Learning Objectives

  1. Define and apply the biological, morphological, ecological, and phylogenetic species concepts, while recognizing that speciation is a process.
  2. Distinguish between sympatric and allopatric speciation.
  3. Define, recognize, and understand the significance of reproductive isolating mechanisms in reducing gene flow between populations, and distinguish between pre-zygotic and post-zygotic barriers to reproduction.
  4. Recognize that polyploidy can create reproductively isolated species in a single generation, while other paths toward speciation are much slower to acquire reproductive isolation.

What is a species? Species Concepts

Speciation is the process by which two new species form. If you find this definition problematic, know that biologists do, too. Biologists have a long tradition of debating whether species really exist, how to define a species, and what it means biologically to say that there are species. The most prominent and relevant definitions for us are framed around:

  • the ability of two individuals to successfully produce viable, fertile offspring (biological species concept or BSC),
  • whether individuals look similar (morphological species concept or MSC),
  • how closely related individuals are evolutionarily (phylogenetic species concept or PSC), and
  • whether the ecological opportunity exists for individuals, who might be physically capable to interbreed in a lab or zoo environment, to interbreed in the wild (ecological species concept or ESC).

Which species concept is most useful depends on circumstance and available data. For instance, in the figure below, branches that don’t reach the top of the diagram represent extinct species (or taxa).

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 are drawn today to illustrate the relationships of species. (b) Modern elephants evolved from the Palaeomastodon, a species that lived in Egypt 35–50 million years ago. (Source: OpenStax Biology)

Speciation is readily illustrated using branching diagrams. (a) The only illustration in Darwin’s On the Origin of Species is a diagram showing speciation events leading to biological diversity. The diagram shows similarities to phylogenetic charts that are drawn today to illustrate the relationships of species. (b) The two species of modern elephants evolved from the Palaeomastodon, a species that lived in Egypt 35–50 million years ago. Other branches from this lineage did not survive to present day (they are extinct). (Source: OpenStax Biology)

Mastodons are no longer living, so it becomes impossible to know if mastodons from different populations were able to interbreed (BSC). We can look at their morphologies by comparing teeth, bones, tracks, and sometimes fur, and that gives us a basic idea of whether mastodons were of the same species (MSC), but we don’t have lots of complete fossils to examine and we know of living species that look the same yet cannot interbreed. We are left with a combination of fossil and DNA evidence that allows us to construct a phylogeny, which shows us that the combination of factors (fossil morphology, DNA comparison, geographic location) can be combined using an mathematical algorithm that groups species based on phylogeny (PSC). The MSC and PSC are also more useful for analyzing asexually-reproducing organisms, such as bacteria and archaea, where the BSC isn’t relevant because bacteria and archaea reproduce asexually.


Sympatric and Allopatric Speciation

Watch this Crash Course Biology video for a 10 minute overview of speciation that hits all the salient points.

Speciation is all about gene flow – or lack thereof. The less gene flow, the more likely speciation is to occur. There are two different types of speciation, based on the mechanism that prevents gene flow: allopatric speciation and sympatric speciation.

Allopatric speciation can occur when two populations are isolated from each other (allopatry), creating the absence of gene flow. In the figure below, geographic isolation occurs when a beetle population is divided by a body of water that prevents interbreeding between the two populations. Small changes occur in each isolated population over time, and if changes occur that prevent successful production of fertile offspring, then when the isolating ‘barrier’ is removed, the two populations can no longer interbreed. Sympatric speciation occurs when two populations in the same location become unable to interbreed.

What was once a continuous population is divided into two or more smaller populations. This can occur when rivers change course, mountains rise, continents drift, or organisms migrate. The geographic barrier isn’t necessarily a physical barrier that separates two or more groups of organisms — it might just be unfavorable habitat between the two populations that keeps them from mating with one another (University of California Museum of Paleontology’s Understanding Evolution (http://evolution.berkeley.edu)


The evolution of reproductive isolation establishes “good” biological species

Both allopatric and sympatric speciation are the product of reproductive isolation, which reduces gene flow between populations. Reproductive isolating mechanisms prevent two individuals from distinct species from interbreeding to produce viable and fertile hybrid offspring. They can affect successful mating either before or after the egg unites with the sperm to form a zygote. Pre-zygotic reproductive isolation prevents mating or fertilization and might include behavioral differences in mating song or dance, differences in when and where individuals attempt to mate, or sperm-egg incompatibility.

Pre-zygotic reproductive isolation barriers include:

  • Behavioral: Birds sing different songs, and females only respond to the song of males from their own species.
  • Habitat: Walking stick insects breed on their preferred plant species only.
  • Temporal: Diurnal versus nocturnal mating separates mating in time.
  • Mechanical: Matings are not compatible at the genitalia or the egg-meets-sperm level.

Male damselfly species have evolved species-specific shapes to their penises (four examples shown above) that are lock and key compatible only with females from the same species. Reproductive organ incompatibility keeps the species reproductively isolated. (From OpenStax Biology.)

Post-zygotic reproductive isolation prevents development or reproduction of the offspring and includes developmental failures and spontaneous abortion early on up to fully formed adult (viable) offspring that are themselves sterile (infertile).

With renewed or continues contact between two populations undergoing speciation, there are three possible outcomes:

  1. Individuals hybridize readily.
  2. Individuals do not hybridize at all.
  3. Individuals hybridize but offspring have reduced fitness.

Because speciation is a process, all the ideas above can be incorporated into any scenario about defining species. In the short video below, identify the evidence for the different species concepts and reproductive isolation, whether this is case of allopatry or sympatry, and what follow up questions you’d like to ask the researchers about these butterflies: