Natural Selection

Even though “natural selection” is not a topic that lends itself to take-home projects, there are so many interesting case studies/examples to explore…and some fun group activities to test our understanding of the topic. So, unlike many of our other science programs, this one was heavier on content and media, with a group activity at the end to round out our understanding of the content.

The Peppered Moth (1848 and on)

I started our discussion for the day by telling the story of the Peppered Moth. This story is one of the simplest and best representations of natural selection at work. Peppered Moths were originally ivory colored, with black specs – allowing them to blend in perfectly on the bark of trees covered with white lichen that were dominant in their environment. Around 1848, moths that were entirely black in color began to appear in the moth populations. Why was this happening? Well, scientists believed that the moths were adapting to their changing environment and as a species were selecting traits/characteristics that would help them to survive. As humans relied more and more on machines and large factories, the trees that were originally mostly white with dark specs were actually becoming completely black due to the soot and black smoke in the air. When the moths were still ivory with black specs, predators like birds were able to pick them off too easily from the trees. The only way for the moths to survive was to find a way to blend into the now mostly black trees…hence, the population started to create all black moths. By 1895 (less than 50 years from when the first black moths started to appear), the population of Peppered Moths was approximately 98% all black! Normally, it would take some species hundreds or thousands of years to make such an adaptation to their environment (which is why studying the Peppered Moths is so interesting for scientists). And in another fascinating turn of events, today, the populations of Peppered Moths are once again becoming mostly ivory with black specs. Why do you think that might happen? Well, scientists believe that now that humans are using cleaner burning fuels in factories, the trees are once again reverting to a cleaner, more natural state (with mostly white bark)…and thus the Peppered Moth must once again change color to blend into its environment to avoid predators.

Next, our group watched a highlight from a documentary by hhmi BioInteractive, “Galapagos Finch Evolution” (embedded below).

This video details the work of evolutionary biologists Rosemary and Peter Grant and their 40-year project to painstakingly document the evolution of Darwin’s famous Galapagos finches. Their pioneering studies have revealed clues as to how 13 distinct finch species arose from a single ancestral population that migrated to the islands 2 million to 3 million years ago. Also of note in the video is their research showing how quickly finch populations adapt to changes in their environment. For example, if there is a drought on an island one year, within one generation (as soon as the next baby finches are born), the finch population is already physically changing to survive better in a dryer environment.

The Galapagos are a unique environment because of the way they are isolated islands. The environment and the animals have essentially adapted to each others’ preferences and needs. For example, the most common honey bee on the island prefers yellow flowers, so most flowers are yellow. The most common bird species on the islands – by far – are finches, and each type of finch has adapted to their particular part of the island environment. You can see the differences most easily by looking at the different types of beaks each finch has and how they use their particular beak to obtain food. Some finches have narrow, pointy beaks, perfect for getting nectar out of flowers or bugs out of tiny crevices. Other finches have more rounded beaks best used for pounding on trees or logs to extract food. One type of woodpecker finch even creates a straw tool to help him pull worms and grubs out of holes that he creates in trees!

One important part of learning about how species adapt to their changing environment through natural selection is also understanding what happens when they are unable to adapt. According to the Yale Peabody Museum of Natural History (New Haven, CT), there have been 3 species of Connecticut birds in the past 100 years that have become extinct due to a variety of reasons: the Labrador Duck (extinct 1878), the Passenger Pigeon (extinct 1914), and the Heath Hen (extinct 1932). For each bird, the reasons for extinction were a little different.

The Labrador Duck was unable to maintain its regular diet made up almost entirely of mollusks (which humans began to eat), and was not able to adapt and find another food source. The Passenger Pigeon was, among other things, aggressively hunted for its meat (yes, we used to eat pigeon meat…). And the Heath Hen was heavily hunted, and then despite preservation efforts, was unable to survive due to a series of illnesses, etc. that hit the population. [Interestingly enough, the Heath Hen is the first species that our country ever tried to actively save with preservation efforts!].

Tardigrade

We also discussed the ever-popular species that “cannot be killed”: the Tardigrade (aka “Moss Piglet” or “Water Bear”). Such a weird, curious looking critter that at most will get to be 1.5 millimeters in length. All different kinds of Tardigrades are found near water, usually living on moss or lichen. Scientists have done their darnedest to see what kinds of environmental extremes the Tardigrade can endure. According to Nicola Davies in her book, Extreme Animals: The Toughest Creatures on Earth, scientists have frozen, boiled, squashed, zapped, and poisoned Tardigrades at extreme levels…and still they survive unharmed! And I bet you thought that Cockroaches were the toughest creatures around!

Our handmade “beaks”

Now, for the main group activity of the day! Linking back to our discussion about the different finch species on the Galapagos, our SEC scientists were provided with different tools that represented different types of bird beaks (see PBS’s Fetch program description for “Eat Like A Bird“). Our scientists were given one of the following tools to use: plastic spoons, plastic forks, tongs made from craft sticks, toothpicks, or clothespins. About twelve feet from the starting line were several large foil cookie sheets containing a variety of “food” items: crumpled paper, lima beans, kidney beans, rubber bands, paper clips, mini marshmallows, and M&Ms. When I started the clock, each team ran a relay race for about 2 minutes. Each scientist was asked to run to the “food” trays, use their tool/beak (no hands allowed!), and grab a single item of food to carry back to their cup. When time was up, each scientist tallied the food items in their cups so we could see what items they were able to collect with their particular tool (in other words, their “beak”). When all was done, our scientists found that the most difficult to use tools/beaks were the toothpicks, followed closely by clothespins, both tools only able to collect a limited number of specific items. The tongs, forks, and spoons were able to collect a wide variety of “food” items.

Our thoughts? Well, a bird that has a narrow/pointy beak similar to our toothpicks probably has a limited selection of food for their regular diet. And if that food item were to disappear from their environment (like the mollusks disappeared for the Labrador Duck), the species could very well have a difficult time adapting and might become extinct. Birds that have beaks shaped like tongs or spoons have a greater selection of food sources and thus could adapt better to changes in their environments.