7.1 Introduction to Natural Selection

Evolution is change in a population's gene pool (all the alleles its members carry) from one generation to the next, for example a change in how common an allele is. Natural selection is one of several mechanisms that cause it (others, like genetic drift, are in 7.4). Charles Darwin laid out the idea in On the Origin of Species in 1859.

Natural selection follows from a few observations that are easy to check:

• Organisms produce more offspring than the environment can support, so individuals compete for limited resources like food, water, space and mates.
• Individuals in a population vary in their traits.
• Many of those differences are heritable, passed from parents to offspring through genes.
• Individuals whose traits suit the environment better are more likely to survive and reproduce (differential survival and reproduction).
• So the favorable heritable traits become more common in the next generation, and the population changes over time.

In biology, fitness is measured by reproductive success: how many surviving, fertile offspring an individual contributes to the next generation compared with others in the population. It's not about strength or speed. A strong animal that never reproduces has a fitness of zero, while a small, plain one that raises many offspring has high fitness. Survival matters only because it usually gives an organism more chances to reproduce.

An adaptation is a heritable trait that increases fitness in a particular environment, such as camouflage coloring, a beak shape suited to available food, or the ability to digest a new food source.

Selection happens to individuals, because each one survives or doesn't and reproduces or doesn't. But an individual's genes don't change during its life in response to the environment. What changes is the population: the proportion of individuals carrying certain alleles goes up or down over generations. Phrases like 'the giraffe stretched its neck so its offspring had longer necks' describe Lamarck's discredited idea of inheriting acquired traits, and they lose points on the exam.

Both the biotic environment (living factors such as predators, prey, competitors and pathogens) and the abiotic environment (nonliving factors such as temperature, rainfall and pH) can change. When they do, the rate and direction of evolution can change too. A trait favored one year may be neutral or harmful the next.

Medium ground finches on the Galápagos island of Daphne Major show this. During a severe drought in 1977, small soft seeds ran out, and birds with deeper beaks that could crack large hard seeds survived better. The next generation had deeper beaks on average. In later years with different conditions, smaller beaks were favored instead. Selection tracked the environment year by year.