In How New Species Form populations stopped exchanging genes and drifted apart. Zoom out to the fossil record and trees built from data, and patterns appear in the pace of speciation, in bursts of new species, and in extinctions.
- Gradualism: species change slowly and steadily over long times.
- Punctuated equilibrium: long periods of little change (stasis), broken by relatively rapid change, often when a new species branches off. Niles Eldredge and Stephen Jay Gould proposed it in 1972, from patterns in fossils. Both patterns occur in the record.
- Adaptive radiation: one ancestor gives rise to many species that fill different niches, often after reaching new land (Darwin's finches on the Galápagos, honeycreepers in Hawaii) or after a mass extinction opens niches.
- Extinction: most species that ever lived are extinct. A steady background rate is punctuated by five mass extinctions. The largest, at the end of the Permian about 252 million years ago, wiped out most marine species. After the end-Cretaceous extinction about 66 million years ago, which ended the non-bird dinosaurs, mammals radiated into many new forms.
Key idea
Speciation can be gradual or come in bursts after long stable periods (punctuated equilibrium). Adaptive radiation fills open niches quickly, often after a mass extinction. Extinction is a normal part of life's history, at a background rate and in rare mass events.
Why it works
Why would change come in bursts? A widespread, well-adapted species can stay stable for a long time: stabilizing selection and gene flow hold it in place. A small population cut off at the edge of the range can change faster (with drift and new selection) and may become a new species in a geologically short time. Opening niches has a similar effect: with no competitors, small variations that let individuals use a new food or habitat pay off quickly.
Worked examples
Worked example: What does "rapid" mean?
A snail species shows no change in shell shape for 4 million years, then a new form appears over about 40,000 years and stays stable. Is that gradual or punctuated?
- 40,000 years is 1% of the 4 million years of stasis.
- On that scale, the change is a short burst between long stable periods.
Answer: Punctuated equilibrium. "Rapid" means tens of thousands of years, quick only against millions of years.
Worked example: Reading a diversity curve
Counts of marine animal families in the fossil record fall from about 400 to about 200 over a short interval, then climb to 300 over the next 20 million years. Describe the two events.
- A drop of 200 out of 400 is a loss of half the families: a mass extinction.
- The climb afterward is a radiation into niches left empty.
Answer: A mass extinction (about 50% of families lost), followed by an adaptive radiation.
Common mistake
Thinking punctuated equilibrium means species appear overnight. "Rapid" here means tens of thousands of years, quick only against millions of years of stability. Neither pattern needs anything beyond ordinary mechanisms: mutation, selection, drift and the isolation of populations.