Ecology and Evolution

"A parent struggling to stay awake for the 2:00 AM feeding of her month-old baby or a tree swallow staring into the gaping mouths of his five featherless, pink chicks might be tempted to envy insects, whose parenting duties generally end with the laying of the last egg.  But the life of an insect parent, or rather parent-to-be, is not carefree.  Ecologists have found that for many insects, actions taken by the mother before the birth of her young are as vital to their survival (and ultimately their reproductive success) as is postnatal care in mammals and birds.  Over the last twenty years, in the course off studying the goldenrod gallfly, Eurosta solidaginis -- one of North America's most abundant but least-noticed insects--we have observed just how important these maternal preparations can be.  Along the way, we have uncovered a complex set of relationships involving the insect, the plant on which it lays its eggs, and several predators and parasites."  Arthur E. Weis & Warren G. Abrahamson in Natural History, September 1998 pp. 60-63.

    The goldenrod, gall fly, and natural enemies system has been used to study important concepts in ecology and evolution.  Three of the topics which we have studied, using this system, are host-race formation, tri-trophic interactions, and selection in nature (illustrated by gall-size selection).
 

 

Host-race Formation


    Host-race formation is the formation of specialized populations of organisms which use different hosts.  In our system, the host races are populations of gall flies which produce galls on different species of goldenrods.  Host-race formation is considered to be important because it represents a step along a path to sympatric speciation.  Most often, speciation is thought to occur allopatrically, which means that the individual populations which evolve into new species are geographically isolated (separated in space).  Sympatric speciation is the evolution of new species from populations that are not geographically isolated.  This would seem to be impossible since biologists agree that for speciation to occur there must reproductively isolated populations within a species.  If this were not true, then the species would be made-up of one interbreeding population and no genetic differentiation could occur.  The most obvious form of reproductive isolation is geographic isolation.  What better way to keep populations from interbreeding, then to keep them separated?  However, what if populations could be reproductively isolated is some other manner?  In our system, the host races of gall flies mate, lay eggs, and their larva develop on different species of goldenrod.  This host-plant preference is very strong.  Because the gall flies nearly always mate on their own host goldenrod, each host race of flies is reproductively isolated from those that use other species of goldenrod.  This allows the host races to be reproductively isolated, even though they occur in the same fields.  If the host races of flies continue to be isolated for long enough they could potentially evolve into separate species.  This would be an example of sympatric speciation.
 

 

Tri-trophic Interactions

     Trophic levels are the levels that energy passes through on its way through an ecosystem.  Energy is collected from inorganic sources by organisms on the first trophic level (these organisms are called primary producers).  Plants are the most common example of these organisms, since they collect energy in the form of light and convert it (along with chemicals) into sugars.  The second trophic level is made up of the organisms which feed on the primary producers, these organisms are called primary consumers or herbivores. The upper trophic levels are occupied by predators and it is very rare for there to be more than one or two levels of predators above the herbivores.  This is because it takes a large number of producers to support a few herbivores and many fewer top predators.  As you move up through the trophic levels the number of organisms at each level becomes much smaller, making it more difficult for an organism on an even higher level to find enough food to survive.
    Tri-trophic interactions occur when a change at one trophic level indirectly affects trophic levels which are more than one step away. An example of a tri-trophic interaction would be the following: suppose that there were a grassland where the major herbivore was a species of vole which eats grass seeds and that this vole was able to reach population levels which allowed the vole to eat nearly all of the seeds.  Further suppose that the main predator of this vole was a species of hawk and that this hawk was capable eating enough voles to reduce the voles population to nearly zero (at least to the point that voles could no longer eat very many of the seeds).  So, if the population of hawks is high, the population of voles is low and the grass produces lots of seeds.  However, if the population of hawks is low, the vole population will be high, and the grass will disperse few seeds.  This is an example of a type of tri-trophic interaction.
    In our system there are several examples of tri-trophic interactions. One tri-trophic interaction we have studied is the interaction among goldenrod, gall flies, and the Eurytoma wasp.  If the goldenrod is of a genotype that produces small galls, then the wasp's ovipositor can reach the gall fly larva's chamber and the larva is killed.  If the goldenrod is of a genotype that produces large galls, then the wasp's ovipositor can not reach the chamber and the gall fly larva survives.  This interaction gets even more complicated and is discussed in greater detail in the next section.

 

Gall-size Selection


    Another area of research in our lab has been on the selection of gall size.  First we need to define some terms.  The first is natural selection.  Organisms that are better adapted to their environment are able to produce more offspring which causes their genes to become more common in the population.  Natural selection requires that there be some variability among individuals in the population, otherwise no differentiation would occur.  This variability is generated through recombinations and mutations, which are changes in the genetic constitution of an individual.   Another important term is fitness.  A species' fitness can be defined as its ability to pass its genes on to the next generation.  Many factors can affect a species fitness and these factors may vary from species to species. Individuals that are favored by natural selection are said to have a high fitness.
     In the ball gall system one good measure of gall fly fitness is probability that a larva will survive (survivorship).  In some cases this is greatly influenced by the size of the gall that a larva induces in the plant.  But what size gall is the best?  As it turns out, this depends on several different factors.  In places where the Eurytoma gigantea wasp (a very effective predator of gall fly larvae) is abundant, there is strong selection for large galls.  This is because large galls have walls that are too thick for the wasp's ovipositor to penetrate, protecting the gall fly larva.  In this case, flies that produce large galls may have high fitness, while those that produce small galls may have low fitness.  In areas where downy woodpeckers are abundant the story is different. Downy woodpeckers are also very efficient predators of the gall fly larvae and preferentially choose large galls over small galls.  It's assumed that they choose large galls because those galls are likely to contain larger larva, making a better meal for the effort.  So, in areas with lots of downy woodpeckers, flies that produce large galls may have low fitness and flies which produce small galls may have high fitness.

    Finally, in the case where both Eurytoma gigantea wasps and downy woodpeckers are common the two competing selection pressures produce a net selection for intermediate (medium) sized galls.  Other factors that have been shown to affect gall size are the ability of the flies to produce large or small galls and the effect of the plant on gall size.  In fact, if you look carefully in many goldenrod fields you will see that the galls on each goldenrod clone are nearly the same size.  While one clone contains all large galls, another will have all small galls, still others will have galls of intermediate size.  Researchers in our lab have found that while gall size is heritable in Eurosta, the heritability of gall size in the goldenrod is the strongest controller of gall size.  To put it another way, although gall flies do have some influence over gall size and this influence is passed down from parent flies to their offspring, the genetically-determined influence of gall size by the plant is a greater factor in determining gall size.

 
 
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