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Critter Comparison

Science Snack
Critter Comparison
Observe physical and behavioral differences in locally collected creatures.
Critter Comparison
Observe physical and behavioral differences in locally collected creatures.

Take an inventory of a variety of traits in a group of living specimens and discover that even small inherited differences have the potential to impact a population’s long-term survival and evolution.

Note that this Snack can be easily scaled from a single group of three or four participants to a large class made up of many smaller groups.

Tools and Materials
  • Samples of individuals from a wild population of organisms, such as land snails, pill bugs, wood lice, small fish, beetles, or similar (alternatively, you can order organisms from a biological supply house); you’ll need one set of at least five specimens of the same organism for each group of three to four participants
  • Plastic boxes or other clear enclosures (to contain organisms)
  • Tools for measuring (rulers, tape measures, scales, stopwatches, etc.), one set for each group
  • Tools for observation (magnifying glasses, flashlights, etc.), one set for each group
  • Tools for recording results (paper, pencils, calculators, etc.), one set for each group
  • Tools to conduct behavioral experiments (spray bottles filled with water, food items, black paper, etc.), one set for each group
  1. Collect sample organisms the night before or on the morning of your observations. Look in open fields, in gardens, ponds, under rocks, and near decomposing vegetation. Place specimens in clear containers for observation and experimentation.
  2. Divide participants into small groups of three or four.
  3. Give one set of specimens to each group of participants.
  4. Distribute tools and materials to each group of participants.
To Do and Notice

Begin by observing specimens in their enclosures. Do they all appear identical? How do their physical traits differ? Do you see any behavioral differences among them? Do you think these traits are inherited (genetic), caused by their environment, or influenced by both genetics and the environment?

Choose three or more anatomical traits that may be genetically inherited, and quantify (measure) the trait among your specimens, or design a behavioral experiment to assess variation. If you were studying snails, for example, some possible traits to explore might include length of foot, length of antennae when extended, color and pattern of shell, food preference, attraction or repulsion to water or light, and speed of movement, among many other possibilities. For example, the length of the entire salamander is being measured in the photo below.

Of the heritable traits you’ve studied, are any variants more prevalent in the population? Can you think of how some trait variants could be advantageous to the individual in its environment? Are there any traits you’ve observed that could be a disadvantage to the animals?

Conduct measurements and experiments on at least five specimens of the same organism. Record your observations and experimental results, and share with other groups.

What's Going On?

Genetic variation is the raw material of evolution and exists among individuals of the same species within a population. While some traits are due to behavior or the environment (tattoos, dyed hair, sunburn), genetically inherited traits are caused by a change in the DNA of an individual (a mutation). Depending on the environment, this variation could be advantageous, detrimental, or simply neutral.

It’s important to remember that individuals don’t evolve—populations do. If environmental conditions change (climate, food availability, introduction of a new type of predator or disease, for example), natural selection will favor those individuals whose genetic variations allow them to survive and reproduce fertile offspring. This favorable genetic variation will be passed along to offspring, who will thrive, reproduce, and likewise pass along this trait. Over time, the genes for this trait will become more predominant in the population, as individuals without this favorable genetic variation become fewer and reproduce less.

With the passage of time, natural selection—and possibly other evolutionary mechanisms—may cause a new species to emerge from an original population.

Going Further

Genetic variation within a species confers resilience. Confronted by a novel disease, new predator, or serious change in habitat, a population of organisms with little genetic variation can be at high risk of extinction.

Habitat destruction, global climate change, human poaching, and other factors have decreased populations of many organisms to dangerous levels. Many of these at-risk species—including cheetahs in Africa, Tasmanian devils in Australia, North American Prairie chickens and Channel Island foxes in the United States—are so few in number that they lack the genetic diversity that might help them survive.

Even cultivated crops can suffer from low genetic diversity: 99 percent of commercially sold bananas worldwide are of one variety, the Cavendish, and are nearly genetically identical. These bananas are being attacked by a fungus to which they have no resistance, so the long-term outlook for the beloved Cavendish banana is dim.

Consider conditions where you live: Which animals or plants in your area are endangered? What factors have led to this? Has their genetic diversity been studied? What do you think will happen to them in the next ten years? The next fifty years?

Teaching Tips

This Snack is an excellent way to introduce students to the idea of genetic variation and its significance in evolution.

This activity works best with several groups of explorers and with many specimens to observe. More specimens represent a larger population and a greater chance of recording variation.

When discussing whether a variation is due to genetics, environment, or a combination, ask students to consider the effects of age and sex on their specimens.

This exploration can also be conducted by comparing easily seen physical traits among students, such as skin color, hair color, eye color, and so on. Traits in humans that are environmental or behavioral can be easily seen, such as hairstyles, piercings, eye color changes due to contact lenses, and more. Most visible human traits are polygenic (influenced by multiple genes), and there is often an environmental or behavioral component as well. Most genetic variation is invisible, as it occurs at the cellular and biochemical level.


The following websites provide fundamental and extensive information about biological evolution, and include teacher resources and evolution in the news:

University of California, Museum of Paleontology Understanding Evolution

Genetic Science Learning Center; Conservation Genetics

National Center for Science Education

Public Broadcasting System (PBS) Evolution

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