Natural Selection and the Mechanism of Evolution

Evolution is the change in the inherited characteristics of populations over generations, and natural selection is its main mechanism: individuals with traits that suit their environment survive and reproduce more successfully and pass those traits on. In brief: natural selection needs four conditions: variation among individuals, inheritance of that variation, more offspring than can survive and differences in survival and reproduction (fitness). Over generations, helpful alleles become more common in the population. Individuals do not evolve; populations do.

Natural selection explains both the diversity of life and the close fit between organisms and their environments. This guide explains variation and mutation, selection pressure, adaptation and fitness, shows how selection works in a worked example with antibiotic resistance and peppered moths, gives the historical background from Darwin and Wallace and corrects the typical misconceptions.

What is evolution?

In biology, evolution means a change in the frequency of alleles in a population from one generation to the next. Over long periods, such changes can produce new species, and all living things are related by common descent. The word does not mean progress toward a goal. Evolution has no plan, and populations change in response to the environment of the time.

Evolution acts on populations, not individuals. A giraffe does not grow a longer neck in its lifetime to reach leaves. Instead, over many generations the proportion of giraffes with long-neck alleles in the population can increase if long necks improve survival and reproduction.

Where does variation come from?

Individuals in a population differ in many traits: size, color, speed, disease resistance, behavior. Part of this variation is inherited. The sources of inherited variation are:

• Mutation: random changes in DNA that create new alleles. Most are neutral or harmful, and a few are beneficial in a particular environment. Mutations are random with respect to need: they do not occur because the organism needs them.
• Sexual reproduction: crossing over, independent assortment and random fertilization shuffle alleles into new combinations.
• Gene flow: migration of individuals between populations brings new alleles.

Variation caused only by the environment, such as a scar or the muscles built by exercise, is not inherited and does not contribute to evolution.

How does natural selection work?

Natural selection follows from four observations and two inferences.

1. Variation: individuals in a population differ in their traits.
2. Inheritance: some of this variation is passed from parents to offspring.
3. Overproduction: populations produce more offspring than the environment can support. For example, a single oak may produce thousands of acorns, and only one needs to survive to replace it.
4. Struggle for existence: resources are limited, and there is competition, predation, disease and harsh conditions, so many offspring die.
5. Differential survival and reproduction: individuals with heritable traits that are better suited to the environment tend to survive longer and leave more offspring.
6. Change over generations: the beneficial alleles become more frequent in the population, so it becomes better adapted.

The environmental factors that affect survival and reproduction are called selection pressures. They include predators, food supply, climate, disease and competition for mates. The word “natural” distinguishes this process from artificial selection by humans.

What are fitness and adaptation?

In evolution, fitness is the measure of an organism’s success in passing its genes to the next generation. It is not about strength or health, but about the number of surviving offspring. A small, weak animal that has more surviving offspring has higher fitness than a strong animal that has fewer.

An adaptation is an inherited feature that increases fitness in a particular environment. Adaptations have been shaped by natural selection over many generations.

Selection depends on the environment. A trait that is helpful in one environment may be harmful in another, and if the environment changes, different traits may be favored. Sexual selection is a special case in which traits are favored because they increase success in attracting mates, such as the tail of a peacock, even if they reduce survival.

Steps of natural selection

Worked example: peppered moths

The peppered moth (Biston betularia) in England is a well-known case. Before the Industrial Revolution, most moths were pale with dark speckles, which camouflaged them on lichen-covered tree trunks. A dark (melanic) form was rare. Around the mid-nineteenth century, the dark form became common in industrial areas, where soot killed the lichens and darkened the bark. After clean-air laws in the 1950s and later, the pale form increased again.

1. Variation: pale and dark forms exist, and color is inherited (a single gene with a dominant allele for dark).
2. Selection pressure: birds eat moths that they can see. On dark trees, pale moths are easier to see.
3. Differential survival: dark moths survive better on dark bark, so a higher proportion reaches adulthood and breeds.
4. Result: the frequency of the dark allele increases in the polluted areas.

Suppose the dark form is 2 percent of a population of 1 000 moths in the first generation. If each generation the dark moths have a survival advantage of 20 percent relative to pale ones, we can estimate the change. A relative advantage means that dark moths have 1.2 times the chance of surviving. In generation 1: 20 dark and 980 pale. After selection and reproduction: 20 × 1.2 = 24 dark and 980 pale, a proportion of 24 ÷ 1 004 = 2.39 percent. After 10 generations at this rate: 20 × 1.2¹⁰ ≈ 124 dark and 980 pale, which is 124 ÷ 1 104 ≈ 11 percent. After 20 generations: 20 × 1.2²⁰ ≈ 767, which is 767 ÷ 1 747 ≈ 44 percent. The advantage compounds each generation, and a small difference in survival produces large changes within a few decades. The numbers are illustrative and ignore dominance, population size and other effects.

Bernard Kettlewell’s experiments in the 1950s supported the explanation, although later critics raised concerns about his methods. New experiments by Michael Majerus and colleagues, published in 2012, confirmed that bird predation is the main cause of the changes.

What did Darwin and Wallace propose?

Charles Darwin developed his theory after the voyage of HMS Beagle (1831 to 1836), in which he observed the variation of species such as the finches and tortoises of the Galápagos Islands. In 1858 Alfred Russel Wallace sent Darwin an essay that outlined the same idea, and the two men’s papers were presented together to the Linnean Society of London. Darwin published On the Origin of Species in 1859.

Darwin did not know how inheritance works. The synthesis of Darwin’s ideas with Mendelian genetics in the 1930s and 1940s, called the modern synthesis, established the current understanding of evolution, which was later extended by molecular biology.

Which mistakes are common?

Mistake 1: Saying that organisms evolve because they need to

Mutations occur at random. Selection favors those that happen to be useful.

Mistake 2: Saying that individuals evolve

Individuals do not change their genes during life. Populations change in the frequency of alleles.

Mistake 3: Equating “survival of the fittest” with the strongest

Fitness means reproductive success. It can be due to camouflage, cooperation or fast breeding, not just strength.

Mistake 4: Thinking that evolution is “just a theory”

In science, a theory is a well-tested explanation supported by large amounts of evidence. Evolution is among the best-supported theories in science.