Project Description

Variation and Inheritance: Foundations of the Theory

Years of observations aboard HMS Beagle, followed by rigorous debate with colleagues and 20 years of experiments, led Darwin to propose the elegant yet revolutionary idea now known as the Theory of Evolution. The basic concepts underlying what he called his “one long argument” are surprisingly easy to understand. Variation, inheritance, selection, time, and adaptation are the foundations of the theory that unifies all of biology.

All species of organisms on Earth are variable. They may vary in traits such as size, color, shape, and pattern, as shown in the snail shells below. They may also vary in traits that are harder to see, such as behavior.

More than 1,000 seeds were removed from this sunflower (Helianthus annuus) and approximately 10% were measured to make the graph below. Most seeds are between 9 and 10 mm long, but some are larger and some are smaller. Seed size may affect the growth of the plant that sprouts from it.

In many cases variations are the result of small differences between individuals in their DNA. These differences are the result of mutations, which are mistakes that occur when DNA is copied as cells divide. DNA is the genetic material found in every cell that contains all of the instructions for life.

Mutations and Variation: In this population of brown mice, a mutation has occurred in the genetic material that controls fur color. The mouse with the mutation has tan fur instead of brown. Inheritance: When tan mice reproduce, the mutation for tan fur is passed to some of the offspring.

Offspring often inherit variations in traits from their parents. For example, eye color in humans is passed from parents to children. Only mutations present in eggs and sperm are passed from parents to offspring.

Variations in color and pattern on the shells of Arabian cowries (Cypraea arabica) provide camouflage for these marine snails.

Adult Florida fighting conchs (Strombus alatus) vary greatly in color.

Curator’s Corner

How do you tell if a trait is genetically inherited or environmentally influenced?

Achillea millefolium. Walter Siegmund, CC BY-SA 3.0, via Wikimedia Commons

In the Sierra Nevada Mountains of California, western yarrow (Archillea millefolium) plants grow at many different elevations. Plants at high elevations tend to be very short while plants at low elevations are taller. Are the high-elevations plants short because of poor growing conditions or because of a genetically based trait difference?

The test: Scientists grew seeds from low- and high-elevation plants in a greenhouse near sea level.

The result: The offspring of the high-elevation plants were still short, showing that this is largely a genetically determined trait and not just due to poor conditions at high elevations.

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