Adaptation, Niche and Mimicry: The Language of Evolution

Built for the job

A snowshoe hare is brown in summer and white in winter. Its huge hind feet work like snowshoes, so it can run across deep snow where a lynx might sink. It looks as if the hare was designed for a Canadian winter.

Biologists explain this fit without a designer. Over many generations, hares whose inherited traits matched snowy winters survived and had more young than hares that did not. That long process is evolution, and this unit is about how it works and how we know it happened.

Evolution has its own vocabulary. This lesson builds the first set of words you will use all unit: adaptation, niche, camouflage and mimicry.

What is an adaptation?

An adaptation is an inherited trait that helps an organism survive and reproduce in its environment. Two words in that definition matter most.

• Inherited means the trait is passed from parents to offspring in their DNA. A hare does not grow big feet because it practises running on snow. It is born with them.
• Survive and reproduce means the trait increases the chance of leaving offspring. In evolution, surviving only matters because it gives you a chance to reproduce.

Biologists sort adaptations into three kinds.

• Structural adaptations are body parts, shapes or colours: a beaver's flat tail and orange, ever-growing incisors; a porcupine's quills; a snowy owl's feathered feet.
• Behavioural adaptations are inherited actions: Canada geese migrating south in the fall; black bears hibernating; a killdeer pretending to have a broken wing to lead a predator away from its nest.
• Physiological adaptations are processes inside the body: the wood frog makes glucose that acts as a natural antifreeze so it can survive being partly frozen in winter; the snowshoe hare changes coat colour as day length changes.

Some traits fit more than one category. The hare's white winter coat is structural (fur colour), but the switch to white is controlled by hormones responding to day length, which is physiological. When you classify, say what feature you are looking at.

Adaptation is not the same as acclimatization

If you move from Toronto to a town high in the Rocky Mountains, your body slowly makes more red blood cells to carry oxygen in the thinner air. That is acclimatization: a change in one individual during its life. It is not passed on to your children.

Evolution happens to populations, not individuals. An individual is born with the genes it has. What changes over generations is how common each inherited trait is in the population. This is the single most important idea in the unit, and it is the root of most mistakes about evolution.

Niche: a species' job and address

A habitat is where an organism lives. A niche is much more: it is the species' whole role in the ecosystem. A niche includes:

• where it lives (habitat, including microhabitat),
• what it eats and what eats it,
• when it is active (day, night, season),
• what conditions it can tolerate (temperature, salt, light),
• how it affects other species and the environment.

A useful saying: the habitat is the address; the niche is the job. Two species can share a habitat but not a niche. In an Ontario forest, a red-tailed hawk and a great horned owl both hunt small mammals, but the hawk hunts by day and the owl mostly at night. Ecologists have found that when two species try to use exactly the same niche, one usually out-competes the other. This is called the competitive exclusion principle. Competition for niches is one of the pressures that drives evolution.

Camouflage and mimicry

Camouflage helps an organism blend in with its background: a snowshoe hare in snow, a walking stick insect on a twig, a flounder on sand.

Mimicry is different. In mimicry, an organism resembles another species, and this fools a predator (or prey).

• In Batesian mimicry (named after the naturalist Henry Walter Bates), a harmless species resembles a harmful one. Many hoverflies have yellow and black stripes like wasps but cannot sting. Birds that have been stung by a wasp tend to avoid both.
• In Müllerian mimicry (named after Fritz Müller), two or more harmful species resemble each other. Bumble bees and yellowjackets both sting and both wear yellow-and-black warnings. A predator only has to learn the pattern once, so all the species benefit.

For many years, the viceroy butterfly was taught as a harmless Batesian mimic of the bad-tasting monarch. Later experiments found that viceroys can also taste bad to birds, so many biologists now describe the pair as Müllerian mimics. This is a good example of science changing its explanation when new evidence appears.

Survival of the fittest: what "fit" really means

The phrase survival of the fittest was coined by the philosopher Herbert Spencer, and Charles Darwin later used it too. In biology, fitness does not mean strength or speed. It means reproductive success: how many offspring an organism leaves that survive to reproduce themselves. A small, plain bird that raises six chicks is fitter than a large, beautiful bird that raises none.

So a more accurate phrase is the survival and reproduction of the best-suited. You will see in the lesson on Darwin how this idea becomes the theory of natural selection.