THE KEYS EXPERIMENT AND EVOLUTION
SOME BASIC IDEAS AND A GUIDE TO THE GUIDE
MEYLAN-2013
There is little in the faculty guide on the Keys Experiment, 2 entries. There are 25 entries under Darwin.
New background is provided here for the Keys Experiment - the Darwin section is a primer and a guide to the guide.
Naz will suggest specific activities to engage students in these readings.
General approaches: One approach to the material would be to use the Keys Experiment to consider how science works
(resource 13 part 2, 17 and 21) and Origin of species to ask students to do science by considering observations that either corroborate
or suggests rejection of a hypothesis that should explain much of what we see (everything we see?) in the natural world.
A comparative aspect of these readings might include the concept that science hopes to find broadly applicable explanations for
phenomena. The Key's experiment is part of a body of work, Island Biogeographic Theory that originally sought to explain why
biologists found different numbers of species on different islands and the relatively consistent pattern in what they found. Island
Biogeographic Theory is a fundamental part of ecology and now plays in important role in conservation biology. It has proven to be a
valuable human endeavor (it explains a lot!).
But what about Origin of Species? It also seeks to explain a large set of observations. Does it explain more or less than Island
Biogeographic Theory? How much can it explain?
If your group is the type to ask, “Why are we reading this particular text?” I suggest having them read the 8-page intro by Julian
Huxley.
The Keys Experiment.
The Keys Experiment was designed to test ideas of "Island Biogeographic Theory". This is a big idea and may help to explain why
these scientists thought it would be worthwhile to do these very large scale experiments (that might be controversial for your
students). One of the two items on the resource page is a consideration of the problematic aspects of this study (killing things!).
Island Biogeographic Theory is quite intuitive. Bigger islands can support more species that smaller islands. Islands further from the
coast have fewer species reaching them per unit time (and contributing to their diversity). MacArthur and Wilson sought to quantify
these relationships and understand the interactions of island size and distance from sources in explaining the diversity of species on
different islands. The figure below (and in the powerpoint) summarizes the relationship. The x-axis is number of species. Rate of
immigration at left is considered based on proximity of the source area (mainland) and determines the rate of arrival of new species.
Closer islands get more arrivals. Whether or not arriving species survive depends on suitable habitat. Large islands have a better
chance of providing suitable habitat and thus have a lower extinction rate of arriving species. So, a large island close to the mainland
has more species (D) than the other scenarios. The theory has far-reaching implications: when we consider that protected areas in
many places are islands in a sea of development, we can see that Island Biogeographic Theory might help us understand how the size
and proximity of protected areas might influence their value over time. The second item under Keys Experiment is also # 20 in the
Darwin resources and provides good back ground.
Formulation of the theory relied on counting species on islands. Wilson wanted to do experiments where the species were all gone
and they could watch the reestablishment of the fauna on islands that had been cleared off. The island of Krakatau was cleared by a
volcanic explosion and the biota returned. They needed a range of islands of different sizes and distances from source areas
(mainland) that were cleared off, like Krakatau. The Keys experiment simply sought to test the ideas shown in the figure above. By
removing the animals from islands of different sizes and at different distances from source areas, and then following their
repopulation, the authors were attempting to see if they could generate results that would look like those shown above in the graph.
They also wanted to test the idea that the species that made up the diversity in each case was dynamic, not static.
The Eckerd Experiment:
A cleared off piece of habitat was generated on the Eckerd campus in 1962-1963 when sand was pumped out of Boca Ciega Bay to
make new land all along the west side of campus (see powerpoint slides). For the last sixty years a portion of this island of habitat has
accumulated a biota and now constitutes the "Palm Hammock". A great diversity of plants now exists there. It is a result of the same
processes that produced the results in the keys experiment: dispersal and succession. How did those species get there? Were source
areas near or far? Was the size of the "island of new habitat" large or small? What would determine if a newly arrived species would
survive or not? How many species of plant now inhabit the "Palm Hammock." Deb Hilbert has documented 156 in a formal botanical
study for her Ford Project.
Origin of species.
I would suggest spending some time going over the basics of the concept of natural selection. I use five steps (below) to discuss the
concept. Then go on to Origin of Species and give the students a chance to test their understanding of natural selection.
-Item #4 in the faculty guide is the most comprehensive in terms of having discussion points for all four chapters.
-Item #1 also should engage students and would be particularly useful in the first day of Darwin.
-You might consider have students assigned to group that are responsible for finding support for the five steps listed below from the
various parts of the readings (would work best on Day 2).
What is Darwin’s hypothesis? [It is a mechanistic hypothesis for evolution: how evolution happens – it is not a hypothesis
about the origin of life. However, it does explain why once life appeared it might have done all that it has .
There were many observers who preceded Darwin who considered the possibility that plants and animals changed through time.
Some even proposed mechanisms that might cause the change. If evolution is change through time, Darwin did not invent or discover
evolution. What he did was to come up with the best explanation for how evolution occurs. There are a number of good references in
the faculty guide to the context of the development of Darwin’s mechanistic hypothesis (#3, #15).
