Variation in Biology (CIE IGCSE Biology)
Revision Note
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PhilExpertise
Biology Project Lead
Types of Variation
What is Variation in Biology?
- Variation is defined as differences between individuals of the same species
- Phenotypic variation is the difference in features between individuals of the same species
- Some of these differences are caused by differences in genes, which is genetic variation
- Phenotypic variation can be divided into two types depending on how you are able to group the measurements:
- Continuous Variation is when there are very many small degrees of difference for a particular characteristic between individuals and they are arranged in order and can usually be measured on a scale
- Examples include height, mass, finger length etc. where there can be many ‘inbetween’ groups
- Discontinuous Variation is when there are distinct differences for a characteristic
- For example, people are either blood group A, B, AB or O; are either male or female; can either roll their tongue or not - there are no ‘inbetweens’
- When graphs of these data are plotted, continuous variation gives smooth bell curves (a result of all the small degrees of difference), whereas discontinuous gives a ‘step – like’ shape
Height is an example of continuous variation which gives rise to a smooth bell-shaped curve when plotted as a graph
Blood group is an example of discontinuous variation which gives rise to a step-shaped graph
Phenotypic Variation
- Phenotypic variation can be caused in two main ways:
- It can be genetic - controlled entirely by genes
- Or it can be environmental - caused entirely by the environment in which the organism lives
Genetic Variation
- Examples of genetic variation in humans include:
- blood group
- eye colour
- gender
- ability to roll tongue
- whether ear lobes are free or fixed
Whether earlobes are attached (lobeless) or free (lobed) is an example of genetic variation
Environmental Variation
- Characteristics of all species can be affected by environmental factors such as climate, diet, accidents, culture and lifestyle
- In this instance ‘environmental’ simply means ‘outside of the organism’ and so can include factors like climate, diet, culture, lifestyle and accidents during lifetime
- Examples include:
- An accident may lead to scarring on the body
- Eating too much and not leading an active lifestyle will cause weight gain
- Being raised in a certain country will cause you to speak a certain language with a certain accent
- A plant in the shade of a big tree will grow taller to reach more light
Genetic and Environmental Causes
- Discontinuous variation is usually caused by genetic variation alone
- Continuous features often vary because of a combination of genetic and environmental causes, for example:
- tall parents will pass genes to their children for height
- their children have the genetic potential to also be tall
- however if their diet is poor then they will not grow very well
- therefore their environment also has an impact on their height
- Another way of looking at this is that although genes decide what characteristics we inherit, the surrounding environment will affect how these inherited characteristics develop
Mutation
- Mutations are genetic changes
- Most mutations have no effect on the phenotype as the protein that a mutated gene produces may work just as well as the protein from the non - mutated gene
- Rarely, mutations lead to the development of new alleles and so new phenotypes and if they do, most have a small effect on the organism
- Occasionally, the new allele gives the individual a survival advantage over other members of the species
- For example:
- A bird develops a mutation leading to a change in feather colours
- This makes it more attractive to birds of the opposite sex
- Which causes the bird to breed more frequently and have more chances of passing on the mutated phenotype to the next generation
- Mutations can also lead to harmful changes that can have dramatic effects on the organism - for example, sickle cell anaemia in humans
- Mutations happen spontaneously and continuously but their frequency can be increased by exposure to the following:
- Ionising radiation (e.g. gamma rays and X - rays) - which can damage bonds and cause changes in base sequences
- Some non-ionising radiation (e.g. ultra-violet) - can also damage bonds and cause changes in base sequences
- Certain types of chemicals - for example chemicals such as tar in tobacco
- Increased rates of mutation can cause cells to become cancerous, which is why the above are linked to increased incidence of different types of cancer
Mutation: Extended
- Genetic variation in populations can be caused by a variety of sources:
- Mutations
- Mutations are random genetic changes to the base sequence of DNA
- New alleles form through these random changes to DNA
- Meiosis
- New allele combinations form through segregation
- Random mating
- Which partnerships form for sexual reproduction
- Random fertilisation
- Which sperm and egg combinations occur during sexual reproduction
- Mutations
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