Concepts
Evolution
Evolution. Charles Darwin. Survival of the fittest. Only the strong survive. We all know what evolution is, or at least we think we know. Oh, and it is only a theory. Charles Darwin published his theory for how natural selection drives evolution in 1859 and there has been much scholarship on it ever since. Instead of going back to Darwin’s work “On the Origin of Species” we will use noted University of Chicago professor Jerry Coyne’s definition of evolution: the theory of evolution can be summarized as so, “Life on earth evolved gradually beginning with one primitive species - perhaps a self-replicating molecule - that lived more than 3.5 billion years ago; it branched out over time, throwing off many new and diverse species; and the mechanism for most (but not all) of evolutionary change is natural selection.”1 Organisms change over time. Not just in our lifetimes as we are born, grow and die, but over generations as we, as a species, adapt to our surroundings. Organisms cannot decide to evolve nor does evolution involve any effort on the part of the organisms. How and why does evolution occur?
Natural Selection
The major engine of evolution is natural selection, “A process in nature in which organisms possessing certain genotypic characteristics that make them better adjusted to an environment tend to survive, reproduce, increase in number or frequency, and therefore, are able to transmit and perpetuate their essential genotypic qualities to succeeding generations.”2
The website Understanding Evolution from the University of California has a simple illustration of natural selection. Imagine there is a population of beetles some brown and some green. The green ones get eaten by birds because they are more readily seen. More brown ones will survive to reproduce thereby creating more brown beetles because color is hereditary. Over time, the green beetles will disappear and only brown ones will remain.3
We hear the phrase “survival of the fittest” and we assume that the fittest is the strongest, but fitness is not necessarily physical. The Understanding Evolution site defines fitness as: “Fitness is a handy concept because it lumps everything that matters to natural selection (survival, mate-finding, reproduction) into one idea. The fittest individual is not necessarily the strongest, fastest, or biggest. A genotype's fitness includes its ability to survive, find a mate, produce offspring — and ultimately leave its genes in the next generation.”4 (A genotype, in case you were wondering, is the genes that make up an organism, the genes that cause all of an organism’s traits). Do students see “survival of the fittest” in their lives?
And what causes these genetic differences between individuals? The cause is mutation, defined by the Understanding Evolution site as, “a sudden departure from the parent type in one or more heritable characteristics, caused by a change in a gene or a chromosome.”5 Most mutations are either harmful or neutral, but on rare occasions, a mutation might prove useful or beneficial to the organism. If the mutation is beneficial, as the organism reproduces the trait will become more prevalent in the next generation and eventually spread throughout the population. We call this beneficial mutation that has spread to become a typical part of the species an adaptation. It is important to remember that mutations are changes in traits that already exist in the species; they almost never create brand new features. If your species has four appendages, two can evolve into wings but you cannot grow a set of wings where appendages are not present already. You cannot have six limbs; your developmental plan limits your appendages to four.6 The bottom line? All evolution starts with mutation of preexisting features.
Adaptations can be structural or behavioral. An example of a structural adaptation is how the beaks of some bird populations elongated over time, allowing better retrieval of flower nectar in response to living in an environment with this food resource. A behavioral adaptation is the way an organism acts in response to its environment in order to survive. A behavioral adaptation would include how some animals (whales, caribou, wildebeests, etc.) migrate every year in search of food or to birth their young in an environment with more abundant food.
Some adaptations are called exaptations: adaptations that evolved for one purpose but are used for another. This idea was put forward in 1982 by noted paleontologists Stephen Jay Gould and Elizabeth Vrba in the journal Paleobiology. We cannot necessarily tell why a feature was adapted just by looking at how it is used today. An example of an exaptation is the feather, which evolved on dinosaurs most likely for insulation and warmth but is now beneficial for flight.7
Other adaptations become vestigial: “(of certain organs or parts of organisms) having attained a simple structure and reduced size and function during the evolution of the species.”8 These vestigial adaptations are no longer beneficial but are not detrimental to the organism, either. Among vestigial traits that we have as humans are our appendix, our coccyx or tailbone, the Arrector Pili or goosebumps and wisdom teeth. In whales, you can see vestigial hind legs that are no longer connected to the spine and flightless birds such as the kiwi still have small wings.
Common Ancestor
Natural selection means that all living things (except maybe viruses but that is for another unit) share a common ancestor, even though we do not currently share any obvious traits with them. How is this possible? The evolutionary biologist John Maynard Smith used a word game to help describe how this works, going from “word” to “gene” in four mutational steps. We can use this example of transitioning between words to show how a variety of species can come from a common ancestor with this simplified form.
