Shoshanah Jacobs
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Dear participants in my teaching demonstration scheduled for March 19th, 2019: Thank you for coming to this temporary website.

I have really enjoyed adopting a 'flipped' course approach to teaching where the face-to-face class time is reserved for assessment (formal and informal), feedback, and active learning. Below is a sample of what a student could be required to do in preparation for a class on the topic of Natural Selection. It is estimated that this preparation would take up to 1.5 hours and that 6 class hours would be needed to address the topic in a second year course in Evolution. This one presented as a demonstration would be the second of those classes.   

This website page was built and completed before February 27, 2019.

​Use the 'zoom' function on your web browser to change the orientation of the site such that the text becomes full page as you zoom in. 

Evolution 
​    Natural Selection 

Review of key concepts from last week: 
   Evolution is an umbrella term that describes the change of allele frequencies within a population over time. The way in which those frequencies can change (a.k.a. the 'mechanisms') can vary. Over the past week we have looked at 'random' mechanisms of evolution. By 'random' we mean that survivorship is random with respect to the phenotype of the individuals who survive and who pass on their genetic material. These random mechanisms include gene flow (but see the exception!) and genetic drift. 

  We began building a concept map of these terms associated with evolution. Last week, we focused on the 'random' branch. This week and for the next couple of classes, we will focus on the 'not random' branch. 
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Natural selection is a process by which individuals from a population have higher survivorship for 'not random' reasons with respect to their phenotype that is linked directly with an aspect of the environment. Survivorship can vary in its expression; from death (and therefore complete removal from the breeding population) to the reduced reproductive output of an individual. 

To study natural selection, we'll be considering several cases that will focus your attention at different levels of organisation and different time scales for you to appreciate the complexity of the mechanism. 

1) The colour of the shells of marine turtle: this is a fictitious case study to consider the way in which predation can cause natural selection of one trait of the prey. Here we will isolate both a single trait and a selection pressure to ensure that we understand the mechanism fully.
2) The lures of freshwater mussels: You'll be asked to read about different freshwater mussel species that have either generalised lures or specialise lures. This is a case of diversity of adaptation across many species of mussels. Here we will appreciate the different selective (and non-selective) pressures that result in diversity as well as the costs and benefits of each.  

To get us started though, let's consider the polar bear and its white fur.
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A mother polar bear and her two very healthy cubs. (photo: Jacobs, used with permission) Based upon the proportions of the features of the head and snout of the mother, can you guess the sex of the cubs? 
Polar bears evolved from brown bears. Mitochondrial DNA evidence suggests that the modern polar bear is actually a hybrid of an extinct polar bear species and brown bears. If the phylogeny of extant bears was made using mitochondrial DNA alone, 'polar bears' would be nested within a clade of brown bear populations. Contemporary hybridization events of modern polar bears and brown bears yields a phenotypic intermediate of the two species with viable offspring. As we learn more about the evolution of polar bears, our understanding of their history changes. It is an exciting time to be a polar bear biologist!

Polar bears have many adaptations to living in the Arctic that brown bears do not. Polar bears are more streamlined, have smaller ears, a conical face and head, fur around the pads of their feet, more fat, differences in their use of dens, different diets, different means of hunting, to name a few. But it is their white fur that makes them most obviously different. The mutations that led to the trait of 'white fur' afforded bears an advantage in hunting in a snow and ice-covered environment.  The white fur colour increases their ability to remain hidden from view of seals and other potential prey so long as they are hunting on a snow/ice covered background. The environment, or the context, is key.
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Look! A polar bear on the ice! Though polar bears are actually a more 'cream' colour they are very difficult to spot on the ice. Sometimes, all you can see is a black nose and two black eyes. (Photo: Jacobs, used with permission). ​

Imagine if a bear had white fur but lived in a temperate forest as it ambushed fast moving prey? Would it be able to sneak up on this prey? Likely not! In the context of a dark background of trees, bare soil, and rocks, a white bear would not have an advantage in hunting. But it isn't as simple as just evolving white fur. The adaptation of 'white fur' had to occur along with a few other changes to bear biology. It isn't likely that polar bears evolved white fur and then were selected to become active hunting carnivores. That's because 'white fur' would not have been an advantage without the context of being a carnivore. C.R. Harington (2008) of the Canadian Natural History Museum suggests that there may have been an intermediate bear that they call the "Arctic coastal bear". They imagine a dusky grey scavenger bear who fed on whale carcasses and other marine mammals washed ashore on grey dusky beaches. This bear, they suggest, may have been selected for white fur as it wandered out on the ice offshore in search of seals. Further research (maybe yours!) may reveal the story behind the evolution of white fur. But for now, Harington's mechanism works to explain how a dark brown omnivorous bear may have evolved by natural selection to a white carnivorous bear. 
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A mother and two year old cub on the ice. (Photo: Jacobs, used with permission)

Perhaps some of you are thinking about the white bears of the forest on the Pacific Coast of Canada. These bears, Ursus americanus kermodei, are a subspecies of the black bear. And, indeed, it has a really difficult time remaining hidden within the very dark backdrop of the forest. Thankfully, this species doesn't hunt fast moving terrestrial prey where their fur would make them more visible. Interestingly, research shows that individuals of this rare subspecies have greater success in fishing salmon from rivers because their white fur makes it harder for fish to see them from below the water's surface as they are swimming by. So why aren't there more of these white forest bears? Expression of the white trait requires homozygous recessive alleles so selection to fixation may take a very long time or may never happen. It depends! ​
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Photo of Ursus americanus kermodei used with wikimedia licensing https://upload.wikimedia.org/wikipedia/commons/2/2e/Spiritbear.jpg

There are several types of Natural Selection. They are distinguished from each other by the resulting distribution of the variation that exists within the trait. This is because the way in which the alleles are selected can vary. 

