Models of Speciation

NOTE: These are lecture notes for Biology 391, Organic Evolution, at The University of Tennesee at Martin. Anyone outside of UT Martin wishing to use these notes or to contact me for additional information should first read the information obtained by clicking here.

Goals: introduce the different ways speciation can occur based on the geographic location of the groups that evolve to be different species, and examine some of the kinds of genetic change that lead to new species and a technique for studying such genetic differences.

Related Textbook Material: Freeman and Herron (2001) Chapter 12

Lab Manual Questions over this material are in Lab Manual Chapter XIII

The Lecture:

Speciation, the evolution of two or more new species from one pre-existing species, will occur if different groups within a species evolve to become different from each other -- so different that they would be considered different species. How different that is depends on which species concept you are using. Remember that the biological species concept says they have to be so different that they cannot reproduce with each other to produce healthy, fertile offspring. The phylogenetic species concept says they have to have evolved different derived character states from each other.

Whether different groups within a species evolve differences depends on the main forms of evolution we've already studied -- natural selection, genetic drift, and gene flow. These affect speciation as follows.

Remember that gene flow tends to make populations similar to each other. Since speciation requires that groups evolve differences from each other, gene flow tends to prevent speciation from occurring.

Remember that genetic drift and natural selection tend to make different populations different from each other. Since speciation requires that groups evolve differences from each other, genetic drift and natural selection can result in the evolution of differences among populations and therefore can cause speciation to occur.

The different models of speciation we will look at in this lecture are all based on determining situations in which there will be little or no gene flow between groups so that speciation can occur because of natural selection and genetic drift within groups, or at situations in which the effects of natural selection may be strong enough to counteract gene flow so that speciation occurs.

The main models of speciation are categorized based on geography -- that is, what are the geographic locations, with respect to one another, of the groups that evolve to be different species. There are three geographic categories of speciation. These are:

Allopatric speciation: speciation that occurs when the groups that evolve to be separate species are in different geographic locations and are isolated geographically from each other so that individuals cannot move between the different locations.
Parapatric speciation: speciation that occurs when the groups that evolve to be separate species are geographic neighbors; they are in different areas, but the areas are next to each other and individuals can move between the areas.
Sympatric speciation: speciation that occurs when the groups that evolve to be separate species occur together in the same geographic area.

Note that these definitions are based on geographic location -- they do not consider the process through which speciation occurs. We will look now at the processes that can occur to cause each of the geographic categories of speciation listed above.

During allopatric speciation there is a geographic barrier between areas that prevents gene flow between areas from occurring. As a result, there is no gene flow to keep populations similar to each other. Natural selection and genetic drift occurring in each of the geographically isolated populations will cause those populations to evolve to be different from each other -- eventually they will be so different that they evolve to be different species.

The question we should ask about allopatric speciation is: how do different populations of a species become geographically isolated from each other? There are two main causes of geographic isolation:

Vicariance events are events that split the range of an existing species by creating some kind of geographic barrier within the range of that species. These are typically geological events. Examples include:

the formation of glaciers during the ice ages, which divided the ranges of species into smaller areas isolated by glaciers (huge mountains of ice.)
continental drift -- all the continents on earth were once a single land mass; as they separated from each other the species on them became separated into different populations on the different continents.
changes in water position and level -- rising water levels have flooded low lying areas and turned high areas into islands isolated from each other by water; rivers changing course can divide species that can't disperse over water into isolated populations.

Dispersal events, also called founder events are events during which a small number of individuals from the original geographic range of a species move to a new area, previously unoccupied by that species, and start a new population there. For example, a small number of individuals may find their way over a mountain range, or be blown out to an island during a storm. After this dispersal, there is no further contact between the original population and the new population. The form of allopatric speciation that occurs after a dispersal (founder) event is called founder effect speciation. It is not currently clear how important founder effect speciation is as a form of speciation; some evolutionary biologists have argued that it is an extremely common form of speciation while others have argued that it is rare. We will consider the theoretical arguments on both sides of this issue in the next lecture.

The basic process of allopatric speciation is similar for either cause of isolation of the populations -- once the populations are isolated, gene flow is prevented and differences between the isolated populations evolve through drift and selection.

