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Variation and Evolution

1. Variation

Variations are differences between organisms of the same species.

A species is a group of organisms that can interbreed and produce fertile off spring.

Not all organisms within the same species look alike. They all vary (differ) and this is very important for survival and for evolution.

Causes of variation

Genes- organisms vary because of genes inherited from their parents. This is called genetic variation. For example, you and your siblings look similar but not the same even if you are from the same parents. This is because you inherited different combinations of genes from your parents.

  • Genetic variations can be inherited by offspring.

Environment- what we eat, weather conditions and lifestyle may also cause variations. These are called environmental variations. For example, identical twins have the same genes, but one may end up taller then the other or even smarter than the other, also they don't think the same. These are environmental variations.

Other examples include: scars, tattoos, sun tan, and muscles from body building.

  • Environmental variations cannot be inherited by offspring.

Some variations are due to the effect of both genes and the environment.

Continuous and discontinuous variation

a. Continuous variation- some variations have a wide range, from one extreme to another with many intermediates in between.

These are called continuous variations. Examples include mass (weight) and height. For each of these, we can see that there are very few people who show the extremes (very short, very tall), while most of us are in between.

A graph to show these will have a bell shape. These variations may be due to both genes and the environment.

b. Discontinuous variation

These are variations where you either have one or the other, there are only 2 (few) alternatives, no in betweeners. Examples include:

  • gender (you are either male or female),
  • tongue rolling (you either can do it or you can't) and
  • earlobes (you either have them or you don't)

These variations are usually caused by genes only.



Zebras and horses can breed and produce an offspring called a Zorse. The zorse is infertile. Are zebras and horses in the same species?



Make a list of variations that are caused by both genes and the environment.