Species exist as populations
Organisms belong to the same species when they are sufficiently similar and can breed together to produce fertile offspring. Members of a species normally share the same chromosome number.
Different species may have different chromosome numbers, which can prevent any hybrid offspring from forming viable gametes. For example:
- a horse has 64 chromosomes, so its gametes contain 32
- a donkey has 62 chromosomes, so its gametes contain 31
- a horse–donkey hybrid therefore has 63 chromosomes
- with an odd chromosome number, chromosomes cannot pair normally during meiosis, so the hybrid is generally unable to produce functional gametes and is infertile
Members of one species do not usually exist as isolated individuals. A species instead occurs as one or more populations.
Population: members of one species present in the same particular area at the same time, with the potential to interbreed with one another.
Same species
The organisms belong to the same species and are potentially able to reproduce with one another.
Same place
They occupy the same defined space rather than being geographically separate groups.
Same time
They occur in that space during the same period, making interbreeding possible.
Appearance alone does not always determine whether organisms are members of the same species. Variation can cause members of one species to look different, and differently appearing organisms can still be capable of successful interbreeding.
Exam Tip: When defining a species, do not rely only on physical similarity. State that members can interbreed and produce fertile offspring; visible variation can occur within a species.