Now let's consider parapatric speciation. Since individuals can move between the populations in this case, gene flow occurs. We have seen that gene flow tends to make speciation unlikely. For speciation to occur in such a situation, there must be strong selection to counteract the effect of gene flow. It is argued that parapatric speciation may occur where there is an abrupt change in the environment over a geographic border. The result is that forms that have high fitness in one area have low fitness in the neighboring area, and hybrids between these forms do not have high fitness in either area. For example, there are areas where there is a sudden change in soil type from one place to the next. Plants that have high fitness in one soil type have low fitness in the other. This results in strong natural selection against the forms from other areas and this may counteract the effects of potential gene flow enough so that the forms in the different areas evolve, through natural selection, into different species.

Note that the form of selection occurring in this case is disruptive selection in that one extreme form has high fitness in one environment, the opposite extreme has high fitness in the other environment, and intermediates do not do well anywhere so have the lowest fitness.

Note that parapatric speciation differs from allopatric in the process that occurs -- for parapatric speciation to occur there must be strong disruptive selection since that is what counteracts the effects of gene flow. For allopatric speciation, gene flow is not possible because of geographic barriers, and genetic drift or weaker forms of natural selection can lead, over time, to speciation -- strong disruptive selection is not required. Because of the requirement for an environment that will result in strong disruptive selection across a geographic boundary, parapatric speciation is generally thought to be less common than allopatric speciation.

Finally, let us consider sympatric speciation. Sympatric speciation was initially considered to be much less common than allopatric speciation because if groups are occurring within the same area it seems likely that gene flow will occur between them, and this will tend to prevent speciation. It has been recognized, however, that there are situations in which sympatric speciation can occur. Two main ways sympatric speciation can occur are:

Ecological isolation in which different groups occur in the same geographic area but do not contact each other because of some ecological difference that prevents gene flow between the groups. For example, parasitic forms of a species that occur, and reproduce, in or on different host species will not have gene flow between them. Flower forms that attract different pollinators and are only pollinated by certain specialist pollinators will not have gene flow between them. Species with small body size may specialize on different parts of the habitat -- for example, some on small plants, some on treetops -- and as a result not contact each other. In any situation like this, it is possible for there to be different forms within the same geographic area that do not have gene flow between them. In the absence of gene flow, natural selection and genetic drift will tend to make the groups different over time, and speciation can occur. Note that the process of speciation in this case is similar to what occurs in allopatric speciation; the difference is that for allopatric speciation the barriers to gene flow are geographic, while in this case there are ecological barriers to gene flow within the same geographic range
strong disruptive selection within an area could potentially result in sympatric speciation if selection for the extreme forms and against intermediates is sufficiently strong so that crosses between the extreme forms result in few surviving offspring and therefore little gene flow between the two extremes. Note that this process of speciation is similar to what occurs during parapatric speciation; the difference is that for parapatric speciation the disruptive selection occurs across a geographic boundary, while in this case it is within one area.

With regard to all of these speciation models, an unanswered question in evolutionary biology is how much is speciation driven by strong selection, such as active selection for different forms of traits in different groups, versus how much occurs gradually through drift and gradual accumulation of minor differences between groups as a result of weak selection. One approach that is being taken to study such questions is based on analyses of genes that affect quantitative traits that differ between closely related species. The approach involves mapping the location of quantitative trait loci (genes responsible for quantitative traits) on chromosomes; this is referred to as QTL analysis. QTL analyses are based on making hybrids between two species, and then finding genetic markers that are statistically associated with particular parental phenotypes. Your textbook discusses an example of this in two species of monkeyflower, one which is pink and pollinated by bumblebees, and the other of which is red and pollinated by hummingbirds (this study inspired the red monkeyflower example in the first computer assignment.) Bradshaw, Schemske, and others have created hybrids between these species by cross-pollinating them by hand and have found genetic markers associated with traits important in pollination, such as color of the flowers, shape of the flowers, and nectar production. Their results suggest that a relatively small number of genetic loci that are associated with these important adaptive traits and that differ between the species. They suggest that speciation may occur as a result of selection for these important adaptive traits that result in reproductive isolation (because they attract different pollinators.) We do not know at this point how general this result is.

Study Tips:

make yourself a table of the different forms of speciation and how they occur
make yourself a table describing the roles of gene flow, natural selection, and genetic drift in speciation; note which make speciation more likely, and why, and which ones make speciation less likely, and why.
note that the distinction between allpatric, parapatric, and sympatric speciation is based on geographic distribution of the groups that evolve to be different species, not on the processes that lead to speciation

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