Trade-offs are a pervasive feature of life; no organism can be superior in every aspect. A common trade-off is that between growth and reproduction: Changes that increase growth often result in declines in reproduction and vice versa. The following simulation exercise illustrates this growth/reproduction trade-off and its consequences.
Consider an organism that can allocate resources either to growth or to reproduction. At what point should it cease growth and start reproducing? That is the question that we will explore in this exercise, and one that many organisms face in nature. Bigger individuals usually can produce more offspring than smaller ones, so delaying reproduction could benefit an organism. On the other hand, organisms face risk of death due to predation and other causes; waiting too long to start reproducing may result in being eaten before one can reproduce. Thus, a balance exists between these two competing pressures, and at some point, reproductive output is maximized. Because the onset of reproduction is usually at least partially heritable, natural selection will shape populations to increase their reproductive output.
This simulation exercise revolves around a hypothetical fish, the Doriefish. Doriefish can either grow or they can reproduce; they cannot do both, and once they stop growing, they are unable to resume growing. If a Doriefish is growing, it will gain 5 grams, and up to a maximum of 50 grams. Fish that reach at least 10 grams are able to reproduce, but at the cost of no longer growing. The number of offspring that an individual fish can produce is its weight in grams divided by 5. Thus, a 40-gram fish that is reproductive will produce eight offspring per time period, but a 10-gram fish will only reproduce two offspring in the same time period (provided it is reproductive). Remember: Once a Doriefish is reproductive, it can no longer grow.
Predation from larger fish is the major source of mortality in Doriefish. The mortality rate varies, however, because some habitats have more predators than do others. As we will see below, changes in predation risk influence the relative benefits of reproducing at different times. In these exercises, you will explore this trade-off between growth and reproduction by making decisions about when to start reproduction. In each simulation exercise, you will be given ten Doriefish, each with an initial weight of 5 grams.
This simulation is open-ended. Feel free to explore the consequences of different set-points of size under different predation intensities.