A mechanistic hypothesis in five steps: Ask students to explain and give examples from the readings or
from their own experience A summary figure of these five steps.
1) All species over reproduce. That is, they produce more offspring than it would take to replace
themselves. [This is easily seen in nature and your students should be able to come up with examples. The interesting cases are large mammals
like elephants and whales, but the observation still holds.]
2) There is variation in this over-reproduction.
[This is another point that students should be able to address. Sexual reproduction
is all about mixing of genes and there should be some students in every section who should be able to add to the biology of this aspect of the
discussion. There is the interesting issue that some organisms don’t reproduce sexually and only vary due to mutation. Evolution is slower when
variation is small. This explains why very little change took place before the appearance of sexual reproduction, but once sexual reproduction
appears, the diversity of life explodes. You will have to explain to students that this is why sex is so great!]
3) Among the variants some will “do better” than others in a given environment. [This should be intuitive to
students. We measure who is doing better by Darwinian fitness. Darwinian fitness is measured by the number of offspring that reach the next
generation. (see slide in powerpoint). This is the place to bring in the important point that “Survival of the fittest” is a bad substitute name for natural
selection. Survival alone is not the key; it is survival and reproduction. Natural selection is not really a matter of survival but differential
reproductive success -- leaving more copies of successful traits].
4) Those that do better will leave more offspring. [Individuals that survive to reproductive age can reproduce.
But there will be
variation in their ability to leave offspring behind. If energy is used in ways that are unnecessary, individuals who avoid that use of energy could put
it into more offspring. This is why the tapeworm that lives in an ocean of digested food in your small intestine has lost its digestive tract].
5) The traits of those individuals that leave more offspring will become more common in the population .
[Individuals who reproduce more leave more copies of their genes in the next generation. We can now consider evolution to result from changes in
gene frequencies – but genes were not known to Darwin].
Pattern vs. process Darwin’s is a process hypothesis. If it is a valid hypothesis, observations that we make on the results of
evolution (evolutionary pattern) should be consistent with the process hypothesis. For example, Darwin predicted the existence of
transitional fossils. Perhaps your students would be interested in looking at transitional fossils: Archaeopteryx, Tiktallik, or the fossil
record in general.
Those “trees”. Phylogenetic and evolutionary trees are the ultimate pattern. If life has a single origin and evolved from that
single origin, then there is one history of live. Willi Hennig has supplied a method to reconstruct this one history of life based on the
recognition of evolutionary novelties. Everything that has hemoglobin has hemoglobin because it shares a common ancestor that had
hemoglobin. Same for four limbs, a breathing nose, the amniote egg, three middle ear bones, etc. etc. Morphologists started to use
this idea very successfully in the 1970’s. Now molecular biologists have picked it up and are also using evolutionary novelty to
reconstruct the history of life. If Darwin is right, and there is one history of life, data from any source, morphology, genetics,
behavior, etc. should produce the same tree.
Anagenesis and cladogenesis – how can evolution lead to new species? Anagenesis is gradual change within a lineage that
does not lead to splitting (Humans getting taller since we started wearing armor). Cladogenesis is splitting (speciation) normally
caused by some kind of isolation of a part of the group that had previously shared a gene pool. Allopatric or geographic speciation is
the most obvious case (think about the Galapagos). If you take a small tortoise species and some of them drift out to the Galapagos,
when they get there they will be under different selective pressure and are very likely to evolve into a different species that, if it came
into contact with the parental species, could no longer exchange genes with it.
Gradual change versus punctuation. An early view of natural selection was that all aspects of an organism might be
changing at all times – constant gradual change. Punctuated equilibrium suggests that some aspects of organisms may remain constant
for long periods and then change significantly over short periods. This change still would occur by natural selection, just over
relatively short periods of time. Mosaic evolution is the idea that some aspects of an organism may remain constant while others
change through time.
Why do we all need to understand evolution? That species change through time can no longer be debated. All you have
to do is look at the AIDS virus. Evolution is going to become a more important part of the world we live in for reasons that I am
hoping that you and your students might discover by using the separate sheet: Why does every human need to understand evolution?
Evolution, intelligent design and science. Observers seek explanations for what they see. The more universal the
explanation the more powerful. Gravity is a great example; it explains how any two bodies in the universe should be attracted to one
another. Similarly, we might expect an explanation for the diversity of life to explain all of the diversity. Can evolution explain all of
life’s diversity? Can intelligent design explain all of life’s diversity? Do you want it to? Do you want it to explain the existence of
pathogens, parasites, fly larvae that eat their mother from the inside while she is still alive, adult mammals that purposefully kill
offspring? Do you want to use evolution to explain some aspects of life and ID to explain other aspects? How will scientists feel
about multiple explanations? (See also faculty guide 8, 11, )
Summary of guide:
Keys see both items – Wilson one is also #20 in Darwin resources. Also #13 part 2
Darwin day 1: (1, 2, 3, 4, 13 part 2, 15)
Darwin day 2: (4, 12, 13 part one, 15, 25)
for added detail: ( 7, 9, 14, 16, 18, 19)