Coaching Centre
Evolution review
A summary of the whole Evolution chapter, key words, and a mixed test covering Darwin and Wallace, natural selection and allele frequency, speciation, fossil and living evidence, evolution happening today, and human evolution.
Use this page to check the whole chapter. Read the summary, make sure you can explain every key word, then do the test without looking back.
Chapter summary
| Topic | What you need to know |
|---|---|
| 2A Darwin, Wallace and natural selection | Lamarck's idea that characteristics acquired during life are inherited is rejected. Darwin's Beagle voyage (1831–36): Galápagos finches, tortoises and mockingbirds varied between islands. Artificial selection (wild mustard → kale, cabbage, broccoli, cauliflower). Malthus: more offspring than can survive. Wallace in South-East Asia: Wallace's Line. Joint papers 1858; On the Origin of Species 1859. Four conditions: variation, inheritance, overproduction and struggle, differential survival and reproduction. |
| 2B Natural selection and allele frequency | Evolution is a change in allele frequencies in a population over generations. Selection pressures; individuals do not evolve, populations do. Allele frequency = copies of the allele ÷ total copies. Peppered moth: dark form rose in soot-covered industrial areas and fell after clean air laws (bird predation confirmed by Majerus; cortex gene mutation identified in 2016). Directional, stabilising and disruptive selection. Sexual selection. |
| 2C How new species form | A species: individuals that can interbreed to produce fertile offspring. Isolating mechanisms before mating (habitat, timing, behaviour, structure) and after mating (hybrids die or are infertile, e.g. the mule). Allopatric speciation: a barrier splits a population, different mutations and selection pressures, genetic differences build up until they can no longer interbreed. Sympatric speciation. Adaptive radiation (Darwin's finches, Australian marsupials). |
| 2D Evidence from fossils | Fossils form mainly in sedimentary rock; the record is incomplete. The order of fossils in the rock layers; the geological time scale (Paleogene and Neogene replace "Tertiary"). Transitional fossils: Tiktaalik, Acanthostega, Archaeopteryx, the whale series from Pakicetus to modern whales. Predictions tested by digging in rocks of the right age. Australian fossils: Steropodon from Lightning Ridge, Riversleigh, megafauna such as Diprotodon. Living fossils. |
| 2E Evidence from living things | Homologous structures (pentadactyl limb) show common ancestry; analogous structures (bird and insect wings) show convergent evolution. Vestigial structures. Embryo similarities (Haeckel exaggerated them). Comparing protein sequences (cytochrome c) and DNA: fewer differences = more recent common ancestor. Evolutionary trees. |
| 2F Evolution in action | Grants' Galápagos finches: beak depth rose after the 1977 drought. Resistance: insecticides (DDT and malaria in Sri Lanka), antibiotics, rabbits and myxoma virus, RHDV. Cane toads and red-bellied black snakes. Tusklessness in Gorongosa elephants after ivory poaching. Lenski's E. coli experiment. Resistance spreads by selection of existing variation, not because organisms "try" to adapt. |
| 2G Human evolution | Humans and chimpanzees share a common ancestor about 6–7 million years ago. Bipedalism came before large brains (foramen magnum, pelvis, Laetoli footprints). Hominins form a branching bush: australopithecines (Taung child, Lucy), Homo erectus, Neanderthals, Homo sapiens about 300 000 years ago. New finds: H. floresiensis, H. naledi, H. luzonensis, Denisovans. Interbreeding shown by DNA. Sunda and Sahul; Madjedbebe at least 65 000 years; Lake Mungo. Humans are still evolving (adults digesting lactose, malaria protection). |
Putting it together
The whole chapter rests on one mechanism: natural selection acting on inherited variation changes allele frequencies, and over long enough times those changes produce new species. Variation comes from mutation and sexual reproduction (Chapter 1). More offspring are produced than can survive, so individuals whose inherited features suit the environment leave more offspring, and their alleles become more common. When populations are isolated, they change in different directions until they become separate species. Repeated over millions of years this produces the branching tree of life. Several independent lines of evidence all fit that tree: the order of fossils in the rocks, transitional forms, homologous structures, and the amount of difference in proteins and DNA. And because selection happens whenever there is a strong pressure, we can watch it now, in drought-hit finches, resistant bacteria and insects, and in our own species.
Key words
adaptation · adaptive radiation · allele frequency · allopatric speciation · analogous structure · antibiotic resistance · artificial selection · bipedalism · common ancestor · convergent evolution · cytochrome c · directional selection · disruptive selection · evolution · extinction · fertile offspring · fossil · gene pool · geological time scale · hominin · homologous structure · hybrid · isolating mechanism · mutation · natural selection · pentadactyl limb · population · Sahul · selection pressure · selective breeding · sexual selection · speciation · species · stabilising selection · Sunda · sympatric speciation · transitional fossil · variation · vestigial structure · Wallace's Line
Test yourself
- Which statement is Darwin's explanation rather than Lamarck's?
Selection acts on variation that already exists and is inherited.
- In a population of 500 moths, 200 alleles out of 1000 are the dark allele. The frequency of the dark allele is
500 moths carry 1000 copies of the gene; 200 ÷ 1000 = 0.2.
- A horse and a donkey can breed to produce a mule, but horses and donkeys are separate species because
A species is defined by producing fertile offspring.
- A bat's wing and a human arm contain the same set of bones. This is best explained by
Same bone plan, different use: homologous.
- Species A and B differ by 3 amino acids in cytochrome c; A and C differ by 15. Which is most likely?
Fewer differences means less time since the lineages split.
- After a drought in 1977 the average beak depth of medium ground finches increased. Which explanation is correct?
Selection acted on existing inherited variation.
- Which discovery is the best evidence that several hominin species lived at the same time?
That overlaps with early Homo sapiens and Neanderthals.
- Tiktaalik is called a transitional fossil because it
It shows intermediate features between two major groups.
Label the animals on the evolutionary tree.
Which kind of evidence for evolution is each example?
Put the stages of allopatric speciation in order.
Evolution is a change in frequencies in a population over many generations. Breeding plants or animals for chosen features is called artificial . A barrier that splits a population can lead to speciation. Structures with the same basic plan because of shared ancestry are . The first Australians crossed the sea from Sunda to .
Golden staph bacteria resistant to the antibiotic methicillin (MRSA) are now common in hospitals. Explain how this resistance evolved using the conditions for natural selection, and explain why doctors tell patients to finish a full course of antibiotics and avoid using them for viral infections.
A population of golden staph contains huge numbers of bacteria, and random mutations mean a few have alleles that let them survive methicillin, for example by changing the protein the drug attacks. This variation is inherited, because each bacterium divides to make copies with the same DNA. When methicillin is used, it is a selection pressure: sensitive bacteria die, while the resistant ones survive and reproduce, every 20 minutes or so in good conditions. Over many generations the frequency of the resistance allele rises until most of the population is resistant. The bacteria did not choose to become resistant; the drug selected those that already were. Stopping a course early leaves alive the bacteria that were hardest to kill, which can multiply and spread, so finishing the course gives the best chance of killing them all. Antibiotics do not affect viruses, so using them for a cold adds selection pressure for resistance in the bacteria that live on us without any benefit. Using antibiotics only when needed slows the spread of resistance.