Knee-Deep in Green Peppers

• green peppers (1 per group of 3 or 4 students; do NOT use hot peppers)
• gloves (for protecting sensitive skin)
• scissors or knife to cut open the peppers
• plates or napkins
• worksheet (1 per group)

1. Purchase sweet green peppers, one for every 3 or 4 students. Green peppers have lots of seeds, so counting will be easier in groups. DO NOT USE HOT PEPPERS!
    
2. Make copies of the “Knee-Deep in Green Peppers” worksheet for each group.
    
3. Have scissors or knives available to cut open the pepper and plates or napkins to count the seeds on.
    
4. Some teachers have reported skin irritation from handling peppers, so rubber or latex gloves are advised.

1. Divide students into groups of 3 or 4.
    
2. Give each group a worksheet, a green pepper, scissors or a knife, and a plate or napkin.
    
3. Tell the students to cut the pepper open carefully and count all the seeds. When they finish counting, have them answer all the questions on the worksheet.

Charles Darwin and Alfred Russel Wallace independently developed theories of natural selection in the 1800s. They were each influenced in their thinking by a variety of different experiences, readings, and observations. On one point, however, they both drew inspiration from the same source: the 1798 writings of political economist Thomas Malthus.

In An Essay on the Principle of Population, Malthus observed that the human population was capable of growing faster than our food supply could be increased. He pointed out that if human population growth were uncontrolled, our population would inevitably outgrow our food supply, leading to famine and general misery. The reason this had not already occurred was that population growth was slowed by forces such as disease and war.

While Malthus was concerned with the social impacts of this situation, Darwin and Wallace each realized that the simple fact that every population, human or otherwise, has the capacity to outgrow its resources if left unchecked has important implications for evolutionary theory.

In On the Origin of Species, Darwin makes the following calculation: “There is no exception to the rule that every organic being naturally increase at so high a rate that, if not destroyed, the earth would soon be covered by the progeny of a single pair .... The elephant is reckoned the slowest breeder of all known animals, and I have taken some pains to estimate its probable minimum rate of natural increase; it will be safest to assume that it begins breeding when thirty years old, and goes on breeding till ninety years old, bringing forth six young in the interval, and surviving till one hundred years old; if this be so, after a period of from 740 to 750 years there would be nearly nineteen million elephants alive, descended from the first pair.”

This brings up an obvious question: why is the Earth not covered in millions and millions of elephants? The answer may also seem obvious: because not every elephant can survive. There are not enough resources to support such a large population—not enough food, not enough fresh water, and not enough space. Competition for those resources as well as predation, disease, and a variety of other factors act as checks on population growth.

As Darwin and Wallace realized, the question of why the Earth is not covered in elephants is not the important point. The less obvious—and far more interesting—question is what determines which elephants will survive?

In his autobiography, Charles Darwin wrote: "In October 1838, that is, fifteen months after I had begun my systematic inquiry, I happened to read for amusement Malthus on Population, and being well prepared to appreciate the struggle for existence which everywhere goes on from long- continued observation of the habits of animals and plants, it at once struck me that under these circumstances favourable variations would tend to be preserved, and unfavourable ones to be destroyed. The results of this would be the formation of a new species. Here, then I had at last got a theory by which to work".

As other activities in this curriculum highlight, a remarkable degree of variation exists among individuals of a single species. While many differences have little or no effect on an organism’s success, differences in some traits can greatly affect an individual’s fitness. Fitness is the reproductive success of an organism. An individual with higher fitness produces more viable offspring (therefore passing on a higher proportion of genes to the next generation) than an individual with lower fitness. The traits of individuals with higher fitness become more and more common as generations go by, while the traits of individuals with lower fitness may eventually be lost from the population. This is the process that Darwin called natural selection.

In order for natural selection to occur, there must be variation among individuals and a “struggle for existence” in which not all individuals can survive. Without variation, there would be no differences in fitness. Without a Malthusian “struggle for existence,” differences in fitness wouldn’t matter. All individuals would be able to survive and pass on their genes, and the population would look the more or less the same from one generation to the next. As Darwin realized, natural selection is an unavoidable consequence of the existence of variation and the fact that more offspring are produced than can possibly survive.

This activity uses green peppers to provide a concrete example of high reproductive potential. This sets the stage for understanding the “struggle for existence” concept that was proposed by Malthus and that influenced both Darwin and Wallace in developing their ideas about evolution.

Darwin, Charles. (1872). On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life (6th ed.) London: John Murray.