Evolution and inheritance, Year 11: fossils, resistant bacteria and classification

They show that living things have changed over a very long time

HintCompare the remains found in the oldest rocks with those in younger layers above.

The whyFossils in older rocks are different from those in younger rocks, and from the species alive today. Placed in order of age, they show how much, or how little, organisms have changed. The record has gaps, because few organisms are ever fossilised.

Characteristics are passed on to offspring in genes

HintThink of the small sections of DNA studied earlier in this topic.

The whyNatural selection works only if helpful characteristics can be inherited. The later discovery that characteristics are passed on in genes showed how this happens, and so supported the theory.

Natural selection can be seen changing a population within a short time

HintBacteria reproduce so fast that scientists can watch it happen.

The whyWhen an antibiotic is used, resistant bacteria survive and reproduce while the others die, so the resistant strain becomes more common. This is evolution by natural selection, observed directly within months or years.

genes; fossil; resistance

HintOne is in every cell, one is in rocks, and one is seen in hospitals.

The whyThe three are independent of each other: one explains how characteristics are inherited, one shows change over a very long time, and one shows natural selection happening now. Agreement between them is what makes the case strong.

Data from several sources now supports it

HintScientific ideas stand or fall on evidence.

The whyA scientific theory is accepted when evidence supports it and no evidence contradicts it. For evolution, data is now available from genetics, from the fossil record and from bacteria.

Fossils

HintCoal, oil and gas are called this kind of fuel.

The whyFossils may be hard parts that did not decay, parts replaced by minerals, or preserved traces such as footprints. From them we learn how much or how little organisms have changed as life developed on Earth.

One or more of the conditions needed for decay was absent

HintDecomposers need warmth, oxygen and moisture.

The whyDecay is carried out by microorganisms, which need suitable conditions. In ice it is too cold for them, and in a peat bog there is too little oxygen and the water is acidic. Without decay, parts of the organism can survive.

minerals

HintThe bone slowly turns to rock.

The whyHard parts such as bones and shells decay slowly. As they do, minerals from the surrounding water and sediment take their place, leaving a copy in rock of the original shape.

A trace left by the organism

HintIt records an activity, not a body.

The whyFootprints, burrows and the marks left by small roots can be preserved when the soft ground hardens into rock. They are the third way fossils form, and they show how an organism moved or lived.

Many were soft-bodied, and geological activity has destroyed most of the traces

HintNo shells, no bones — and the rocks themselves have been remade since.

The whySoft bodies decay quickly and seldom fossilise. The few traces that did form are in very old rocks, most of which have since been destroyed by geological activity. This is why scientists cannot be certain how life began.

reproduce

HintEach division is a new generation.

The whyEvolution happens across generations. Bacteria can divide very often, so many generations pass in a short time, and a helpful mutation can spread through a population quickly.

MRSA

HintIts second letter stands for the word 'resistant'.

The whyMRSA is a type of bacterium that often lives harmlessly on the skin but can cause a serious infection if it gets inside the body. People staying in hospital, especially for surgery, are at higher risk. It is resistant to several commonly used antibiotics.

It survives and reproduces, so its population rises

HintWith its competitors gone, nothing holds it back.

The whyMutations produce new strains, and some are resistant. The antibiotic kills the non-resistant bacteria, leaving the resistant ones to multiply. The resistant strain then spreads, because people are not immune to it and there is no effective treatment.

So that all the bacteria are killed and none survive to form resistant strains

HintA few of the toughest may still be alive when the symptoms stop.

The whyStopping early can leave some bacteria alive. If any survive, they can reproduce, and a mutation among them may give a resistant strain. Finishing the course is one of three ways to slow resistance; the others are not prescribing antibiotics inappropriately and restricting their use in agriculture.

Agriculture (farming)

HintThink of where large numbers of animals are kept together.

The whyEvery use of an antibiotic gives resistant bacteria an advantage, wherever it happens. Restricting agricultural use is one of the three measures named at GCSE. New antibiotics are costly and slow to develop, so they are unlikely to keep up with new resistant strains.

class; family

HintOne is also a group of pupils; the other is also a group of relatives.

The whyCarl Linnaeus grouped living things by their structure and characteristics. Each group is divided into smaller ones, so a kingdom holds many phyla and a genus holds one or more species. Many pupils learn the order with a sentence whose words start k, p, c, o, f, g, s.

The genus, then the species

HintThey are the two smallest groups in the Linnaean system.

The whyBinomial means 'two names'. The first word is the genus and has a capital letter; the second is the species and does not. Scientists everywhere use the same name, whatever their language.

Woese

HintCarl was his first name, as it was for Linnaeus.

The whyCarl Woese used evidence from chemical analysis to divide living things into three domains: Archaea, Bacteria and Eukaryota. It is an example of a classification system changing as new evidence becomes available.

Archaea

HintIts name comes from the Greek for 'ancient'.

The whyThe three domains are Archaea, Bacteria (the true bacteria) and Eukaryota, which includes protists, fungi, plants and animals. Archaea look like bacteria under a microscope, but chemical analysis shows that they are a separate group.

A and B

HintLook for the most recent common ancestor.

The whyOn an evolutionary tree, each branch point is a common ancestor. The more recently two species shared one, the more closely they are related: here 5 million years ago against 20 million. Trees are built from classification data for living organisms and fossil data for extinct ones.