Ecology
Ecology is very complex, representing a nearly-infinite number of interactions between and among species, and between species and their environments. What I will say here will be a far-too-brief summary that fails to do justice to the complexity of the real world. Nonetheless, I hope that it will provide some sense of how ecological issues impact evolution.
Competition
I will define competition, here, as virtually any type of interaction between species or between individuals that can result in a gain for one and a loss for the other. Included in this rather vague and probably quite imprecise definition are predator-prey relationships, parasites and diseases, and the more traditional types of competition, such as fighting over nesting sites or over food. When looked at from this viewpoint, just about every living thing faces some kind of competition in its life.
If there is competition, then it is likely that the individuals that compete more successfully will flourish, and the individuals that compete less well will not.
In a sense, this is what evolutionary selection is based upon. If a new species invades an ecosystem, it may out-compete native species that require the same resources. There are many examples of exotic, invasive species that have done, or are doing exactly this, as a result of human transport of plants and animals from one place to another. There are also many examples of exotic species that have not out-competed native species, and survive in their new environments only because humans take care of them. For these species, the natives have the competitive advantage.
These competitive relationships are extreme examples of the basic principle. In natural environments (rather than human-manipulated), differences in competitive advantage are probably much less. In an ecosystem that is in a relatively stable equilibrium, the newcomers tend not to be migrants from another continent, but instead are merely genetic variants of the species already present.
Much of the genetic variation in populations results from reassortment of alleles at meiosis, so that different individuals carry different allele combinations (usually referred to imprecisely as gene combinations). In general, however, the original origin of a new allele is mutation. Without mutation to create new alleles, there would be no gene variants to reassort at meiosis. Although most traits depend upon the interactions of multiple genes, I will restrict this discussion to the effects of newly-arisen mutations.
Most new mutations interfere with gene function. An individual carrying a mutation that is deleterious, and therefore showing a trait that is disadvantageous, is likely not to be particularly robust. Such an individual will be out-competed by other individuals in the population, and will leave few offspring. Sooner or later, that particular mutation will be lost from the population. In short, even something as "obvious" as the death of a sick, mutant animal is natural selection in action.
Occasional new mutations enhance gene function, or alter it in some way. Under some conditions, such mutations provide their bearers with a competitive advantage. These individuals may leave more offspring than other individuals. In time, as these offspring out-compete the offspring of other individuals, and so forth for a number of generations, the new genetic variant may become the primary variant in the population.
Arms Races
We traditionally think of natural selection operating within a population by this kind of competitive advantage (or disadvantage) of some genetic variants relative to others. We can also think of it in terms of inter-species interactions, the classic example of which is the "arms race," studied in detail by Butch Brodie.
In the US, garter snakes eat salamanders. On the west coast, the salamanders have acquired a genetic variation that enables them to produce tetrodotoxin, a neurotoxin that inactivates sodium channels in neurons. Production of this toxin provides individual salamanders with a competitive advantage relative to others of their species, in that they are less likely to be eaten. It also provides them with a competitive advantage relative to the snakes, in that snakes that attempt to eat them are likely to spit them out before any harm is done to the salamander.
Some snakes carry mutations in the gene for the sodium channel, rendering it less sensitive to blockage by tetrodotoxin. These snakes can tolerate poisonous salamanders, although they cannot move as rapidly as other snakes. These mutations provide a competitive advantage for the snakes, relative to the poisonous salamanders, which they can eat. In regions where the salamanders do not produce the toxin, however, the sodium channel mutations confer a competitive disadvantage, because the snakes cannot escape their predators as easily.
Here, the competitive relationship is both within populations of salamanders and snakes, as well as between species.
If we think if competition among individuals of a population, where individuals compete for food, shelter, mates, etc. we have to ask why there is competition in the first place. The short answer is that resources are limited. For any species in any particular region, there is only a finite amount of food, or water, or minerals, that they need to survive. Once plants have covered all of the available space in which they can grow, that space becomes a scarce resource for which plants must compete. Once deer have eaten enough of the plants in a region, food plants become a scarce resource for which deer must compete. In general, there is always one, and often more, limited resources for any population.
When a population reaches the number of individuals--the population size--at which resources are limiting, then that population cannot increase in size any further. At least, it cannot do so and maintain a stable number of individuals. There are many instances of populations expanding rapidly, consuming the available resources as they do, and then crashing when the resources are exhausted.
In most natural populations, there is a tendency to overshoot, and consume resources too extravagantly, and then decline as individuals die from lack of whichever resource was overused. There is rarely a stable population size that hovers at just the right balance between population expansion and resource over-utilization. Nonetheless, the concept of such a stable population is valid, and is referred to as the "carrying capacity" of the environment. That is, the carrying capacity is the maximum population size, the maximum number of individuals, that an environment can support. If a population expands beyond the carrying capacity, it subsequently crashes.
The concept of carrying capacity is important to consider when thinking of the impact of competition on evolution. If a population could expand forever, then all genetic variants in that population should be able to be maintained. However, populations cannot expand forever, because resources are limited. Therefore, if one genetic variation is selected for, other genetic variants are selected against. If one genetic variation out-competes other variations, then as individuals with the successful variation have more offspring, they get a greater share of the resources. Individuals without that genetic variation get a smaller share of the resources, have fewer offspring, and may die out altogether.
Not everything in life is competition. It is easy to think of instances in which different species help one another. Bees, hummingbirds, and some other animals help some flowering plants by spreading their pollen. In return, the plants help the animals by providing nectar as a source of food. There are many examples of specific plant/animal pollination relationships. Similarly, edible fruits are a form of cooperation. Plants produce edible structures surrounding their seeds, and thereby attract the "animal seed-dispersal service," in which animals eat the fruits, and scatter the seeds to new locations.
These situations can be seen to result from competition within each species (the bird that is attracted to the fruit obtains more nutrients than the bird that is not attracted), but cooperation between species. The tree that produces visible cherries to sacrifice to birds gets its seeds scattered far and wide, and has more successful offspring.
Like the arms races, these are examples of plant/animal co-evolution, in which the mutations that occur in one species, thereby affecting its traits, influence the success of individuals of the other species. That is, when we refer to "natural selection" as resulting from selection "by the environment," the term "environment" refers not just to the geological and meteorological aspects of the world, but also to the other plants and animals with which every individual interacts.
Unlike the snake/salamander arms race, which involves fewer genes, the coevolution of pollination relationships, or of fruits and fruit-eating relationships, is complex. In most instances, the genes involved affect morphology, rather than the production of a chemical compound, or the function of a gene that has a long history of study in a number of organisms. As a result, we are largely ignorant of the specific genes involved.
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Last updated: 31 December 2005
Comments: Jose
Bonner, OSO
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