Once upon a time...
Peter and Rosemary Grant (at right, inset) have been traveling to the Galapagos Islands since the 1970s. During that time, they've spent most of their time on a single island called Daphne Major (also at right).Daphne Major, as Weiner describes it, is "small and lonely," and there is only one way onto it. At low tide, the Grants have to leap from a rowboat onto a small ledge as most of the island's 'shore' consists of concave 3-story cliffs all the way down to the water of the Pacific Ocean.
What might live on such an isolated tip of an old volcano 600 miles (1000 km) from mainland South America? Well, birds, for one. There are iguanas too, but in this story I want to talk mostly about a group of birds, a group of "finches," in fact, and also organisms such as the plants on which the Daphne Major finches regularly feed. Since Prof S has been talking about individuals, species, and populations in lecture, I want to introduce these terms into today's story in the hope that you will slowly (but surely) become more comfortable with them. More importantly, I want you to learn to use them how biologists use them when they speak 'biologese.'
If you travel, as the Grants have, throughout most of the islands and islets in the Galapagos archipelago, you will likely run across 13 different species of finches spread throughout them. Well, at least this is the number of species that most taxonomists currently agree on.What does it mean to say that there are "13 different species of finches" in the Galapagos Islands?
It means that while these different 'groups' of birds share many common characteristics (e.g. behaviors and physiology), they either (1) CAN'T produce fertile offspring even if they tried, or they (2) COULD produce fertile offspring IF they tried, but they DON'T usually try!
An important point! Reason "(1)" above is my own simplified version of the Biological Species Concept, which Prof S discussed in Lecture 7 this past Wednesday. Reason "(2)" above is my own known simplified version of the Ecological Species Concept, which Prof S has not discussed (at least so far) in our course. I'll say more about this in a minute...
Lots of bird taxonomists currently lump these 13 species of finches into four "groups of species" or genera according to the species's behavior & diet. Click HERE to see a high-quality image with pictures of individuals from 11 of the 13 species--you'll probably get some sense of how four main groups are possible just by looking at their beak and head similarities/differences.
- Group A: The finches that live in trees and eat fruits and bugs.
- Group B: The finches that live in trees, but are strict vegetarians (i.e., no bugs!).
- Group C: The finches that live in trees, but they don't look and act like the other groups of tree finches...they actually look and act more like a different genera of birds (like the "warblers").
- Group D: The finches that spend most of their time hopping on the ground.
- Geospiza difficilis: the sharp-beaked ground finch.
- Geospiza scandens: the cactus finch.
- Geospiza conirostris: the large cactus finch.
- Geospiza magnirostris: the large ground finch.
- Geospiza fortis: the medium ground finch.
- Geospiza fuliginosa: the small ground finch.
What IS a species?
IF, we define species according to the Biological Species Concept (which Prof S talked about in Lecture 7), we would say, for example, that when a female Geospiza fortis tries to mate with a male Geospiza fuliginosa, they might produce an baby bird BUT it would be sterile. In other words, it would be impossible for the baby bird they produce to then go and mate (with either a G. fuliginosa or a G. fortis, it doesn't matter) and produce "viable offspring" (i.e., babies that can have babies).
Funny thing is, on Daphne Major, if a female G. fortis tries to mate with a male G. fuliginosa they can actually produce viable offspring, yet scientists still consider them as two different species. How is this possible? Well, that's because many biologist define species not by the Biological Species Concept (which I'll call BSC), but instead by the Ecological Species Concept (which I'll call ESC). The ESC looks at species a little bit differently. The ESC calls a group of finches a "species" when that group of finches could but--for any number of different reasons--choose not to reproduce on somewhat of a regular basis. So, you see, biologists actually have a number of different ways of talking about species, of which I have shared just two. And yes, they have more ways to talk about species--in fact, I know of at least two additional ways, but they're not really that important for our story, which we need to get back to...
