return to
Lines of Evidence
for Past Change

The Importance of Variation

1. Variation within a population


Each team examined a population of fossils - all the same species, a single population, collected at the same time. All showed variation within the population. Teachers were asked to consider the following:

  • What are some similarities, what are some differences?

  • What features (characters) can we use to measure similarities or differences

  • What are your population "subgroups" and how did you determine them?


Sand dollars

bell curve distribution of size

color variation

ratio of height/width consistent


Clams/bivalve (A)


curve of ridges vary


size 1-4 cm - wide variation of size


Snails (A)


4-6 cm size

color: pale to gray

spire, spiral in different directions

hatch marks of different types


Clams/bivalves (B)


1-6 cm sizes

two different colors - gray and pink




1.75 cm - 3.5 cm in length

different shades

consistency in striping distance and number

texture varied from smooth to grainy, but grainy only on big ones


Snails (B)


four types: varied in size from .5 cm - 3 cm

three very different looking, like moon snails and were all < 1 cm

the rest were conical and similar, but striations running in different directions

opening different shapes - some more elongated, some more round

Why is variation important within a population?

With variation, individuals will be different from one another and those differences allow some to survive better than others. With variation, selection has something to work on.

More on Variation: Variation and Selection | Activity: Candy Dish Selection | Variation and Genetic Drift

Return to: Lines of Evidence for Past Change | Dynamic Earth Homepage | UCMP Homepege