Variation

• Variation is the differences found within the same species
• Variation can be caused by both sexual reproduction and environmental influences
• Individuals in a population are usually similar but not identical
• Some variation is genetic, some is environmental, and some is a combination of both
• Genetic causes of variation:

  • Children inherit half of their features from each parent
  • Sperm and egg cells contain half of the genetic information needed for an individual
  • When sperm and egg cells join at fertilization, a new cell (zygote) with all the genetic information is formed
  • Examples of genetic variation in humans include blood group, skin colour, and eye colour
• Environmental causes of variation:

  • Characteristics of animal and plant species can be affected by factors such as climate, diet, accidents, culture, and lifestyle
  • Examples of environmental variation include weight gain or loss due to food intake and plant growth in response to light availability
• Genetic and environmental causes combined:

  • Some features vary due to a combination of genetic and environmental causes
  • Identical twins inherit the same features but differences in lifestyle can lead to variations in weight, for example
• Some of the features of the different organisms in a species show continuous variation, and some show discontinuous variation.

  Continuous variation:

  For any species, a characteristic that changes gradually over a range of values shows continuous variation. Examples of such characteristics are:

  • height
  • weight
  • hand span
• Height ranges from that of the shortest person in the world to that of the tallest person. Any height is possible between these two extremes. So it is called continuous variation.

  The shape of this graph is typical of a characteristic with continuous variation. The more people you measure, and the smaller the range of categories you use, the closer the results will be to the curved line. The shape seen in this graph is called a bell shaped curve, and is the result of a variable being normally distributed.

  
• Discontinuous variation

  A characteristic of any species with only a limited number of possible values shows discontinuous variation. For example:

  • gender (male or female)
  • eye colour
  • blood group
• Human beings have one of four blood groups, A, B, AB or O. There are no values in between (intermediate values), so this shows discontinuous variation.

  
• Asexual reproduction

  Only one parent is needed in asexual reproduction. There is no fusion of gametes, so genetic material does not mix. This means that the offspring produced through this process are clones, ie they are genetically identical to the parent.

  Examples of organisms that use asexual reproduction include:

  • bacteria
  • production of spores by fungi
  • some plants, such as strawberries, use runners
  • formation of tubers in potatoes and bulbs in daffodils
• Mitosis

  Asexual reproduction uses the process of mitosis to create the identical copies (clones) of the parent cell.

  It therefore leads to offspring that show no genetic variation.

  
• Sexual reproduction requires two parents, a male and a female, each of which produce gametes
• Fertilisation is the fusion of the nucleus of a male gamete with the nucleus of a female gamete
• In humans, each gamete has half the total of 46 chromosomes, which are referred to as haploid (23 chromosomes)
• When two gametes combine, they merge to have 46 chromosomes, referred to as diploid
• This fusion produces a new cell called a zygote, which matures into an embryo
• The number of cells increases by mitosis as the embryo develops
• Cells begin to differentiate or specialise as the embryo develops
• The offspring is genetically different from its parents, showing variation