Looking closely at Geospiza fortis on Daphne Major...
Glance back up at the top of the page and look again at the picture of Daphne Major. Can you imagine all of the individual Geospiza fortis--the medium ground finches--on the entire island (who from now on I'll simply call the "Forts"). In Lecture 7, Prof S introduced the the term "population" like this:- Population: All of the individuals of a single species in a defined area or "habitat" that will potentially interbreed.
Considering Prof S's definition above, one way to ask this same question is this: Do you think all of the individuals of a single species found in this habitat can interbreed? Here's another--albeit less-scientific!--way to ask this same question: Do you think it's possible (or likely) that all of the male and female Forts found on Daphne Major can find each other and have sex with one other if they want to?
To be honest with you--but I can't say for sure--I think the Grants would answer this question as, 'Yes, we consider all of the Forts on this Daphne Major to be a single population.' (I should have asked them this when I called them at Princeton in 1996--the day after they got back from their most recent trip to Daphne Major--and talked at length with both of them on the phone!)
Lets just say for the sake of own our BS110 story that there is only one population of Forts on Daphne Major. It doesn't matter if the Forts sleep on one side of the island or the other, near the ocean, or up inside of the dormant crater. When they want to reproduce, they can (and do) find each other.
So do you feel like you're getting a good sense of a what a species is and what a population is so far? Good, I thought so. But before we see how Mother nature can mess with these birds in some mind-bending ways, lets look at this single population of Forts on Daphne Major in even greater detail...
Variation in a single population
I've written extensively about variation in my Blogs this semester, so you should be getting quite confident in your ability to talk about variation across different scales. Do the Forts in this population show variation? In other words, do the Forts look different on the outside? (And in the words of Prof S, do the Forts show "phenotypic variation?")
DURING THERE: If you read one of my earlier Blogs ("Learning the Language of Variation"), then you know how to talk about variation at the molecular and the cellular scales (you use terms like base pairs, DNA, gene/allele, chromosomes, etc.). If you read the Lecture 7 Blog ("Hey, Four Eyes!"), you know that Prof S has been recently trying to teach you how to talk about variation on larger scales, like in individuals and in groups of similar individuals or populations.
In the Lecture 7 Blog, I told you that I like to talk about individual variation as "DURING THERE." That is, during an individual organism's time on earth, why/how does the individual show phenotypic variation? This is an easy question to answer when we think about a single Fort finch...
Like humans, Forts go through a number of stages from sperm/egg to death. We often call this their "life cycle." Young Forts are often called "juveniles," and they are called "adults" as soon as they develop the ability to reproduce (that is, when they reach "sexual maturity"). This is different than with humans, isn't it? Biologically speaking, many male and female humans develop the capability to reproduce sometime in their early teen years.
Legally speaking, however, in the U.S. we don't usually consider humans to be "adults," until they're 18...well past when they could be considered a biological 'adult.' Not so in the bird world.Imagine the beak of a single Fort...
Does the beak size of a single Fort change during its life cycle? Absolutely, Forts have smaller beaks when they are young "fledglings" than when they are adults (fledglings are even younger than the juvenile stage). Therefore, we say that a single Fort's beak size shows variation during or over the course of its life span.
Believe it or not, the Grants and their graduate students have spent 30+ years measuring the beaks of every Fort on the island multiple times over the course of each Fort's life span. If you click on the image to the right, you can get a good idea as to the types of data that the Grants collect for each of the Forts found on Daphne Major. If they want to try to answer the all-important--at least in the sciences--why and how questions, biologists have to do A LOT of this type of "During there..." or descriptive work.
Is there phenotypic variation in the Daphne Major Forts population?
