Evolution and Classification

6. Inheritance, Variation and Evolution

Evolution is a change in the inherited characteristics of a population over time, driven by natural selection. The theory holds that all species alive today evolved from simple life forms that first appeared over three billion years ago.

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193 questions12 subtopicsFoundation and Higher

What evolution and classification covers

Every subtopic below has its own questions, mark schemes and notes.

4.6.2.2Evolution by natural selection

4.6.2.3Selective breeding

Selective breeding, also called artificial selection, is humans choosing which plants or animals breed together so that a useful or attractive characteristic is kept in the population. Its main drawback is a reduced gene pool.

4.6.2.4Genetic engineering

Genetic engineering transfers a gene for a desirable characteristic from one organism's genome into another organism, so that it too shows the characteristic. It has clear benefits and real concerns, and the process detail is Higher tier only.

4.6.2.5Cloning

A clone is an organism genetically identical to another. Plants are cloned by tissue culture and cuttings; animals by embryo transplants and adult cell cloning.

4.6.3.1The theory of evolution

Darwin's theory was only gradually accepted, and rival hypotheses such as Lamarck's were around at the same time. How scientific ideas are tested and eventually accepted matters as much here as the biology.

4.6.3.2Speciation

Speciation is the formation of a new species. It happens when populations of one species become isolated, natural selection acts differently on each, and eventually they can no longer interbreed to produce fertile offspring.

4.6.3.3The understanding of genetics

Gregor Mendel worked out the basic rules of inheritance from pea plants in the mid 19th century, but their importance was only recognised after his death, once chromosomes, genes and DNA had been discovered.

4.6.3.4Evidence for evolution

Darwin's theory is now accepted because evidence has been found that he did not have: genes explain inheritance, fossils show gradual change, and antibiotic resistance shows natural selection happening now.

4.6.3.5Fossils

Fossils are the remains of organisms from millions of years ago, found in rocks. They form in three ways, and the record they leave is incomplete — which is one reason nobody can say for certain how life began.

4.6.3.6Extinction

Extinction is when no individuals of a species remain. The fossil record contains many species that no longer exist, and there are five standard causes to learn.

4.6.3.7Antibiotic-resistant bacteria

Bacteria mutate randomly, and some mutations make them less affected by an antibiotic. Because bacteria reproduce so quickly, resistant strains spread fast — which is natural selection happening in front of us.

4.6.4Classification of living organisms

Organisms were traditionally classified by Linnaeus according to their structures and characteristics. New evidence from microscopes and chemical analysis led Woese to propose the three-domain system in 1990.

Key terms

The definitions examiners expect, in the wording that earns the mark.

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

Natural selection — The process by which individuals better suited to their environment are more likely to survive, reproduce and pass on their genes.

Phenotypic variation — Differences in the characteristics shown by individuals of the same species.

Mutation — A random change in DNA, which can produce a new genetic variant and so a new characteristic.

Selective breeding — Humans artificially selecting which plants or animals breed, so that the genes for a chosen characteristic remain in the population.

Gene pool — The number of different alleles present in a population.

Inbreeding — Breeding together individuals that are closely related.

Genetic engineering — Transferring a gene for a desirable characteristic from one organism's genome into another organism.

Vector — Something used to carry a gene into the cells of the target organism — usually a bacterial plasmid or a virus.

Plasmid — A small circular piece of DNA found in bacterial cells.

Gene therapy — Inserting working genes into the cells of a person with an inherited disorder, to treat it.

Clone — An organism genetically identical to another organism.

Tissue culture — Growing new plants from a few cells placed in a growth medium containing hormones.

Adult cell cloning — Replacing the nucleus of an unfertilised egg cell with the nucleus of an adult body cell, then stimulating it to divide.

Hypothesis — A proposed explanation that can be tested by looking for evidence to support or disprove it.

Acquired characteristic — A feature that develops during an organism's lifetime rather than being inherited.

Species — A group of similar organisms that can reproduce to give fertile offspring.

Speciation — The development of a new species.

Geographical isolation — Populations of one species being separated by a physical barrier so they cannot interbreed.

Reproductively isolated — No longer able to interbreed to produce fertile offspring.

Hereditary unit — Mendel's name for what determines a characteristic and is passed from parent to offspring; now called a gene.

Dominant — An allele that is expressed even when only one copy is present.

Recessive — An allele that is only expressed when two copies are present.

Fossil record — The collection of fossils found in rocks of different ages, showing how organisms have changed over time.

