B4.1 Adaptation to environment

Why do cacti have spines instead of leaves, and why do Arctic foxes have small ears? Living things are matched to their surroundings with extraordinary fit, and that fit is no accident: it is the product of natural selection acting over many generations. For B4.1 the key theme is that the environment determines which adaptations are favourable, so similar environments around the world tend to produce organisms with similar features. Understanding this lets you explain not just individual adaptations but the broad patterns of life across the planet — the biomes — and predict how species are distributed by the conditions they can tolerate.

Adaptation and the match of form to function

An adaptation is a feature that increases an organism’s chance of survival and reproduction in its environment. Adaptations arise through natural selection: variants whose features happen to suit the local conditions leave more offspring, so over time those features become common in the population. It is important to be precise — individuals do not change to suit their environment during their lifetime; rather, the proportion of well-suited individuals increases across generations.

The syllabus groups adaptations into three useful categories:

• Anatomical (structural): physical features such as a thick coat of fur or the spines of a cactus.
• Physiological: internal functioning, such as the ability to produce very concentrated urine or to tolerate high salt levels.
• Behavioural: what an organism does, such as migrating, hibernating or being active only at night.

Abiotic factors and biomes

Abiotic factors are the non-living physical and chemical conditions of an environment — temperature, water availability, light intensity, soil type, salinity and so on. These factors set the challenges that organisms must be adapted to meet. A biome is a large group of ecosystems that share a similar climate and therefore similar communities of organisms, even on different continents.

The two abiotic factors that most strongly determine which biome forms in a region are temperature and rainfall (water availability). Hot, wet regions support tropical rainforest; hot, dry regions become desert; cold regions with low rainfall form tundra. Because the same climate poses the same problems, unrelated species in matching biomes on different continents often evolve similar solutions — an idea explored further as convergent evolution.

Adaptations of plants to contrasting environments

The syllabus expects detailed examples of how plant form fits a particular habitat. Plants of hot, dry deserts (xerophytes) show adaptations that conserve water:

• Reduced leaves or spines, lowering the surface area for water loss.
• A thick waxy cuticle and sunken stomata that reduce transpiration.
• Water-storage tissue in swollen stems, and extensive or very deep root systems.

By contrast, plants adapted to waterlogged, salty conditions, such as mangroves, face the opposite problem: low oxygen in the mud and high salt. They show aerial roots (pneumatophores) that take in oxygen from the air, and mechanisms to exclude or excrete excess salt. The contrast highlights the general rule that adaptations are solutions to the specific abiotic challenges of a habitat.

Adaptations of animals and the role of behaviour

Animals show the same three classes of adaptation. A useful exam example is heat exchange in relation to body shape. Allen’s rule observes that animals in cold climates tend to have shorter extremities (ears, limbs, tails) than relatives in hot climates, because reduced surface area limits heat loss — compare the small ears of the Arctic fox with the large ears of the desert fennec fox, which help to lose heat.

Physiological adaptations include the thick blubber of marine mammals for insulation and the production of concentrated urine by desert mammals to save water. Behavioural adaptations are equally powerful: migration lets animals leave a region when conditions become harsh, while hibernation and being nocturnal allow animals to avoid extreme temperatures. Together these examples show that survival in a habitat usually depends on a combination of anatomical, physiological and behavioural features.