Reduction in Gene Flow: Dietary Preferences

Speciation takes place when groups that were part of one species become reproductively isolated from each other.  Once the groups have become reproductively isolated from one another, speciation may result from each population becoming more and more adapted to their local environment (natural selection).  The gradual accumulation of random genetic mutations (a process known as drift) can also contribute to speciation, but at a much slower rate.   In the classic model of speciation, the process was only complete when no gene flow occurred between the divergent groups at all.  However, more recent research has shown that species can maintain their distinctness even when small numbers of hybridizations occur.

One way for two groups to become reproductively isolated from one another is by developing different dietary preferences.  This can happen when groups specialize on different parts of a resource (large versus small seeds, or insects that live on the outer tips of tree branches versus inner foliage near the trunk).  This results in individuals of different groups encountering one another only rarely simply because they are foraging in different habitats or different parts of the same habitat.  Diet-driven habitat isolation is different from the patterns of spatial separation I covered in my last post because it is not an geographical accident or some inherent physiological tolerance that is separating members of the two groups.

Dietary preferences can arise by mutation or in response to competition.  Favorable mutations may allow one group to utilize a whole new type of food which opens new habitats for that group to evolve into.  Such dietary innovations can lead to the evolution of different morphological features that further aid in the use the new resource.  These different morphologies can in turn lead to further reproductive isolation, and this process can become a self-reinforcing cycle.  This cycle is also supported by hybrids frequently having morphologies that are intermediate between the two groups.  These intermediate morphologies will likely be inferior to either of the groups and will result in hybrids leaving fewer, or no, offspring.  Natural selection will then favor adaptations to avoid hybrid matings because these matings will be wasted reproductive effort.  Competition can lead to differing dietary preferences by favoring the individuals in a population that are best suited to utilizing the extremes of a resource.  This can occur when competition for food resources is high.  Then if one group is able to utilize one end of a continuum and another group is able to utilize the other end, these populations may become favored because they avoid much of the competition.  Again, hybrids may do poorly because of intermediate morphologies that have evolved in response to the extreme ends of the food continuum, and also because they may have to deal with stronger competition from the other group.

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Reduction in Gene Flow: Spatiotemporal

Speciation takes place when groups that were part of one species become reproductively isolated from each other.  Once the groups have become reproductively isolated from one another, speciation may result from each population becoming more and more adapted to their local environment (natural selection).  The gradual accumulation of random genetic mutations (a process known as drift) can also contribute to speciation, but at a much slower rate.   In the classic model of speciation, the process was only complete when no gene flow occurred between the divergent groups at all.  However, more recent research has shown that species can maintain their distinctness even when small numbers of hybridizations occur.

Three major spatial patterns are commonly discussed when talking about speciation.  They are allopatric distributions, parapatric distributions, and sympatric distributions.

Allopatry (in ancient Greek allos means other and patri means fatherland) is when the two diverging groups live in different geographic locations which are separated by some geographic barrier such as a large body of water or a desert.  The barrier has to be large enough to stop animals from crossing it for allopatric speciaiton to occur.  In this situation, the barrier itself is what reduces gene flow between the populations.

Parapatric speciation (in ancient Greek para means beside and patri means fatherland) is when two groups are found in different parts of a continuous habitat with ranges that overlap only along a relatively narrow contact zone.  Their different ranges result in low levels of contact between members of the different groups, but there is no physical barrier stopping individuals from mixing.  Here, the different ranges play a partial role in reducing gene flow between the groups, but this alone would not be enough to allow speciation to take place.  In parapatric speciation, some other barrier must be reducing gene flow between the groups.  These barriers may be behavioral such as if the two groups preferring to feed or breed on different species that themselves do no overlap, or if the two groups develop different mating signals which the other group does not respond to.

Sympatric speciation (in ancient Greek sym means together and patri means fatherland) is when two groups become reproductively isolated while occupying the same geographic area at the same time.  In this mode of speciation, there is absolutely no geographic barriers to gene flow.  This means that any barriers to gene flow must have at least some behavioral component.  These behavioral differences much be significant if they are to prevent members of the two groups from interbreeding since the two groups live in very close proximity to one another.

Temporal patterns of speciation generally have to do with dispersal or migratory behaviors.

If two groups of a species migrate to different locations to breed, or if the chose mates during the non-breeding season and these non-breeding sites are in different locations, then this might be a form of allopatric speciation.  Here reproductively significant activities (selection of mates) take place in part of the year when the members of the different groups are in different locations.  Individuals would then choose a mate from those available which would be limited to other individuals who migrated to the same location, and so gene flow between the groups is prevented.  Different migratory patterns could be when one group is resident in a particular range and the other is migratory, or when one group migrates along a north and south pathway and another migrates along an east and west pathway, or when one group is an altitudinal migrant which moves up and down slope.  Gene flow can also be prevented if hybrids between members of the different groups display some behavior that results in high mortality.  For example, if one group is resident and the other migratory, a hybrid might have only a weak tendency to migrate and so not go vary far.  The area that they end up in might be very unsuitable to spend the non-breeding season in, and so these hybrids may have much higher mortality rates than purebred individuals.  This hybrid inviability would result in reduced gene flow between the groups.  Dispersal behavior can have a similar effect on gene flow.  If members of different groups have different dispersal tendencies (direction or distance), then intermediate dispersal behaviors may not be advantageous and so result in hybrid inviability as well.

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