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INTRODUCTION

In 1831 Charles Darwin began a five-year journey as ship's naturalist on the H.M.S. The Beagle (Fig 1). During this time he visited South America, Australia, South Africa, and islands of the Pacific and South Atlantic. He later published his travels in The Voyage of the Beagle where he introduced many themes that later became crucial to the arguments presented the more-familiar The Origin of Species by Means of Natural Selection or the Preservation of Favoured Races in the Struggle for Life (published in 1859 and more commonly known as "The Origin of Species"). Majors in the sciences and those interested in philosophy should read this monograph. You won't find it easy reading because the language is often archaic and the arguments are sometimes difficult to follow, but it represents one of the most important contributions made to Western culture.

Although Darwin is often referred to as "the father of evolution", he was not the first to introduce the idea of changing species. Maupertuis and Diderot in the mid 18th century, for example, wrote of evolution and the ideas of changing life are part of many religions. Darwin's contribution was to provide a mechanism through which evolution could function. Briefly, the Darwinian argument is as follows:

Variation exists within a species. Although we may consider all houseflies as being more-or-less alike, on closer examination you find that they are nearly as recognizable as one person is from another.

Some of this variation has a genetic basis. Evolution can act only on traits that are passed genetically from one generation to the next. Just as an animal or plant breeder has no interest in non-genetic traits, evolution can not work on differences caused by trauma, parasitism, and other environmental variation.

The reproductive potential of organisms is vast. Darwin calculated that a single pair of elephants could have 19 million descendants within 750 years if each animal lived to be 100 and each pair had six calves. Calculations for other organisms produce similar increases in population size. Elephants are not the most common beasts, the oceans are not overflowing with fish and we aren't nose-deep in ragweed (although it sometimes seems that way). Therefore something must happen to all these extra offspring and, unless species other than man practice birth control, most of the young must die before they reproduce.

Because individuals differ from one another, some should be more capable than others in eluding predators, coping with environmental extremes, or in competing with members of their own or other species. Those that are more capable should leave more offspring to the succeeding generation. Since some aspects of coping must be tied to genetic attributes, the favorable genes are passed on to the next generation. The genetic makeup of the population changes and evolution is said to occur. This varying reproductive success of individuals based on their different genetic constitutions is natural selection.

Often the concept of natural selection is simplified to "survival of the fittest". Fitness in evolutionary terms has an exact meaning related to the number of surviving offspring produced by an individual in comparison to less well-endowed individuals. Evolutionary fitness is therefore more than just the ability to run quickly or fight off competitors.

Evolution is not a historical process; it is occurring at this moment. Populations constantly adapt in response to changes in their environment and thereby accumulate changes in the genes that are available to the species through its gene pool. In today's lab you will explore some of the evidence for evolution and will examine a few of the mechanisms through which evolution acts.

The response of the peppered moth (Biston betularia) to industrial pollution in England is a well-known example of selection in natural populations. In brief, the peppered moth is found in two forms (or "morphs"): a mottled form and a dark-colored melanic morph. A single gene controls the expression of this trait and that the melanic gene is dominant over the light gene. During the mid 1800s the mottled form predominated the English countryside. By 1898, however, the situation was reversed and the melanic (dark) form comprised the greater percentage of the population.

Researchers noted that the spread of the melanic form paralleled an increase in industrial pollution. They hypothesized that the melanic form was better camouflaged than lighter morphs when they settled on soot-darkened tree trunks (Fig 4b). These observations suggested that the light forms were removed from the population by birds because they were so conspicuous. Additional support for this hypothesis came from non-industrial regions (and areas upwind from polluters) where the mottled form greatly outnumbered the melanic moths. Here the selection was reversed and the mottled moths had the advantage in hiding from birds on the lichen-covered trunks (Fig 4a).

The hypothesis was tested by releasing an equal number of melanic and mottled forms in an unpolluted area and then observing birds feeding from a blind. Birds had the same difficulty as the researchers in recognizing the mottled moths against the lichen background and ate only 26 of the light forms while 164 of the poorly-camouflaged melanic moths were captured. In another series of experiments it was found that the melanic form had the advantage in polluted areas. Recent advances in controlling pollution have returned many areas of Great Briton to their previous state. With this, the peppered moth population is shifting again toward the mottled form.

In reality, the Peppered Moth story is not as cut-and-dry as it may seem. Recent research has shown that the moths don't normally spend much time sitting on trees in the wild, suggesting that selection by birds based on camouflage is more complicated than we originally anticipaed. Nonetheless, something is happening that is related to selection. As you run this model keep in mind that the whole story is not yet know.

In this portion of the laboratory you will simulate changes in a population of peppered moths due to selection in a polluted environment. If necessary, review Mendelian and population genetics (Box 1). An outline of the general procedure follows:

PROCEDURE FOR PEPPERED MOTH SIMULATION.

Go here to see a slide show demonstrating how to use the model.

Adjust the upper window on this page so that you can see the entire simulation Then choose a low-pollution environment by pressing the "Low" button. Note that the tree bark background changes to an unpolluted state.

When you're ready, press the "Do It!" button. Dark (melanic) and light (peppered) moths are randomly scattered over the tree bark background. Melanic forms are represented by the dark blue spots, the light moths by the gray dots. Feed on the moths as quickly as possible (you only have 10 seconds) by moving the cross-hair cursor over your prey and left-mouse clicking on it. If you have successfully fed, the prey will turn red and the After selection counter will show that you've removed either a light or dark prey. The relative fitness panel will also show the change in fitness. For a discussion of survival rates and relative fitness, see Box 2.

After your 10 seconds has expired, the mouse reverts to the normal pointer and you can no longer feed. The yellow data sheet will show the new frequency of the p (dark) and q (light) genes in your population (note that they did NOT start at .5 each as a default. Why?).

Erase the "Finished" message box by clicking on it and generate your next generation based on the current gene frequencies. The program figures out the proportions for the next generation by using the Hardy-Weinburg equilibrium formula (p² + 2pq + q² = 1.0; Box 1) It then uses these frequencies to figure out how many light-colored (mm) and dark-colored (MM or Mm) moths will make up the new population.

Press the "Do-it!" button when you're ready to start feeding. Feed as above.

Continue until one or the other genes have been eliminated from the population. Transfer your data for the Low pollution environment to the clipboard and paste your data into Excel (or the spreadsheet of your choice) for analysis.

Re-run the low pollution situation a total of 5 times. Then do the same for the medium and high-pollution environments.

For an explanation on using Excel for this application, watch this: Listen to me!

Analysis of Results and Questions.

For each of the pollution levels, graph the average frequencies of the p and q alleles vs. generations. Do the same for relative fitness. Attach these results to your paper. Include both the data sheet and graphs.
Is this model realistic? Did things work out as you expected? Discuss your results from question 1.
If a population started with a frequency of .8 melanic and .2 of the light genes, what would be the expected frequencies of each of the moth types in the next generation (Do these calculations by hand and show your work)?
If the next generation's population size was 500 individuals, how many would be expected to be MM, Mm, and mm? How many would be dark and how many would be light (more hand calculations; the calculations are shown here)?
If 10 of the dark moths in question 4 were eaten and 10 of the light moths were eaten, what would be the relative fitness of each morph (hand calc)?
What do you think would happen to the relative fitness values if the environment were changing over time?
Search the internet and find two more examples of natural selection.Include the address with your results. Briefly summarize each.