Topic 3 focuses on organisms, and you are supposed to understand that every individual organism is unique in some capacity and that not all their traits are identical due to genetic variation. However, organisms can be grouped together via shared traits for successful classification. At the most specific level of classification, organisms are classified as species. The biological definition of a species is a group of organisms that can interbreed and produce fertile offspring.

However, this definition does not work in several situations:

Asexual reproduction in prokaryotes and some eukaryotes does not involve interbreeding.
Extinct groups of organisms that look morphologically similar, but it is not known if they can interbreed.
Morphologically different organisms that are listed as separate species but can interbreed to form hybrid fertile offspring.
When populations form a geographical ring, where adjacent populations can interbreed but populations further away from each other cannot.

As a result of these issues, there are other definitions (concepts) of species that explain different aspects of species:

Agamospecies - differentiates between asexual species based on morphology or cytology.
Biospecies - differentiates between species based on reproductive isolation.
Ecospecies - differentiates between species based on their ecological role (niche).
Evolutionary species - differentiates between species based on their evolutionary line.
Genetic species - differentiates between species based on their gene pools.
Morphospecies - differentiates between species based on their phenotype.
Taxonomic species - differentiates between species based on their taxonomy.