First, I want to summarize the most important points from Chapter I and Chapter II of the Biology Bedtime Story. In this summary, I want to tell you what I was doing and what I wasn't doing. Second, I want to try and put a little bit of organization around the Lecture 9 slides so that you can maximize your study time effectively. Here goes...
Did the Forts live happily ever after?
When we last left our finch population, we saw that the drought of 1977 absolutely tore them apart.By 'tore apart,' I mean to say that they experienced one of the most severe selection pressures that Mother nature could throw at them. Was this a natural selection event? Yes it was. Absolutely.
As you'll recall from the bar graph at the bottom of the Chapter 2 Blog (also below right), the drought seemed to select against the allele combinations for the below average and average beak sizes. However, the drought also seemed to select for the allele combinations for above average beak sizes. Could this same change in the frequency of phenotypes in this Fort population have happened through genetic drift?
If you read my revised entry about genetic drift in the Chapter II Blog, then your answer should be, "Yes." It is possible that even without a natural selection event like a drought the beak phenotypes could change simply because of which birds happen to mate, and which gametes randomly join together in that process. This situation, however, even though it might result in a similar re-distribution of beak phenotypes, would not be considered natural selection.So, my story about the Forts on the island of Daphne Major was meant to illustrate a natural selection event. Nature, in the form of a drought, selected certain phenotypes to surive the 18 month drought that started in 1977. Some of you may be wondering why those Forts with the above average beaks survive.
Well, once the drought began many of the plants on the island began to decrease their seed production. The Forts, in competition with each other as well as the other ground finches on the island, began looking around the soil and under rocks for any seeds they could find. Just like you and your friends when you are given a bowl of pistachios, the Forts cracked open and ate all of the seeds that were easiest to crack.
After a while, only the difficult seeds were left to eat! Peter and Rosemary Grant took pictures of the one seed that none of the birds seemed to want to eat, but they had to once all the rest of the seeds on the island were gone. Here it is at right. It's called a Tribulus seed and it comes from a caltrop plant. Nasty buggers, arent' they? Believe it or not there's small round seeds housed inside each of those spiky, woody sections which fit together sort of like a pinwheel.
Are you surprised that it was the Forts with the largest beak size/strength that had the most success opening these seeds during the drought? I suspect you aren't. The thing that might surprise you, however, is the difference in beak length/depth between those Forts that survived and those Forts that died. Are you ready for this? It turns out that the average difference was only about 1 mm. Those Forts that survived the drought had beak lengths/depths that were on average about 1 mm longer/deeper than those that died.
Was this a speciation event?
One thing I was to make clear is that my story was meant to illustrate a natural selection event, NOT the creation of a new species. In my story, as I explained in Chapter II, we saw a case of directional selection (you should know about the other types of selection too), but I said nothing about the creation of a new species. After the 1977 drought we still only had one species, Forts. The frequency of their phenotypes had changed, but there wasn't a new species of finch after the drought. The above average beak Forts could still reproduce with the below average and average beak Forts and produce viable, fertile offspring.
So this leads us directly to Lecture 9, in which Prof S spent nearly the entire period talking to you about what it takes to make a new species. He called this "speciation."
I'm not going to go into the details of what counts as a new species mostly because I talked a little bit about 2 species concepts in the Chapter I Blog (I talked about the Biological Species Concept & Ecological Species Concept). Plus, Prof S went over 4 species concepts in class and I don't really think there's anything tricky about them. What I do think is important is that you understand the Biological Species Concept (BSC) the best. That concept seems to me the one that Prof S favored in all of the disussions we've had thus far about species. So, according to the Biological Species Concept, how can you create a new species?
Well, according to the BSC you would have to create (at minimum) two populations from a single population. In addition, the two populations would have to lose the ability to produce viable, fertile offspring with each other.
So how does nature pull off something like this?
In Lecture 9, Prof S gave you two terms that had the word "speciation" in it: allopatric speciation and sympatric speciation. Using both your lecture notes and your textbook or other resources, my advice to you would be to get to know how these two types of speciation operate. How are they similar? How are they different? What conditions are necessary for each of them to occur? You should be able to talk through these two types of speciation pretty fluidly. The lecture slides in which these two terms appeared are copied below:
One thing I would like to point out, however, is that the order of the Lecture 9 slides may have confused you a little bit. Notice that the top of the slides above ask the question: How do these barriers arise? The "barriers" that Prof S was talking about in these two slides are the prezygotic and postzygotic barriers to gene flow (you should know the differences between these two types of barriers--Prof S spent a good deal of time on them in class--6 of 16 slides, in fact).
I want to make it clear that what Prof S was saying with these two slides is that allopatric speciation is one way that prezygotic and/or postzygotic barriers can arise. Sympatric speciation is another. Some of you may have been confused that the 'effects' (the two types of barriers to gene flow) were discussed first and the 'causes' (the two types of speciation) were discussed second.
That's all I have time to discuss now...
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