e.g., behavioral strategies of predators, or size of beans of
same color/pattern. These variations may
have influenced survival and reproductive success, but the simulation does not
make assumptions about their heritability, and we did not track differences in
success. You may wish to speculate on
some of these phenotypic differences.)
The
rules were simple. After prey population
was established for each generation, predators could pick up prey items with
their mouthparts and place it in their stomach; scraping or shoveling directly
into the stomach without lifting it from the ground was not allowed -- food has
to be picked up with the mouthparts (you are not sea cucumbers). Once prey entered another predator’s stomach
it is no longer available to other predators. Each
generation lasted two minutes.
At the
end of each generation, each predator reproduced according to individual
hunting success, with the most successful predators producing more offspring,
the least successful producing none.
Prey phenotype reproduction was
proportional to each phenotype’s survival (‘births’ apportioned among surviving
individuals so as to bring prey population back to ‘carrying capacity’ each
generation). As some of you noted the spatial distribution of the prey
population varied somewhat from generation to generation (presumably habitat
changed a bit…); this may well have affected the relative success of predator
hunting strategies.
THE
RESULTS OF THIS YEAR’S SIMULATION are in this
spreadsheet.
HERE
IS YOUR ASSIGNMENT
1.
Make some graphs to illustrate
the dynamics of predator and prey populations over generational time
(i.e., scale on the horizontal axis should represent generations);
design these to show important comparisons
and differences as clearly as possible.
Follow the usual rules about graphs (e.g., make sure you label axes,
make a legend as necessary, etc.). Explain what you see as the main patterns shown in each graph.
You can do the graphing digitally using whatever software you
prefer, or print some graphpaper from a site like
www.printfreegraphpaper.com
2. Interpret the patterns: Which prey type(s) seem to be best and which
least adapted to the environment? Propose a hypothesis as to how prey characteristics (phenotypic traits) might have caused differences in fitness or 'adaptedness'. Which
predator seems best and which least adapted to feeding on this type of
prey? Offer a hypothesis as to why (put it in terms of FITNESS differences).
3. You may have noticed other patterns that aren't
reflected in the raw data on population numbers (e.g., differences in predator behaviors or
diffeences related to the way prey were distributed within their
habitat....). If so, do you have hypotheses for how they might have affected results?
4. How did total number of
prey killed change over generations? What do you think this pattern indicates? (You may want to make a graph of the changes
in the total number of prey killed vs. time).
5. How would you expect the results to
change if the exercise were done on gravel?
On a tile floor?
HAND IN WORK BY MONDAY 20 OCT