Lecture 8 (6/5/09): A Biology Bedtime Story...Chapter II

Look at these 3 words: fuliginosa, magnirostris, fortis.

If you have absolutely no idea what these words means, then you need to go back and begin our story from the beginning ("A Biology Bedtime Story...Chapter I").

For the rest of you, read on...

A Recap of Where We've Been

When we last left our story we said that we were going to follow the population of Forts on Daphne Major in the years leading up to, during, and immediately after the severe drought of 1977-78. In particular, we said that we were going to track the Forts' beak size, which we know varies quite widely throughout the population. This is to say that there are Forts with larger beaks, Forts with smaller ones, and Forts with every beak size in between. In other words, Forts are highly variable with respect to their beak size.

Now, the fortis population numbers that I'm about to use in the rest of this story are completely made up to make certain things more clear. Please know that I didn't take them straight out of The Beak of the Finch by J. Weiner, I took them instead (and adapted them) from an activity created by Presada Calrabi called "Evolution: A Natural Experiment," which was in a book that I can no longer find.

Daphne Major Census: 1976

Lets pretend that at the beginning of the wet season in 1976 there was an easy way to classify the population of Forts on Daphne Major according to a particular body part: Their beaks. Rather than use actual quantitative measurements for things like beak length, beak depth, and beak width, lets just lump all of these measurements into single category and call it "Beak size/strength." Imagine that we can then capture all of the Forts on Daphne Major and classify their beak size/strength as Average, Below Average, or Above Average. Simple, right?

As we learned in Chapter 1, 1976 was a a fairly normal year in terms of rainfall and food availability. If we were performing a census on Daphne Major at the beginning of 1976, here's what our census might look like (click on it to enlarge it):

A census is just an official count or survey of a population, typically recording various details of individuals. In this case, lets walk through the numbers...

• The table says there are 50 adults breeding pairs for each beak phenotype. Remember, these are all Forts with phenotypes for either average, below average, or above average beak size/strength. So, 50 breeding pairs of each phenotype means a total of 300 adult Forts.

• The table also says that these breeding pairs produce a total of 300 offspring (100 offspring produced by each of the different beak phenotypes).

• Of the 100 below average phenotype chicks born this year, 60 survive. Of the 100 average phenotype chicks born this year, 80 survive. Of the 100 above average phenotype chicks born this year, 55 survive. This means that a total of 195 Fort chicks survive.

Let's assume that about 10% of the adults typically die every year (old age, predation, accidents, etc.). So by the end of 1976, how many Forts make up the population of Forts on Daphne Major? The table at right summarizes the numbers.

As you can see, a total of 465 Forts survive 1976.

Lets also assume that chicks become become adults and breed the year after they are born. This means that there are 465 adult Forts that survive 1976: 150 below average beak size/strength, 170 average beak size/strength, 145 above average beak size/strength.

How do we talk about what happened in 1976 in terms of ideas in BS110?

Well, it's clear that the Forts demonstrate phenotypic variation within their population. We should also say that beak size/strength is heritable, which simply means that Forts' beak size/strength is passed on from parents to their offspring through certain alleles of genes. To see this fact you have to put on your "In There..." glasses. You need to be able to 'see' things like alleles, chromosomes, base pairs, and DNA when looking at a table of data like this. You also need to be able to see that things like Punnett Squares, meiosis, crossing over, independent assortment, and gametes are lying just beneath the surface of this table. Can you see them?

Is there genetic variation in this population? Yes. There are alleles present in the Fort population that actually code for three beak phenotypes (average, below average, and above average size/strength). How do we talk about the frequencies of alleles (or allele frequencies) in this population? Well, frequency basically means "how often." So how often do the alleles for average, below average, and above average size/strength beaks occur? We can calculate this as follows: 150 out of surviving 465 birds have the allele combination that gives them below average beaks (32.3% of the surviving Fort population); 170 out of 465 surviving birds have the allele combination that gives them average beaks (36.6% of the surviving Fort population); 145 out of 465 surviving birdshave the allele combination that gives them above average beaks (31.2% of the surviving Fort population). When we put it in percentages, it seems that no one allele combination is too much more frequent than the others, right?

What about the fitness of this population? According to Prof S, the fitness of this population of Forts is determined by measuring the number of Fort chicks produced that can then produce offspring themselves. In 1976, these Forts seemed pretty 'fit,' don't you agree? 195 Fort chicks survived who we assume all have the potential to develop into adults by 1977, where they can then start reproducing themselves.