1) Stabilizing selection: here the 'average' of the trait yields higher survivorship. Trait variation is therefore narrowed towards the centre of its original distribution. 

2) Directional selection: here it is one extreme of the trait that yields higher survivorship and variation is narrowed towards either end of the distribution. 

3) Disruptive selection: here it is both extremes of the trait that yield higher survivorship and the 'average' is removed from the population. 

Can you think of examples of this? Can you think of scenarios where one type of selection is more likely to occur? What might you need to know about the context (environment) in order to predict what type of selection would likely occur? ​
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Each of these individuals is a freshwater mussel of the same species. There is considerable variation present here, both in colour and pattern. Take a moment to think about why such variation exists?

Now try to think of a completely different reason. How might you go about testing which hypothesis is correct? Where would you go? What sampling would you do? How could you determine whether the variation in colour and pattern is adaptive or the result of genetic drift? 

What would you say is the 'average' colour? What would be an 'extreme' colour? How about 'average' pattern? Try to line them up in an order so as to create a distribution. 
​Used with permission under the wikimedia license. https://commons.wikimedia.org/wiki/File:Coquina_variation3.jpg

Draw it out:  (Modified from Jacobs et al 2017, with permission)

How could you illustrate the difference between stabilizing, disruptive, and directional selection? One way is to graph the frequency of the variation that exists within the trait.

Let's work with the following trait: shell colour of sea turtles. 

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Beside each colour type labelled A to E is the number of individuals of the corresponding colour in a population of turtles where the total population is N=90. 

1) Draw a graph to represent the above distribution.
2) Write a sentence to describe what is represented.

Now imagine that after many generations, the population is recounted and the effects of stabilizing selection have occurred. 

3) Draw a graph to represent stabilizing selection of the above distribution. 

Now imagine that after many generations, the population is recounted and the effects of disruptive selection have occurred. 

4) Draw a graph to represent disruptive selection of the above distribution. 

Now imagine that after many generations, the population is recounted and the effects of directional selection have occurred. 

5) Draw a graph to represent directional selection of the above distribution. ​
Consequences of Natural Selection

It depends! There are many potential consequences of natural selection. 
Take a moment to think about upon what it might depend.
​ 
When we learn about natural selection, we tend to focus on one trait and the phenotypic variation that is expressed within that trait (as we have done above with the sea turtle example). It is technically possible that natural selection in the wild focuses on one trait, but it is more likely that selection is happening on many traits, at different rates with different implications for survivorship (or fitness). And if that wasn't enough, there are many random mechanisms acting upon individuals as well!  Just because a sea turtle is perfectly camouflaged from aerial predators, doesn't mean that it will survive a mechanical beach combing operation, for example. In fact, in this case perfect camouflage may be maladaptive if the operator of the machine could have avoided the turtle had they seen it. Similarly, the colour of a baby sea turtle's shell may be perfectly camouflaged on the sand yet makes the turtle highly visible once it enters the water. Which environmental background is most important to hide in? The sea perhaps, because it spends most of its time there? But what if it never makes it there because it wasn't well camouflaged on the sand? When we study phenotypes in the wild, we can sometimes learn more about the answers to these types of questions and the dynamics at play. 

In a population of infinite abundance, we would anticipate that the effects of random mechanisms would be very small. Genetic drift can have a much greater impact on small populations than it does on very large ones. (If you're grappling to visualise this: Imagine that you have 1000 people flipping a coin and calling out 'heads' or 'tails'. It is more likely that you will get a very near 50:50 split among heads and tails within this large group than you would if you only had 10 people flipping coins. With only 10 people, a 7:3 split is more likely than it would be in a population of 1000.) Now swap out the sidedness of the coin for an allele.) So predicting the impact of natural selection must take into account competing evolutionary mechanisms that can vary with population size. An advantageous mutation may be removed from the population due to genetic drift especially if it is a small population. 

The weight of influence of the random mechanisms must not be discounted. If we can marvel at the remarkable 'ingenuity' of nature to adapt to complex problems, think for a moment about all of the other traits that evolved and were eliminated due to random events or circumstances that had nothing to do with their fitness. It is truly awesome. 

And yet even once an adaptation has evolved, it is not without cost. The reason that adaptations have evolved is not because the costs are eliminated but, rather, because the benefits to overall fitness have outweighed the costs, for the moment in that context. Studying traits that have evolved by natural selection can yield a lot of interesting information about the environments in which those species evolved. But beware! Not every trait that you see evolved adaptively by natural selection. Make sure that you ask yourself about your assumptions. 

One dichotomy of  natural selection that is informative to explore is that of adopting a 'generalist' strategy vs a 'specialist' strategy. And there is no better system to learn about these strategies than the fresh water mussels. You remember about those, right? BIOL*1070 first introduced them to you in the first case study. If you need a refresher or you've transferred from another school or program, check out this video (closed captioning available on video and slides available upon request): 
We'll be diving a bit deeper into Natural Selection in class. We'll be adding to the recipe for evolution that we began last week to include the component necessary for adaptation. Before next class, please complete the homework that was assigned at the end of last class. Here it is again: 
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The citation of the article is: ​International Journal for Parasitology 49 (2019) 71-81 and the title is: Ecological correlates and phylogenetic signals of host use in North American unionid mussels. 

Can't find all the answers to the questions? There are some great resources online. You can even get started here: 

https://en.wikipedia.org/wiki/Generalist_and_specialist_species

Use your online searching skills to find what you're looking for. Unsure about the credibility of a source? Check out the resources posted on courselink about how to assess credibility (in the Content section under Useful tools). 


Come to class ready to share your answers to the homework questions, ready to spend our time together working through problems, and practicing our ability to apply what we know. 


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