- lamp -> lame -> came -> cane -> cone
- lamp -> lame -> lime -> time -> tire
If we take the word “lamp” and change the p to an e (like a mutation of the genetic code) we get “lame”. Then the word mutates again to “came”. A further mutation takes us to “cane” and finally to “cone”. We have mutated the word lamp to cone. None of the letters remain the same but it is still a word. The word correlates to an organism (and the fact that it is a word and not a nonsense word means it is viable) and the letters to DNA. If, instead, we change the word “lame” to “lime” and then continue to mutate and evolve to “tire” (as in example 2), the closest common ancestor to tire and cone is lame but they still come from the word lamp. So whatever single cell organism that started the whole process (lamp in this example) used the same building blocks in its DNA as modern organisms (cone and tire) and, over almost 4 billion years, life has slowly evolved into the variety of species we have today.
Theory
We should also discuss what a theory is. In our everyday usage, a theory is an idea or a guess about what we might think about something. A scientific theory is different, it is “a coherent group of propositions formulated to explain a group of facts or phenomena in the natural world and repeatedly confirmed through experiment or observation.”9 A scientific theory is not a “guess”, it is a collection of facts. The ideas that underlie these facts are tested through observation and experimentation.
Facts are introduced in 1st grade through our Reading and Writing curriculum. We learn about “Fact and Opinion” but, unfortunately, many students see fact and opinion as a dichotomy. Something is either a fact or an opinion. When they come up against a “fact” that is not true they have a problem identifying it. I will make sure that when we are discussing facts that I stress that facts are true, that they are observed and/or proven. Information is not a fact just because it is not an opinion. It is a fact if it is true. We can receive information that is not an opinion but is also not a fact, either.
David Young in The Discovery of Evolution states that the theory of evolution seeks to explain the large diversity of plants and animals on Earth and how this diversity came about, by providing accurate information on the changes in the history of life that account for the diversity. Inferences about what happened in the past are based on evidence that is found in the present, i.e. the fossil record. The process of natural selection is the mechanism that provides well-adapted organisms. Evidence for natural selection is seen in the processes of life that are being played out, in the natural world, at the present time.10
Time
Time is a big part of my students understanding of this process. In our organisms science unit, we see the life cycle, if we are lucky, of the guppy. This plays out in weeks as we get male and female guppies segregated from one another. We combine them in an aquarium and a few weeks later (if they aren’t eaten by the adults!) we see smaller versions of the adults swimming around in the tank. Eventually, guppies die and we witness the life cycle from birth through adulthood to death. Children understand the concept of a year, we have 12 months, 365 days, 24 hours in a day, 60 minutes in an hour, and 60 seconds in a minute. They may not be able to tell time but they understand that it exists and moves forward, however slowly that may feel. When we talk about evolution, we will be talking about numbers that are beyond the comprehension of most 1st graders and quite possibly their teachers, as well.
There is a YouTube video (see references for a link to the video) that gives a physical representation of time using blocks. It starts with 1 block the size of a wooden toy block, then ten, then 100 organized in a flat just like in the base 10 blocks that we use in Math. It then starts adding more blocks to show larger and much larger numbers. One million blocks are the size of a house and one billion the size of an apartment building.
Fossils
If present day organisms (or, for that matter, extinct organisms) have evolved from early life forms we should see some proof of this in the fossil record. While we may have heard the term before, what exactly is a fossil and how do the remains of organisms become one?
Fossils “refer to the physical evidence of former life from a period of time prior to recorded human history. This prehistoric evidence includes the fossilized remains of living organisms, impressions and moulds of their physical form, and marks/traces created in the sediment by their activities.”11
How are fossils made?
Over 99% of species that ever lived are extinct and very few are preserved as fossils and very few of those fossils are found.12 Why is that? There are a variety of ways that fossils can be formed. Let’s look at what happens when an animal dies and is turned into a fossil. Fossils are most easily formed using sediment and water, so if an animal dies and is quickly swept into a body of water before its carcass is scavenged, there is a better chance of getting a more complete fossil specimen. As the body sits on the bottom, the soft tissue of the body decomposes and the hard material, bone, shell, and/or teeth, is covered with mud and silt. Over time, the sediment that covers the bones hardens into sedimentary rock. As the now encased bones decay, the organic material in the bone is replaced by minerals in a process called petrification. Sometimes the bones decay completely away and the resulting void is filled with minerals creating a stone replica of the bone. As the earth changes, the layer of rock gets buried deeper and deeper but then tectonic activity and environmental activity such as rain, wind, and ice change the landscape again and the fossil rises to the surface and it is exposed such that an observant individual can find it.13
In order to become fossilized, animals must die in a watery environment and become buried in the mud and silt. Because of this requirement most land creatures never get the chance to become fossilized unless they die next to or within a lake, stream or ocean. Animals also need bone or other hard material in order to make a fossil, which is why there are few fossils of soft-bodied creatures such as jellyfish.
