Evolution and Natural SelectionAQA GCSE Biology: Revision notes

Evolution is a change in the inherited characteristics of a population over time, brought about through natural selection, and may result in the formation of an entirely new species. Evolution occurs because variants (individuals with different characteristics) that are best suited to their environment are more likely to survive, reproduce and pass those characteristics on — over many generations, this shifts the characteristics of the whole population.

Charles Darwin proposed the theory of evolution by natural selection, based on the following reasoning:

1. Individuals within a species show variation in their characteristics
2. Individuals with characteristics best suited to their environment are more likely to survive and breed
3. These advantageous characteristics are then passed on to the next generation

Darwin published his ideas in On the Origin of Species in 1859. However, the theory was only gradually accepted, because:

• It challenged religious ideas about how life was created
• There was insufficient evidence available at the time to fully support it
• The mechanism of inheritance (how characteristics were actually passed on) was not understood until about 50 years after publication, when genetics developed

Alfred Russel Wallace independently proposed a very similar theory of evolution by natural selection at around the same time as Darwin.

• Wallace also carried out important work on warning colouration in animals (bright colours that warn predators of danger)
• He developed ideas about speciation — how new species form
• In 1858, Wallace and Darwin published joint writings on natural selection, which prompted Darwin to finally publish On the Origin of Species in 1859

Before Darwin and Wallace, Jean-Baptiste Lamarck proposed a different theory: that characteristics an organism acquired or developed during its lifetime (e.g. through use or disuse of a body part) could be passed on to its offspring. This theory of the inheritance of acquired characteristics is now known to be incorrect in the vast majority of cases — changes that happen to an organism's body during its life do not alter the DNA passed on in its gametes.

Evolution is supported by several types of evidence:

• Fossils — may form from parts of organisms that have not decayed, from parts replaced by minerals as they decayed, or as preserved traces such as footprints; the fossil record is incomplete, because many early organisms were soft-bodied and left few traces, and many traces have been destroyed by geological activity
• Antibiotic resistance in bacteria — bacteria reproduce very rapidly, so mutations can quickly produce new, resistant strains; strains resistant to an antibiotic survive and reproduce, spreading resistance through the population

To slow the development of antibiotic-resistant bacteria: doctors should avoid prescribing antibiotics inappropriately, patients should always complete their full course of antibiotics, and agricultural use of antibiotics should be restricted. Developing entirely new antibiotics is costly and slow, so preventing resistance is important.