Adaptation to Environment

An adaptation is an inherited characteristic that increases an organism's chance of survival and reproduction in its particular habitat. Adaptations arise through natural selection: variants better suited to the environment leave more offspring, so the favourable allele becomes more common over generations.

Biologists classify adaptations into three categories:

1. Structural (anatomical) adaptations — physical body features.

• Polar bear: thick fur and a layer of blubber for insulation.
• Cactus: spines (reduced leaves) and a thick waxy cuticle to cut water loss.
• Camel: large flat feet to spread weight on sand.

2. Physiological (functional) adaptations — internal biochemical or metabolic processes.

• Kangaroo rat: produces highly concentrated urine to conserve water.
• Antifreeze proteins in some fish blood prevent freezing in polar seas.
• Many desert mammals can tolerate large rises in body temperature during the day.

3. Behavioural adaptations — what the organism does.

• Desert animals are often nocturnal or burrow to avoid daytime heat.
• Migration of birds to follow food and favourable temperatures.
• Huddling in penguins to reduce heat loss.

The categories often overlap — a single organism shows a combination of all three working together.

Xerophytes are plants adapted to dry (arid) habitats, where the challenge is reducing transpiration while still photosynthesising.

• Reduced leaf area — leaves reduced to spines (cactus) or needles, lowering the surface area for evaporation.
• Thick waxy cuticle — a waterproof barrier limiting cuticular water loss.
• Sunken stomata in pits, and leaf hairs (trichomes) — both trap humid air next to the stomata, reducing the diffusion gradient for water vapour.
• Rolled leaves — marram grass rolls its leaves so stomata face inward into a moist, sheltered chamber.
• Water-storage tissue — succulent stems (cacti) store water; many have shallow, wide-spreading roots to absorb brief rain.

Hydrophytes are plants adapted to water (e.g. water lily): air spaces (aerenchyma) for buoyancy and gas exchange, and stomata on the upper leaf surface (the side in contact with air).

Halophytes tolerate high salt, such as mangroves in salty coastal mud. Adaptations include salt-excreting glands, salt-filtering roots, and aerial roots (pneumatophores) that take in oxygen from waterlogged, anaerobic mud.

Cold-climate animals must conserve body heat.

• Insulation — blubber (whales, seals) and thick fur or feathers.
• Small surface-area-to-volume ratio — large, rounded bodies and short extremities lose heat slowly (Allen's and Bergmann's rules). A large animal has less surface per unit volume, so less relative heat loss.
• Counter-current heat exchange — in the legs of wading birds and the flippers of whales, warm arterial blood flowing outward runs alongside cool venous blood returning, so heat is transferred back into the body and not lost at the extremities.

Desert animals must avoid overheating and conserve water: nocturnal/crepuscular activity, burrowing, concentrated urine and dry faeces, and large ears (e.g. fennec fox) that radiate excess heat.

Biomes are large geographic regions with characteristic communities of organisms. The terrestrial biome that forms in a region is determined chiefly by two abiotic factors: temperature and rainfall (precipitation).

• Hot + wet → tropical rainforest (high productivity, broad leaves).
• Hot + dry → desert (sparse, xerophytic vegetation).
• Cold + dry → tundra (low shrubs, permafrost).
• Temperate + seasonal rainfall → grassland or deciduous forest.

Organisms in a given biome share adaptations suited to its climate, because the same abiotic factors act as selection pressures.

Convergent evolution occurs when unrelated species independently evolve similar adaptations because they face similar environmental pressures (the environment "shapes" the solution).

• Cacti (Americas) and Euphorbia (Africa) — unrelated plant families — both evolved succulent, spiny, water-storing forms in deserts.
• Sharks (fish), dolphins (mammals) and extinct ichthyosaurs (reptiles) all evolved a streamlined fusiform body and fins/flippers for fast swimming.

These similar features are analogous structures (similar function, different evolutionary origin), in contrast to homologous structures (shared ancestry, e.g. the pentadactyl limb), which result from divergent evolution.