Extinction and Vulnerable Species: Environmental Change and Natural Selection

When the environment changes faster than evolution

In the 1800s, passenger pigeons flew over Ontario in flocks so large they were said to darken the sky for hours. They were one of the most abundant birds in North America. Hunting on a massive scale and the clearing of the forests they depended on drove them to extinction. The last known passenger pigeon, named Martha, died in the Cincinnati Zoo in 1914.

Evolution helps populations adapt to change, but only if there is enough genetic variation, enough time and enough individuals. When the environment changes too quickly, species can be lost. This lesson looks at extinction through the lens of natural selection.

Background and mass extinctions

Extinction is a natural part of the history of life: scientists estimate that the vast majority of species that have ever lived are now extinct. Most go extinct at a slow, steady background rate, as environments change and competitors evolve.

Five times in the last 500 million years, extinction rates soared in mass extinctions, when a large fraction of species disappeared in a geologically short time. Causes included major climate change, changes in ocean chemistry, massive volcanic eruptions and, for the end-Cretaceous event 66 million years ago, the impact of a large asteroid. Each mass extinction was followed by adaptive radiation of the survivors into the emptied niches.

Many scientists warn that extinction rates today are far above the background rate because of human activities, and some describe the current situation as a possible sixth mass extinction.

Adapt or disappear: what decides?

When the environment changes, a population can move (to a place that still suits it), adapt (by natural selection) or decline, possibly to extinction. Whether natural selection can save a population depends on:

• Speed of change vs generation time. Selection works one generation at a time. Species with short generations (insects, mice, bacteria) can pass through many generations of selection while a slow-breeding species passes through only a few.
• Genetic variation. Selection can only favour alleles that exist. Populations that have lost variation (through bottlenecks or small size) may have no individuals with helpful alleles.
• Population size. Small populations have fewer individuals that might carry rare helpful alleles, and they are more affected by drift and random disasters.
• Niche breadth. Specialists with narrow niches are hit hard when their one food or habitat disappears; generalists can switch.
• Reproductive rate. Species that produce few young (large mammals, many birds) recover slowly from losses.

Environmental change in Canada: case studies

• Snowshoe hares and shorter winters. Hares turn white in the fall based on day length, not snow. With climate warming, snow arrives later and melts earlier in many places, so more hares are white on brown ground and easier for predators to see. Researchers have found that some populations show variation in the timing or degree of colour change. If that variation is heritable, natural selection could favour hares that change colour later or stay brown; if the change is too fast, populations may decline.
• Polar bears and sea ice. Polar bears hunt seals from sea ice. As Arctic sea ice forms later and melts earlier, bears must go longer without food. With long generations and few cubs, they cannot evolve quickly. Under SARA they are listed as a species of special concern.
• Little brown bats and white-nose syndrome. A fungal disease introduced to North America has killed huge numbers of hibernating bats. The little brown myotis is listed as endangered. Scientists are studying whether a few surviving bats carry traits that help them resist the disease, a case where natural selection is acting right now.
• Boreal caribou. Woodland caribou in the boreal forest need large areas of old forest. Roads, logging and other development fragment that habitat and make it easier for wolves to hunt caribou. The boreal population is listed as threatened.

Protecting species: science and Indigenous knowledge

In Canada, COSEWIC (a committee of experts) assesses the status of wildlife species, and the federal government decides whether to list them under the Species at Risk Act (SARA). Listed species and their critical habitat receive legal protection, and recovery strategies are prepared. Ontario also has its own species-at-risk law (the Species Conservation Act, 2025, which replaced the Endangered Species Act, 2007, in March 2026).

COSEWIC's assessments are required to consider both scientific knowledge and Aboriginal traditional knowledge. First Nations, Inuit and Métis knowledge holders have detailed, long-term observations of the land, water and animals, often going back many generations. Many Indigenous communities run Indigenous Guardians programs that monitor wildlife, water and land on their territories. The Mi'kmaw Elder Albert Marshall described Two-Eyed Seeing (Etuaptmumk): learning to see with the strengths of Indigenous knowledge in one eye and Western science in the other, and using both eyes together.