Biology Date 2-22-13______
LAB - NATURAL SELECTION
This game was invented by G. Ledyard Stebbins, a pioneer in the evolution of plants. The purpose of the game is to illustrate the basic principles and some of the general effects of evolution by natural selection.
Natural selection acts at the level of individuals. It is the individual organism that lives or dies, reproduces or fails to reproduce because of its characteristics. When more individuals with a particular trait survive then the overall population will change over time — it will be made up of more and more individuals with that successful characteristic. This change over time in the population is evolution.
For example, let’s imagine that it is a dry year and food is scarce. There is a flock of birds. The birds in the flock that have the larger, sturdy beaks are the only ones that can eat what are usually hard-to-crack seeds. So those large-beak birds get more food than the smaller beak birds and they therefore survive more. Since they survive more they also get to reproduce more
— lay more eggs and have more babies. Now more of the birds in the next generation will inherit the large beak from their parents. So the next generation flock will be made up of more large-beak birds. If this drought stayed for many years then over time this bird species may end up being made of mostly large-beak birds and very few small beak birds. We would therefore say that this flock of birds had evolved over time.
Evolution by natural selection, as first proposed by Charles Darwin, includes four conditions:
1. Variation: Variation means that there are differences between the individuals in a population. In this lab, variation is simulated by different colored paper dots. For the purposes of this lab, these dots are assumed to be different colored butterflies of the same species — a species that has a range of colors in one population living in an area together.
2. Inheritance: The variations that exist within the population must be inheritable from parents to offspring. The characteristics can be passed on in genes. Darwin clearly recognized that this was the case, although he did not know about genes or DNA. In this lab, inheritance is "true breeding" — that is, offspring inherit the exact color of their parents, for instance red butterflies only reproduce red butterflies.
3. Overproduction: As a result of reading a famous essay of his time — Essay on the Principle of Population by Malthus — Darwin realized that in natural populations more offspring are born than can possibly live to reproduce. In this simulation, overpopulation is modeled by having only part of each generation's offspring survive to be able to reproduce. The rest of the individuals are eaten by a predator.
4. Differential Survival and Reproduction: Given the three conditions described above, certain individuals will survive and reproduce more often than others, and these individuals and their offspring (the ones with the successful traits) will therefore become more common over time. This, in a nutshell, is evolution by natural selection.
In natural environments, one of the most noticeable forms of natural selection is predation. Predators eat other organisms, while prey are eaten by them. In our natural selection “game” (actually a simulation), we will study a closely related phenomenon — the evolution of protective coloration. Many animals, especially insects, are very well camouflaged against
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being seen or found by their predators, especially birds. In some cases, the insects mimic some part of their habitat, such as a leaf. The question under investigation in this game is, how do mimicry and protective coloration evolve?
DIRECTIONS: HOW TO PLAY THE GAME