Extinction and the Pace of Evolution

Essential Question

Why have the rates of speciation and extinction changed over Earth’s history?

More than 99% of the species that have ever lived are extinct. Extinction is a normal part of evolution, but at times it has happened far faster than usual. In this lesson, you’ll look at the five largest mass extinctions, see how life recovers, compare two ideas about the pace of evolutionary change, and compare past extinction rates with those of today.

Background and Mass Extinctions

• Extinction: the loss of a species
• Background rate: the low, steady normal rate
• Mass extinction: a large share of species lost quickly
• Five large mass extinctions are known

Extinction happens when the last individual of a species dies. Species have always gone extinct at a low background rate. A mass extinction is a short period in which a large share of all species disappear. The fossil record shows five major ones, shown in the graph with approximate dates and losses. The largest, at the end of the Permian about 252 million years ago, wiped out an estimated 96% of marine species. The most recent, about 66 million years ago, ended the reign of the non-avian dinosaurs.

Causes of Mass Extinctions

• Large changes in the environment
• Causes include eruptions, impacts, and climate change
• Species that cannot adapt or move are lost

Cause	How it affects living things	Example
Large volcanic eruptions	Ash and gases change the climate and acidify the oceans	Eruptions in Siberia near the end of the Permian
Asteroid impact	Dust blocks sunlight, plants fail, food webs collapse	An impact about 66 million years ago
Climate change	Habitats shift or shrink faster than species can adapt	Cooling at the end of the Ordovician
Falling sea level	Shallow-sea habitats shrink	Marine losses in the Ordovician and the Devonian

Each mass extinction was linked to a large and fast change in the environment, and often to more than one cause at the same time. Large volcanic eruptions, an asteroid impact, changes in climate, and falling sea levels have all been proposed for different events. The details are still being studied. What the events have in common is that environmental conditions changed faster than many species could adapt, and species with narrow tolerances, small ranges, or specialized diets were most at risk.

Recovery and Adaptive Radiation

• Survivors enter empty niches
• New species appear by adaptive radiation
• Recovery takes millions of years
• Mammals diversified after the dinosaurs

Mass extinctions also create opportunities. When many species disappear, niches open, and surviving groups can diversify to fill them by adaptive radiation. The graph is a simplified model of how the number of families drops sharply and then rebuilds over millions of years, even passing the level before the event. After the dinosaurs went extinct, the surviving mammals diversified into many new forms, including the ancestors of whales, bats, and primates.

Gradualism and Punctuated Equilibrium

• Gradualism: steady, slow change
• Punctuated equilibrium: long stasis, then rapid change
• Both patterns are found in the fossil record
• The rate depends on how fast the environment changes

Scientists have two models for the pace of evolutionary change. Gradualism says species change slowly and steadily. Punctuated equilibrium says species remain nearly unchanged for long periods (stasis) and change rapidly, in geological terms, when new species form. Both patterns appear in the fossil record. Evolution can speed up when the environment changes quickly, or when a small group is isolated in a new habitat, and slow down when the environment is stable.

Extinction Today

• Background rate: about 1 extinction per million species per year
• The present rate is estimated to be far higher
• Main causes are human activities

Many scientists warn that extinction rates today are far above the background rate, and some call it a sixth mass extinction. Estimates vary, but studies often find present rates 100 to 1,000 times the background rate. Unlike earlier mass extinctions, the main causes today are human activities: habitat loss as land is cleared, overharvesting of animals and plants, invasive species brought to new places, and climate change. Because new species take thousands to millions of years to form, species lost now cannot be quickly replaced.

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Worked Examples

Try each problem on your own first, then open the solution to check your work.

Example 1: Time Between Mass Extinctions

The table gives approximate dates of the five largest mass extinctions. Find the time between each pair of mass extinctions in order, and the average time between them.

Mass extinction	Millions of years ago
End-Ordovician	445
Late Devonian	372
End-Permian	252
End-Triassic	201
End-Cretaceous	66

Show the solution

1. Subtract the dates: 445 − 372 = 73; 372 − 252 = 120; 252 − 201 = 51; 201 − 66 = 135 (millions of years).
2. Average: (73 + 120 + 51 + 135) ÷ 4 = 379 ÷ 4 = about 95 million years.
3. Notice: the gaps vary from 51 to 135 million years, so mass extinctions do not happen on a regular schedule.
4. Answer: 73, 120, 51, and 135 million years; the average is about 95 million years.

Example 2: Stasis or Steady Change?

The graph shows the average shell width of a lineage of marine snails over 10 million years, using sample data. (a) What was the average shell width from 10 to 6 million years ago? (b) How much did it change between 6 and 5 million years ago? (c) Does this pattern fit gradualism or punctuated equilibrium? Explain.

Show the solution

1. (a) Early shells: about 20 mm, with almost no change.
2. (b) Change: about 28 mm − about 20 mm = about 8 mm, in 1 million years.
3. (c) Pattern: punctuated equilibrium. The snails stayed the same for millions of years (stasis), changed quickly in a short time, and then stayed the same again. Gradualism would show a slow, steady rise.
4. Answer: (a) About 20 mm. (b) About 8 mm. (c) Punctuated equilibrium: stasis, a quick change, then stasis.

Example 3: Today’s Extinction Rate

Suppose the background extinction rate is 1 species per 1,000,000 species per year. How many extinctions would you expect in 100 years in a group of 10,000 species? In the last 100 years, 12 species in this group went extinct (model numbers). How many times higher is the observed rate than the background rate?

Show the solution

1. Expected: 10,000 species × (1 extinction ÷ 1,000,000 species per year) × 100 years = 0.01 per year × 100 years = 1 extinction.
2. Compare: 12 observed ÷ 1 expected = 12 times as many.
3. Answer: 1 extinction is expected; the observed rate is 12 times higher.

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Practice

Read a graph of mass extinctions, interpret a pattern of evolutionary change, calculate an extinction rate, and explain how extinction can lead to new species. Try it on your own first, then check your work with the answer key.

• Extinction and the Pace of Evolution Practice Worksheet (PDF)
• Extinction and the Pace of Evolution Practice Answer Key (PDF)

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Lesson Slides

• 4.9 Extinction and the Pace of Evolution Slides (PDF)

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Big Ideas

• Mass extinctions remove a large share of species quickly
• Large environmental changes cause mass extinctions
• Survivors diversify into empty niches
• Gradualism and punctuated equilibrium
• Extinction rates today are far above the background rate

Back to the essential question: extinction is normal, but mass extinctions happen when the environment changes faster than species can adapt. Recovery brings adaptive radiation, as surviving groups fill empty niches, and the fossil record shows both slow change and long stasis with rapid change. Today, human activities are raising extinction rates far above the background rate. This wraps up our unit on evolution and natural selection.

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