Variation in Biology (Cambridge International IGCSE Biology)

What is variation in biology?

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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 ‘in-between’ 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 ‘in-betweens’

• 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 are inherited, the surrounding environment will affect how these inherited characteristics develop

Mutation

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• 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
  • this 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

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• Genetic variation in populations can be caused by a variety of sources:

  • mutations: mutations are random genetic changes to the base sequence of DNA, forming new alleles
  • 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