Chapter 17: Variation and Evolution

Chapter 17: Variation and Evolution


Variation : differences amongst members of the same species.

Variation amongst species occurs by two mechanisms:

  • Sexual reproduction: produces variation due to meiosis (see Chapter 13: The Cell Cycle)
  • Mutation: produces variation due to changes in the DNA

Mutation: changes in the structure or amount of DNA in a cell.

Mutations can occur spontaneously – for no apparent reason – or by means of an agent, called a mutagen.

Mutagen: agent that increases the rate of mutations.

Examples of mutagens:

  • Cigarette smoke
  • UV light
  • Asbestos
  • Radon gas
  • X-rays

Mutations can be of two types:

  1. Gene mutations
  2. Chromosome mutations

Gene mutation: changes in the structure of a single gene; e.g. sickle cell anaemia.

Chromosome mutation: changes in the structure or number of chromosomes; e.g. Down’s syndrome.


Evolution: genetic changes in species, over a long period of time, to produce new species in response to environmental stresses.

Species: group of similar organisms that can interbreed to produce fertile offspring.

Speciation: formation of a new species following many changes in the structure of an organism until the new species cannot interbreed to produce fertile offspring with the original species.

Theory of Natural Selection

Natural selection: process by which particular traits become more common in a population due to that trait being advantageous to the species.

Examples of natural selection:

  • Sickle cell anaemia (SCA):

SCA is a recessive condition where the haemoglobin in red blood cells does not form properly due to a gene mutation. People will only suffer from SCA if they are homozygous recessive for the SCA gene. SCA causes the red blood cells to take up a sickle shape. They cannot carry as much oxygen as normal red blood cells. People with SCA have to have regular blood transfusions. However, people who are carriers of the defective gene (heterozygous) show increased resistance to malaria. This gives them an advantage over homozygous dominant and homozygous recessive people. This is an example of natural selection in humans where people who are heterozygous for the SCA gene have an advantage.

  • MRSA

MRSA stands for multi resistant Staphylococcus aureus. It is a species of bacterium that is resistant to all known antibiotics. Overuse of antibiotics has given a chance to certain strains of bacteria to become resistant to antibiotics. Strains that develop mutations that give them the ability to survive in the presence of antibiotics are naturally selected for. They will reproduce and pass on this trait to their offspring.

Charles Darwin and Alfred Russell-Wallace

Darwin and Russell-Wallace were two English naturalists who both independently came up with the Theory of Natural Selection. Darwin was first to publish his findings, hence he received the most recognition.

Darwin’s theory of natural selection made a number of observations and conclusions:


  1. Species produce many more offspring than is necessary (overbreeding).
  2. There is a limited supply of resources to allow survival of the species.
  3. Numbers of species remain relatively constant over long periods of time.
  4. All species show variations among their members and these variations are inherited.


  1. There is a ‘struggle for existence’ (competition) between species and between members of the same species for food, space, shelter and mates.
  2. Organisms most suited to their environment, due to favourable characteristics, will survive and reproduce at a faster rate than those less well suited.
  3. The characteristics that make an organism most suited to its environment will accumulate among the population.

Evidence for evolution

  • Paleontology: study of fossils.

Paleontology is a source of evidence for evolution as fossils show a detailed timeline of the evolution of many organisms.
Fossil: preserved remains of an organism, part of an organism or an imprint left by that organism.

  • Comparative anatomy: study of the similarities and differences in the structure of living organisms.

Comparative anatomy is a source of evidence for evolution as certain structures of organisms from different species show similarities; e.g. the pentadactyl limb is an appendage with five digits that many different mammals possess (humans, dolphins, whales, horses, bats,…).

  • Comparative embryology: study if the similarities and differences in the structure of the embryo from different species.

Comparative embryology is a source of evidence for evolution as the structure of the early embryo from many different species is very similar.

  • Comparative biochemistry: study of the similarities and differences in the chemistry of living organisms.

Comparative biochemistry is a source of evidence for evolution as all living organisms possess DNA and/or RNA and its molecular structure is essentially the same for all living organisms.