Posts Tagged ‘Dispersal’

OR_25 May 20 2014_Imnaha PackAllow me to introduce you to OR-25, the most recent wolf visitor to the great state of California. This very handsome 3-year-old fellow recently left his Imnaha Pack in eastern Oregon. He decided to walk south and, just last week, crossed into California where he has been hanging out in Modoc County. His arrival, along with OR-7 who visited for most of 2012 before settling just north of the California/Oregon border to have babies and the Shasta Pack that has established itself in Siskiyou County in 2015, may indicate that wolves are starting a trend of dispersal and range expansion into this state. If this expansion continues, we who live in California may be lucky enough to encounter these long-lost members of our state’s wilderness. I am certainly hoping for it!

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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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I have been seeing lots of Cattle Egrets around Davis the past couple of weeks.  Before a couple of weeks ago, I did not see any moving through the skies over Davis, but now I am seeing them frequently.  They have been in groups that range in size from five or six up to flocks of 25!  Very beautiful birds.  From what I know of their annual schedule, some should be starting to molt (if they started breeding early in the season and have already fledged their young) or still have young birds to feed (if they started breeding later in the season).  So, I am guessing that the groups I am seeing are family groups of adult and juvenile birds or perhaps a few family groups that have merged together.  Cattle Egrets are very successful dispersers.  Originally native to Africa and southern Europe, they are now found on every continent except Antarctica and also on many remote islands, and they have done so without human intervention crossing from Southeast Asia to Australia in the 1930s and crossing the Atlantic Ocean from the west African coast and making landfall in northeastern South America in the late 1800s.  The first recorded breeding pair was in Florida in 1953.  In South Africa, where their population is generally sedentary, individuals or small groups of juvenile birds have been frequently observed to go long distances when leaving their natal area in search of new breeding areas.  This behavior is common to many of the Egrets and Herons, but it is especially well developed in the Cattle Egret and it seems to be one of the major factors that have allowed them to cover the globe.  In fact, the dispersal of individuals across large bodies of water is probably still occurring because these birds are regularly seen be passing ships far out at sea.  With so many examples of species being introduced by humans into new areas, I think it is really cool and interesting to see an example of dispersal happening naturally, as it has been happening for millions of years.

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