Thanks to work of the Grants, we can actually answer that question: 'Yes, across the population on the island, the beaks of the Forts are variable.' How variable are they? That's a good question, and one the author of The Beak of the Finch describes like this:
"...the medium ground finch, fortis, sometimes shades into the species above it, magnirostris, or the species below it, fuliginosa. The very biggest specimens of fortis are just as big as the very smallest specimens of magnirostris, and so are their beaks. At the same time the very smallest specimens of fortis are just as small as the biggest fuliginosa, and so are their beaks. Within each of these three species, the beak of individual birds are variable...You can't distinguish these three species by their plummage, and usually not by their build or body size either. You have to tell them apart by their beaks." (p. 42-43)Can you imagine? What a logistical research nightmare for biologists hoping to study any one of three species of ground finches on a single island!
Well, this is actually the case on Daphne Major where all 3 species of ground finches can be found hopping around on the soil, looking for food, reproducing, and trying to avoid predators. Oddly enough, the finches do these things under the watchful eyes of Peter and Rosemary Grant and the rest of their "Finch Unit" crew. After 30+ years of practice watching and measuring the ground finches, the staff at the Charles Darwin Research Station on a nearby island have an unsurprising saying which goes something like this: Only God and the Grants can recognize the difference between species of Galapagos finches.
Looking ahead...
In the Grants' first four years doing research on Daphne Major, between 1972-1976, all of the ground finch species on the island had it pretty good. There was enough rain so that the plants on the island produced lots of food (mainly in the form of seeds) for all of the ground finches to eat (seeds are the preferred food of magnirostris, fortis, and fuliginosa). In the language of BS110, there was very little "competition" for "resources" (like seeds) within and between the different "species" of ground finches. In the language of economics: When it came to the ground finches favorites foods, supply outstripped demand.
Just how good were things between 1972-1976?
Well, when the Grants first arrived on Daphne Major during the wet season (there are only 2 seasons in the Galapagos: wet and dry), they began collecting finch data and they estimated about 1500 Forts to be living on the island (i.e., the total population of the species on Daphne Major). At the end of the dry season, just before the next wet season began, the Grants estimated that about 9 out of every 10 of those Forts survived the dry season (about 1350 of them). That's a 90% survival rate following the dry season, when food is typically harder to find than in the wet season. This logic makes sense, right? Less rain = Less plant growth = Less seed production by plants = Less food for the ground finches = Less raw 'materials' for cell processes like cellular respiration. Can you see in this last sentence how "Out There..." is coming into this survival equation? Changes in the environment (Out There...) lead to some interesting changes inside of the cells of an organism (In there...)...
In 1977, however, what nature had been accustomed to giving Daphne Major somewhat regularly between 1972-1976, i.e., rainfall, it decided to take away...rather abruptly. Begining sometime in 1977, no rain fell on Daphne Major for about 18 consecutive months. The Mags, Forts, and the Fulis were forced to deal with the fact that sometime in 1977 the island's plants stopped producing seeds. All 3 species of ground finches had to rely on finding and cracking open seeds that had fallen to the ground and been ignored or overlooked in previous years. In other words, the only food that existed on Daphne Major in late 1977 were the seeds left lying in the dirt and under small rocks from years like 1976 and 1975.In the language of BS110, the ground finches of Daphne Major experienced a severe "selection pressure" or a "selective force" starting in 1977. Finding enough food to eat and survive each day was a "struggle" for survival. In the next Blog, we'll look specifically at how the Forts dealt with this selection pressure or 'force.' We will look things like the Forts overall birth & death rates, as well as the measurements--thanks to the Grants!--of the beaks of those Forts that died (and survived) the 18 month drought of 1977-78.
By doing this, we can then map onto our story those terms that Prof S used extensively in Lecture 8, terms like:
- genetic variation (including phenotypic & genotypic variation)
- heritability
- allele frequencies
- genetic drift
- founder effects & bottleneck effects
- genetic diversity
- fitness & relative fitness
- immigration/emigration
- gene 'flows' & gene 'pools'
- inbreeding
- selection (including stabilizing, directional, and destabilizing selection)
Looks like my summer reading list just got a little bit longer...
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