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Lab 6- Natural Selection and Genetic Drift
Welcome to Lab 6- Natural Selection and genetic drift
Welcome everyone! This is your new, online Lab 6: Natural Selection and genetic drift.
The purpose of this lab is to help you understand these key evolutionary processes:
• Natural Selection
• Genetic Drift
We will also be learning how to model evolutionary processes; this is vital when studying the genetics of populations, such as in fragmented habitat or on islands. The content of this lesson primarily relates to lectures 10 and 11. Please read through the entire Moodle lesson before completing the exercises.

You will be be provided with a worksheet (found here) that you complete as you work through this lesson. Once you have filled out your worksheet, you will need to submit it into the Lab 6 Assignment dropbox found on the 'laboratories and tutorials' page below this Moodle lesson.
It is estimated that this online lesson will take 1 - 3 hours to complete. 
Lets begin!
This lab will put you in the mindset of an evolutionary biologist studying Koala populations. You will simulate two evolutionary mechanisms, natural selection and genetic drift. 
There are two exercises in this lab class: modelling natural selection and modelling genetic drift. Each page will walk you through the experiments you will be running, and the exercises laid out in your worksheet. Make sure you answer the questions in the worksheet as you work through this lesson.
This lab requires a small amount of preparation on your end; as we cannot catch a group of koalas, we will instead substitute them with marked cards instead.
For this lab, you will need the following:

• A Dice roller, using a 4 sided and 6 sided die. For an online version,https://www.google.com/search?q=dice+roller

• Paper to make cards / You can either make your own cards by hand or print the templates we've provided in the link. You will need:
o 45 Arboreal Koala cards 
o 45 Intermediate Koala cards 
o 45 Terrestrial Koala cards 
Now you are ready to simulate your koala population! Follow the instructions in this lesson and write down your results in your workbook when required.
1. Make a bank of koala cards, and create your original population. This consists of 10 Terrestrial, 10 Intermediate, 10 Arboreal. Shuffle thoroughly and deal randomly into 15 pairs. This represents the koala parents for the next generation

An example of the 15 breeding pairs of Koalas.
2. Using the rules in the previous page, work out the offspring for each pair. Using these, and the following rules, you should end up with about 45 cards.

15 mating pairs, with offspring.
3. For Intermediate and Arboreal/Terrestrial offspring, use a six-sided dice.  A die roller is linked in the first page, or use your own dice if you have one. 
•  Let the numbers 1, 3 and 5 on the die represent Intermediate Koalas and the numbers 2, 4, and 6 represent the Terrestrial and Arboreal Koalas. Roll the die and let the outcome determine the habitat preference of the offspring
4.  For Intermediate / Intermediate, use a four-sided die. 
•  Let the number 1 on the die to represent Terrestrial. Numbers 2 and 3 represent Intermediate and number 4 represents Arboreal. 

We have had our first round of births- now, it is time for our first round of deaths.
1. Now model mortality. Individuals that spend more time on the ground are more likely to be killed- so for every Arboreal death, 2 Intermediate and 3 Terrestrial die. To simulate this, roll a six-sided dice 15 times. At each roll, remove a koala according to the following- 
•  A Terrestrial if the dice is 1, 2 or 3.
• An Intermediate if the dice is 4 or 5.
• An Arboreal if the number is 6.
NOTE: If you do not have any of the specific Koala rolled left in your population, re-roll until you can remove a koala. 15 koalas must be removed from the population each year.

Your pairs will look something like this. Keep the rules in mind when deciding the habitat preference of the offspring!
Keep in mind your punnet squares when choosing the right dice for your offspring calculations- why are you using a four-sided dice for Intermediate/Intermediate vs a six-sided dice for all others? 
1. After one year of births and deaths has been simulated (so you have gone through the cycle once), sort the koalas into separate piles based on their habitat preference. The total number of cards should be 30. 
2. Shuffle the cards, and repeat these cycles 10 times using the population you just simulated. Remember- you want to track how your population changes over time! Record the results of each year in the table provided in your worksheet.
3. Complete the questions in your worksheet before continuing to Exercise 2.

Exercise 1- Questions and review
Now you have your results, think about what they mean for the wider population.

