Evolutionary Ecology: Natural Selection, Adaptation, Speciation and Coevolution

Ecology and evolution are two sides of the same coin. Ecological interactions such as competition, predation and climate determine which individuals survive and reproduce, and that drives evolution. Evolution, in turn, shapes the traits that determine how species interact. Evolutionary ecology studies this two-way relationship.

What you will learn

How natural selection works and what it requires
Types of selection and examples of adaptation
How new species form
Coevolution between predators and prey, hosts and parasites, and mutualists
Examples of rapid evolution happening today

Natural selection

In 1859, Charles Darwin published On the Origin of Species, presenting the theory of evolution by natural selection, which Alfred Russel Wallace had arrived at independently. Natural selection requires three conditions:

Variation: individuals in a population differ in their traits.
Inheritance: some of that variation is passed from parents to offspring.
Differential survival and reproduction: individuals with some traits leave more offspring than others, often because resources are limited and not all young survive.

When these conditions are met, advantageous traits become more common over generations. Evolution is therefore defined as a change in the genetic make-up of a population over time. Individuals do not evolve; populations do.

Darwin’s idea was later combined with genetics in the twentieth century to form the modern synthesis, and today it is supported by evidence from fossils, comparative anatomy, genetics and direct observation.

Types of selection

Directional selection favours one extreme. When drought hit the Galápagos island of Daphne Major in 1977, only large, hard seeds remained. Medium ground finches with bigger beaks survived better, and the average beak size of the next generation increased measurably.
Stabilising selection favours intermediate values. Human babies of average birth weight have historically had the highest survival.
Disruptive selection favours both extremes. In the African seedcracker, a finch, birds with either small or large beaks do well, because they specialise on soft or hard seeds, while intermediate birds do poorly.
Sexual selection favours traits that improve mating success, even if they reduce survival: the peacock’s tail, the red deer’s antlers and the elaborate displays of birds-of-paradise.

Adaptation

An adaptation is a trait that has evolved because it improves survival or reproduction in a particular environment. Examples include:

Structural: the thick fur and small ears of Arctic foxes; the long neck of the giraffe; the camouflage of snow leopards.
Physiological: antifreeze proteins in Antarctic fish; the ability of camels to tolerate large water loss; the high-altitude haemoglobin of bar-headed geese.
Behavioural: migration, hibernation, tool use by chimpanzees and New Caledonian crows, cooperative hunting by wild dogs and orcas.

Two general patterns relate body shape to climate. Bergmann’s rule notes that within a group, animals in colder climates tend to be larger, which reduces heat loss relative to body mass. Allen’s rule notes that animals in colder climates tend to have shorter limbs, ears and tails. Compare the small-eared Arctic fox with the huge-eared fennec fox of the Sahara.

How new species form

Speciation is the formation of new species. It usually requires populations to be separated so that they stop exchanging genes:

Allopatric speciation occurs when a population is split by a geographical barrier, such as a river, a mountain range or the sea. Chimpanzees and bonobos, for example, are separated by the Congo River.
Sympatric speciation occurs without a geographical barrier, for example when populations specialise on different foods or breeding times.
Adaptive radiation is the rapid evolution of many species from a common ancestor, each adapted to a different niche. Darwin’s finches, Hawaiian honeycreepers, the lemurs of Madagascar and the cichlid fish of Africa’s Rift Valley lakes are famous examples.

Coevolution

Coevolution happens when two or more species influence each other’s evolution.

Predators and prey. Cheetahs evolved extreme speed; gazelles evolved speed, agility and endurance. This escalation is often called an evolutionary arms race.
Hosts and parasites. Common cuckoos lay eggs that mimic those of their hosts; hosts evolve better egg recognition, and cuckoos in turn evolve closer mimicry.
Plants and herbivores. Milkweeds produce toxins; monarch butterfly caterpillars evolved to tolerate the toxins and store them, becoming poisonous to birds themselves.
Mutualists. Darwin predicted in 1862 that a Madagascan orchid with a nectar spur about 30 cm long must be pollinated by a moth with an equally long tongue. The moth, Morgan’s sphinx moth, was described in 1903, and its pollination of the orchid was finally filmed in the 1990s.

Rapid evolution in the modern world

Evolution can be fast enough to observe in a human lifetime, and people are often the selective force:

Peppered moths in England became mostly dark during the Industrial Revolution, when soot blackened trees, and returned to mostly pale after clean-air laws.
Elephants without tusks. During the Mozambican civil war, intense ivory poaching in Gorongosa National Park favoured tuskless females. The proportion of tuskless females rose sharply, and a 2021 study identified the genes involved.
Antibiotic and pesticide resistance evolves in bacteria and insects within years.
Fish getting smaller. Heavy fishing that removes the largest fish has favoured cod and other species that mature earlier and at smaller sizes.
Urban evolution. Species living in cities, from lizards to blackbirds, show evolutionary changes in behaviour, body shape and song.

Case study: the Grants’ finches

For more than 40 years, Peter and Rosemary Grant studied Darwin’s finches on the tiny island of Daphne Major in the Galápagos, measuring and tagging nearly every bird. They documented natural selection on beak size during droughts and wet years, the effects of competition between species, hybridisation, and in 1981 the arrival of a single large cactus finch from another island that founded a new, reproductively isolated lineage within a few generations. Their work provided some of the most detailed evidence of evolution happening in real time.

Try it yourself

Pick an animal you like and list three of its adaptations: one structural, one physiological and one behavioural. For each, describe the environmental challenge it solves.

Common misconceptions

“Evolution means organisms try to adapt.” Variation arises randomly; selection acts on what already exists.
“Evolution always produces perfect designs.” Evolution works with existing structures and trade-offs, so many traits are compromises.
“Humans evolved from chimpanzees.” Humans and chimpanzees share a common ancestor that lived around 6 to 7 million years ago.

Key terms

Natural selection: differential survival and reproduction of individuals with heritable traits.
Adaptation: a trait that improves fitness in a given environment.
Speciation: formation of new species.
Adaptive radiation: rapid diversification from a common ancestor.
Coevolution: reciprocal evolutionary change between interacting species.

Quick quiz

Test what you have learned. Scoring 60 percent or more marks this module as complete.

Evolutionary Ecology: Natural Selection, Adaptation and Coevolution