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Variation & antibiotic resistance

Living things of the same type belong to the same species. For example, humans are one species and dogs are another species. Individuals of the same species can reproduce to make more individuals of the same species. Two individuals belonging to different species cannot normally reproduce together. If they do, their offspring is usually infertile and unable to reproduce.

Some of the features of the different species show continuous variation, and some features show discontinuous variation.

Continuous variation

A characteristic of any species with a range of values that changes gradually shows continuous variation. Examples of such characteristics could include:

  • height
  • weight
  • foot length.

For example, human height ranges from that of the shortest person in the world to that of the tallest person. If you record the heights of a group of individuals and draw a graph of your results, it usually looks something like this: 


Discontinuous variation

A characteristic of any species with only a limited number of categories shows discontinuous variation.

Eye colour, hair colour etc show discontinuous variation – there are distinct groups but some are more common.
• Discontinuous variation is controlled by a single gene or small number of alleles. It is not usually affected by the environment.
 

Here are some examples:

  • gender (male or female)
  • blood group (A, B, AB or O)
  • eye colour.


CAUSES OF VARIATION

1. MUTATIONS – Changes to your GENES & CHROMOSOMES à different proteins & amino-acids being made & therefore variation arises – inherited (ie. In your genes)
2. MEIOSIS – Formation of GAMETES caused by cells dividing twice (haploid) - inherited (ie. In your genes)
3. FUSION OF GAMETES – ½ genes (characteristics) from mother & ½ from father in sexual reproduction. Fertilisation is RANDOM – inherited
The new cell is called a ZYGOTE and contains 46 chromosomes – 23 from the mother and 23 from the father 
4. ENVIRONMENT – External influences  -
• The characteristics of animal and plant species can be affected by factors such as climate, diet, culture and lifestyle.
•Too much food à fat
•A plant in the shade of a big tree will not be able to photosynthesise as quickly as one in the sunshine à smaller.
•Examples:
–Height - CONTINUOUS
–Weight - CONTINUOUS
–Skin colour - CONTINUOUS