Where are fossils found?
There are three kinds of rock that are found on the earth’s surface. These rocks are igneous rocks, which cool from a molten state such as volcanic rocks; metamorphic rocks, which have been changed due to temperature or pressure such as marble; and sedimentary rocks, which are formed through an accumulation of cementing of mineral grains or by chemical precipitation at the Earth’s surface.14 The first two rock types are not formed by processes that are favorable to preserving fossils. Fossils are much more likely to be found in sedimentary rocks.
What can be made into a fossil?
Bones are not the only items that can become fossils. Eggs, footprints, skin and feather impressions, and feces also can become fossils although all these are rarer owing to their more delicate nature. Impressions made by plant material such as stems and leaves can also be fossilized.
The time it takes to fossilize a bone is not well established. Paleontologists, people who collect and study fossils, generally agree that it takes at least 10,000 years and usually much more for bone to turn to a fossil.15
As the conditions to create fossils are not easily attained, it is highly unlikely that all the transitional forms from ancestor to descendant will be found. But there is enough evidence in the fossil record to show how certain species alive today evolved.
Transitional Forms
As was noted earlier, if modern organisms evolved from earlier organisms we should find fossils showing the transitions in the organisms as they adapted to their environment. If we get to point z from point a, there should be points b through y in between. Scientists have been discovering these transitional forms - “fossils or organisms that show the intermediate states between an ancestral form and that of its descendants”16 connecting the dots to present day organisms from their earlier ancestors. We have seen how hard it is to create fossils so we cannot expect to have every transitional form from point a to point z. Scientists can infer what occurred using the fossils that have been found and the traits they show as points along a continuum. As evolutionary biologist Neil Shubin explains in the first chapter of his book Your Inner Fish, using what is known about geology, scientists at least have an idea of where to look in the world for other forms that predate or postdate those that have already been discovered. We will look at two species where we can see transitional forms today, whales and birds.
Why Whales?
Whales are mammals that live in the ocean. If you look at a whale skeleton you will find vestigial hind legs. These vestigial rear legs are evidence that whales evolved from land animals. But locomotion from legs to flukes is not the only thing that evolved. How do we know what has evolved? We have transitional forms.
In 1978, paleontologist Phil Gingerich found a 52 million year old skull, dubbed Pakicetus, which showed characteristics shared by both wolf sized carnivore land animals and Archaeocetus, the oldest known whale. These characteristics were related to hearing and the difference between directional hearing underwater by whales and the hearing associated with land animals. Pakicetus had an ear region that was the intermediate between the two. This ear region is evidence that Pakicetus was semi-aquatic.
Pakicetus courtesy of Nobu Tamura http://sinops.blogspot.com
Using a variety of scientific methods, scientists have been able to trace the evolution of Pakicetus from a semi-aquatic animal to Ambulocetus, which spent more time in estuaries comprised of a mix of fresh and saltwater, then to ancestors who moved to more of a marine environment and finally to the whales we have today. As these animals moved to saltwater, other changes occurred.
Ambulocetus courtesy of Nobu Tamura http://blogspot.com
When we look at a whale we notice that it has a blowhole at the top of its head. This blowhole has evolved from nostrils that were found at the front of the skull.17 As the ancestors of the whale transitioned to a more aquatic life, it became more useful to have their nostrils towards on top of their heads rather than in front of their skull.
Individuals who did not need to raise their head to breathe expended less energy, it also took less time to breathe and therefore an organism would spend less time at the surface where there may have been predators lurking. These individuals would have been more fit for their environment and would have reproduced more than individuals with nostrils at the front of their skull, explaining why this trait (and others) was passed on to future generations. (See reference section for web addresses for illustrations of these concepts.)
Dinosaurs to Birds
Archaeopteryx courtesy of H. Raab
When I was a child, dinosaurs fired my imagination. My favorite was Triceratops, with its three horns and a big armored plate around his head. Dinosaurs were seen in books, at museums - if you were lucky enough to live near one, or as pets on animated TV series The Flintstones. Students now have seen them in the Jurassic Park films and maybe as Jack and Annie have a run in with them in The Magic Tree House TV series. My students are not very interested in them; my non-fiction books go unread and in 10 years of teaching I have had not one “expert” on the field in my class. I hope to change that by just looking out the window and talking about the familiar animals flying in the sky.
