1The idea in one paragraph

Evolution is change in the heritable characteristics of a population over generations. We know it has happened because of evidence of several independent kinds: the base sequences of genes and the amino acid sequences of proteins, the speed of change under selective breeding, and structures such as the five-digit limb that are built on one plan in animals that use them for quite different jobs. Structures that look alike can also mislead, because unrelated lineages can evolve similar solutions to the same problem. New species appear only when an existing species splits, which needs two things: the populations must stop interbreeding, and then selection must push them in different directions. At HL, you also learn the different routes to isolation, how one ancestor can radiate into many species, what keeps species apart once formed, and how plants can make a new species in a single step.

2What evolution is, and what it is not

Evolution is change in the heritable characteristics of a population over time.

Two words in that sentence do the work.

Heritable. Only changes that are passed on in the genes count. A weightlifter who builds large muscles does not have children born with large muscles, because training changes the body, not the DNA in the gametes. The belief that characteristics acquired during a lifetime are passed on is called Lamarckism, after Jean-Baptiste Lamarck, and it is not evolution. The definition exists partly to rule it out. In Darwinian evolution, variation arises first, by mutation and sexual reproduction, and the environment then favours some variants over others.

Population. An individual does not evolve. Its genes are fixed at fertilisation. What changes is how common each allele is across the population, from one generation to the next.

Nature of science: why it is "only" a theory. In science a theory is not a guess. It is a well-tested explanation that predicts and explains a broad range of observations. The theory of evolution by natural selection explains fossils, sequences, anatomy, the spread of antibiotic resistance and much more, and it is unlikely ever to be falsified. Yet it cannot be formally proved true, because no one can check it directly against every past event. It is accepted because it works, which is why the guide calls it a pragmatic truth. Calling it a theory is a statement about how science labels knowledge, not a sign of doubt.

3Evidence from sequences

Every organism stores its genes in DNA, using the same four bases and almost the same genetic code. Many genes, such as those coding for the proteins of respiration, are found in organisms as different as yeast and humans. If all life descends from common ancestors, two predictions follow: related organisms should carry similar sequences of the same gene, and the number of differences should be smallest between the closest relatives and grow with distance, because mutations accumulate in each lineage after it splits.

Both predictions hold. Figure 1 shows the pattern using invented numbers of the kind real data give.

Read it the way a Paper 1B question will ask you to.

• Chimpanzee differs from human at 1 position, rhesus macaque at 3: primates cluster together, as anatomy already suggested.
• Dog, chicken, frog and tuna differ at 10, 14, 19 and 22 positions: the order matches the order in which these groups are thought to have branched off from the lineage leading to humans.
• Yeast differs at 45 positions, but it still makes the same protein doing the same job. That is the striking part: a fungus and a mammal share a working gene, which only makes sense if they inherited it from a very distant common ancestor.

Sequence data are powerful because they are numerical and independent of anatomy and fossils, yet give the same pattern.

4Evidence from selective breeding

Selective breeding, or artificial selection, is humans choosing which animals or plants reproduce, generation after generation, so that a wanted trait becomes more pronounced. Figure 2 shows one wild plant, wild cabbage, Brassica oleracea, turned into six vegetables by selecting for different parts.

Likewise every dog breed, chihuahua to Great Dane, descends from wolves domesticated at least 15,000 years ago, and maize was bred from a wild grass, teosinte, in Mexico over roughly the last 9,000 years.

Why this is evidence for evolution. The variation between breeds, and between a breed and its wild ancestor, is heritable, and it built up quickly: thousands of years, not millions. It shows directly that selecting from heritable variation changes a population's characteristics, and that large changes can arise fast. Natural selection works the same way, with survival and reproduction in the wild doing the choosing, over far longer periods. Note the limit: selective breeding shows that the mechanism works; it does not by itself show that wild species arose this way.

5Homologous structures: the pentadactyl limb

Homologous structures are structures with the same basic plan, inherited from a common ancestor, even though they may now do different jobs. The classic example the guide names is the pentadactyl limb, the five-digit limb of amphibians, reptiles, birds and mammals.

Every pentadactyl limb has the same bones in the same order:

• one upper bone, the humerus;
• two lower bones, the radius and ulna;
• a group of small wrist bones, the carpals;
• a row of long bones, the metacarpals;
• the finger bones, the phalanges, in five digits.

