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Selection and Speciation 1
Selection and Speciation
How can changes in a population result in the formation of a new species?
Why?
Have you ever wondered how the great diversity of life on Earth has come about or how a single new spe- cies forms? Environmental pressures may cause populations to change over time or evolve. This is because an organism’s ability to live to adulthood in its current environment will determine its reproductive suc- cess and ability to pass on its genes. But changes within a population can occur without creating a new species. At what point do scientists start thinking of a new name for a species?
Model 1 – Three Types of Selection
0 25 50 75 100 125 150 0 25 37.5 50 62.5 75 87.5 100 112.5 125 137.5 150 Number of Individuals Body Size (g)
B Disruptive Selection
0 25 50 75 100 125 150 175 Number of Individuals Body Size (g)
A Directional Selection
Number of Individuals Body Size (g)
C Stabilizing Selection
Original population Population after selection
- What variables do the graphs in Model 1 compare?
- What are the three types of selection illustrated in the graphs in Model 1?
2 POGIL™^ Activities for AP* Biology
- According to the graphs in Model 1, there is variation in the body mass in the original popula- tion. Using your knowledge of genetics, describe how this variation is possible.
- Refer to graph A of Model 1. a. How is the population that has experienced selection different from the original population? b. Fitness is defined as the relative ability of an individual (or population) to survive, reproduce, and pass on genes. Which individuals in the original population appear to display better fitness? c. As a group, propose some characteristics of the environment that could lead to the population changes illustrated in graph A.
- Refer to graph B of Model 1. a. How is the population that has experienced selection different from the original population? b. Which individuals in the original population appear to display better fitness? c. As a group, propose some characteristics of the environment that could lead to the population changes illustrated in graph B.
4 POGIL™^ Activities for AP* Biology
- In each of the following examples, describe the likely outcome due to the environmental pressure and state the type of selection. Justify your choice. a. Finches with a small beak cannot crack open seeds. b. Human babies with very high or very low birth weights have a higher mortality rate. c. A population of seed-cracker finches feeds on seeds available in two sizes, small or large. d. Overfishing occurs in two rivers in British Columbia, Canada, where larger salmon are prefer- entially caught.
Selection and Speciation 5
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Natural selection , the improved fitness of certain individuals in the population that allows for survival and reproduction, is the primary mechanism by which populations change over time. Other mechanisms include the introduction of a mutation in the population and artificial selection (the effect of humans purposefully breeding animals to select for desirable traits). But at what point does a change in the popu- lation develop a new species? Data has been collected showing that the average height of an American adult is increasing, but that does not mean the American population has evolved into a new species.
Model 2 – Reproductive Isolation
Mating results in viable, fertile offspring. A B Organisms A & B are the same species. Mating results in viable, but infertile offspring. C D Organisms C & D are NOT the same species. Mating results in weak offspring that does not live to reproduce. E F Organisms E & F are NOT the same species. Organisms live in separate geological areas. Mating is not possible. G H Organisms do not have compatible reproductive organs. Mating is not possible. I J Organisms feed from different sources. Not likely to mate. K L Organisms G & H are NOT the same species. Organisms I & J are NOT the same species. Organisms K & L are NOT the same species.
Selection and Speciation 7
- Could disruptive selection lead to the creation of a new species? Justify your reasoning using what you’ve learned from Model 2.
- Could stabilizing selection lead to the creation of a new species? Justify your reasoning using what you’ve learned from Model 2.
8 POGIL™^ Activities for AP* Biology
Extension Questions
Model 3 – Genetic Drift
1st Generation 2nd Generation 3rd Generation 4th Generation
- Refer to Model 3. Fill in the table below with the number of alleles of each type in each generation. 1st Generation 2nd Generation 3rd Generation 4th Generation Light Medium Dark
- Compare the frequency of the three alleles in the 1st generation of the small population illus- trated in Model 3.
- In the scenario illustrated in Model 3 two of the light alleles were lost (through the death of the individual who had these two alleles) before reproduction occurred in the 1st generation. How did this affect the distribution of the three alleles in this small population?
- The phenomenon illustrated in Model 3 is called genetic drift. It mainly occurs in small, isolated populations. Propose an explanation for why the light allele disappears from the population by the fourth generation.
