CIE IGCSE Biology: 复习笔记：18.1.1 Variation

• 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

• 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:
  • Gamma rays, x - rays and ultraviolet rays - all types of ionising radiation which can 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

• Mutations are random genetic changes to the base sequence of DNA
• 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:
  • Gamma rays, x - rays and ultraviolet rays - all types of ionising radiation which can 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

Summary of Sources of Genetic Variation in Populations

• Mutations
  • New alleles form through 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

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