Variation and evolution

Variation – differences in the characteristics of individuals in a population

Cause of variation

• genes they have inherited (genetic causes)
• conditions in which they have developed (environmental causes)
• combination of genes and the environment

See topics below for more notes

6.2.2 Evolution

Evolution

• Change in the inherited characteristics of a population over time via natural selection
• Form new species

Explain process of natural selection (6)

1. Individual organisms within a particular species show wide range of variation of characteristics controlled by different alleles caused by mutations
2. Individuals with characteristics most suited to the environment are more likely to survive & breed successfully (aka survival of the fittest)
3. The beneficial characteristics / alleles which enable individuals to survive are passed on to their offspring (to next generation)
4. Over many generations, frequency of alleles for this beneficial characteristic increase within the population

Why many ppl didn’t accept his theory? (3)

• They believed God made all living things
• Insufficient evidence & proof
• Mechanism of inheritance & variation not discovered

Why the theory of evolution by natural selection was only gradually accepted? (3)

• Challenged the idea that God made all the animals and plants that live on Earth
• Insufficient evidence at the time the theory was published to convince many scientists
• Mechanism of inheritance and variation was not known until 50 years after the theory was published

An organism during its lifetime can be inherited (Jean-Baptiste Lamarck)

• Changes that occur in an organism during its lifetime can be inherited
• If an individual continually repeated an action, characteristic that allow it to do this would develop further
• But we now know that in vast majority of cases, this type of inheritance cannot occur as changes in environment does not pass on to offspring

Use Lamarck’s theory to explain how elephant’s trunk evolved (2)

• Ancestor continually stretched its nose to reach food
• This characteristic of stretching its nose develop further during its lifetime
• This characteristic then passed on to offspring

Explain how 2 different species could have developed from a common ancestor (6)

• 2 populations separated by geographical barrier
• Different environmental conditions cause alleles to mutate differently in each population
• Individuals with characteristics more suitable for environment are more likely to survive and breed successfully
• Beneficial characteristics / alleles which enable species to survive are passed on to their offspring
• Over many generations, frequency of alleles for this beneficial characteristics increases within each population
• Eventually 2 types cannot interbreed successfully

Suggest how scientist prove they come from same species

• Interbreeding successfully between island types
• DNA analysis

Suggest why 2 species have not become more different over time (2)

• Similar environment so similar adaptations
• Original ancestor already well adapted

6.2.3 Selective breeding

Selective breeding (artificial selection)

• Process by which humans breed plants and animals for particular genetic characteristics
• By choosing parents with desired characteristics from a mixed population & breed them together

Why – to choose characteristics for usefulness or appearance

• Food crops – disease resistance
• Animals – produce more meat or milk
• Domestic dogs – gentle nature.
• Flowers – large / unusual

Advantages	Disadvantages
Produces organisms that are useful to us eg ↑food production	Selective breeding can lead to inbreeding (some breeds are particularly prone to disease or inherited defects eg pedigree dogs have hip problems) ↓no of alleles in a population – ↓variation of species – if environment changes, organisms can’t cope & die

6.2.4 Genetic engineering

Genetic engineering

• Process which involves modifying the genome of an organism by introducing a gene from another organism to give a desired characteristic
• Plant crops – resistance to diseases, produce bigger better fruits (See GM crops)
• Bacterial cells – produce useful substances eg insulin to treat diabetes

Describe process of genetic engineering (6)

1. Use enzyme to isolate required gene
2. Insert isolated gene into a vector (bacterial plasmid / virus) which is cut opened using enzyme
3. Use vector to insert gene into required cells
4. Transfer gene to animals / plants cells or microorganisms at an early stage in their development so they develop with desired characteristics

Genetically modified crops (GM crops)

Advantages	Disadvantages
Resistant to disease Resistant to insect attack / herbivores ↑yields /produce bugger & better fruits – ↓starvation in less developed countries Improve nutritional value eg Golden rice contains carotene – ↓ chance of vitamin A deficiency	Affect population of wild flowers & insects – ↓ biodiversity Haven’t fully explored effects on GM crops on human health Shouldn’t interfere with God’s creation

GM mosquitoes

Advantages	Disadvantages
↓mosquitos spreading malaria ↓ppl get malaria ↓healthcare needed Better economically for developing countries	↓crops reproduce ↓crop yield High cost of GM production Disrupt food chain

Figure 1 shows how scientists produce GM crop plants. The scientists use a GM-bacterium can invade plant cells.

Explain why scientists add glyphosate to agar? (2)

The resistance gene will be transferred from bacteria to some plant cells

6.2.5 Cloning (biology only)

Plants – tissue culture & cutting

Tissue culture

• Using small groups of cells from part of a plant to grow identical new plants
• Importance – preserving rare plant species or commercially in nurseries
• 
• Process
  1. Remove leaf from plant
  2. Scrape off small group of cells (tissue) sample from part of plant onto agar jelly.
  3. Grow tissue in agar with nutrients & plant hormones to stimulate plant cells to divide to form big mass of identical plant cells called callus.
  4. Use different mixture of hormones & conditions to form tiny identical plantlets.
  5. Plantlet clones grown on.

Advantages / Why use tissue culture	Disadvantages
↑ yields Quick Little space Grow all year	Expensive

Cuttings

• An older, but simple, method used by gardeners
• To produce many identical new plants from a parent plant
• Quick & cheap

Process

1. Cut off a branch from parent plant
2. Plant stem in damp compost
3. Keep cuttings in moist & warm conditions
4. After a few weeks, new roots develop & new plant produced

Animals – Embryo transplant & Adult cell cloning

Embryo transplant

Process

1. Split cells apart from developing animal embryo before they become specialised
2. Each cells grows into identical embryo in lab
3. Transplant identical embryos into host mothers
4. Identical cloned calves born

Advantages	Disadvantages
Produce more animals Ppl can transfer high-quality embryos around the world Produce medically useful compounds	Expensive Need skilled labour

Adult cell cloning

Process

1. Remove nucleus from unfertilised egg cell
2. At the same time, take nucleus from adult body cell eg skin cell of another individual of same species
3. Insert nucleus from adult cell into egg cell
4. Give egg cell an electric shock to stimulate it to divide to form an embryo
5. When embryo has developed into ball of cells, insert it into womb of adult female to continue its development

Advantages	Disadvantages
Produce animals with desirable characteristics eg cows produce large quantity of milk Produce large no of cloned medically useful animals Save animals from extinction Clone pets/prized animals so DNA continue after original died	Reduce variation in a population – less able to survive if environment changes Expensive Can help infertile couples but can also be abused Unethical Possible operation complications