Evolution and its evidence

Evolution is a change in the inherited characteristics of a population over time, through a process of natural selection, which may result in the formation of a new species.

The theory of evolution by natural selection states that all species of living things have evolved from simple life forms that first developed more than three billion years ago.

Explaining how evolution occurs is a four-step answer, and questions are marked on having all four:

1. There is variation within the population, caused by mutations. 2. Individuals with the phenotypes best suited to their environment are more likely to survive. 3. Those survivors are more likely to breed successfully and pass on the alleles for that phenotype. 4. Over many generations, the proportion of the population with that phenotype increases.

Write it in that order for any example you are given — antibiotic resistance, moth colour, beak shape — and the marks follow.

If two populations of one species become so different in phenotype that they can no longer interbreed to produce fertile offspring, they have formed two new species. That is the definition of speciation, and the test in it is fertile offspring — not merely whether they can mate.

Charles Darwin proposed the theory of evolution by natural selection as a result of observations on a round-the-world expedition, backed by years of experimentation and discussion, and linked to developing knowledge of geology and fossils.

His reasoning, in three steps:

• Individual organisms within a particular species show a wide range of variation for a characteristic. • Individuals with characteristics most suited to the environment are more likely to survive to breed successfully. • The characteristics that have enabled these individuals to survive are then passed on to the next generation.

Darwin published his ideas in On the Origin of Species in 1859, and there was much controversy surrounding these revolutionary new ideas.

The theory was only gradually accepted, for three reasons the specification lists:

• it challenged the idea that God made all the animals and plants that live on Earth • there was insufficient evidence at the time to convince many scientists • the mechanism of inheritance and variation was not known until 50 years after the theory was published.

That third reason is the scientific one, and it is the one worth understanding: Darwin could see that characteristics were inherited, but not how. Without genes, the theory had a hole in the middle of it.

Other theories existed, including that of Jean-Baptiste Lamarck, based mainly on the idea that changes which occur in an organism during its lifetime can be inherited — a blacksmith's strong arms being passed to his children, or a giraffe stretching its neck and its offspring being born with longer ones. We now know that in the vast majority of cases this type of inheritance cannot occur.

A study of creationism is not required.

Bacteria can evolve rapidly because they reproduce at a fast rate.

Mutations of bacterial pathogens produce new strains. Some strains might be resistant to antibiotics, and so are not killed. They survive and reproduce, so the population of the resistant strain rises. The resistant strain will then spread, because people are not immune to it and there is no effective treatment.

MRSA is resistant to antibiotics.

Notice that this is the four-step natural selection answer with different nouns in it — variation from mutation, survival of the best suited, reproduction, and a rising proportion in the population.

To reduce the rate at which antibiotic resistant strains develop:

• doctors should not prescribe antibiotics inappropriately, such as for non-serious or viral infections • patients should complete their course of antibiotics, so all bacteria are killed and none survive to mutate and form resistant strains • the agricultural use of antibiotics should be restricted.

And the reason this matters so much: the development of new antibiotics is costly and slow, and is unlikely to keep up with the emergence of new resistant strains. We cannot simply invent our way out of the problem at the rate it is being created.

The theory of evolution by natural selection is now widely accepted, because the evidence Darwin lacked has since arrived.

It has been shown that characteristics are passed on to offspring in genes, which supplies the mechanism his theory was missing. There is further evidence in the fossil record, and in the knowledge of how resistance to antibiotics evolves in bacteria.

Antibiotic resistance is the strongest of the three for one reason: it happens fast enough to watch. Bacteria evolve in days, so natural selection can be observed directly rather than inferred from the distant past.

Fossils are the 'remains' of organisms from millions of years ago, which are found in rocks. They may be formed:

• from parts of organisms that have not decayed, because one or more of the conditions needed for decay are absent • when parts of the organism are replaced by minerals as they decay • as preserved traces of organisms, such as footprints, burrows and rootlet traces.

We can learn from fossils how much or how little different organisms have changed as life developed on Earth.

The fossil record is incomplete, and knowing why is examined. Many early forms of life were soft-bodied, which means they left few traces behind, and what traces there were have been mainly destroyed by geological activity. This is why scientists cannot be certain about how life began on Earth.

