MI BIG
Evolution III.4

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All students will explain how scientists construct and scientifically test theories concerning the origin of life and evolution of species.
All students will compare ways that living organisms are adapted (suited) to survive and reproduce in their environments and explain how species change through time.

Overview

Biological evolution is continually being challenged by individuals and organized groups. Many feel threatened by its perceived attack on their belief systems. Teachers often feel uneasy presenting a topic knowing parents or local groups may challenge them. The issue can be defused by a better understanding of the nature of science, its process and limitations. Science attempts to explain questions about the natural world. It has no means to observe, analyze, or assess the supernatural. Students need to be able to examine the evidence for evolution and reflect on what it means to them.

Essential Background Narratives

Explain how scientists construct and scientifically test theories concerning the origin of life and evolution of species.
Compare ways that living organisms are adapted to survive and reproduce in their environments and explain how species change through time.

Explain how scientists construct and scientifically test theories concerning the origin of life and evolution of species.

Are there patterns of similarity among organisms alive today? Are there relationships among organisms that lived in the past and organisms alive today? Where did humans come from in the history of the world? What is the evidence that might give us some clues to these questions? Many students come to the science classroom with strongly held beliefs that don't allow them to openly examine the scientific explanations to these questions. The challenge in the science classroom is to help students distinguish between a scientist's way of thinking and understanding from those practiced by some theologians, poets, or philosophers.

Students need working definitions of key terms with clear examples to provide a framework for their thinking. This should help them demarcate where science begins and ends. Words such as fact, theory, hypothesis, and law are terms students encounter in casual conversation in everyday life. In science these terms have a very specific meaning. A fact is an observation that has been repeatedly confirmed but facts can change. It was a scientific fact for many years that human cells had 24 pairs of chromosomes. Improved techniques of microscopy revealed that they actually have 23 pairs.

People commonly use the word theory to mean a "guess" or "hunch". "My theory is..." in common usage would be better stated as "My hypothesis is....". In science, a hypothesis is a testable statement about the natural world. A theory, in science, is a well-substantiated explanation of some aspect of the natural world. It is a powerful idea that represents our best explanation at this time. The abundance of supporting evidence makes the subsequent abandonment of a theory unlikely. Theories can be continually refined and even replaced with an alternative theory in light of new and compelling evidence.

Finally, laws are generalizations that describe phenomena whereas theories explain phenomena. The laws of thermodynamics describe what will happen under certain circumstances; thermodynamic theories explain why these events occur. A clear understanding of these terms should provide students the structure needed to begin the study of evolution.

It is important for students to be able to distinguish between evolution and the proposed mechanism, which accounts for its occurrence. Evolution is the historical change in life forms that is well substantiated and is generally accepted as fact by scientists. Students should explore the similarities of vertebrate's limb structures such as the forelimb of a chicken or bat, human, whale, cat, and lizard. They may also study similarities in the early development of vertebrate embryos. Students need an opportunity to observe, from the fossil record, kinds of organisms that once lived on earth but now are extinct such as the Petoskey stone's, Hexagonaria percarinata, trilobites, or crinoids. They can examine the apparent relatedness of fossils to one another and to organisms living today. This will begin to lay the groundwork for evidence of common ancestry.

Students are naturally drawn to questions of their own ancestry. Students can perform a simple comparison for similarities and differences when given pictures or models of a gorilla, a modern human, and a hominid fossil skull. Patterns will emerge as students discover intermediate traits and forms. Further study of the relationships of humans to other selected animal groups can be done by looking at DNA and blood protein similarities. Vestigial structures such as the appendix, tailbone, wisdom teeth, and ear muscles also give evidence to common ancestry with selected animal groups.

Patterns and evidence of change, as well as recognizing the diversity and apparent relatedness of species, needs to be firmly established before delving into possible mechanisms for evolution. To better understand natural selection students can survey the examples of artificial selection occurring today with common pets like cats and dogs, or numerous agricultural products.

Compare ways that living organisms are adapted to survive and reproduce in their environments and explain how species change through time.

Students need to understand the key components of natural selection. Variation in heritable traits can give some organisms within a species a distinct advantage over others. This variation is derived from new gene combinations or mutations. If a trait, such as sharp claws or teeth for capturing prey, or coloration for camouflage, gives an organism an advantage to survive and reproduce, those advantageous traits will be passed on to it's offspring. The advantaged offspring, in turn, are more likely to survive and reproduce. The frequency of the advantageous trait will therefore increase in the population. Some traits may give no selective advantage or disadvantage to an organism in a particular environment. If the environment changes, however, a particular trait may become advantageous. The environment may also change enough that only a few or possibly no organisms have the traits necessary for survival and the species will then become extinct.

It is important for students to understand the level at which adaptation takes place. Students should understand that organisms cannot willfully change to better fit the environment and pass those changes on to offspring. This involves a common language use of the word adapt, not a biological definition. A Lamarckian vs. a Darwinian explanation of the mechanism for change in the evolution of a giraffe's neck is a good example for students to distinguish the different levels of adaptation first proposed. Students should begin to understand that while natural selection acts on the individual, the resulting adaptation is at the population level. To further student understanding on adaptation, examples of antibiotic and pesticide resistance can be studied.