Key concepts in Variation

As stated previously, for evolution to occur, the variation has to be heritable. If all the individuals in a population are genetically identical, and they produce offspring that are genetically identical, evolution hasn't occurred, because the next generation is the same genetically as the current generation.

Variation exists in natural populations, and there's often a lot of it:

1 Variation within a species: All species have variation. Just thinking of the variations within our own species is a good place to start. It would take at least 100 books this size to list all the ways that people are genetically different from one another. An important thing to remember, though, is that the variation within a species doesn't need to be uniformly distributed across all population in that species. The different populations will all have genetic variability, but they all won't always have the same genetic patterns. For example, a species may contain individuals of many different heights, but not all populations within that species will have individuals of all heights; some populations may be made up of, on average, taller individuals than others.

1 Variation within populations: It's the variability within a population that allows the population to respond to natural selection. Take the finches in the Galapagos. Within one finch population that was studied, there was existing variation in beak size. When other birds arrived on the island and began eating the food these finches relied on, this existing variation allowed that species to evolve to better utilize a smaller seed resource.

1 Variations between populations: It's the variation between populations that tells us that two populations have evolved in different ways. The populations are genetically different from one another, meaning that gene flow between the populations hasn't been able to overwhelm the different evolutionary forces they experience. That different alleles have increased or decreased in the different populations could be the result of natural selection (discussed in Chapter 5), random factors (the topic in Chapter 6), of both. If two populations continue to become genetically different, they can become too different to interbreed, a topic addressed in Chapter 8.


In evolution, the relevant variation occurs at the group or species level. You cannot talk about variation unless you have more than one individual. If you have variation in the genetic composition of the group or species, the next generation can be different. That's evolution.

A variety of mechanisms can cause the genetic makeup of the next generation to differ from the genetic makeup of the current generation. Natural selection (Chapter 5) and random factors (Chapter 6) are two immediate examples. These mechanisms change the genetic makeup of a population from one generation to the next: natural selection, because some genetic types have better reproductive success than others, and genetic drift, just because sometimes stuff happens — a fire kills many blue-eyed deer, leaving fewer blue-eyed deer to make more blue-eyed deer.

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