Natural Selection and Genetics Lab
Earth Science Extras
by Russ Colson
The Modern Synthesis
The Modern Synthesis of Evolution combines the concepts of Darwin 's natural selection with an understanding of genetics
In the Modern Synthesis of Evolution:
Adaptation is the reason or cause for change-so a species will become better suited to its environment
Natural Selection is the mechanism for change. Less well suited individuals are less likely to survive and thus move the average individual of a population toward a more suited form. Evolution, in this model, happens in the population, not in the individual. In Darwin 's model, an individual is not able to adapt to its environment (unlike in Lamarck's model). Individuals can of course adapt in some ways, but that adaptation is in learned traits and adjustment to physiology, not in innate biology, and that adaptation is not inheritable by its offspring.
Gene is the unit of evolution--that part of the biology which can be passed to offspring and can be operated on by natural selection. For evolution to occur, there must be variability in the gene pool of a population as well as the potential for changes in genetic material.
Evolution, in the modern synthesis, involves all of these, and is a slow and gradual process proceeding from random variations accompanied by adaptation to changing environments through natural selection. In the modern synthesis, evolution is not abrupt as in saltation or directional as in orthogenesis.
Natural Selection
To be scientifically valid, a theory must not only be based on a solid and extensive set of observations and experiments, but must make predictions that can be tested. One of the famous tests of natural selection came in the late 1800's with the adaptive change in coloration of peppered moths near Manchester England . During the industrial revolution, tree trunks, formerly a light gray-green color, became darkened by soot. Peppered moths, predominantly light colored with dark spots, became more visible against the dark tree trunks, and easier prey for predators. Over a period of 45 years, light colored peppered moths became less common, and darker peppered moths became more common.
Note that this observation is not proof that any particular species evolved from another (other observations address that question). Rather, this observation is proof that natural selection operates in nature in much the way Darwin predicted, thus providing a valid test of his theory of natural selection.
As of this writing, a Natural Selection game, based on the peppered moth observations, can be played at the following address, if you would like to try it out for fun.
http://peppermoths.weebly.com/
A Simulation Experiment with Data Analysis
The following activity is a simulation of Natural Selection. A simulation is not the same as an experiment in that a simulation does not provide observational evidence for natural selection, but rather provides a framework for understanding it. I adapted this activty based on a classroom lesson posted on https://www.biologycorner.com/
Although this is a simulation, and not an experiment, we are going to take the opportunity to compile, analyze, and interpret data as we would do for an experiment. Analyzing and interpreting data is one of the most important practices of science. As you watch the videos below, be sure to record the data, as you will need to have it available to think about in answering the ensuing questions. You might want to have pencil and paper handy and pause the video as needed.
Natural Selection Simulation Video-1
Looking for Patterns
The practice of science might be briefly summarized as the process of looking for patterns in the natural world and then trying to understand what those patterns mean or what causes them. Take a few minutes to examine your data and see what patterns you can spot in them. Of course, with such a small amount of data, only repeated once, we don't know which patterns might be statistically significant. Later on in this lesson, we are going to repeat our simulation to get a better idea of which patterns are 'real,''but for now just look for patterns that seem significant.
When you've done the work of trying to find patterns, test yourself against the pattern questions below.
Before trying to interpret these patterns, we are going to watch two more simulations, each a repeat of the first simulation, but with some variable changed. Again, keep track of data, and try to identify the new variables and interpret what effect those variables have on the simulation. Again, as you watch the video, be sure to record the data, as you will need to have it available to think about in answering the ensuing questions. You might want to have pencil and paper handy and pause the video as needed.
Natural Selection Simulation Video-2
After watching these two videos and jotting down the results, consider the numbers, again looking for patterns, including patterns that either match or don't match your previous observations and any new patterns that you see.
When you have considered the data, check your pattern recognitions against the following question.
Interpretting Patterns
After finding patterns, the next step in science is to interpret those patterns. Let's do some interpretation of the patterns we see.
Graphing data often helps us to both recognize and interpret patterns in data. Consider the graph fo the simulation data below and figure out what it tells you. The "Y" axis lots percentage red of prey, meaning the percent of the total prey captured that was red.
Once you've tried to interpret the graph, test yourself against the question below.
When you've thought through the graph and what it tells you, test yourself against the question below.
Further Considerations
Although we didn't really track changes in the predator in the way that we tracked changes in the prey population with predation cycle, several "predator" variables might be selected for over time (that is, we might expect changes in traits of the predator with time just like we see changes in prey traits with time). For example, in this simulation, if we had a large number of predators, and less successful predators 'die', then we might expect selection for speed, size of sticky tape, stickiness of sticky tape, how long sticky tape retains its stickiness, length of pencil, hand-eye coordination, ability to see low contrast objects against a similar background, willingness or ability to take up more space around the game board (the bully gene), willingness to dump other's squares on the floor before they can collect them (getting to the altruism-in-evolution question, which is a very interesting puzzle in its own right!), etc. One trait of the predator listed above might be predicted (by Darwin 's theory) to become more significant with time, at least under the dim light conditions. Which trait is this? Think of an explanation.