Daphne Major Census: 1977, the Drought

At the start of the 1977 the Fort population seemed to be in a relatively decent state: only 10% of the adults were dying and lots of chicks were born. Not only that, but the many chicks that survived 1976 and entered the 1977 wet season as adults were ready to breed themselves. If we took another census of the Fort population on Daphne Major at the start of 1977, we could imagine that it might look something like this:
Lets briefly walk through the numbers in this table, which lists mostly data about "pairs" of birds instead of individuals.

• The table shows that there was a 10% mortality rate among the adults from the year before. You can see this because 50 adult pairs from the year before has dropped slightly down to 45 adult pairs. Remember, these are all Forts with phenotypes for either average, below average, or above average beak size/strength. So, 45 breeding pairs of each beak phenotype means a total of 270 adult Forts (45 pairs x 2 individuals x 3 beak phenotypes).

• The table also shows that there are "new adult pairs." These are the chicks who survived 1976 and are now adults themselves! They pair up and add even more adults to the total Fort population, 184 new adults to be exact (30 pairs + 40 pairs + 22 pairs x 2 individuals).

• The last line of the table shows the total number of adult breeding pairs of each phenotype at the beginning of 1977. There are 75 total breeding pairs of birds with the below average beak phenotype (150 individuals). There are 85 total breeding pairs of birds with the average beak phenotype (170 individuals). There are 67 total breeding pairs of birds with the above average beak phenotype (134 individuals).

Had 1977 been anything like 1976, we would expect all of these of breeding pairs of Forts to have lots of chicks throughout the wet season. However, I've already told you that 1977 was the beginning of an 18 month drought on Daphne Major. Take a look at this census data from the end of 1977:


Stunning...isn't it? Now, I told you that I've invented these numbers to illustrate some important ideas. However, in the Grant's actually finch data from 1977 a similar drama unfolds in term of the plummeting numbers of each of the three Fort beak phenotypes. The table above does not show pairs--it shows the number of individuals still found on Daphne Major by the end of the dry season in 1977. NO CHICKS HATCHED that year. Only 2 below average beak Forts survived (98.7% mortality rate). Only 3 average beak Forts survived (98.3% mortality rate). Only 27 above average beak Forts survived (80% mortality rate). Compare that to the 10% mortality rate that we saw in adult Forts during 1976!

How do we talk about what happened in 1977 in terms of ideas in BS110?

[The following discussion about genetic drift was modified/corrected/revised on 6/8/09]

Is genetic drift occurring here? The short answer is, "NO." But there has been some confusion about this term as it was presented in class. In Lecture 8, Prof S presented genetic drift as "a change in allele frequencies in response to...unpredictable events [that often] happen to populations."

Unfortunately, many of us are tempted to look at the drought of 1977 and think to ourselves, "The drought was definitely an unpredictable event...and the drought definitely led to a change in the allele frequencies of the Fort population...therefore, this must be a case of genetic drift!" At least that's the logic I used when I went back over my own notes from Lecture 8 and initially wrote this Blog. As I said above, this is unfortunate. Why is it unfortunate?

This is unfortunate because--at least when it comes to thinking about the concept of genetic drift--this is the wrong way to think about unpredictability. Before we tease this idea apart any further, lets first establish one thing that we should all agree on at this point: The drought led to a change in the allele frequencies in the Fort population. Why am I so confident that we can accurately make this statement? Because of the following data:

• Before the drought 150 out of 465 Forts had the allele combination for below average beaks (32.2%). After the drought, 2 out of 32 surviving Forts had this same allele combination (6.25%).

• Before the drought 170 out of 465 Forts had the allele combination for average beaks (36.6%). After the drought, 3 out of 32 surviving Forts had this same allele combination (9.375%).

• Before the drought 145 out of 465 Forts had the allele combination for below average beaks (31.2%). After the drought, 27 out of 32 surviving Forts had this same allele combination (84.375%).
  

When we put this story in 'before/after' drought percentages like this, it should be clear that one particular allele combination--the one coding for above average beak size/strength--occurs in the Fort population much more frequently AFTER the drought compared to before the drought. Therefore, the drought has in fact led to a "change in the frequencies of the alleles" in the Fort population. Can you see this too? Unfortunately, this is only one part of the complete definition of genetic drift. In order for this instance to be a case of genetic drift, Prof S also said that this had to be an "unpredictable" (or "random") event that led to this change in allele frequency.

I know what you're thinking...you're still thinking that the drought was an "unpredictable" or "random" event! This is a classic case in science where finding the right words to use (and the right order to put them in!) makes all the difference. Sure, from a certain perspective, the drought could be seen as an random or unpredictable event. However, what about the phenotypic response of the Forts to this event? Is their phenotypic response random? Is their phenotypic response unpredictable? In other words, as a scientist, if you knew a drought was coming to Daphne in 1977, would you predict that the allele combinations for beak size/strength would change in response to a severe drought? SURE YOU WOULD! Thus, in our drought episode, biologists don't see the changes in the frequencies of the allele combinations for beak phenotype as random or unpredictable. They see these changes in the allele frequencies as entirely non-random and predictable!