That is a good illustration of a limit on scientific knowledge: not that the theory is weak, but that the evidence for one particular period was destroyed long before anyone looked for it.

You should be able to extract and interpret information from charts, graphs and tables, such as evolutionary trees. Read a tree from the bottom up: each branch point is a common ancestor, and the closer two organisms branch from each other, the more recently they shared one. Distance along the bottom is time, not similarity.

Three named fossils trace the change from an ape-like ancestor to something recognisably human, and the exam expects the names, the ages and one or two features of each.

Ardi — Ardipithecus ramidus — lived 4.4 million years ago. The skeleton has long arms and short legs, and a big toe that stuck out to the side for grasping branches, so Ardi still climbed. But the structure of the feet and pelvis show she could also walk upright. Brain size was about the same as a chimpanzee's.

Lucy — Australopithecus afarensis — lived 3.2 million years ago. Her feet were arched, like a human foot and unlike an ape's, and her big toe was in line with the others: she walked upright all the time and was not adapted for climbing. Her brain was still close to chimpanzee size.

The fossils found by Louis and Mary Leakey and their team in Tanzania are more recent again. Homo habilis lived between about 2.4 and 1.4 million years ago with a brain larger than Lucy's, and Homo erectus, about 1.6 million years ago, had a brain around twice the size of a chimpanzee's and a skeleton much like a modern human's.

Stone tools tell the same story independently, which is what makes them useful evidence. The oldest are simple pebble tools, around 2.5 million years old, made by striking one stone with another to give a sharp edge. By about 1.6 million years ago the tools are hand axes, shaped on both sides and clearly designed. By 300,000 years ago flint tools were being made for particular jobs, and by around 50,000 years ago there were arrowheads and fish hooks. More complex tools appear alongside larger brains.

Tools are dated in two ways. The first is by the rock layers they are found in: layers form on top of one another, so a tool in a deeper layer is older, and the age of the layer can be measured. The second is by dating the fossils found in the same layer as the tool.

The pentadactyl limb is evidence of a different kind. A human arm, a bat's wing, a whale's flipper and a bird's wing all do completely different jobs, and yet all four are built from the same arrangement of bones — one long bone, then two, then a group of small bones, then five digits.

That is the argument. If each of these animals had been created separately for its own way of life, there would be no reason for the same bone pattern to appear in all of them. The pattern is explained if they inherited it from a common ancestor and it was then modified for flying, swimming or grasping.

Traditionally, living things have been classified into groups depending on their structure and characteristics, in a system developed by Carl Linnaeus.

Linnaeus classified living things into kingdom, phylum, class, order, family, genus and species. Each level is a subdivision of the one before it, so the groups get smaller and the organisms in them more alike as you go down.

Organisms are named by the binomial system of genus and species — Homo sapiens, Panthera leo. The genus comes first with a capital letter, the species second in lower case.

Classification systems have changed as the evidence available changed. As evidence of internal structures became more developed, due to improvements in microscopes, and as the understanding of biochemical processes progressed, new models of classification were proposed.

That is the impact of developments in biology on classification systems, and it is the point of this sub-topic. Linnaeus could only classify by what he could see; later biologists could look inside cells, and later still could compare the chemistry. Better evidence produced a better grouping — which is how scientific methods and theories develop over time.

Due to evidence available from chemical analysis, there is now a 'three-domain system' developed by Carl Woese. In this system organisms are divided into:

• archaea — primitive bacteria usually living in extreme environments • bacteria — true bacteria • eukaryota — which includes protists, fungi, plants and animals.

The striking part is that archaea and bacteria look almost identical under a microscope, and Linnaeus would have had no way to separate them. Their chemistry shows they are as different from each other as either is from us — which is why the new evidence forced a new system rather than a small adjustment.

Evolutionary trees are a method used by scientists to show how they believe organisms are related. They use current classification data for living organisms, and fossil data for extinct ones.

To interpret one: work from the bottom up. Each point where the tree splits is a common ancestor of everything above it. Two organisms that branch apart near the top shared an ancestor recently and are closely related; two that split near the bottom have been separate for far longer. It is the branching pattern that matters, not how near the tips happen to be drawn to each other.