Nat selection lab

8/10/2019 Nat selection lab

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NS: NATURAL SELECTIONIntroduction

In his book, On the Origin of Species, Charles Darwin proposed natural selectionas a mechanism for

evolutionary change. Darwin noted two major trends in most populations; one, more offspring are producedthan

survive to reproduce, and two, that members of a population differ by small variations in form and behavior

(phenotypic variation). He postulated that the individuals compete with one another for their biological needs and

that some of the variants would be more successful in filling these needs than others. The less successful forms

would most likely die, thus lessening the chances of passing on to their offspring those characteristics that made

them less successful. Darwin hypothesized that such differential survivalis the driving force behind natural

selection; that is, there would be a gradual decrease in unsuccessful traits (ones that are selected against) because

their bearers would not contribute as many offspring to future generations. Generation after generation of natural

selection on variable and heritable phenotypes would thus result in evolutionary change.

Survival is only one (very important) component of Darwinian fitness. An organism can be great at surviving but if

it is sterile or has no success in reproduction, its representation in the next generation is still zero. The concept of

differential reproductiontakes into account the other major component of fitness, which is reproductive success.

An organisms fitness is (for our purposes) the number of offspring that it leaves in the next generation. In order to

have high fitness it must both survive and be successful in reproduction. If an individual has characteristics that

help it to achieve high fitness, those characteristics are likely going to have a greater abundance in the next

generation.

This laboratory exercise is designed to acquaint you with the modern view of natural selection. Please remember

that this lab was designed as a simulation; natural systems are much more complex and difficult to investigate.

Objectives

Upon the completion of this exercise, you should be able to:

1) define natural selection and describe the factors that result in its occurrence

2) distinguish between differential survival and differential reproduction as two potentially independent

components of natural selection.

3) calculate, using data provided, evolutionary change across a single generation

4) speculate the effects of long-term selection favoring an optimal phenotypic value (stabilizing selection)

5) speculate the effects of environmental change on the way natural selection shapes the phenotypes of populations.

Simulation of Foraging Success as Affected by Beak Length

Procedure

In todays lab, we will be simulating foraging of birds with variant beak morphology i.e.,referring to the physical

shape of the beakbill length, width, depth, pointiness are all examples of dimensions of morphological variation

for a birds beak. Each student will play the role of a bird through three replicates in generating a data set that we

will use to demonstrate two different versions of natural selection by differential survival, and then one version of

natural selection by differential reproduction.


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1) In our model, the foragers possess a basic apparatus (beak) for acquiring food items, but of varying length.

The beak will consist of a length of tongue depressor and a paper clip that is slightly bent and mounted on one end

of the tongue depressor.

2) The foragers will use their beaks to extract food items (dried beans) from their habitat (inverted lidded cups with

holes in their bottoms). The number of beans extracted in one minute will represent the energy available for the

foragers continued survival and/or reproduction.

3)Rules for foraging:Foragers maneuver their beaks by holding onto the wooden end of the apparatus opposite of

the paper clip with one hand. The other hand may be used to rotate the cup or to hold it tight to the desktop, but

tilting or shaking the cups is not allowed. The bottom of the cup must remain flush with the desk the entire time.

4) We will replicate this exercise a couple more times (three runs), but with a random exchange of beaks in

between. This will make our results more repeatable and reliable by reducing the effect of anomalous variation in a

single trial and also by making it less likely for particularly good and bad foragers to skew the data. For example, if

one forager is just really good at extracting beans while another is terrible, it would bias the results if the good

forager just happened to have a beak of length X while the lousy forager had a beak of length Yin the end both

foragers will have contributed to the data set with three different beaks.

5) After the three replicate runs are complete, the instructor will assist in the compilation of the data into an Excelfile. We will make three graphs from this file. One will be a histogram showing the distribution of beak sizes in our

population of birds. The second will be a histogram showing the distribution of success rates in foraging. The third

graph will be a scatterplot showing the relationship between beak size and foraging success.

A) Based on your observation of the beak lengths that you saw around the class, what do you predict regarding

the frequency distribution of beaks of different lengths? How would this look in a histogram?

B) Based on your personal success rates with beaks of different lengths and what you observed from others

around you, what do you predict regarding the frequency distribution of success rates? In other words did everyone

have about the same rate of success, or were there some birds that had great success while others had poor

success? How would this look in a histogram?


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C) Based on your experience with this experimental system simulating bird foraging, what do you predict

regarding the relationship between beak length and success rates? In the third graph, sketch an XY scatter plot

showing your prediction.

Answer all three of the questions above before the instructor makes the three graphs available. Staple printouts of

the three graphs to this sheet.

