B2.3 Life diversity · KS3 Science

Variation

Variation is the differences between organisms of the same species.
Some variation is inherited (eye colour, blood group), some is caused by the environment (scars, accent, language), and some is caused by both (height, body mass).
Continuous variation can take any value in a range (height, mass) and is shown on a histogram or line graph. Discontinuous variation falls into distinct groups (blood group, tongue rolling) and is shown on a bar chart.
A species is a group of organisms that can breed together to produce fertile offspring.

Characteristics are passed on in genes. A gene is a section of DNA, and DNA is found in chromosomes in the nucleus.
Human body cells have 23 pairs of chromosomes (46). Sex cells have 23, so a baby gets half its chromosomes from each parent.
DNA has a double helix shape. Watson and Crick worked out its structure in 1953, using X-ray images made by Rosalind Franklin.

Selective (artificial) breeding: humans choose parents with useful characteristics, breed them, and repeat over many generations (cows that give more milk, wheat with bigger grains, dog breeds).
Natural selection (Charles Darwin): there is variation in a species; organisms compete; those best adapted to the environment are more likely to survive and reproduce; they pass on their genes, so the useful characteristic becomes more common.
Over many generations, natural selection causes evolution. Fossils give evidence of how species have changed.
Peppered moths: when soot darkened tree trunks, dark moths were better camouflaged and eaten less, so they became more common.

A species becomes extinct when there are no individuals left, for example because of a changing environment, new predators or diseases, competition, or habitat destruction.
Humans affect natural selection too: overusing antibiotics has led to resistant bacteria such as MRSA.
Gene banks and seed banks store genetic material to protect species.
Maths in science: 5 out of 20 = 5/20 = 1/4 = 25%.

Variation: Differences between organisms of the same species.
Continuous variation: Variation that can take any value in a range, such as height.
Discontinuous variation: Variation that falls into distinct groups, such as blood group.
Species: A group of organisms that can breed to produce fertile offspring.
Gene: A section of DNA that controls a characteristic.
Chromosome: A long strand of DNA in the nucleus.
DNA: The molecule that carries genetic information.
Selective breeding: Humans choosing parents with useful characteristics to breed.
Natural selection: Organisms best suited to their environment surviving and passing on their genes.
Evolution: The gradual change in a species over many generations.
Extinction: When there are no living members of a species left.
Fossil: The remains or traces of an organism preserved in rock.