After thinking about this question a bit, test your ideas agains the multiple choice question below.
Mendel Genetics
Mendel studied 7 traits of pea plants which he observed to occur in either one form or another (not in intermediate forms). These traits were flower color (purple or white), flower position (axil or terminal), stem length (long or short), seed shape (round or wrinkled), seed color (yellow or green), pod shape (inflated or constricted), and pod color (yellow or green). For the exercises below, we consider only pod color.
Prior to his experiments, Mendel developed varieties of the peas that reliably produced either one or the other of these characteristics in all offspring (e.g. always produced green peas in all descendants when crossed with another 'all-green' variety).
Thinking about Evidence
Consider the following simulated results for several generations of pea plants (labeled as g0 for generaton zero, g1 for generation one, and so on). The x means that he crossed the indicated types of peas. Remember, for generation zero, both the green and yellow peas were ones that Mendel had developed so that all offspring were always either green or yellow when crossed with another green or yellow pea.
g0 = green pea producing x yellow pea producing
g1= 16 yellow producing, 0 green producing
g2 = yellow x yellow cross from g1: 96 yellow producing, 32 green producing, with all pairs producing the same proportional result
g3: yellow x yellow cross from g2: 5/6th are yellow, 1/6th are green
green x green cross from g2: all are green
From results like these simulated results, Mendel drew the following three conclusions. For each conclusion, explain the experimental evidence on which that conclusion is based. Remember, "theory" is not evidence, so don't invoke anything in your argument that is not present in Mendel's experimental data.
Mendel Conlclusion 1: inheritance of each trait is determined by "factors" that are passed on to descendents unchanged.
When you have thought through the evidence, test yourself against the multiple choice question below.
Mendel Conclusion 2: A trait may not show up in an individual but can still be passed on to the next generation.
When you have thought through the evidence, test yourself with the multiple choice questions below.
Mendel Conclusion 3: An individual inherits one "factor" from each parent for each trait.
When you have thought through the evidence, test yourself with the multiple choice questions below.
Today, we associate Mendel's "factors" with genes or alleles. Phenotype refers to how the trait is manifest in a particular individual. Genotype refers to the actual genetic makeup of an individual. Thus, an individual's phenotype does not directly reflect its entire genetic makeup. That is, the phenotype is not an average of the genotypes, but rather one or the other of the genes will be dominant and determine the phenotype. The other gene is recessive.
What is the evidence in Mendel's data to support the idea that one gene is recessive and the other dominant (rather than the genes combining)?
Your argument should include the observation that g1 contains no green peas. It should include the observation that one-fourth of the descendents of g1 are green, and that any time a green is crossed with a green,you always get green--no yellow.
The idea that traits "blend" to make a different intermediate trait was the predominant view in the 1800's. Mendel's work disproved this idea, at least for specific traits in peas. Mendel's work also argued against the idea that traits are modified by life experiences (like Lamarcks idea of inheritance of acquired characteristics or like Darwin 's incorrect view of pangenesis--pangenesis was one of Darwin's idea of inheritance of acquired traits that has been graciously forgotten with time).
Thnking about Theory
Phenotype refers to the traits as manifest in an organism's appearance. Genotype refers to the genetic material contained in the organism. Consider that the initial parent pea plants had the genetic alleles (genotype) YY (always produces yellow seed offspring) and GG (always produces green seed offspring). and that the offspring always inherit one allele from each parent,as Mendel concluded. Write the genotype (i.e. GG, YG, YY) for each of the offspring in the following table which gives proportions of offspring with given phenotype. To solve this thought puzzle, you need to consider all the possible combinations of offspring genotypes for any given pair of parents.
Notice that for 2 parents, each of which has 2 alleles for a trait, there will always be 4 possible outcomes, each with 25% chance of occuring: The 1rst parent's 1rst allele with 2nd parent's 1rst allele, the 1rst parent's first allele wth the 2nd parent's second allele, the 1rst parent's 2nd allele with the 2nd parent's 1rst allele, and the 1rst parent's 2nd allele with the 2nd parent's 2nd allele. For the exercises below, we will always list a dominant allele first, so that options are YY, YG, and GG (that is, GY is reported as YG).
last updated 4/8//2020. Text and pictures are the property of Russ Colson, Museum display is a photo taken at the Smithsonian Museum of Natural History, Washington DC, 2010
