Organisms live in a habitat. They are part of an ecosystem within which they interact with both their biotic environment (the other living organisms) and with the abiotic environment (physical and chemical factors). Members of a single species form a population and, with other populations, make up a community.
Each organism has an ecological niche that describes its role within the ecosystem:
- what it feeds on or other nutritient needs
- what feeds on it
- competition with other organisms
- its temperature, water and other requirements.
It is unusual for more than one species to occupy the exact same ecological niche - when this does happen, interspecific competition often leads to the extinction of one of the species - the competitive exclusion principle.
If it is to survive and reproduce, an organism must have adaptations for each of the above points. Adaptations may be behavioural, physiological or morphological.
A behavioural adaptation is an aspect of the behaviour of an organism that helps it to survive and reproduce - e.g. attracting a mate.
A physiological (biochemical) adaptation is one in which there is appropriate functioning of the organism or its cellular processes - e.g. ability to respire anaerobically.
A morphological (anatomical) adaptation refers to any structure that enchances the survival of an organism - for example, the spines on a cactus that prevent grazing.
All the individuals of any species tend to vary from each other. The variation can be environmental or genetic. Processes such as independent assortment of chromosomes, crossing over during gamete formation and the random mixing of chromosomes at fertilisation produce variation, as does mutation (spontaneous change in the DNA code).
This means that, within most species, there is considerable variation among individuals. If a population is variable, it is logical that some are better adapted…