D) Did your predictions match the actual outcomes? If not, how were the actual outcomes different from what

you predicted?

E) What is the name given to the bell-shaped frequency distribution seen for beak lengths? How can the

average beak length be estimated by looking at the graph? How can the average beak length be calculated precisely

from the data? What was the average beak length?

F) Based on an examination of Graph 3, is there a particular beak length that seemed to have the highest

success rate? Was this optimalbeak length the longest beak, the shortest beak, or a beak that was somewhere in

between the longest and the shortest?

G) Was the optimal beak length the same as the average beak length in the population?

Natural Selection via Differential Survival

Provided there is: A) any kind of nonrandom relationship between success rate in foraging and beak length, and B)

a relationship between success rate in foraging and likelihood of survival, then natural selection is going to be at

work, shaping the future phenotype of the population. In the case of our exercise, its probably going to be the case

that beaks that are too short will result in low success rates because the forager will not be able to reach to the


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bottom of the cup. Possibly the beaks that are too long will have a lower success rate because it is harder to

manipulate the beak that is too long, just like its hard to print precisely if you are holding a long pencil from the

eraser end. Its hard to say what any class results are going to actually look like, but this is how I envisioned the

outcome of this exercise when I wrote it.

As I mentioned before, differences in fitness could be realized by differential survival alonei.e., some survive

and some dont, but all of the survivors have the same rate of success in reproduction. There are a couple of ways

you can envision this happening. The easiest way is to say Okay, anyone with a foraging success rate greater than

X survives and makes babies, while everyone else just dies. Well start with this model, which well call Brutally

Deterministic.

Well apply this model to our population by identifying the top 33% foragers, and having them leave babies that

have similar phenotypes. But instead of having the babies be exactly the same as the parent, well incorporate some

ne variation, as might arise from sexual recombination or mutation. In the table below, write in the phenotypes for

the top 33% of foragers in the Survivorcolumn, and in the Offspringcolumn enter the phenotypes of the

survivors three offspring: one with exactly the same beak length, one with a beak that is smaller by two

millimeters and one with a beak that is larger by two millimeters. If natural selection is favoring either larger or

smaller bills, we should be able to see a change in average beak length from the first to the second generation.

Brutally Deterministic Natural Selection

Average phenotype of all offspring = _______________

Average phenotype from previous generation = _______________


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Evolutionary change

(Av. phenotype generation 2 Av. phenotype generation 1) = ______________

While youre busy doing this, your instructor will be applying an algorithm to the Excel data table to generate a

less severe and somewhat more realistic mode of natural selection via differential survival. As you might suspect,

nature does not operate like some kind of axe-wielding accountant who says, You didnt get enough beans, you

must die. In real situations in nature, its going to be more like, Theres no guarantee for success or guarantee for

failure. If you got a lot of beans, youll have a better chance to survive than someone who got fewer beans, but in

the end you might still end up dying while the few-beans individual survives.

The instructor will put up the list of survivors on the board, and your job will be to complete a similar table and

calculate the average phenotype in generation 2 under this form of natural selection, which Im going to call More

Stochastic. The term stochasticrefers to the way that what actually happens in real systems is never as pre-

determined (deterministic) as we made things in the previous version. For example if the fire risk in your area is

extremely highsay 90% riskthat doesnt mean its definitely going to burn. Theres a chance that your area

wont burn while another area does burn even though its risk was assessed at a lower valuesay 30%.

More Stochastic (and realistic) Natural Selection



Evolutionary change



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Darwin saw natural selection as being very much the same as the artificial selection that cattle breeders use to

increase, say, the milk yield in their stock. Under artificial selection, the scheme is typically more similar to our

Brutally Deterministic model, i.e., the cows that give the most milk are allowed to breed while the rest are turned

into burgers. A significant difference of natural selection as opposed to artificial selection is the strength of

selection during any given generation. The filters deciding who succeeds and who doesnt under natural

conditions are far less severe compared with those seen in selective breeding by humans. For reasons discussed as I

introduced the More Stochastic model, real-life natural selection is weak from generation to generation, but despite

its weakness it is able to effect great changes, provided that the same kind of weak selection is applied over

hundreds to thousands of generations.

H) Was the evolutionary change seen over one generation greater for Brutally Deterministic or More

Stochastic?

Natural Selection via Differential Reproduction

As I noted in the introduction, survival is one component of an organisms overall Darwinian fitness. It is also

possible for there to be no differences in survival and still have evolution by natural selection. This would occur if

the organisms success in reproducingand this may include variation in the number of mates one has, the numberof babies produced with each mate, and the success in rearing the babies to their point of independencewas

variable and influenced by a particular phenotype, such as beak length.

