A doctor prescribes an antibiotic that used to cure an infection easily, and this time it barely works. Nobody designed a stronger bacterium, yet the bacteria have changed. The process behind this is the same one that shaped the snowshoe hare's winter coat and the giraffe's neck: natural selection. It is not a force with goals. It is simply what happens, automatically, whenever individuals differ in inherited traits and those differences affect how many offspring they leave.
The four conditions for natural selection
- Variation: individuals in a population differ in their traits (colour, size, resistance to a toxin), and much of that variation is caused by differences in their genes. New variation arises from mutation and from the shuffling of alleles in sexual reproduction.
- Overproduction: organisms produce more offspring than the environment can support. A single cod or frog can release thousands of eggs, yet a stable population means that, on average, only enough survive to replace the parents.
- Selection: because resources are limited, individuals compete, and those whose inherited traits suit the current environment are more likely to survive and reproduce. This is differential survival and reproduction, not a random lottery.
- Inheritance: survivors pass their advantageous alleles to their offspring. Over many generations, those alleles, and the traits they produce, become more common in the population.
A classic example is the peppered moth, Biston betularia, in England. The species has a light, speckled form and a dark form, and the difference is inherited. During the Industrial Revolution, soot darkened tree trunks near cities. Light moths resting on dark bark were easy for birds to spot, while dark moths were hidden, so dark moths survived and reproduced more, and the dark form became common in industrial areas. After clean-air laws reduced soot, trunks became lighter again and the light form became common once more. The environment changed, so the direction of selection changed.
| Pattern of selection | Which individuals are favoured | Effect on the population | Example |
|---|---|---|---|
| Directional | One extreme of the trait | The average shifts toward that extreme | Dark peppered moths favoured on soot-darkened trees |
| Stabilizing | Intermediate values | Variation narrows around the average | Human birth weight: very low and very high birth weights have historically had lower survival |
| Disruptive | Both extremes | The population may split into two distinct groups | Birds where only very small and very large seeds are available favour small and large beaks over medium ones |
Adaptations
- An adaptation is an inherited trait that improves an organism's survival or reproduction in its environment. Adaptations can be structural (a body feature, such as the thick fur of an Arctic fox), behavioural (an action, such as seasonal migration in Canada geese) or physiological (a body process, such as the wood frog's ability to survive being partly frozen in winter).
- Camouflage lets an organism blend into its surroundings. The snowshoe hare moults into a white coat in winter and a brown coat in summer, matching snow and forest floor.
- Warning colouration advertises that an organism is dangerous or bad-tasting, such as the black-and-white stripes of a skunk. Predators learn to avoid it.
- Batesian mimicry: a harmless species resembles a harmful one. Many hoverflies have yellow-and-black stripes like wasps but cannot sting; predators that have learned to avoid wasps also avoid the hoverflies.
- Müllerian mimicry: two or more harmful species resemble each other. Many stinging bees and wasps share similar yellow-and-black patterns, so a predator that learns from one species avoids all of them, and every species benefits.
Antibiotic and pesticide resistance
Bacterial populations are enormous and reproduce quickly, so natural selection can be seen within years. In a large population, a few cells may already carry a gene that lets them survive an antibiotic, either because of a random mutation or because they received the gene from another bacterium. When the antibiotic is used, susceptible cells die and resistant cells survive and multiply, so the population becomes mostly resistant. MRSA (methicillin-resistant Staphylococcus aureus) is a well-known example. Using antibiotics when they are not needed, including for viral illnesses that antibiotics cannot treat, and routine use in livestock increase how often resistant bacteria are selected. The same logic explains pesticide resistance in insects and herbicide resistance in weeds: repeated spraying kills susceptible individuals and leaves resistant ones to breed.
Artificial selection
In artificial selection (selective breeding), humans, not the environment, choose which individuals reproduce. Every breed of dog descends from wolves; modern corn was bred from a wild Mexican grass called teosinte; and kale, cabbage, Brussels sprouts, broccoli, cauliflower and kohlrabi were all bred from one wild mustard species, Brassica oleracea, by selecting for large leaves, large buds, flower clusters or thick stems. Canola was developed in Canada from rapeseed by breeding for an oil and meal that are healthier for people and livestock. Artificial selection works exactly like natural selection (variation, selection and inheritance), which is why Darwin used it as evidence that selection can change species.
- Higher crop yields and faster-growing livestock increase food supply.
- Crops can be bred for disease resistance, drought tolerance or better nutrition.
- Disease-resistant varieties can reduce the need for pesticide sprays.
- Uses existing natural variation, with no need to insert genes from other species.
- Selecting from a few individuals reduces genetic diversity.
- Genetically uniform crops (monocultures) can be wiped out by a single disease, as in the Irish potato famine of the 1840s.
- Traits chosen for human preference can harm the animal, such as breathing problems in some flat-faced dog breeds.
- Wild relatives and older varieties, which hold useful genes, may be lost if they are no longer grown.