Edexcel IGCSE Biology 复习笔记 3.2.6 Variation

• Variation is defined as differences between individuals of the same species
• 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 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

• Meiosis creates genetic variation between the gametes produced by an individual
• This means each gamete carries substantially different alleles
• During fertilization, any male gamete can fuse with any female gamete to form a zygote
• This random fusion of gametes at fertilization creates genetic variation between zygotes as each will have a unique combination of alleles
• Zygotes eventually grow and develop into adults
• 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