What are the Different Types of Speciation?
What is a Species?
Before we can discuss the different types of speciation that may occur, we must first pose the question, "What is a species?" Defining what exactly a species is is more different than one might think. After all, where does one species stop and another begin? There are many different breeds of dogs with different phenotypes... is each breed a different species? See how hard of a question this is?
Well, there are three main schools of thought regarding the definition of a species. They are the biological species concept, the morphological species concept, and the phylogenetic species concept.
Biological Species Concept (Recognition Species Concept)
Currently, the biological species concept is the most widely accepted definition of a species amongst biologists. It defines a species as a group of interbreeding populations that are reproductively isolated from other groups. This is why the different breeds of dogs are not considered separate species since they can all interbreed.
However, there are some problems with this concept since it is difficult to apply it to species that are extinct or are asexual.
Morphological Species Concept (Phenetic Species Concept)
The morphological species concept defines species in accordance with their morphological characteristics such as appearance. This was the concept that defined species in the past and it is nice because it can be applied to extinct and asexual species.
However, members of a population be drastically different in appearance, yet still genetically very similar. Should these be classified as the same species or different?
Phylogenetic Species Concept
According to the phylogenetic species concept, a species is the smallest monophyletic group within a phylogenetic tree. The problem with this method is that it requires a phylogenetic analysis for the species and that speciation is determined by the method that was used to construct the tree.
How Does Speciation Take Place?
Next, before we can discuss the types of speciation, we need to consider the evolutionary forces that cause speciation to take place. Of the four mechanisms of evolution, only three apply to speciation: genetic drift, mutation, and natural selection. Gene flow does not apply since it has the opposite effect, since it causes breeding populations to become more similar rather than different.
Two Main Types: Geographic Isolation and Selection
Now that we've got all of that out of the way, we can discuss the two major types of speciation: those that based on geographic isolation and those that are based on selection.
Geographic Isolation (Allopatric):
Geographic isolation can result in speciation by first eliminating gene flow between different populations. Then, gradually, divergence occurs due to selection, mutation, and genetic drift. Geographic isolation occurs through vicariance, dispersal, and distance. Vicariance results due to the formation of a barrier between two groups of a population. Dispersal occurs when a subsection of a population moves away into a new region. Lastly, a large distance between to populations can prohibit gene flow and migration between them which can cause speciation due to geographic isolation as well.
Selection Based (Parapatric):
Selection can cause speciation due to a reaction to the environment. For example, if two populations live in two different habitats in the same geographic region, the selection of traits that favor the habitat they reside in may make them unable to interbreed after a time. Gene flow can still occur up until this point and selection must be strong enough to overcome it.
What is the difference between Pre-Zygotic Isolation and Post-Zygotic Isolation?
Pre-Zygotic isolation occurs before the egg is fertilized (and becomes a zygote), while Post-Zygotic isolation occurs after the zygote is formed.
Pre-zygotic isolation deals with geographic isolation, temporal isolation, ethological isolation, gametic isolation, or mechanical isolation.
- Geographic isolation - The populations live in separated geographic regions.
- Temporal isolation - The populations may mate in different seasons.
- Ethological isolation - Members of different populations may not be attracted to each other.
- Gametic isolation - The gametes from two different populations may not be able to produce a viable zygote.
- Mechanical isolation - The reproductive parts of the different populations may not fit together (ie. lock and key).
Post-zygotic isolation may arise due to the mother's body rejecting the zygotic, sterility of the hybrid, or just a general lower fitness of the hybrid.
What is Peripatric Speciation?
Peripatric speciation is a form of Allopatric speciation (not Parapatric!). It occurs when an isolated population has a very small number of individuals and it is very rapid. Since the population is small, genetic drift has a great effect and this can cause speciation to quickly occur. These changes cause a genetic revolution and the population finds a new adaptive peak.
What is Sympatric Speciation?
Sympatric speciation is a form of selection based speciation which occurs with no different in geographic location (unlike in Parapatric speciation, even the habitat is the same). Sympatric speciation requires heterozygotes to have low fitness and selection to be very strong and disruptive.
What Are Hybrid Zones?
Hybrid zones are where the ranges of two interbreeding species meet. With sympatric/parapatric speciation, the hybrid zone is where the speciation is actually occurring. This is known as a primary hybrid zone. With allopatric/peripatric speciation, the hybrid zone happens with secondary contact. This is known as the secondary hybrid zone.
There are three possible outcomes of hybridization:
- If the fitness of the hybrid is greater than the fitness of the parents then it can become a new species. The hybrid zone is stable in this case.
- If the fitness of the hybrid is equal to the fitness of the parents then the parent species rejoin into one species. This hybrid zone can be wide and long lasting.
- If the fitness of the hybrid is less than the fitness of the parents then the parents will continue to diverge into separate species and the hybrid zone will be small and short.