Adaptive Radiation, Speciation and Extinction

1Overview

Earth holds millions of species, from bacteria to blue whales, and all of them trace back to shared ancestors. How did one lineage become many? And why do species also disappear? This lesson covers the remaining parts of the official natural selection content: adaptive radiation, extinctions, and the way selection pressure can lead either to adaptation or to extinction. The big idea of the course, that DNA is the basis for the diversity of living things, is tested here on the scale of whole species.

2What is a species?

A species is commonly defined as a group of individuals that interbreed and produce fertile, viable offspring. Members of different species may look alike but cannot produce fertile offspring together. The definition is a useful starting point, though it works best for sexually reproducing organisms.

Speciation is the process by which one ancestral species splits into two descendant species. Its key ingredient is reproductive isolation: two populations stop exchanging genes, and each then accumulates its own changes through mutation and natural selection.

3How new species arise

Barriers to reproduction

Reproductive barriers come in two groups.

• Prezygotic barriers prevent mating or fertilization. Examples include different breeding seasons, different habitats, different courtship behaviour and incompatible reproductive structures.
• Postzygotic barriers act after fertilization. Hybrid offspring may fail to develop, or may be sterile.

Allopatric speciation

In allopatric speciation, geographic separation stops gene flow between populations. The barrier may appear because a few individuals disperse to a new place (an island, for example) or because a physical barrier arises (a rising mountain range or a changing river). With no interbreeding, the groups diverge genetically, partly because each lives in a different environment with different selection pressures. The northern and Mexican spotted owls are an example of separated populations.

Sympatric speciation

In sympatric speciation, new species arise within one shared location. In plants this can happen quickly through polyploidy, the presence of extra sets of chromosomes, which can instantly make an individual unable to breed with the parent population. It can also arise when a population splits by habitat or resource use, as with cichlid fish in Lake Apoyeque in Nicaragua.

4Adaptive radiation

Adaptive radiation is the rapid diversification of one ancestral species into several new species, each adapted to a different habitat or way of life (niche). Britannica describes it as the evolution of a group of animals or plants into many types suited to specialized ways of life, seen best in closely related groups that diversified in a relatively short time.

Adaptive radiations are common where new opportunities appear, and archipelagos far from the mainland are classic settings. A small number of colonists arrive, find habitats with few competitors, and diversify. Well-known examples include:

• Darwin’s finches (Galapagos finches), whose beak shapes suit different foods.
• Hawaiian honeycreepers, whose beaks adapted to different foods.
• Cichlid fish, which have diversified into many species in lakes.
• Australian marsupials, which diversified into many forms.
• Early mammals, which, beginning in the Paleogene about 66 million years ago, diversified into forms adapted to running, leaping, climbing, swimming and flying.

Islands show the result clearly: in Hawaii, over 90 percent of native flowering plants, land mollusks, birds and insects are endemic, meaning that they are found nowhere else.

5Extinction

Extinction is the dying out of a species. It happens when species decline because of environmental forces, whether natural or human-made, or because of evolutionary changes in their members. Extinction is a normal part of the history of life: the large majority of species that ever lived are gone. Two kinds are usually distinguished:

• Background extinction, a steady, low rate of species loss.
• Mass extinction, when many species disappear in a geologically short time. Five major mass extinctions are usually named: the Ordovician-Silurian, Devonian, Permian, End-Triassic and Cretaceous-Tertiary (K-Pg) events.

Recorded examples include the dodo, the thylacine (Tasmanian wolf) and the passenger pigeon, and the golden toad, which was last sighted in 1989 and is believed to be extinct.

Causes of extinction today

Biologists identify five main threats to biodiversity, most linked to human population growth and resource use:

1. Habitat loss, including outright destruction and fragmentation into isolated patches. It has been called the number one cause of species extinction globally.
2. Pollution, for example nutrient runoff that causes eutrophication in water bodies.
3. Overexploitation, removing organisms faster than they can be replaced, as in overfishing or poaching.
4. Invasive species, which outcompete or prey on natives. Increased mobility and trade have driven their spread.
5. Climate change, which causes range shifts, mismatched interactions between species, sea-level rise and ocean acidification.

6Selection pressure: adaptation or extinction

When an environment changes, selection pressure on a population changes with it. There are broadly two outcomes. If the population contains heritable variation that suits the new conditions, individuals with those traits survive, and the population adapts. If it does not, or if the change is faster than selection can keep up, the population shrinks and may become extinct. This is the same selection that produces adaptations in the previous lesson; the difference lies in whether suitable variation exists.

