iGCSE Biology Notes Variation and selection

[q] What is variation?

[a]

[q] What can variation be caused by?

[a]

• Genetic variation is the variation in a species purely caused by their genes
• Phenotypic variation is the observable variation in a species and is usually the result of a combination of genes and the environment they live in

[q] What are the types of variation?

Discontinuous variation

[a]

• Discontinuous variation is where the variation of a trait has a limited number of phenotypes no intermediates like blood group, pea shape or pea colour.
• Usually as a result from characteristics determined only be genes.

[q] What are the types of variation?

Continuous variation

[a]

• Continuous variation is where the variation of a trait can vary between two extremes like weight or height.
• Usually as a result of a combination of both genes and the environment.

[q] What is a mutation?

[a] 

• A mutation is a random change in the base sequence of DNA.
• New alleles are created by mutations
• Mutations can be caused by ionizing radiation and some kind of chemicals.
• Mutations, along with meiosis, random mating and random fertilization all increase the variation in a population

[q] Investigation!

[a] 

• Choose a characteristic to measure in the room (eye colour, glasses or no glasses, straight or curly hair etc)
• Decide whether your trait is continuous or discontinuous.
• Create a table to record your results
• Create a graph to show these results. You should use a bar graph for discontinuous data and a histogram for continuous data.
• When measuring the effect of one variable on another, we usually use a line graph.

[q] What is evolution?

[a] 

• Evolution is the changing of adaptive features of a population over time as the result of natural selection
• It was discovered by these men: Charles Darwin and Alfred Russel Wallace:

[q] What is natural selection? 

[a] 

• Step 1: There is genetic variation in a population
• Step 2: Many offspring are produced
• Step 3: There is a competition for resources and a struggle for survival
• Step 4: Some individuals are more likely to survive as they have more beneficial traits for the environment
• Step 5: These individuals can then mate and pass on those beneficial genes to the next generation

[q] What does natural selection lead to?

[a] Adaptation is the process resulting from natural selection by which populations become more suited to their environment over many generations

[q] Hydrophytes

[a] 

• Hydrophytes are plants adapted to live in water like water lilies
• They have flexible stems, stomata only on the top of leaves and the leaves contain air pockets so they float.

[q] Xerophytes 

[a] 

• Xerophytes are plants adapted to live in dry, arid climates like marram grass
• They have sunken stomata and rolled leaves to reduce transpiration by decreasing the concentration gradient and they have a thick cuticle to prevent water loss by evaporation.

[q] What is antibiotic resistance?

[a] 

• Antibiotic resistance is an example of natural selection.
• There is variation in a population of bacteria
• When humans use an antibiotic, most are killed, but some may be resistant and survive
• They now have no competition so they spread rapidly, reproduce and pass on these beneficial genes, creating a population of antibiotic resistant bacteria

[q] Artificial selection / Selective breeding 

[a] 

• Step 1: Humans breed the plants together that have the desired characteristic
• Step 2: We then look at the offspring and again breed the plants together that have the desired characteristic
• Step 3: Over many generations we can breed organisms that have the desired characteristic
• Examples include cows with more beef, chickens that lay more eggs, plants that grow bigger and more fruit

[q] Natural selection

[a] 

• The characteristics selected are the ones that suit the environment and help them survive
• Slower
• Happens to all wild organisms
• Healthier as less likely to be inbred
• More variation so more likely ro survive new diseases

[q] Artificial Selection

[a] 

• The characteristics selected are what humans think are desirable
• Faster
• Only happens if humans do it
• Can cause inbreeding and increased chance of genetic disorders
• Less variation which means more susceptible to new diseases