The development of understanding of genetics and evolution

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.

Alfred Russel Wallace independently proposed the theory of evolution by natural selection. He published joint writings with Darwin in 1858, which prompted Darwin to publish On the Origin of Species the following year.

Wallace worked worldwide gathering evidence for evolutionary theory. He is best known for his work on warning colouration in animals and for his theory of speciation. He did much pioneering work on speciation, but more evidence over time has led to our current understanding of the theory.

That last point is the impact these two had on biology: not one man's flash of insight, but a theory built and corrected by many scientists over time — which is how scientific understanding generally develops.

The steps which give rise to new species:

1. Two populations of one species become isolated from each other — often geographically, by a river, mountain range or stretch of sea. 2. Conditions are different on each side, and there is variation within each population. 3. Natural selection acts differently on each: different phenotypes are best suited to each environment, so different alleles become common. 4. Over many generations the two populations become increasingly different. 5. Eventually they are so different in phenotype that they can no longer interbreed to produce fertile offspring — they are now two species.

In the mid-19th century Gregor Mendel carried out breeding experiments on plants. One of his observations was that the inheritance of each characteristic is determined by 'units' that are passed on to descendants unchanged.

Why Mendel's discovery was not recognised until after his death is a question in its own right. He was working before chromosomes had been seen, so there was nothing physical to attach his 'units' to; his work was published in an obscure journal and read by few scientists; and his mathematical, ratio-based approach was unlike the biology of the time, so its significance was not obvious. Only when chromosomes were observed behaving the same way did his results suddenly explain something.

This scientific work by many scientists led to the gene theory being developed.

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.

Extinction occurs when there are no remaining individuals of a species still alive.

Factors which may contribute to it:

• the environment changes too quickly for the species to adapt — there is no time for natural selection to act • a new predator kills them faster than they can breed • a new disease spreads through a population with no resistance to it • a new, more successful competitor takes the food or space • the food source itself is lost, often because another species has gone • a single catastrophic event, such as a volcanic eruption or an asteroid collision.

The common thread is speed: species survive gradual change through natural selection, and are destroyed by change faster than a population can respond to.

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.