Genetic diversity is therefore a kind of insurance. A population with many alleles is more likely to contain some that work under new conditions. Genetic diversity has been described as central to the long-term survival of any species. When threats shrink a population, variation is lost, and the population has fewer ways to adapt.

7Linking radiation and extinction

Extinction and adaptive radiation are connected. When species disappear, ways of life (niches) are left open, and surviving lineages can diversify into them. The diversification of mammals in the Paleogene followed the end of the Cretaceous, a period that closed with one of the five mass extinctions. The balance between the formation of species and their loss is what shapes biodiversity at any given time.

Today the balance is shifting toward loss. People are both a cause (habitat destruction, introduced species, pollution, climate change) and the only species able to act in response. Genetic knowledge is increasingly part of the response: for example, researchers use genetic methods to study rare and endangered species, to measure connectivity between populations separated by roads and to monitor habitats using DNA collected from air, water and soil (environmental DNA). The final lessons return to these applications.

The next lesson turns from natural processes to human-directed ones: artificial selection, breeding and agriculture.

8Practice problems with solutions

Problem 1: classify the speciation

A lake is divided by a falling water level into two basins. Fish in each basin evolve separately, and many thousands of generations later they will not interbreed when the lake reunites. This is allopatric speciation: geographic separation came first, then genetic divergence, then reproductive isolation. In contrast, a plant that doubles its chromosome number in one generation and cannot breed with its parent population has become isolated in the same place, which is sympatric speciation by polyploidy.

Problem 2: prezygotic or postzygotic?

Two frog species live in the same pond, but one breeds in spring and the other in autumn. This is a prezygotic barrier (different breeding seasons). Two plant species can cross-pollinate, but the seeds fail to develop. This is a postzygotic barrier (hybrid inviability).

Problem 3: adaptive radiation or not?

Ten unrelated species on different continents each develop a streamlined body for swimming. This is convergent evolution, not adaptive radiation, because the species do not share a single recent ancestor that diversified. Adaptive radiation is the diversification of one ancestral lineage into many forms.

Why extinction matters for genetics

Every species is a distinct collection of alleles. When a species disappears, its alleles disappear with it, including any that could have been useful for crops, medicine or ecosystems. When a population shrinks, it loses alleles even if it survives, leaving fewer options for adapting to disease or climate change. Genetic diversity is therefore treated as part of biodiversity, alongside the diversity of species and ecosystems.

Case study: a species under pressure

Consider an amphibian that depends on a small wetland. Habitat loss shrinks the wetland (threat 1), runoff adds nutrients (threat 2), and an introduced predator preys on tadpoles (threat 4). Individuals with traits that help them avoid the predator or tolerate poorer water survive better, but only if some individuals already have those traits. A small, isolated population is less likely to have them. Conservation actions might include protecting the wetland, controlling the invader and allowing animals to move between populations, which keeps genetic diversity higher. This single example connects mutation, variation, selection pressure, invasive species and extinction.

Summary

• Speciation requires reproductive isolation and time for genetic differences to accumulate.
• Adaptive radiation produces many species from one ancestor, often where new niches are available.
• Extinction has several human-driven causes today, and genetic diversity improves the chance that a population can adapt.

Reading evidence for adaptive radiation

Biologists look for several clues that a group arose by adaptive radiation: the species share a recent common ancestor, they live in different habitats or feed on different foods, and the traits that differ (such as beak shape) match those ways of life. Genetic comparison of DNA can confirm how closely related the species are. Because most of the diversity of Hawaiian birds and plants is endemic, researchers treat islands as natural laboratories, and they are also the places where invasive species and habitat loss can remove species quickly.

Selection pressure and time

The speed of change depends on the strength of the pressure, the amount of heritable variation and the generation time. Bacteria, with short generation times, can adapt to antibiotics within a short span. Large animals with long generations adapt slowly, which makes them more vulnerable to rapid environmental change such as habitat loss. This difference helps explain why different groups face different extinction risks.

Vocabulary check

Students often mix up three terms. Adaptation is a heritable trait that suits an environment. Speciation is the splitting of one species into two. Extinction is the end of a species. Selection pressure links all three: it favours adaptations, can drive populations apart until they become new species, and can remove populations that lack suitable variation.

?Quick check

Try each question first, then reveal the answer.