The development of understanding of genetics and evolution — AQA GCSE Biology

Revision notes

7 short notes, in the order of the specification. Each one in short:

1. Theory of evolution

   Triple only

   The theory of evolution by natural selection states that individuals in a species vary, that those with characteristics most suited to the environment are more likely to survive to breed, and that these characteristics are passed on. It was only gradually accepted because it challenged religious belief, evidence was insufficient and the mechanism of inheritance was unknown.

2. Speciation

   Triple only

   Speciation is the formation of new species. A population becomes isolated, for example by a geographical barrier, so the groups cannot interbreed. Mutations cause genetic variation, and natural selection acts differently in each environment over many generations. Eventually the populations are so different in phenotype that they can no longer interbreed to produce fertile offspring, forming two new species.

3. The understanding of genetics

   Triple only

   Gene theory states that characteristics are inherited through genes located on chromosomes. In the mid-19th century, breeding experiments on plants showed that each characteristic is determined by units passed on unchanged. Later, chromosome behaviour during cell division was observed, these units were linked to chromosomes in the early 20th century, and DNA structure was determined in the mid-20th century.

4. Evidence for evolution

   The evidence for evolution by natural selection comes from three main sources. It has been shown that characteristics are passed on to offspring in genes. The fossil record shows how organisms have changed over time. Antibiotic resistance in bacteria shows natural selection happening in a short time. This evidence is why the theory is now widely accepted.

5. Fossils

   Fossils are the remains of organisms from millions of years ago, found in rocks. They form from parts of organisms that have not decayed, from parts replaced by minerals as they decay, or as preserved traces such as footprints and burrows. The fossil record is incomplete because many early life forms were soft-bodied and geological activity destroyed most traces.

6. Extinction

   Extinction occurs when there are no remaining individuals of a species still alive. Factors that may contribute include a change to the environment over geological time, new predators, new diseases, new, more successful competitors, and a single catastrophic event, such as a massive volcanic eruption or an asteroid collision. Each can reduce numbers until none are left.

7. Resistant bacteria

   Antibiotic-resistant bacteria are strains that are not killed by antibiotics, such as MRSA. Bacteria evolve rapidly because they reproduce at a fast rate. A mutation produces a resistant strain. When antibiotics are used, non-resistant bacteria are killed but resistant ones survive and reproduce, so the resistant strain spreads. Appropriate prescribing and completing the course of antibiotics slow this down.