2.3 Speciation

Kari Moreland

2.3 Speciation

Speciation is the evolutionary process by which populations evolve to become distinct species. It occurs when genetic differences accumulate to the point that individuals from different populations can no longer interbreed and produce fertile offspring. Darwin envisioned this process as a branching event and diagrammed the process in On the Origin of Species (Figure 2.3.1a). Compare this illustration to the diagram of elephant evolution (Figure 2.3.1b), which shows that as one species changes over time, it branches to form more than one new species, repeatedly, as long as the population survives or until the organism becomes extinct.

Figure 2.3.1 The only illustration in Darwin’s On the Origin of Species is (a) a diagram showing speciation events leading to biological diversity. The diagram shows similarities to phylogenetic charts that today illustrate the relationships of species. (b) Modern elephants evolved from the Palaeomastodon, a species that lived in Egypt 35–50 million years ago.Image by Open Stax, CC BY 4.0

Types of Speciation

For speciation to occur, two new populations must form from one original population, and they must evolve in such a way that it becomes impossible for individuals from the two new populations to interbreed. Biologists have proposed mechanisms by which this could occur that fall into two broad categories:

Allopatric Speciation

Allopatric speciation occurs when a population is divided by a physical barrier, such as a mountain range, river, or large distance, which prevents individuals from different groups from interbreeding. Over time, these geographically isolated populations experience different environmental pressures so they o accumulate genetic differences that eventually lead to reproductive isolation. Even if the physical barrier is later removed, the populations may no longer be able to interbreed successfully, indicating that they have become distinct species.

Figure 2.3.3 A population becomes separated by a geographic barrier; reproductive isolation develops, resulting in two separate species. Image by Andrew Z. Colvin CC BY-SA 4.0

Figure 2.3.4. In allopatric speciation, a species population becomes separated by a geographic barrier, whereby reproductive isolation evolves, producing two separate species. From this, if a recently separated population comes in contact again, low fitness hybrids may form, but reinforcement acts to complete the speciation process. Image by Andrew Z. Colvin,CC BY-SA 4.0

A well-known case of allopatric speciation can be seen in the Galápagos finches that Charles Darwin observed. Today, there are around 15 distinct finch species across the Galápagos Islands, each with unique physical traits and beak shapes adapted to specific food sources like seeds, insects, and flowers. These birds are believed to have evolved from a single ancestral species that initially colonized the islands. As different groups settled on separate islands, they became geographically isolated. Over time, genetic variations emerged within each group. Traits that improved survival and reproduction in each unique environment became more common, eventually leading to the development of multiple new species.

Sympatric Speciation

Sympatric speciation occurs without any physical separation between populations. Instead, reproductive isolation arises within a shared habitat due to behavioral or ecological differences. For instance, individuals may begin to exploit different resources, develop distinct mating preferences, or undergo chromosomal changes that prevent successful reproduction with others in the population.

Figure 2.3.5 In sympatric speciation, reproductive isolation evolves within a population without the aid of geographic barriers. Image by Andrew Z. Colvin CC BY-SA 4.0

For example, imagine a species of fish that lived in a lake. As the population grew, competition for food also grew. Under pressure to find food, suppose that a group of these fish had the genetic flexibility to discover and feed off another resource that was unused by the other fish. What if this new food source were found at a different depth of the lake? Over time, those feeding on the second food source would interact more with each other than the other fish; therefore, they would breed together as well. Offspring of these fish would likely behave as their parents and feed and live in the same area, keeping them separate from the original population. If this group of fish continued to remain separate from the first population, eventually,y sympatric speciation might occur as more genetic differences accumulated between them.

This scenario does play out in nature. For example, Lake Victoria in Africa is famous for its sympatric speciation of cichlid fish (Figure 11.19). In this locale, two types of cichlids live in the same geographic location but they have come to have different morphologies that allow them to eat various food sources.

Figure 2.3.6 Cichlid fish from Lake Apoyeque, Nicaragua, show evidence of sympatric speciation. Lake Apoyeque, a crater lake, is 1800 years old, but genetic evidence indicates that the lake was populated only 100 years ago by a single population of cichlid fish. Nevertheless, two populations with distinct morphologies and diets now exist in the lake, and scientists believe these populations may be in an early stage of speciation. Image by Open Stax, CC BY 4.0

Overview

Feature	Allopatric Speciation	Sympatric Speciation
Geographic Separation	Yes	No
Cause of Isolation	Physical barriers	Behavioural, ecological, or genetic factors
Common in	Animals and plants	More common in plants, some animals
Example	Squirrels separated by a canyon	Fish in the same lake choose different habitats

Rate of Speciation

The rate of speciation can vary widely depending on environmental conditions, genetic factors, and ecological pressures. In some cases, species evolve gradually over millions of years through a slow accumulation of changes – a pattern known as gradualism. In contrast, other species may appear relatively suddenly in the fossil record, following long periods of little change. This pattern, called punctuated equilibrium, suggests that speciation can occur in rapid bursts, often triggered by environmental shifts or the colonization of new habitats.

Figure 2.3.7 In (a), gradual speciation, species diverge at a slow, steady pace as traits change incrementally. In (b), punctuated equilibrium, species diverge quickly and then remain unchanged for long periods of time. Image by Open Stax, CC BY 4.0.

Knowledge Check

Text Description

“Formation of New Species” from Principles of Biology by Lisa Bartee, Walter Shriner & Catherine Creech is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

“Speciation” from Introductory Biology: Ecology, Evolution, and Biodiversity by Erica Kosal is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

“11.4 Speciation” from Biology and the Citizen by Colleen Jones is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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Types of Speciation

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