Fossil — The remains of an organism from millions of years ago, found in rocks.

Cast — A fossil formed when an organism decays away inside hardened material, leaving its shape behind.

Amber — Fossilised tree resin, which preserves organisms because it contains no oxygen or moisture.

Extinction — When no individuals of a species remain.

Resistant strain — A form of a bacterium that is not killed by a particular antibiotic.

Superbug — A bacterium resistant to most known antibiotics.

MRSA — A common antibiotic-resistant bacterium, often a problem in hospitals.

Classification — Organising living organisms into groups.

Binomial system — Naming each organism with a two-part Latin name giving its genus and species.

Domain — The largest group in the modern classification system: Archaea, Bacteria or Eukaryota.

Evolutionary tree — A diagram showing how scientists think species are related and where they share common ancestors.

Where marks get lost

The mistakes that come up again and again in this topic.

Populations evolve; individuals do not. An organism cannot change the alleles it was born with.
'Fittest' means best suited to the environment, not the strongest or fastest.
Characteristics acquired during an organism's lifetime, such as muscles built by training, are not inherited — they are not coded for in the genes.
In a 'describe the change' question using a graph or table, quote the actual figures. A bare 'it went up' throws away a mark.
Selective breeding does not alter any genes — it only chooses which existing organisms breed. Altering or transferring genes is genetic engineering.
A bull produces no milk, so a breeder selecting for milk yield judges a bull by the yields of his female relatives.
Say 'over several generations' in any description of the process. Leaving it out usually costs a mark.
Enzymes cut the gene out; the vector carries it in. Swapping those two roles is a common slip.
Only Higher tier candidates are asked to describe the steps of the process, but everyone needs the uses, benefits and risks.
'Some people object' answers need a reason, not just an objection. 'It's unnatural' on its own earns nothing.
There is no fertilisation in adult cell cloning. If your answer mentions sperm, you have described embryo transplants instead.
The host mother that carries a cloned embryo contributes no genes, so the offspring is not related to her.
The embryo in an embryo transplant must be split before the cells specialise — say so, it is a marking point.
A clone has the same sex as the animal that donated the body cell, because it has the same chromosomes.
Know the difference precisely: for Lamarck the change happens in the individual's body during its life; for Darwin the proportions in the population change over generations.
'Not enough evidence' is a marking point in its own right — do not give only the religious objection.
Learn the dates: 1858 Darwin and Wallace publish jointly, 1859 On the Origin of Species, 1866 Mendel publishes, 1953 the structure of DNA.
The word 'fertile' is essential in the definition of a species. A horse and a donkey breed, but the mule is sterile, so they are two species.
Geographical isolation starts the process; reproductive isolation is the end point that makes them separate species.
A full speciation answer needs the variation and the natural selection steps, not just 'they were separated and became different'.
Mendel did not know about genes, DNA or chromosomes — say 'hereditary units' when describing what he concluded.
The order of developments is often examined: Mendel publishes (1866), chromosomes observed (late 1800s), units linked to chromosomes (early 1900s), DNA structure determined (1953).
In the 3:1 cross, three quarters of the offspring are tall, not three.
Evidence for evolution has to involve inherited change across generations. An environmental effect on individuals, such as plants growing taller with fertiliser, is not evidence.
In undisturbed rock, deeper layers are older — the deepest fossil is the oldest.
Finding no fossils of an organism is not evidence that it never existed, because soft-bodied organisms rarely fossilise at all.
Preservation questions want the reason decay microbes cannot work: no oxygen or moisture, too cold, or too acidic.
Extinction means none left anywhere, not 'very rare' or 'none left in this country'.
Questions asking for reasons usually want distinct causes from the list of five — do not give the same cause twice in different words.
Never write that antibiotics make bacteria mutate or 'teach' them to resist. The mutation happens first, at random.
Bacteria become resistant to antibiotics; people do not become resistant or immune to antibiotics.
Antibiotics do not kill viruses, so prescribing them for a viral illness helps nobody and speeds up resistance.
Order matters in staging questions: mutation, then survival of the resistant during treatment, then reproduction and passing on the gene, then the gene becoming common.
Learn the order kingdom, phylum, class, order, family, genus, species — a mnemonic such as King Philip Came Over For Good Soup helps.
The three domains sit above kingdom; they do not replace it.
Looking similar is not the same as being closely related — sharks and dolphins look alike but are only distantly related.
On an evolutionary tree, trace back from both species: the first branching point they share is their most recent common ancestor.

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