EvoBeaker Snails

Assignment 17 Darwinian snails: What makes populations evolve?

Goals:

1. To learn the assumptions and requirements behind natural selection.
2. To understand how natural selection leads to evolution.
3. To further practice your skills in experimental design and data interpretation.

Objectives: Students will be able to

test and interpret the effects of potential agents of natural selection, including predation, genetic variation, heritability, and differential survial and reproduction,
explain how and why populations, rather than individuals, evolve,
design and conduct experiments to test hypotheses about natural selection, and
evaluate real data about snail populations in terms of requirements for natural selection.

In Assignment 6, you discussed the history of major ideas that led to our understanding of how organisms change over time. In your discussion you probably began to talk about the topics of mutation, natural selection, and genetic drift. If you discussed these topics, a possible summary of your discussion may read as follows:

Mutation produces random (chance) variation in the genetic makeup of a species or a population--that is, individuals of the same species living in the same area. Natural selection sorts out these random changes according to their value in enhancing the individual's reproduction and survival. Such selection ensures that variations that make a species better adapted to its environment will pass on to future generations. At the same time, natural selection eliminates variations that make a species less able to survive and reproduce. (Coyne, 1995)

In addition to mutation, natural selection, and genetic drift, you probably also have read about other related topics including speciation, gradualism, and punctuated equilibrium. As a review, speciation is a phenomena that "occurs when gene flow has effectively ceased between populations where it previously existed and is brought about by isolation mechanisms." (Hale and Margham, 1991, p. 498) An isolation mechanism is any feature or event that separates different populations of animals so they can no longer interbreed. What do you think some examples of isolation mechanisms could be?

Over the next two weeks, you will study how a species can change over time by using computer simulations. These simulations involve many of the concepts and terms that you already worked with in Assignment 6 and read about in the introduction to this assignment. You will design and conduct several experiments that mimic natural selection. While the activity will demonstrate the process of changing organisms over time in a few minutes, it accurately models the actual process of evolution that occurs over decades to millions of years.

It may be difficult, initially, to understand how organisms could possibly change their form and function over time. Nevertheless, we know that they do change. If your grandfather happens to be a farmer, ask him if the herbicides he used years ago remained effective. The answer will probably be 'no'. It was not the herbicide that changed, however. It was the genetic make-up of the plant population that changed. Some plants that he sprayed were resistant to the herbicide due to their unique genetic make-up (just as your genotype is unique to you). Over the years, plants that were resistant became more common in the population while the non-resistant plants became fewer and fewer. Other more natural selection pressures act continuously on all organisms, leading to the survival of those who have adaptive genotypes and the loss of individuals with less adaptive genotypes.

To review the basic mechanisms of natural selection, click on the icon below and view the short video about the evolution of hummingbirds. Select Video 4 once the web page opens.

How does evolution really work?

Have you ever wondered whether all individuals of a species are equally vulnerable to predators in the wild? Do predators feed equally on juveniles and adults of a species, or on individuals of different colors? You will examine questions like these while conducting a simulation on the impacts of predators on snails with different shell thicknesses. The simulation is based on Dr. Robin Seeley's research on the the effects of green crab predation on evolution of periwinkle populations.

Before conducting any of the SimBio labs, be sure to read the instructions carefully, keeping careful records of all observations that you make and data that you collect. You will need to use your time efficiently in order to complete each lab exercise. Remember, the objective of this activity is to help you understand how selection acts to cause change in allele frequencies in populations. Make sure that you are able to demonstrate your understanding of natural selection by explaining why your hypotheses and prediction(s) were supported or rejected. Make sure that you check your Schedule of Assignments so that you are aware of any optional or required assignments for each SimBio exercise.