Speciation

Learning Objectives

  1. Define and apply the biological, morphological, ecological, and phylogenetic species concepts.
  2. Distinguish between sympatric and allopatric speciation.
  3. Define, recognize, and understand the significance of reproductive isolating mechanisms in reducing gene flow between populations.
  4. Distinguish between prezygotic and postzygotic barriers to reproduction.

What is a species? Species Concepts

Biologists have a long tradition of debating how to define a 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), whether individuals look similar (morphological species concept), and how closely related individuals are evolutionarily (phylogenetic species concept).

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). 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. 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, where the BSC isn’t relevant since there is no interbreeding!

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)

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)

One additional species concept that may ecologists find useful is the ecological species concept, where individuals who might be physically capable to interbreed in a lab or zoo environment would never actually encounter each other in the wild, because they occupy different ecological niches. We’ll review an interesting example of this in class.

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.

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)

Sympatric speciation occurs when two populations in the same location become unable to interbreed. Both of these are the product of reproductive isolation, which reduces gene flow between populations. Reproductive isolating mechanisms can be pre-zygotic or post-zygotic, which is a jargon-rich way to say before or after sperm and egg unite to form a zygote. Pre-zygotic reproductive isolation might include behavioral differences in mating song or dance, differences in when and where individuals attempt to mate, or sperm-egg incompatibility. Post-zygotic reproductive isolation includes developmental failures and spontaneous abortion early on up to fully formed adult (viable) offspring that are themselves sterile (infertile).

A short, fun video contrasting morphological and biological species concepts:

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