In Figure 3 the human arm is built for grasping and fine movement. The bat's metacarpals and phalanges are enormously long and support a skin membrane for flight. The dolphin's bones are short and broad, with extra phalanges, inside a stiff flipper for steering in water.

Why this is evidence. If each limb had been designed from scratch for its job, there would be no reason for a flipper, a wing and a hand to share one bone plan. A wing could be built from anything. The shared plan makes sense only if all of them are modifications of a limb inherited from a common ancestor, adapted in different directions. That is divergent evolution: one inherited structure, different functions.

6Convergent evolution and analogous structures

The opposite case matters just as much. Analogous structures have the same function but different evolutionary origins. They arise by convergent evolution: unrelated lineages face the same challenge and natural selection arrives at similar solutions independently. Figure 4 puts both on a family tree.

Examples you can use (you need at least one):

• The wings of a bird and of a butterfly. Both fly, but a bird's wing is a modified pentadactyl limb with bones and feathers, while an insect's wing is a thin sheet of cuticle with no bones at all. Their last common ancestor had no wings.
• The camera eyes of an octopus and of a human. Both have a lens focusing light on a retina, but they develop differently. In the human eye the nerve fibres run in front of the retina and leave a blind spot where they exit; the octopus eye has no such blind spot.
• The streamlined bodies of a shark and a dolphin, a fish and a mammal shaped alike by fast swimming.

Homologous: same structure, different function, one origin (divergent). Analogous: same function, different structure, separate origins (convergent).

Analogous structures are not evidence of close relationship; they show natural selection shaping organisms to their environment, and they warn that classifying by appearance can group unrelated organisms, as happened with the figwort family in A3.2.

7How new species arise: splitting, isolation and selection

Speciation is splitting. A new species appears when one existing species splits into two or more. This is the only way new species have ever appeared. Speciation increases the total number of species on Earth, and extinction is the only thing that decreases it.

Be careful with one trap. A single lineage can change a great deal over time, until its descendants look very different from their ancestors. That is evolution, but it is not speciation, because no split has happened: there was one species and there is still one. Figure 5 sets the two side by side.

What a split needs. Two things, in order.

1. Reproductive isolation. The populations must stop interbreeding, so that alleles no longer flow between them. Otherwise any difference that arises in one population is diluted by interbreeding with the other. The simplest way is geographical isolation: a mountain range, a sea, or a river divides them.
2. Differential selection. The two populations experience different conditions, so natural selection favours different alleles in each, and they diverge. Given enough time, the differences in body, behaviour or chromosomes become so large that even if the populations met again, they could not produce fertile offspring. At that point they are two species.

The guide's example: bonobos and common chimpanzees. Chimpanzees and bonobos are each other's closest living relatives, yet they are separate species, divided by the Congo River in central Africa. Great apes cannot swim across a river that size, so once populations were established on both banks, gene flow between them stopped. Genetic estimates put the split at roughly one to two million years ago. Figure 6 lays out the argument.

The two banks offered different conditions. North of the river, common chimpanzees share the forest with gorillas and compete with them for plant food on the ground. South of the river, where bonobos live, there are no gorillas, and a widely discussed explanation is that food was more reliable there, allowing larger, more stable foraging groups. Selection on each side favoured different traits. Today bonobos are more slender, live in groups in which females hold high status, and show far less aggression between groups than common chimpanzees. The river supplied the isolation; the different environments supplied the differential selection. Both were needed.

12Linking questions

How does the theory of evolution by natural selection predict and explain the unity and diversity of life? Unity: all organisms inherited the same genetic code, shared genes and body plans from common ancestors, so sequences and homologous structures are shared. Diversity: variation, selection in different environments, isolation and splitting produce new species, and adaptive radiation fills vacant niches. The theory predicted that sequences would match the tree built from anatomy, before sequencing existed, and they do.

What counts as strong evidence in biology? Evidence is strongest when independent kinds agree: fossils, anatomy, sequences and observed selection all point to the same relationships. A single line of evidence can mislead, as analogous structures show. Testable predictions that could have failed but did not, such as the chromosome 2 fusion in A3.1, count for more than observations collected to fit an idea already held.