What does the professor say about major environmental events on the Galapagos Islands?

Listen to part of a lecture in an evolutionary biology class.
Professor: Today we’re going to look at a classic study in the evolution of natural populations and examine how the authors of this study interpreted the data, but before getting into the data itself we need some background.
    The study was concerned with evolutionary changes in a population of ground finches on the Galapagos Islands. You’ve probably heard of these ground finches that live on the Galapagos Islands, a group of small islands in the Pacific Ocean. They’re called Darwin Finches. There’re 13 species of these birds. Generally speaking, they’re small, blackish or dark brownish in color and they’re poor flyers. In fact, many can’t even fly from one island to another, so they aren’t present on all of the Galapagos Islands, but are kind of confined to one or two islands. There’s really nothing out of the ordinary about the finches with one exception: that exception is their beaks. Their beaks are the most visible difference between the 13 species, and are what makes them so interesting to evolutionary biologists. The sizes and shapes of their beaks range from large, broad, almost parrot-like beaks to small, very thin beaks.
    So you may be wondering how can we explain these variations and different gradations? Well, it’s primarily a matter of food specialization. The finches with the broad beaks, with the parrot-like beaks, usually feed for at least part of the year on either large, hard fruit or large seeds and other types of hard foods. The finches with the small beaks feed mostly on insects, on juicy, pulpy fruit or on small types of seeds. They’ve adapted, you see, to fill various ecological niches. In this case, the niches are various sources of food.
    When Charles Darwin traveled to the Galapagos Islands in 1835, he was immediately struck by the perfect gradations in the size and shape of the beaks.
    There’s a famous passage in his diary, which later on, biographers identified as showing a moment that changed Darwin’s thinking ... pushed him in the direction of a theory of evolution. Darwin had noted it was possible to imagine a point in history when one species of finch had somehow made it to the Galapagos from South America, and maybe from that point forward finch beaks evolved for different purposes. I should say here that even though the finches may have provided the spark for Darwin’s theory of evolution, it was 24 years later when he published his work on the subject. It certainly took a long time for the idea to crystalize.
    Ok, anyway, because of Darwin, many evolutionary biologists worldwide have studied the finches: all aspects of their behavior and their biology. But perhaps the longest lasting, most methodical study was done by a couple, Peter and Rosemary Grant, who spent most of their summers from roughly the 1970s well into the 1990s working on one of the smaller Galapagos Islands, an island called Daphne Major, There they observed the finches and their behavior in minute detail, down to the level of individual finches, taking many measurements and getting to know the finch population on that island to a degree no other biologists ever had before. Among the things the Grants were trying to determine were the evolutionary pressures that may have affected the occurrence of certain traits in the entire population over time, Major environmental events like droughts or shortages of a preferred food source could have caused changes to the size, and maybe even shape of the finches’ beaks over time. Had such events occurred in the past? Were events like these still occurring?
    As for the changes themselves that these events might have produced, the Grants didn’t actually expect to find them in live populations of the finches because, like everyone else, they thought that evolution was a process that unfolded over thousands, or hundreds of thousands, or millions of years. But to their surprise, and the surprise of evolutionary biologists everywhere, the Grants did find evidence, barely detectable evidence. We’re talking about changes to the average shapes and sizes of beaks that were only measurable in millimeters, so that’s very small, but they did indicate that evolution was occurring in measurable ways with each finch generation, and these changes correlated well to observable, ecological or environmental, factors. It was a significant finding, so now, what we’re going to spend the rest of today’s class doing is examining some of the data the Grants collected, and consider how they used it.