Species & Taxonomy


The definition of a species is if two organisms can produce fertile offspring. Whilst some members of different species can reproduce to make offspring, the offspring are infertile and therefore classes as hybrids and not a species. E.g. a lion and a tiger can reproduce to make a liger or a tion.

Species & Taxonomy, figure 1

Two other factors than can also be considered too. Firstly, do they have similar physical features, biochemistry and DNA, but this alone cannot be used because individuals within a species do not have identical features, but also can have similar features to members of different species. Different species often look very similar, but this is because they have both evolved over time to have similar adaptations to their environment. Secondly, the must occupy the same ecological niche.

Binomal Naming System

The way species are named is universal. The reason for this is so that different names are not used in different countries, would could lead to errors in estimates of the number of individuals of different species. The name system used in the binomial system, where individuals are named after their genus and species, eg, Homo sapiens and humans. Individuals can be classified according to their physical and biochemical similarities.

The Linnaeus classification system is a hierarchy system that is made up of the following taxa:

  • Domain
  • Kingdom
  • Phylum
  • Class
  • Order
  • Family
  • Genus
  • Species

This hierarchical system uses smaller groups placed within larger groups, but there are no overlaps between each group. Each level is known as a taxon (taxa plural). There are millions of species (that we know about) and we need a system to organise them. The reason for this is to aid us in understanding relationships between organisms and keep track of changes. For this reason the system is universal.

Below is an example using the human classification:

  • Domain = Eukaryote
  • Kingdom = animalia
  • Phylum = chordata
  • Class = mammalia
  • Order = primates
  • Family = hominidae
  • Genus = Homo
  • Species = sapiens

Individuals can also be classified using the phylogenetic system. This is when species are grouped according to their evolutionary origins and relationships. Phylogeny is the study of the evolutionary origins__ __of groups of organisms. It is a way to determine how closely related species are. All organisms have evolved from shared common ancestors (relatives).

This is shown on a phylogenetic tree, see below. Each branch in the phylogentic tree indicates a common ancestor the species evolved from.

Species & Taxonomy, figure 1

The accuracy of classification and phylogentic trees is improving due to advances in immunology and genome sequencing.

The DNA base sequence of organisms for particular genes can be compared as a measure of how closely related organisms are. This is because the more closely related the species the higher the degree or similarity in the bases sequences. Distantly related organisms usually show fewer similarities. This is because mutations accumulate over time.

The sequence of amino acids in the polypeptide chain of proteins can also be compared because this sequence is determined by DNA. The degree of similarity between the amino acid sequence of the same protein in 2 species will show how closely related they are. The common protein that is compared between species is cytochrome C, which is found in the mitochondria because most species have this protein.

Courtship Behaviour

Courtship behaviour is the term given to the different behaviours that animals demonstrate to attract a mate. Each species demonstrates its own unique behaviour and this is essential for successful mating.

Because the behaviour is unique to a particular species and sex of the species the behaviours enables animals to recognise members of their own species and the opposite sex.

Animals will only display the courtship behaviours when they are sexually mature and fertile. Therefore this helps synchronises mating behaviour so that the animals are more likely to mate when the female is releasing eggs.

Courtship behaviour can also result in increased survival of the offspring. This is because the ritual will help form a pair bond between the parents, so they may be more likely to stay together and raise the offspring as a team. It also allows females to select a strong and healthy mate to reproduce with and therefore ensures more successful alleles are passed on.

Courtship behaviours can range from a particular sequence of dance moves, certain sounds, release of pheromones or displays of feathers.

Species & Taxonomy, figure 1

What is meant by a hierarchy?
Your answer should include: smaller / larger / group / no / overlap
What is the human's binomial name?
Your answer should include: Homo / sapien
What is classification useful?
Your answer should include: Organise / organisms
Why is it important that the binomial system is universal?
Your answer should include: share / data