Variation and evolution

Variation

differences in the characteristics of individuals in the population

three causes of variation in a population

• allleles that individuals have inherited e.g. hair and eye colour

• Environment e.g. colour of flowers determined by pH of soil or language in humans

• Combo of genes and environment e.g. height - alleles can make them grow taller + calcium in diet

Reason for massive amount of genetic variation

Mutation

mutations that influence phenotype

• E.g. alleles for tallness may lead to tallness if diet is suitable

• Small no. Of alleles determine phenotype E.g. eye colour

• Very rarely, can lead to new phenotype, which can be beneficial if environment changes

• In the 1950s a virus killed 99% of wild rabbits in the UK

• A mutation occurred allowing the surviving rabbits to gain resistance to the virus

How many species of animals and plants on earth

• 9 million

• This does not include microbes such as bacteria

• All these species are only 1% of the total that have lived on earth

how long ago do scientists believe life developed on earth

• more than 3 billion years ago

• First life forms were very simple e.g. single cells

• All species evolved from simple life forms (evolution by natural selection)

evolution

Change in inherited characteristic of a population through the process of natural selection

Causes of evolution

• introduction of a predator

• Change in environment (e.g. temperature)

If animal has allele that allows them to survive this change (e.g. thicker fur or better eyesight), they are more likely to survive and reproduce, so offspring will inherit this allele. Over generations, this allele will become more common among the population.

what happens when two populations of one species become different in phenotype

• can no longer interbreed to produce fertile offspring

• Two populations become 2 separate species

selective breeding

• dogs - gentle nature

• Cows - more meat, more milk

• Food crops - disease resistance

• Plants - large/unusual flowers

how is selective breeding carried out e.g. cows

• take a mixed population of cows and select the largest male and female

• Breed these together

• Sexual reproduction produces variation in offspring

• Offspring will be a mixture of larger and smaller animals

• Select largest male and female offspring and breed together

• Continue through many generations until all offspring are large

Problem with selective breeding

• if we breed together closely related plants or animals, we can get inbreeding, which can cause some species to be prone to disease or inherited defects

• E.g. dog breeds can develop joint problems, heart disease , epilepsy

genetic engineering

Genes from one organism e.g. humans are cut out and transferred to cells of a different organism e.g. bacteria. Genome of bacteria is now modified and includes a human gene

example of genetic engineering (insulin)

• people with type one diabetes cannot make their own insulin so they need to inject themselves with insulin regularly

• Bacteria have been genetically modified to contain the human insulin gene

• These bacteria can now produce insulin, which can be purified and used for type one diabetes

Genetically modified crops

• produce a greater yield than normal crops

• Can make them resistant to disease or an insect attack

• Can produce bigger and better fruits

• This means that farmers can spray fields to kill weeds without harming the GM crop

concerns about GM crops

• Whether they are harmful to insects or wild flowers

• More research needed on health effects of eating GM crops

Gene therapy

• Genetic modification is being explored as a way to treat inherited disorders in humans

• Unknown long term effects

• If one gene is modified, unknown effects on other genes

steps of genetic engineering

• identify the gene we want to transfer

• Use enzymes to isolate the gene

• Transfer gene to small circle of DNA called a plasmid. We can also use a virus

• Plasmid transfers DNA from one organism to another, called a vector

• Desired gene is transferred to cells of target organism

• Transfer gene in early stage of an organism’s development to make sure all cells receive transferred gene

Advantage of cloned plants

• Genetically identical to original, so we know exactly what clone’s characteristics will be

• If we used seeds (produced by asexual reproduction) then offspring will be different

methods of cloning plants

• Taking cuttings - genetically identical clone of starter plant

• End of small piece is dipped in rooting powder, which contains plant hormones, encouraging it to develop roots

using a tissue culture

• hundreds of clones

• Divide plant into hundreds of tiny pieces

• Each piece contains a small number of cells

• Cells are then incubated with plant hormones, which stimulate plants to grow and develop into fully grown clones

• Sterile conditions so that microorganisms/fungi aren’t introduced

• Commercial plant nurseries - produces plants quickly and cheaply

• Preserve rare species of plant

Cloning animals

• sperm and egg cells w/ desired characteristics

• Fertilisation - fertilised egg

• Allow fertilised egg to develop into an early stage embryo (no cells have specialised)

• Use glass rod to split embryo into 2

• Embryos grow and develop

• 2 embryos transplanted into host mothers

• Grow and develop into two identical offspring

• Cannot be certain of offspring’s characteristics

how to overcome uncertainty of characteristics

Adult-cell cloning

adult cell cloning

• remove cell

• Remove nucleus of cell

• Take unfertilised egg cell from same species

• Remove nucleus of egg cell and throw away

• Insert nucleus of adult cell into egg cell

• Give shell electric shock, divides to form an embryo

• When developed into balls of cells, inserted into mother

• Same characteristics as original adult cell

• Mother gives birth to clone

Speciation

How new species form

what are closely related species seperated by

geographical barriers e.g. wide river

stages of speciation

• a species can interbreed

• Beneficial mutation spreads through whole population

• Change in environment e.g. river changing course and separates population into 2 groups

• No interbreeding between groups

• Natural selection will favour different alleles on either side of river

• E.g. food sources may be different

• No interbreeding between groups, so mutations are not spread through whole population

• Two populations begin to change

• Phenotypes are different, so if river course changes again, species cannot produce fertile offspring

fossils

remains of organisms from millions of years ago found in rocks

how can fossils form

• when parts of an organism have not decayed (conditions for decay are absent e.g. not enough oxygen, too cold, not enough water)

• If an organism decays, parts of organism are slowly replaced by mineral ions in decay process

• Can be preserved traces of organism e.g. footprints or plant roots (spaces where they grew)

problem with fossils

• Early organisms were soft bodied

• Rarely formed fossils

• Many fossils destroyed by changes to rocks in the earth’s crust

• Very few of early forms of life, so scientists cannot be sure how life on earth began

extinction

• No remaining individuals of species that are alive

• shown by fossils

reasons for extinction

• asteroids (dinosaurs)

• Environment changes e.g. changing weather patterns

• New disease/predator killing a species

• Evolution of new, more successful species competing for scarce food or water

resistant bacteria

• e.g. MRSA

• Mutation causing resistance

• If antibiotic used, all bacteria die apart from those with mutation

• Resistant bacteria reproduce, creating a resistant strain

ways of reducing development of antibiotic resistant strains

• doctors not prescribing antibiotics inappropriately

• Restricting use in farming

• Developing new ones (takes a long time, can’t keep up as strains develop very quickly)