So what would a case of genetic drift look like with our Fort population? To see that we have to use our imagination and go back a short ways in time...

Imagine that Daphne Major from 1972-1976 is an incredibly stable place. Imagine that it receives the same amount of rainfall each year, has no new introduction or loss of predators, has no major changes in the plant communities, etc. In other words, imagine that there are no significant changes in the selective forces that might choose certain beak size/strength phenotypes over others. Imagine also that there are, say, 150 Forts on the island: 50 below average (with aa alleles), 50 average (with Aa alleles), and 50 above average (with AA alleles).

Now here's an important question for you: With no new significant changes in selection pressures acting on this population of 150 Forts, is it possible that in 1976 we could find the following distribution of Fort individuals: 100 below average (aa), 25 average (Aa), and 25 above average (AA)? Is this type of allele distribution possible without some major selective pressure on the Forts? What do you think?

The answer, at least in theory, is: "YES, it's entirely possible." How is this possible?

This hypothetical change in allele frequencies between 1972-1976--in our case, the small "a"'s appear in the Fort population in 1976 more frequently than they do in 1972--we said is not due to a drought or any other major selection pressure. In our example (again, a hypothetical one), this change can be viewed as due to the unpredictable or random event which is best summarized by a few (closely related) questions: (1) Which male phenotypes happened to mate with which female phenotypes? (2) Which male gametes randomly came forth from meiosis? (3) Which female gametes randomly came forth from meiosis?

Take an Aa male and an Aa female (imagine that both show the average beak phenotype because of incomplete dominance). During a straightforward episode of meiosis, the male can produce two types of gametes: A and a. Similarly, the female can produce two types of gametes: A and a. If these two Forts mate according to a Mendillian model of genetics, they have a 25% chance of having a baby Fort with AA (above average beak), a 50% chance of having a baby Fort with Aa (average beak), and a 25% chance of having a baby Fort with aa (below average beak). And you thought Punnett Squares wouldn't be useful to you any more!!! What's to stop the Aa parents in this population from producing a bunch of aa offspring? They have a 25% chance of producing this kind of chick every time they mate, right? The answer to the questions is, "Nothing, can stop them." Nothing, that is, except for some generally accepted 'laws' of chance/probability, but if you've ever flipped a coin those are easy to break!

Have you ever flipped a coin and gotten 4 heads in a row? If you did, you temporarily violated the laws of chance/probability (theoretically speaking you should have gotten 2 heads and 2 tails in your 4 flips of the coin). Why should the gametes of finches be any different than a quarter or, say, a pair of dice multi-sided dice? Why couldn't there be four aa babies born in a row--one each year for four years--to a pair of Aa parents? If this type of random or unpredictable event happens without any type of selection pressure for a particular phenotype, and it results in changes in the frequencies of allele combinations present in a particular population, we say that it is an example of genetic drift.

Because of this random pairing of gametes from parent birds, in the example above we can say that the allele combinations for the average (Aa) and above average (AA) beak phenotypes have 'drifted' from the Fort gene pool on Daphne Major (in other words, it's harder to come by the A allele in the Fort population in 1976). We can also talk about this in terms of the Fort genome, which is a collection of all of the different alleles in the Fort population. In this case, we can say that the Fort genome is "less diverse" following our hypothetical example of genetic drift between 1972-1976. Or, following the draught of 1977, we could also say that the Fort genome has "lost" some of its diversity following the natural selection event (i.e., the drought).

[The above discussion about genetic drift was modified/corrected/revised on 6/8/09]

Has there been immigration in this episode of drought? In other words, have any new Forts arrived at the island from other islands? Not that we can see in the data tables, so we have to assume that no new Forts have found their way to Daphne Major from neighboring Galapagos islands.

Has there been emigration? In other words, have any new Forts departed the island for other islands? Well, only that 'Big Island' in the sky :) Again, our data tables don't report any emigrants so we have to assume that the Daphne Major Forts died on the island instead of flying off to a nearby island. Has there been gene flow? Prof S said that gene flow was when "individuals move between populations," and we just said there was no immigration or emigration to or from Daphne Major (which holds the entire Fort population), so there must have been no gene flow!

