Populus 2.10, August 1992
Overview
The Populus software contains a set of simulation models that we use in
teaching population biology and evolutionary ecology at the University of
Minnesota. All of the simulations share a common format, as follows: After
a model is chosen from the menu, the program displays (optionally) several
screens of background material which introduce the mathematics and end with
basic references. Next, there is a screen listing all of the input
parameters; students can move among the parameter boxes and change initial
defaults to values of their own choosing. The program maintains a record
including permissible maxima and minima for each parameter and filters input
values accordingly. Usually there are several possible outputs (e.g., N vs T
graphs and phase planes) which can also be selected from the parameter input
screen; alternatively, students view the different outputs in sequence, by
pressing the space bar between views. Context-sensitive help screens are
available (press ) from the input and output screens of every model, and
by pressing twice one can see a menu of additional help screens which
includes discussions of function and editing key assignments implemented by
the program, printing protocols, video options, etc.
Hardware Requirements
The program is written in Turbo Pascal to run under DOS on IBM and
compatible computers. To implement the full range of Populus features, about
500KB of RAM are necessary, free of memory-resident utilities and drivers.
The program will use LIM expanded memory, reducing the frequency of disk
reads and speeding program execution. Although the code will emulate an xx87
coprocessor if one is not present, many of the models are computationally
intensive and will run much more rapidly on a machine with the floating-point
chip. Since overlays are read during program execution, the program files
(populus.exe, populus.ovr and populus.txt) should be copied to a hard disk
(or a RAM disk emulator). If this is not possible, the program will run from
two 720K (or larger) diskette drives, and will prompt users for a path to the
Populus.ovr file.
Version 2.10 will run on any IBM-compatible video system which permits
graphic displays, including "Hercules," "CGA," "EGA," "VGA," and "IBM8514."
In most cases, the software will recognize and accommodate your video system
automatically. If your display emulates one of the color systems in shades
of gray (as many LCD screens do, with varying success), consider using the
option which forces output to the monochrome mode. The IBM8514 system is
detected as VGA by Turbo Pascal; IBM8514 users who desire full 1024x768
resolution (beautiful!) can select that video mode from the Populus Options
Menu (Alt-O) and save a populus.cfg file to disk so that their preferred
configuration loads automatically each time the program is run.
Populus 2.10 contains a set of commercially produced drivers by Ryle
Design that supports a wide variety of printers, including 9-pin and 24-pin
dot matrix printers, HP laserjets and deskjets, the HP paintjet, and printers
implementing the Postscript page-description language. Print files may also
be saved to disk and sent to the printer later using the DOS Print command.
Set the appropriate printing configuration on the Populus options menu. Note
that Postscript supports graphics-mode printing only. Our introductory
narratives and all outputs are rendered in graphics mode, but the parameter
inputs and help windows are text mode screens. To print them on a postscript
laser printer you will have to set it up as an HP emulator.
Bells and Whistles
For many problems and exercises it is desirable that students be able to
read numerical values accurately from the graphical output. We have included
several new video routines for this purpose. A gridding function (press
) plots horizontal and vertical lines from the major axis ticks, and a
second grids the minor ticks. A third press toggles the feature off.
Our video zooming function (press ) pops up a rectangle whose corners
can be moved (with the cursor keys) to any point on screen. Pressing
then zooms this new rectangle to full-screen size. By zooming in on an
interesting equilibrium region and gridding the output, final frequencies can
be read with any desired degree of accuracy.
For many time trajectories of ecological dynamics, comparison of
arithmetic and logarithmic plots has pedagogical value. Populus plots
arithmetic time trajectories by default but allows you to toggle a semi-
logarithmic plot (press ). Like other video utilities in Populus, the
feature is turned off (and the screen-bottom options line is cleared) with
the same keystrokes that turned it on.
Populus models that display in the lower-right corner of their
input window include a routine that allows on-screen comparison of outputs
resulting from two different sets of parameter values. To implement this
feature, simply toggle . Graphical output from the current parameter
values will be plotted in the normal colors, while output of the previous run
will be shadowed in black. A second press of toggles the graphical
comparison routine off.
Stability analyses are important in helping students to understand the
dynamics of ecological and evolutionary models, and we have provided a rich
set of stability tools in Populus. Many of the genetical simulations begin
from several different starting gene frequencies automatically. Optional
stability analyses are also available with the phase-plane graphs of
ecological dynamics. To implement this feature, press after a phase
diagram is complete; a cursor will pop up and can be moved to any point on
the screen. Pressing will then initiate a trajectory from that
point, and the dynamics can be run forward or backward. Alternatively,
multiple-starting-point stability analyses may be run by pressing after
. The program will then initiate trajectories from the perimeter or
from a gridded pattern of points (the number and position of starting points
is set from the options menu) to illustrate the stability of internal
equilibria.
In addition to these video utilities, Populus now includes several file-
handling routines. Output data from any of the simulations can be saved to
disk for analysis using a spreadsheet, statistical package, or graphics
editor. To do so, call up the options menu after running your simulation,
and specify a path and name for your data file. Similarly, Populus can save
to disk and reload sets of model parameter values; you might use this feature
to save a series of parameter files that show particularly instructive
examples, and ask your students to load, run, and analyze them as part of a
lab exercise.
Finding Documentation
This file contains only a brief introduction to Populus. Full
descriptions of each feature and a listing of the keystrokes necessary to
call and implement it are built into the program, and are accessible from the
Main Help Menu. To see this menu, press (or if you are already in one
of the model parameter or output screens, press twice, to get past the
context-sensitive help). We recommend a tour of the Main Help Menu as the
quickest way to familiarize yourself with the capabilities of Populus.
Sponsorship and Distribution
This software has been underwritten by IBM, the University of
Minnesota, and the Undergraduate Course and Curriculum Development Program of
the National Science Foundation (USE-9150887, USE-919155967); it is not a
commercial venture and we encourage you to distribute it without charge to any
colleague or student who will put it to good use. If you find a bug please
tell us. We will fix it and provide you with an update.
Don Alstad
Department of Ecology, Evolution & Behavior
University of Minnesota
318 Church St. SE Minneapolis, MN, USA 55455-0302
612-625-0488