In 1860 and 1861, first a feather and then a specimen of what was soon called Archaeopteryx, was found in limestone deposits (sedimentary rock) near Solnhofen, Germany. All together 12 specimens have been found, the most recent in 2010. This creature lived in the late Jurassic Period around 150 million years ago. Archaeopteryx is an important fossil because it shows a transitional form between the theropod dinosaur and birds of today. According to the University of California at Berkeley, “Unlike all living birds, Archaeopteryx had a full set of teeth, a rather flat sternum (“breastbone”), a long, bony tail, gastralia ("belly ribs"), and three claws on the wing which could have still been used to grasp prey (or maybe trees). However, its feathers, wings, furcula ("wishbone") and reduced fingers are all characteristics of modern birds.”18 The only obvious feature possessed by Archaeopteryx and not present in theropod dinosaurs was flight feathers. Spencer Lucas, in Dinosaurs: The Textbook, states, “To call Archaeopteryx a “feathered dinosaur”, in other words, to recognize it as an animal with essentially theropod skeleton and avian flight feathers is a reasonable conclusion”.19
We should discuss a bit of what a theropod dinosaur is. The theropod (meaning "beast-footed") dinosaurs are a diverse group of bipedal dinosaurs which include such heavy hitters as Tyrannosaurus rex and Velociraptor. According to the University of California at Berkeley, “Recent studies have conclusively shown that birds are actually the descendants of small nonflying theropods. Thus when people say that dinosaurs are extinct, they are technically not correct.”20
In 1998, fossil finds in Northeastern China made headlines. A Chinese farmer had found fossilized evidence of early birds. These finds continued as the shale and limestone prehistoric lake beds were scoured for fossils. Fossils for Sinornis, a sparrow sized early bird, were found and showed that it still had teeth, a clawed hand, and limited fused bones like its theropod fore-bearers but it flew and perched much like a modern bird. Confusciusornis was discovered and became the earliest known example of a toothless bird but it still had a clawed hand and other dinosaur features.21 As the generations progressed, more birdlike adaptations replaced dinosaur systems so the teeth disappeared, the hand or claw went away, bones become more hollow and lighter and they fused together to make flight more efficient. The breastbone and musculature changed and so did arm length as the major method of locomotion changed from running to flying. Feather length and symmetry changed as natural selection favored the traits that were better for flight and thereby survival. Some features, such as the wishbone and the s-curved neck and long shoulder bones, remained from the theropods giving clues to the ancient ancestry of our modern birds.
Sinornis courtesy of Pavel.Riha.CB
Adding to the proof for the relationship between theropods and birds is the connection found between Tyrannosaurus rex and the modern chicken. In 2003, a 68-million-year old Tyrannosaurus thigh bone was discovered in Montana. What was special about this bone was that it still had collagen in it. After analysis, scientists determined that the collagen make-up in the T-Rex bone is almost identical to that of the modern chicken.
Pokémon
To illustrate what evolution is and what it is not, we will reference the popular game of Pokémon. As I am writing this unit in 2016, there is an international craze for the app Pokémon Go. Many people, from children to adults, are obsessed with capturing Pokémon in their houses, neighborhoods, and city streets. For those reading this after the craze has died down, I will explain the premise of Pokémon.
Pokémon are imaginary creatures which are said to evolve. Players capture their Pokémon with Pokéballs and hold them and, over time, through various ways like gaining experience or trading items they “evolve” new features and skills that help them to battle. Each iteration grows larger and changes features until the “cute” little lizard like creature, Charmander becomes a larger more dangerous Charmeleon and then, later, a large flying dragon, Charizard.22 Although this change is called evolution and our students will likely have referenced Pokémon when first discussing evolution, the creatures have not evolved. Evolutionary biologist Richard Lenski in How Evolution Shapes Our Lives defines evolution as “the process by which life changes from one generation to the next and from one geological epoch to another.”23 Pokémon do not change generationally, they only change as individuals. In evolution, wings will not immediately sprout where none were before. The changes in Charmander and other Pokémon are not caused by mutation, and natural selection does not come into play. The characters undergo a change that is more akin to what happens in a life cycle, many undergoing metamorphoses which my students will study in 2nd grade.
In evolution, traits are passed on to offspring through heredity. Most Pokémon do breed and must be of compatible types but not necessarily of the same species. If two compatible Pokémon breed, the offspring, hatched from an egg, is the same species as the mother. The mother does not pass her physical traits down to the offspring. A Charizard’s offspring would be a Charmander who then would change, if experience is gained, but does not benefit from the physical changes of its parents. Of course, Pokémon being a game and not real life there are all sorts of scenarios that play out when they breed including producing different Pokémon types and inheriting knowledge and stats. For the purpose of illustrating what evolution is and is not, the fact that Pokémon do not keep their physical changes will suffice.
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