What is this population of Fort's relative fitness? In other words, what is this Fort population's fitness relative to the Fort population of 1976? Can you say, abysmal? That should be easy to see. If fitness is determined by measuring the number of Fort chicks produced that can then produce offspring themselves, then the fitness of the 1977 Forts is practically zero since no chicks were born that year. Fortunately, at least two birds of each phenotype survived. So, assuming that there is a surviving male and female in each of the beak phenotypes, there is still a chance that the relative fitness of the population could increase in 1978 if the rains return.

Are we seeing founder and/or bottleneck effects? As you saw in lecture (see slide below left), Prof S defined the founder effect as events where there is a "loss of allelic (genetic) diversity because of small numbers of individuals starting a new population elsewhere."

Did our Forts lose allelic diversity in 1977? YES. Was it because a small number of individuals started a new population elsewhere? NO. So, did we see a founder effect in this population? NO, we did not.

What about a bottleneck event? Prof S defined these as events that result in a "decrease in genetic [or allelic] diversity after catastrophe." Did our Forts lose allelic diversity in 1977? YES. Was it because of a catastrophy? YES. Need I say more?

Daphne Major Census: 1978

One last year's worth of data and then we can wrap up our biological bedtime story. By now, you can read tables like this and keep track of things like individuals, pairs, chicks hatched vs. chicks surviving, etc.

Lets briefly walk through the numbers in this table...

• The table says that at the beginning of 1978 there is now only 1 adult breeding pair of the below average and average beak phenotypes. However, there are 13 breeding pairs of the above average beak phenotype. Remember, these are all Forts! These are all member of the same species who can interbreed anytime they desire. So, 15 total breeding pairs means a total of 30 adult Forts (in 1976 there were 300).

• The table also says that these breeding pairs produce a total of 35 offspring (2 below average chicks + 3 average chicks + 30 above average chicks).

• Of the 2 below average phenotype chicks born this year, 2 survive. Of the 3 average phenotype chicks born this year, 3 survive. Of the 30 above average phenotype chicks born this year, 27 survive. This means that a total of 32 Fort chicks survive.

Lets take the census data from 1976-1978 and combine it in a single frequency histogram (bar graph) to show the TOTAL NUMBER OF BIRDS alive with each beak phenotype at the end of each year. By this time in your school career you should be able to see three stories within this one graph: The story of 1976 is bold in BLUE; the story of 1977 is told in RED; and the story of 1978 is told in GREEN.

If you concentrate on the stories told by each of the colored bars you will see that we've clearly seen a natural selection event between 1976 and 1978.

• If you connected the very tops of the three blue bars with a single line, you could see that the line would be rather 'flat,' with only a small hump in the middle (due to the average beak phenotype). The story behind this line would be this: In 1976 the three different allele combinations for beak size/strength could be found with (nearly) equal frequency in the Daphne Major Fort population.
  

• If you connected the very tops of the three red bars with a single line, you could see that the line would be rather 'skewed' to the right. This time the hump would be more prominent and to the right (due to the above average beak phenotype). The story behind this line would be this: In 1977 the three different allele combinations for beak size/strength could not be found with equal frequency in the Daphne Major Fort population; as a percentage of the total Fort population, you could find more individuals with above average beak size/strength.
  

• If you connected the very tops of the three green bars with a single line, you could see that the line would be quite 'skewed' to the right. This time the hump would be even more prominent and to the right (due to the even greater number of birds with the above average beak phenotype). The story behind this line would be this: In 1978 the three different allele combinations for beak size/strength could not be found with equal frequency in the Daphne Major Fort population; as a percentage of the total Fort population, you could find even more individuals with above average beak size/strength.

I now want to make one last connection with a slide that Prof S used in class (see below). This was the slide that he used to talk with you about 3 types of selection: directional, disruptive, and stabilizing selection. Given the story of the Forts from 1976-1978, the histogram I created above, and the bullets that talk about 'lines' and 'humps' and 'skews,' you tell me: What type(s) of selection did we see occurring with the Fort population on Daphne Major?


I sure hope you answered: Directional selection.

Well, that's the end of Chapter II of our Biology Bedtime Story. Between Chapters I & II, we've seen a story unfold that has asked you to put all three pairs of your Darwinian glasses (In There..., Out There..., and During There...). I've held up my end of the bargain by telling you a story about a single species of finch in the Galapagos Islands and I've mapped nearly all of the important terms from Lectures 7 & 8 onto this story (I guess I forgot inbreeding, but maybe you can insert that one yourself). You've now got your own work to do by continuing to integrate this story with your lecture notes, your experiences in lab, and your textbook.

In future Chapters (e.g. Chapter IV), we will take a look at how the other species on Daphne Major--like the Mags and the Fulis--dealt with drought of 1977.