Biology Lessons Part 2: Population Biology

Lesson 2.8: How Do Populations Change Over Time? Knowledge Mapping Teachers' Guide

Grade Level	Prospective and Practicing K-8 Teachers; may be adapted for use in elementary classes.
Time	Approximately 1 hour.
Objectives	Once you have completed these exercises you should be able to:
	1.	Describe protective coloration.
	2.	Distinguish between microevolution and macroevolution.
	3.	Distinguish between sexual selection and survivability selection.
	4.	Construct nets from text.
	5.	Merge nets.
Exercise 1	Changes in Traits in a Relatively Few Generations: Natural Selection and MicroEvolution
Introduction	1.	Consider these four observations of guppies which have been made in the wild by John Endler at Princeton (1996), in various pools and streams in south America (Venezuela, Margarita Island, Trinidad, and Tobago). The subject is a fish called the guppy and the trait is its spotted coloration which is inherited.
	2.	In pools where there are no predators, male guppies are very brightly colored. They have black, blue, red, yellow, green and irridescent spots. Endler observed that bright colors attract females and increase the chances of mating and reproduction. The gaudier the male, the better his sex life.
	3.	In pools where predators abound, male guppies have smaller, paler spots and they blend into the background better. Pale males are caught by predators less frequently than bright-colored males.
	4.	Guppy predators range from the most dangerous, a cichlid fish which eats about three guppies per hour, to the least dangerous, a fish called Rivulus hartii, which eats about one guppy every five hours. The more aggressive the guppies' enemies, the smaller and fainter the guppies' spots.
	5.	In pools where the predators are prawns, male guppies have bright red spots. Prawns are generally attracted by bright colors but are red-color-blind.
Questions	6.	Which of these observations provide evidence of protective coloration? Protective coloration is the coloration of some animals and plants so that they blend in with their environment. It is especially prevalent in fish. Items 2, 3, and 4 describe aspects of protective coloration arising from intense selection.
	7.	In the guppies, are the selective effects of sexual selection and survivability reinforcing one another or counter-acting one another? Sexual selection is the attractiveness of an individual to mates and its ability to reproduce. Survivability is the ability, in this case, to avoid predators and thus survive.   Sexual selection is selecting for bright colors, and large spots whereas survivability is selecting for pale colors and small spots. The selective pressures are working in opposite directions; the most powerful force in any given environment seems to prevail.
	8.	Supposing you were to take about 200 pale-colored male and female guppies from a stream containing guppy predators and place them in a safe stream where there are no guppy predators. You come back and look at the population one year later (about 15 generations later). What would you expect to see?   The transported males would have larger spots and they would be more brightly colored. In fact, John Endler performed this experiment and this is what he observed.
	9.	One way to think of natural selection is as follows.
	10.	Construct a small semantic network to represent the information you learned in this knowledge mapping lesson. Our semantic network summarizing these ideas, "2.8d Guppy Net"can be downloaded here. Hopefully, the students' nets will be quite similar.
	11.	Now construct a net to represent the simulation you performed with beak size and shape. Include both the specific beak sizes, shapes and outcomes from the experiment, and a conceptual overview of what happened. Our semantic network summarizing these ideas, "2.8d Nat Sel Simulation"can be downloaded here.
	12.	Now merge your two nets together and review and revise as necessary. Your goal is to build a coherent conceptual picture of evolution illustrated by data from Endler's experiments and from your simulation. Our merged and polished net, "2.8d Natural Selection.mrg", can be downloaded here.
	13.	Distinguish between sexual selection and survivability selection. Sexual selection involves an individual's ability to attract mates and reproduce successfully with them; in each species, certain characteristics in the opposite sex are highly attractive and others are less attractive. Sexual selection favors those that are more attractive. Survivability selection refers to an individual's ability to evade dangers and stay alive in order to reproduce. The dangers may be such things as pesticides, antibiotics, disease, or in the case of guppies, predators.
	14.	Macroevolution refers to the large scale changes that have occurred in life forms on earth and that are studied through the fossil record. Distinguish between macroevolution and micro-evolution. Microevolution refers to the evolutionary changes which can be observed and studied within a scientists lifetime. As illustrated by guppie colors and our beak simulation, some heritable characteristics can change quite dramatically within just a few generations.
	15.	How do the short-term changes in populations occur? The populations change primarily through high death rates for individuals having features that are less desirable in a given environment, and high reproductive rates for individuals that are better suited to the environment. This results in significant changes in the gene pool in the next generation.