Variation and evolution

Individuals within a population show differences caused by genes, the environment, or a combination of both.

• •Variation can be caused by inherited genes (e.g. eye colour), environmental influences (e.g. scars), or a combination of both (e.g. height).
• •Genetic variation arises from mutations, which are random changes in the DNA sequence.
• •Most mutations have no effect on the phenotype, but a small number alter the phenotype or create a new phenotype.
• •If a new phenotype is suited to an environmental change, it can lead to a rapid change in the species.

All species of living things have evolved from simple life forms that first developed more than three billion years ago.

• •Organisms show variation caused by random mutations in their DNA.
• •Organisms with characteristics best suited to the environment are more likely to survive and reproduce.
• •The beneficial alleles are passed on to the next generation.
• •If two populations of one species become so different that they can no longer interbreed to produce fertile offspring, they have formed two new species (speciation).

Humans have used selective breeding for thousands of years to produce domesticated crops and animals.

• •Parents with desired characteristics are chosen from a population and bred together.
• •Offspring displaying the desired characteristics are selected and bred together.
• •This process is repeated over many generations until all offspring show the desired trait.
• •Selective breeding can cause inbreeding, reducing the gene pool and increasing the risk of inherited genetic defects and disease.

Genetic engineering alters an organism's DNA to transfer desirable traits across different species quickly.

• •Enzymes are used to isolate and cut out the required gene from a chromosome.
• •The gene is inserted into a vector (such as a plasmid or a virus).
• •The vector transfers the gene into target cells at an early stage of development (e.g. egg or embryo) so that all cells acquire the trait.
• •Crops have been engineered for higher yields or pest resistance, and bacteria have been engineered to produce human insulin.

Cloning creates genetically identical copies of plants or animals using specialized laboratory techniques.

• •Plant clones can be produced using cuttings (cheap and simple) or tissue culture (uses small groups of cells grown on nutrient agar).
• •Embryo transplants involve splitting apart cells from a developing embryo before they specialize, then transplanting them into host mothers.
• •In adult cell cloning, the nucleus of an unfertilised egg cell is removed and replaced with the nucleus from an adult body cell.
• •An electric shock stimulates the egg cell to divide into an embryo, which is then implanted into a surrogate mother.