Variation and Evolution

- variation is the differences in the characteristics of individuals in a population

- variation may be due to differences in the genes inherited (genetic causes), the conditions in which organisms develop (environmental causes) or a combination of both genes and the environment 

- the theory of evolution by natural selection states that all species of living things have evolved from simple life forms that first developed over 3 billion years ago

- mutations occur continuously, very rarely a mutation leads to a new phenotype. If the new phenotype is suited to an environmental change it can lead to a relatively rapid change in the species

- if two populations of a species become so different they can no longer interbreed to form fertile offspring, they have formed two new species

- selective breeding is a process where humans breed plants and animals for desired characteristics 

- desired characteristics include:

    ....disease resistance, increased food production in animals and plants, domestic dogs with a gentle nature, and heavily scented flowers

- problems can occur with selective breeding including defects in some animals due to lack of variation

- genes can be transferred to the cells of animals and plants at an early stage of their development so they develop desired characteristics, this is genetic engineering

- in genetic engineering, genes from the chromosomes of humans and other organisms can be 'cut out' using enzymes and transferred to the cells of bacteria and other organisms using a vector, which is usually a bacterial plasmid or a virus

- crops that have had their genes modified are known as modified (GM) crops. GM crops often have improved resistance to insect attack or herbicides and generally produce a higher yield

- a modern technique for cloning plants is tissue culture using small groups of cells taken from part of a plant to grow identical new plants

    ...it is important for preserving rare plant species in nurseries as well as for commercial use

- taking cuttings is an older and simpler method of producing many identical new plants from a parent plant, it is widely used by gardeners

- transplanting cloned embryos involves splitting apart cells from a developing animal embryo before they become specialised and then transplanting the identical embryos into host mothers

- in adult cell cloning, the nucleas is removed from an unfertilised egg cell

    ...the nucleas from an adult body cell, such as a skin cell, is inserted into the empty egg cell

    ...a small electric shock stimulates the egg cell to divide to form an embryo

    ...when the embryo has developed into a ball of cells, it is inserted into the womb of an adult female to continue its development

- the cells of the embryo contain the same genetic information as the adult skin cells

- modern medical research is exploring the possibility of genetic modification to overcome some inherited disorders

- there are benefits and risks associated with genetic engineering in agriculture and medicine

   ...improved growth rate, increased food value, designed to grow well in harsh climates

- some people have ethical objections to genetic engineering

    ...genes might spread to the wildlife, originally made infertile, possible effects on people