In the case of our simulation, you could imagine that success in foraging (which is influenced by beak length)

determines the amount of resources that could be used for the purpose of reproduction. A bird that collects lots of

beans will therefore have a higher expected success in reproduction relative to a bird that collects very few beans.

Similarly to our survival-based selection, we could model this with either a deterministic or a stochastic system

well use a deterministic one since its easier to manage. Just keep in mind that a stochastic version of selection by

differential reproduction is also possible and it would result in a less dramatic generation-to-generation phenotypic

change.

This time, well need to take into account all of the birds from our original population (class size X 3). Well say

that if the number of beans collected is greater than Z (your instructor will inform you of the actual values), the

number of offspring will be 5. If between Y and Z-1, the number of offspring will be 4, and so forth. Fill in the

following set of rules according to your instructors direction.

Number of beans collected is greater than _______ , 5 total offspring, 1 with exactly the same phenotype, one

smaller by 1 mm, one smaller by 3 mm, one larger by 1mm, and one larger by 3 mm.

Number of beans collected is between _______ and _______, 4 total offspring, one smaller by 1 mm, one smaller

by 3 mm, one larger by 1 mm, and one larger by 3 mm.

Number of beans collected is between _______ and _______, 3 total offspring, one with exactly the same

phenotype, one smaller by 2 mm and one larger by 2 mm.

Number of beans collected is between _______ and _______, 2 total offspring, one smaller by 2 mm and one larger

by 2 mm.

Number of beans collected is between _______ and _______, 1 total offspring, with exactly the same phenotype.

Fewer beans means no offspring.


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In the table below, list each bird that will have at least one offspring in the Survivorcolumn. In the Offspring

column adjacent to each survivor, enter the phenotypes of all of the offspring produced. I have provided a sample

data point, in which the survivor with phenotype 25 is slated to have four offspring.

Results table for Differential Reproduction(Enter between 1 and 5 offspring for each survivor that is allowed to

reproduce).

Survivor Offspring Survivor Offspring Survivor Offspring

25 22, 24, 26, 28



Evolutionary change


Now compare the evolutionary change that results from the three models.

I) Was the direction of evolutionary change (i.e. towards a larger or towards a smaller beak) consistent for the

three models? Is this the expected outcome?


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J) In which model was the selection (and the evolutionary response over one generation) the greatest? In

which model was it the weakest? What determines the relative strength of selection?

Extending this Experience: consider the (likely) possibilities

What you have modeled with your analysis and projections into the next generation from our experimental results

represents a single generation of natural selection. One generationwhich is hardly even an instant of time in the

scale of the earths history.

1) Lets start by thinking about a slightly longer time scale, and consider what would happen if this same system of

determining Darwinian fitness (with the birds ability to extract beans from cups exactly like those that we used in

our simulation) were to persist for the next ten to twenty generations. After one generation, the average beak length

had changed by some amount.

A) In the generation following, would you expect there to be further change? Why or why not?

B) At what point would the evolutionary change in beak length stopin other words what would need to be true in

order for natural selection to preserve the same average phenotype from generation to generation?

2) Okay, so if the average phenotype in the population is close to the optimal phenotype favored by natural

selection such that the most extreme (the smallest and the largest) individuals have a lower fitness relative to the

average, then the population is said to be under stabilizing natural selection, and there is little or no expected

generation-to-generation change in phenotype. [Yes I know that this kind of gives away the answer to the previous

question, but Im counting on your having tried to answer #1 on your own before starting to read #2]. Evolutionarychange is not expected for as long as the environment remains stable and that beak length continues to be optimal

for those birds world.

But what happens if the environment changes? Maybe the bean-bearing cup plant dies out and the birds are forced

to forage on some other kind of food resource. Come up with a description of a new food challenge that would

require the birds to evolve:

A) towards a shorter optimal beak length

B) to a longer optimal beak length

C) to a narrower beak (maybe popsicle sticks instead of tongue depressors with a smaller sized paper clip).


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3) Sometimes there is an opportunity to evolve in a completely different directiona serendipitous benefit coming

out of an unexpected source. Suppose there was a squishy but nutritious larva feeding on the beanskind of like a

mini-marshmallowthat could be most easily extracted if there was a pointy end on the beak (rather than the bent

paper clip). Those beaks that happened to have a little bit of a pointy end sticking out to the side would be best able

to use this resource, right? Then the next generation would have bigger pointy ends, and over time this might

evolve into a totally pointy beak that is optimally suited to extracting the marshmallow larvae.

A) Think about this and speculate another form of evolutionary innovation that might occur from the starting point

of our system of bean extraction.

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Nat selection lab