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)
  • how closely related individuals are evolutionarily (phylogenetic species concept), and
  • whether the individual use or can use the same set of biological resources; in other words, whether they occupy the same “niche” (ecological 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 (biological species concept). 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 (morphological species concept), 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 (phylogenetic species concept).  The morphological and phylogenetic species concepts are also more useful for analyzing asexually-reproducing organisms, such as bacteria, where the biological species concept isn’t relevant at all since there is no interbreeding! The ecological species concept is useful for analyzing cases where individuals in a lab or zoo environment might be physically capable of interbreeding, but 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.

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)

 

Here is a short, fun video contrasting the morphological and biological species concepts:

 

Sympatric and Allopatric Speciation

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 physically 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 (biological species concept).

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 due to reproductive isolation, which reduces gene flow between populations and thus increases the likelihood of speciation occurring. 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 can include:

  • behavioral differences in mating song or dance, meaning individuals don’t even recognize each other as possible mates
  • differences in when and where individuals attempt to mate, meaning that individuals don’t ever encounter each other for mating
  • or sperm-egg incompatibility, meaning that individuals might attempt to mate, but it is not possible for the sperm to fertilize the egg.

Post-zygotic reproductive isolation can include:

  • developmental failures and spontaneous abortion, meaning that the embryo does not develop properly and is therefore inviable (not capable of living)
  • growth and development of a viable (living) fully formed adult offspring that are themselves sterile (infertile), meaning that the offspring are not capable of reproducing.

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

Evolution of pre-zygotic reproductive isolation mechanisms

When interbreeding occurs between two different species (or two different populations in the process of becoming different species), the resulting offspring is called a “hybrid.” In biological terms, a hybrid simply mean something from two different sources; in this case, two different species. One interesting question is how pre-zygotic isolation mechanisms can evolve during sympatric speciation. As with all evolution by natural selection, the key element to focus on is biological fitness. If the hybrid offpsring is less biologically fit (has lower reproductive success) than the non-hybrid offpsring (offpsring that result from within-population mating), then that means the parents of the hybrid offspring had traits that lowered their fitness because of their mating with an individual from the other population. As a result, any trait that reduces cross-population interbreeding will be selected for in those two different populations, and any trait that promotes cross-population interbreeding will be selected against in those two different populations.  In other words, pre-zygotic barriers to reproduction are adaptations that reduce the likelihood of inter-species breeding, and they arise when the hybrid offspring have low biological fitness.

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