A species is the smallest unit of taxonomy. It may be defined as a group of interacting and interbreeding individuals which closely resemble each other and breed to produce fertile offspring and have common ancestry. For example man (Homo sapiens), frog (Rana tigrina), china rose (Hibiscus rosa -sinensis) and sunflower (Helianthus annuus) are different species. Some individuals of different species may cross-reproduce "hybrids" but they are sterile or short-lived, e.g. a mule, which is a hybrid of horse and donkey.

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However, morphological differences called variations may exist within individuals of the same species, e.g. drone, worker and queen of honey-bees. Similarly, the flowers of the 4 o’clock plant (Mirabilis jalapa) may be red, yellow or pink, and those of Bougainvillaea are red, pink, orange or white. These are not different species but variations between the same species.

Fig. 25.3 Morphological variation in honey bees

Population

A population is defined as a group of interacting and interbreeding organisms of the same species inhabiting a particular area at a particular time. A population, however, is not merely a physical group of organisms but forms the unit of an ecosystem which exhibits certain properties not shown by the individual members of the group. For example, it has a birth rate, growth rate and death rate, whereas an individual is born and dies but does not have a birth rate or death rate. Examples of these "collective properties" of a population are cited below :

(1) Density (2) Natality (Birth rate)

(3) Mortality (death rate) (4) Age distribution

(5) Sex-ratio

Density : Is the total number of individuals living per unit area or unit volume at a specific time. It is one of the important parameters of a population.

With reference to human beings, the density of the population is calculated with the help of the following formula :

n = number of individuals

a = area in square kilometers

Population density depends on environmental conditions, natality (birth rate, mortality, death rate), natural resources, emigration (outward movements or flow), immigration (inward movements) and reproduction.

Distribution of individuals in a population may be random or irregularly scattered, clumped or aggregated in groups, or uniform,(i.e. evenly distributed).

There is always interaction between the environment and a population. A population is limited by environmental factors such as food supply, predators and shelter and nesting area. These physical and biological factors which limit growth of a population is called environmental resistance. Without environmental resistance, for instance, oceans would be full of fish which lay about 8 million eggs every year, and the fields would be full of rabbits. When a species is first introduced, environmental resistance is usually low but when its population enormously increases, the environmental resistance also increases in the form of predators, competition between the members for food, space and shelter, etc. For example, when the rabbit was first introduced into Australia and the English sparrow was brought to the United States, initially their populations increased enormously but later on were brought under control by environmental resistance.