What it does
Sudden Speciation demonstrates two phenomena:
(1) the sensitive ecological balance between an animal population and its food source;
(2) the ability for a new species to appear suddenly as a result of a gradual accumulation of advantageous recessive genes.
When there is plenty of food, animals feast and reproduce. When their population passes a critical size, the food supply is consumed too rapidly and some animals starve. When the population declines enough, the food source can recover.
Meanwhile, random mutations that produce a recessive gene may occur. Only individuals with two copies of the recessive gene will express it. If such individuals mate exclusively with each other, they will constitute a new species. But it can take many generations before the recessive gene is sufficiently widespread that more than an occasional newborn carries two copies. Until then, the gene is expressed in so few individuals that the new species goes undetected in the fossil record.
When the proportion of the recessive gene in the population reaches a certain level, enough individuals with two copies appear that they can begin to procreate. If the mutation conveys a survival advantage, the new phenotype can become established and can outnumber the original variety. In the fossil record, a new species will seem to have arisen suddenly. But in the gene pool, it actually developed gradually.
According to a February 21, 1999
article in the Online Post-Gazette, Jeffrey Schwartz of the University of Pittsburgh has proposed that the type of gene most often responsible for sudden speciation is the "homeobox", a gene which controls the development of the embryo in all animal species. Chung-I Wu of the University of Chicago doubts the homeobox is the responsible gene, but agrees that a fast-mutating gene of some kind could produce this effect.
This simulation demonstrates that a new phenotype can appear suddenly when a gene rapidly mutates and its recessive version conveys a survival advantage. It does not serve as evidence for or against Schwartz's theory that the homeobox is the gene doing the mutating.
What to look for
The simulation starts with eight creatures, four male (blue) and four female (pink), all with two copies of the dominant gene for one-eyedness. As the simulation plays out, animals feed, reproduce, and die.
The scorecard displays:
- the current time;
- the number of animals with two dominant one-eyed genes (AA), the number with one dominant gene and one recessive two-eyed gene (Aa), and the number with two recessive genes (aa);
- the number of animals with one eye and the number with two eyes;
- the number of seeds in the field;
- a log showing the population of one-eyed and two-eyed phenotypes after predetermined periods of time.
A heterozygous animal (a one-eyed animal with one unexpressed gene for two-eyedness) displays a red star next to its face.
Each animal displays its remaining energy in a green box beside its feet. Eating a seed adds 1 to an animal's energy stores. Walking consumes 2 units of energy. Mating consumes 4. When an animal has fewer than 2 units of energy in reserve, it is underfed and dies. When it reaches 130 units, it is overfed and dies.
When a female is two squares to the left or right of a male that has the same number of eyes, then if a space is in between and both have sufficient energy (65 units or more), she lays an egg in the space. Both parents protect the egg until it is hatched. The offspring inherits one gene for eye count from each parent.
Pick a seed patch and watch it for a while. Pick an animal and watch it walk, eat, mate, and die. Watch eggs hatch. Observe inheritance and mutation.
To run the simulation at high speed, click the fast forward button (the double green triangle). To run it at slow speed again, click the play button (the single green triangle).
Watch the population tallies of animals and of seeds as they rise and fall. After a few thousand clock ticks, write down the results, reset the simulation, and run it again.
About how many animals and how many seeds can this ecosystem support?
On average, one in four newborns suffers a mutation to one of its two eye-count genes, changing it from the dominant type to the recessive type or vice versa. Eye-count genes also spread through mating.
One-eyed animals wander at random in any of four directions looking for food. Two-eyed animals look around for the patch with the most seeds before they walk. This advantage over one-eyed characters allows them to become established as a successful phenotype that could potentially evolve into a distinct species.
On the scorecard, watch the number of animals with one recessive gene rise even while the population of two-eyed animals remains zero or close to it. After the number of 2-Eyed animals exceeds the number of 1-Eyed animals, stop the simulation and make a graph of the data in the log. Refresh the page and run the simulation again. Does the new species usually appear gradually or suddenly?
Created by Larry Tesler, Stagecast Software, Inc. Art by Marcia Schwertman.
Copyright 1999-2000 Stagecast Software, Inc. All rights reserved. Based on Feed 'n Breed, by Larry Tesler, copyright 1996 Apple Computer, Inc. Thanks to Mark Sheppard, Allen Cypher, Jeff Tribble, and the whole Stagecast team.
Stagecast Creator and SimHost Server are trademarks
of Stagecast Software, Inc.
Java is a registered trademark of Sun Microsystems, Inc.