In your worksheet, answer the following questions. Each answer should be between 1 and 4 sentences in length.

1. What changes occurred in your koala population over the years? Briefly explain why these changes occurred.
2. Compare your results with the data from others students results. The first student found that it took 7 generations for the population to become fixed for the Arboreal/Tree-Dwelling allele (Ht), the second student found that after ten generations they had 29 Arboreal Koalas and 1 Intermediate Koala, while the third student found that after 5 generations the population had become fixed for the Arboreal allele (Ht) as well. The fourth student found similar, and found that it took 6 generations for the population to consist solely of Arboreal Koalas. Did you get the same result? To what extent do you think the end result of natural selection in your koala population was due to chance? Explain.
3. In simulating deaths, why do you think the numbers on the die were assigned to the different types of koalas in the way they were? What does this say about the fitness of the Terrestrial koalas compared to the others?
4. If the ratio of deaths was altered to 4 Terrestrial : 2 Intermediate : 0 Arboreal, predict what changes you would expect in the results. What would this say about the fitness of the Terrestrial koala compared to the others?
5. Did you know that wombats are closely related to koalas? If you think about it, they do look quite similar to each other. Why then do wombats not live up trees?
Genetic Drift: A background
The idea of genetic drift comes from statistics – but you do not need to be an expert in statistics to understand it. 
You may know from lectures on statistics that small samples of a population are not necessarily representative; they can be biased.In the same way, if you take a small sample of genes from a gene pool (i.e. remove a few individuals from a population), the genes carried by the small sample will not necessarily be representative of the total gene pool of the larger population. 
If the small biased sample is then kept separate from the original population, and reproduces and hence establishes its own population, the new gene pool will differ from the ‘parent’ gene pool. This is an evolutionary change! (But note that the concept of ‘fitness’ or ‘suitability to the environment’ is not relevant here).
Genetic drift can result in evolutionary change, but it does not produce adaptations. The fitness of individuals in the new population (derived from the small sample of the original population) may be no different, or may even be less than the fitness of individuals in the parent population. But genetic drift may still lead to evolutionary change, i.e. it can lead to populations with gene pools that are different to that of the population from which they originally came. This is because the new smaller population starts with a biased gene pool. 
There are two ways in which genetic drift can come about: the founder effect and population bottlenecks. The founder effect is due to the establishment of small geographically isolated populations with biased gene pools which are different from the original larger (parent) populations. A bottleneck is a small population left behind when most other members have been wiped out – perhaps as the result of a large scale natural disaster (such as a volcanic eruption, flood or earthquake) which kills most members of the population randomly (not because they are less ‘fit’). The small remnant population is likely, by chance alone, to have a different gene pool to the original population.
A good example of genetic drift at work is seen in the koala populations of Victoria and South Australia. Because of hunting, habitat destruction and disease, Victorian koalas were almost driven to extinction by the 1920s. For example, in 1924 over 2 million koalas were shot for their pelts (skin & fur).
Koalas were widely hunted for their pelts, and only a few remnant populations remained unaffected.
This led to a population bottleneck. In an attempt to rescue the Victorian koala population from extinction, as small founder population of about two or three koalas was released onto French Island in Westernport Bay. As this population grew in size, koalas were reintroduced back to the mainland to areas such as the Brisbane Ranges (north of Geelong) and to Kangaroo Island off the coast of South Australia. 
One of the problems with bottleneck and founder populations however is that they have much less genetic diversity in their reduced gene pools. So, for example, genes which may have been present in the original population for resistance to disease may be absent from a founder or bottleneck population. This has certainly been the case for koalas in south-east Australia. They have little genetic diversity and may be very susceptible to some diseases. This perhaps explains the impact of Chlamydial infections in koalas.
Chlamydia are bacteria which can produce diseases in many animals. For instance in humans, Chlamydia trachomatus causes a sexually transmitted disease. Another type of chlamydia (Chlamydia pstittaci) infects koalas and makes them infertile. Presumably only natural selection can save the koala – any new genes which offer resistance to chlamydial infection will have a dramatic effect on fitness of koalas and would rapidly become established in the population.

 

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  • Posted on : November 09th, 2018
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