4.3, 4.4 - Niches, adaptations and natural selection

Niche and habitat

A habitat describes where an organism lives, whereas a niche describes how it lives in relation to its environment.

A niche includes more than a food source. It includes where and when food is obtained, the temperature or moisture conditions tolerated, and relationships such as predation, competition and pollination. Resources such as food and nesting sites can be used up; conditions such as temperature influence how well an organism functions.

Consider an illustrative woodland community. One bird species feeds on insects among high branches during daylight; another searches the ground for invertebrates. The woodland is their shared habitat. Their feeding positions contribute to different niches. Different niches can reduce competition for a limiting resource, although the species may still overlap in other resource use. Finding two species in the same habitat therefore does not establish that they have identical niches.

The distinction also explains why moving an organism into a place that looks suitable may fail. The place may lack its food, breeding sites or suitable conditions, or contain competitors or predators that prevent successful reproduction. A niche connects the organism's features to these demands.

Three kinds of adaptation

An adaptation has a particular benefit under particular environmental conditions, and that benefit needs a biological explanation.

The word can also describe the evolutionary process by which such features become common. A useful explanation follows the feature through its effect on function to its likely benefit in the stated environment.

The examples are supported by primary studies of penguin huddling, fish antifreeze action and pocket-mouse pigmentation.

The categories describe different aspects of biology rather than three independent causes. Fur colour is an anatomical description of a body covering, but pigment production involves physiological processes. Huddling is a behavioural example; its benefit depends on heat transfer and the birds' physiology. Observing a useful action does not, on its own, establish how much of the behavioural tendency is inherited rather than learned.

An adaptation is not universally best. Camouflage depends on background, and a feature's benefits may come with costs. Selection favours a greater overall contribution to later generations under the conditions that actually occur; it does not produce a perfect organism.

Heritable variation and fitness

Natural selection requires differences between individuals that can affect their contribution to the next generation.

Individuals in a population differ in their phenotypes, their observable features. Some differences arise from different inherited alleles, some from environmental influences, and many from both. An allele is a version of a gene. For selection to produce an evolutionary change through a trait, at least some of the variation associated with reproductive success must be heritable: offspring must tend to inherit the relevant genetic differences.

Mutation is the ultimate source of new alleles. Mutations do not occur because an organism needs a particular useful change. They occur without regard to whether their effects will be beneficial in the current environment. Meiosis and fertilisation produce new combinations of existing alleles; they do not manufacture the allele an individual needs. Selection then acts on the phenotypes produced by the available variation. A mutation may occur before or after an environmental change, but the change does not direct mutation towards the required solution.

Fitness therefore means more than physical strength, speed or lifespan. Survival matters because an organism generally has to survive to reproduce, but an individual that survives for a long time and leaves no offspring passes on no alleles through reproduction. Differences in mating success or number of surviving offspring can also cause selection, even when adult survival is similar.

A feature acquired during an individual's life is not automatically inherited. For example, an injury can alter an animal's appearance without altering the alleles transmitted to offspring. Such a change cannot by itself cause the injured appearance to spread through natural selection. The relevant link is inherited variation → a difference in reproductive success, rather than any visible difference → evolution.