Speciation and extinctionIB MYP Biology: Revision notes
Section 1
What is a species?
A species is a group of organisms that can interbreed to produce fertile offspring. Members of different species cannot do this, or their offspring are infertile. For example, a horse and a donkey can breed, but a mule is usually infertile, so they are separate species.
A new species forms by a process called speciation.
Section 2
Speciation by isolation and selection
In geographical isolation, a population is split by a barrier such as a mountain range, a river or the sea. The groups cannot interbreed.
- The two groups live in different environments.
- Mutations are random and different in each group.
- Natural selection favours different alleles in each group, because the selection pressures are different.
- Over many generations the groups become more and more different.
- Eventually they can no longer interbreed to produce fertile offspring. They are now different species.
Darwin's finches on the Galápagos Islands are an example: birds were isolated on different islands and developed different beaks.
Always name both parts: isolation stops the groups interbreeding, and selection makes them different.
Section 3
Extinction and its causes
A species is extinct when no individuals remain alive anywhere. Extinction is a natural process, but humans have made it much faster. Main causes:
- Climate change: species cannot adapt or move quickly enough
- Habitat loss: forests cleared, wetlands drained, land built on
- New predators: introduced species such as rats, cats or pigs eat native species
- Disease: a new disease spreads and kills a large share of a population
- Hunting: over-hunting by humans, such as the dodo and passenger pigeon
Small populations and species found in one place only (such as on islands) are most at risk.
Do not write that animals 'died out because they could not adapt'. Individuals do not adapt in their lives; the population had no individuals with alleles that let them survive the change.
Section 4
Mass extinctions
A mass extinction is when a large percentage of the world's species become extinct in a geologically short time. The fossil record shows five mass extinctions in the last 500 million years.
The best known is 66 million years ago. An asteroid impact threw dust into the air, blocking sunlight. Plants could not photosynthesise, food chains collapsed, and about three-quarters of species, including the non-bird dinosaurs, died out.
After each mass extinction, the survivors had less competition. Over millions of years, new species evolved to fill the empty niches. Many scientists believe human activity is causing a sixth mass extinction.
Must Know
- Species: can interbreed to produce fertile offspring
- Speciation: isolation, different selection pressures, differences build up
- Extinction causes: climate change, habitat loss, new predators, disease, hunting
- Mass extinctions: five in the fossil record; the survivors diversify afterwards
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Speciation and extinction
- About five million years ago, a mountain range slowly rose through the middle of a forest in South America. A single population of tree frogs lived there. The mountains were too high and cold for the frogs to cross, so the frogs were divided into two groups, one on each side. The climate and the insects available as food were slightly different on the two sides.Explain why the two groups of frogs may eventually become different species.2 marks
- The dodo was a large, flightless bird that nested on the ground on the island of Mauritius. Before humans arrived it had no natural predators. After sailors arrived in the 1500s, dodos were hunted for food. The ships also brought pigs, rats and monkeys, which ate the dodos' eggs, and settlers cleared forest for farming. The last confirmed sighting of a dodo was in the 1660s, and the species became extinct.Explain why the dodo was especially vulnerable to the new predators.2 marks
- Scientists surveyed birds in fragments of rainforest in Borneo that were left when the surrounding forest was cleared. They used the same method for one morning in each fragment. A 1 hectare fragment held 8 bird species, a 10 hectare fragment 21 species, a 100 hectare fragment 45 species and a 1000 hectare fragment 70 species. Each fragment was surveyed only once, and the fragments were different distances from the large forest.Describe the trend in the number of bird species and suggest one reason for it.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).