Learning Objectives
- Explain why speciation is a process, and apply the biological, morphological, ecological, and phylogenetic species concepts to examples
- Distinguish between sympatric and allopatric speciation
- Define, recognize, and explain the significance of reproductive isolating mechanisms in reducing gene flow between populations in the process of speciation
- Explain the roles of gene flow, genetic drift, selection, mutation, and non-random mating in the speciation process
- Distinguish between and identify examples of prezygotic and postzygotic barriers to reproduction
- Explain how the phenomenon of reduced hybrid fitness results in selection for prezygotic barriers to reproduction
What is a species? Species Concepts
As humans, we like to categorize or group things together, probably as a way of knowing the world. This character trait makes it harder for us to embrace processes over the end product. Speciation is the process that generates species, and the process of speciation means that sometimes we observe new species while they are still forming and not yet fully resolved into different species. Because it can sometimes take a long time for new species to fully form or split apart from each other, 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 uses or can use the same set of biological resources; in other words, whether they occupy the same “niche” (ecological species concept).
As befits the name, biologists have a preference for the biological species concept and it’s related concept, the ecological species concept. Imagine making a decision about whether two living individuals are the same species. In the case where biologists can observe that the offspring of the two individuals is both viable (able to survive) and fertile (able to reproduce), then those two individuals are called members of the same biological species. The ecological species concept is useful for analyzing cases where individuals in a lab or zoo environment might be physically capable of interbreeding as per the biological species concept, but those two indivdiduals would never actually encounter each other in nature because they use resources and space so differently. In technical terms, we say those two occupy different ecological niches. We’ll review an interesting example of this in class.
Which species concept is most relevant 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 the morphologies of the members of the elephant lineage by comparing teeth, bones, tracks, and sometimes fur, and that gives us a basic idea of whether the each of these named historic elephant types were of the same species (morphological species concept). However, in the absence of a large collection of complete fossils for each type, the species groupings are probably not accurately representing each species. Instead, we can combine fossil and DNA evidence to construct a phylogeny that uses a mathematical algorithm to combine data on fossil morphology, DNA comparison, and geographic location to decide which specimens are members of the same species and group them accordingly. Species grouped by phylogenetic reconstruction algorithms are classified using the phylogenetic species concept.
Beyond extinct species, the morphological and phylogenetic species concepts are also more useful than other options for analyzing asexually-reproducing organisms, such as bacteria, where the biological species concept isn’t relevant at all since there is no interbreeding!
Here is a short, fun video contrasting the morphological and biological species concepts:
Speciation is a Process
Speciation is the process that results in new species when an ancestral population splits into two or more descendant species which are genetically distinct and unable to interbreed (per the biological species concept). Speciation is all about gene flow—or lack thereof. The less gene flow, the more likely new species will form. There are two different mechanisms of speciation, based on how gene flow is prevented: 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.
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 prezygotic or postzygotic, which is a jargon-rich way to say before or after sperm and egg unite to form a zygote. Prezygotic 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.
Postzygotic 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.
For these different isolating mechanisms, gene flow obviously has a strong impact. Genetic drift, selection, mutation, and non-random mating also affect the speciation process. Genetic drift has the ability to remove an allele from one population that affects mate recognition or other behaviors around mating, while mutation could alternatively create such an allele. Non-random mating is in fact one of the behaviors that mutation and drift have the potential to influence.
Reduced hybrid fitness results in selection for prezygotic barriers
When interbreeding occurs between populations undergoing speciation, the resulting offspring is called a “hybrid.” In biological terms, a hybrid simply means something from two different sources. These two populations will continue to interbreed unless an isolating mechanism arises to prevent interbreeding. Often hybrids are a less good fit for the environment than either parental type, but as long as their parental types can continue to interbreed, hybrids will be produced. Natural selection should act against production of lower fitness hybrids by favoring the reproduction of individuals that do not create hybrids. In other words, selection for a prezygotic isolating mechanism should be strong.
How can prezygotic isolating mechanisms evolve during sympatric speciation? As with all evolution by natural selection, the key element to focus on is biological fitness. If the hybrid offspring is less biologically fit and has lower reproductive success than the non-hybrid offspring, then the parents of the hybrid offspring have traits that lowered their fitness by making them more similar to the other population type. Those traits might include mate preference, mating timing, or location, among many other possibilities. If a variable trait existed that reduces cross-population interbreeding, individuals with that trait will be selected for in that population. Perhaps some individuals make sperm with lower compatibility with the eggs of the other population, or perhaps some individuals prefer to breed in a specific part of the habitat or at a certain time of day. Those versions of the traits would increase in the population if more offspring are produced with those traits. In other words, prezygotic barriers to reproduction are adaptations that reduce the likelihood of inter-species breeding, and they arise when the hybrid offspring have low biological fitness.
Putting it all together
Watch this Crash Course Biology video for a 10 minute overview of speciation that hits the salient points.
UN Sustainable Development Goal
SDG 2 Zero Hunger – Understanding the processes of speciation and genetic diversity is important for developing resilient and sustainable food systems. Knowing (or engineering) genetic resistance of a species to particular pests or other environmental conditions is essential for improving the productivity and sustainability of agricultural systems for a variety of foods we eat.
