Our understanding of inheritance began with the work of Gregor Mendel. He bred pea plants, and later found that when he crossed pink pea plants with white pea plants all the offspring had pink flowers. He also found that certain characteristics can skip a generation, then reappear.

The importance of Mendel’s discovery was not recognised until after his death. In the mid-19th century Gregor Mendel carried out breeding experiments on plants. One of his observations was that the inheritance of. Each characteristic is determined by ‘units’ that are passed on to descendants unchanged. In the late 19th century behaviour of chromosomes during cell division was observed. In the early 20th century it was observed that chromosomes and Mendel’s ‘units’ behaved in similar ways. This led to the idea that the ‘units’, now called genes, were located on chromosomes. In the mid-20th century the structure of DNA was determined and the mechanism of gene function worked out. This scientific work by many scientists led to the gene theory being developed.

New Species

The following are reasons in which new species can arise:

  1. Isolation (When two populations of a species are separated, geographically)
  2. Genetic variation (Different alleles controlling different characteristics)
  3. Natural selection (Certain alleles make it more beneficial for the organism as it helps them survive)
  4. Speciation (Interbreeding cannot take place as the population has become too different)


Extinctions occur when there are no remaining individuals of a species still alive. The following are factors which may contribute to the extinction of a species:

  1. Change in the climate - species cannot adapt to it
  2. New disease - species cannot fight off the disease
  3. New predators - species become the prey and they die off
  4. New competitors - species struggle to survive as they cannot find their own food
  5. Humans - humans may start hunting a specific type of animal for their own benefits
  6. Catastrophe - single event can wipe out an entire species


Traditionally living things have been classified into groups depending on their structure and characteristics in a system developed by Carl Linnaeus. The linnaean system of classification groups living organisms into small yet specialised groups, based on their structure and characteristics. Linnaeus classified living things into kingdom, phylum, class, order, family, genus and species. Organisms are named by the binomial system of genus and species. The five kingdoms are: animals, plants, fungi, protists and prokaryotes. As evidence of internal structures became more developed due to improvements in microscopes, and the understanding of biochemical processes progressed, new models of classification were proposed. However through genetic analysis of DNA sequences in the recent years, the current classification system has suggested that organisms should be classified under three domains, rather than five. Through looking at DNA sequences, scientists have found that the closer the species are in relation, the fewer differences can be found within the DNA sequence.

Carl Woese is one of main scientists has updated the classification system, and has divided organisms in the following groups: Archaea (primitive bacteria - cells that live in extreme environments, has no nucleus and has an unused section of genes), bacteria (true bacteria - bacteria cells with no nucleus and no unused section of genes), and eukaryota (including protists, fungi, plants and animals - they have a nucleus and have an unused section of genes)

What does the linnaean system classify living things into?
Your answer should include: Kingdom / Phylum / Class / Order / Family / Genus / Species
What are the 5 kingdoms?
Your answer should include: Animals / Plants / Fungi / Protists / Prokaryotes
State two ways in which new species can arise.
Your answer should include: Isolation / Genetic / Variation / Natural / Selection / Speciation