3.20: Mechanisms of reproductive isolation

Think about a population that is specializing to fill a particular ecological niche. What is the effect of cross breeding with a population that is, perhaps, on a path to adapting to another niche? Most likely the offspring will be poorly adapted to both niches. This leads to a new selective pressure, selection against cross-breeding. Even small changes in a particular trait or behavior can lead to significant changes in mating preferences and outcomes. Consider Darwin’s finches or Hawai’ian honeycreepers. A key feature that distinguishes these various types of birds is the size and shapes of their beaks. These adaptations represent both the development of a behavior – a behavioral preference to seek food from particular sources and the physical traits needed to harvest that food source effectively. Organisms have to display the behavior, even if it is in a primitive form, to make selection of the physical trait beneficial. This is a type of positive feedback loop, where behavioral and physical traits are closely linked. You can ask yourself, thinking about the ancestor of giraffes, could a long neck have evolved if members of the ancestral population did not choose to eat the higher leaves of trees?

Back to finches and honeycreepers. Mate selection in birds is often mediated by song, generally males sing and females respond (or not). As beak size and shape changes, the song produced also changes.^[157] This change is, at least originally, an unselected trait that accompanies the change in beak shape. It can become a selected trait if females recognize and respond to songs more like their own. This would lead to preferential mating between organisms with the same trait (beak shape). Over time, this preference could evolve into a stronger mating preference, until it becomes a reproductive barrier between organisms adapted to different ecological niches.^[158] Similarly, imagine that the flowers that a particular subpopulation feeds on open and close at different times of the day. This could influence when an organism is active and sexually receptive. You can probably generate your own scenarios in which a behavioral or physical trait has an influence on reproductive preferences and success. If a population is isolated from others, such effects may develop but are initially irrelevant; they become important only when two closely related but phenotypically distinct populations come into contact. Now matings between individuals in two different populations, sometimes termed hybridization, can lead to offspring poorly adapted to either niche. The result is a selective pressure to minimize hybridization. Again, the reproductive isolation of two populations can arise spontaneously, such as when two populations mate at different times of the day or the year or respond to different behavioral queues, such as mating songs. Traits that enhance reproductive success by reducing the chance of detrimental hybridization will be selected. The end result? what is known as reproductive isolation.^[159] As reproductive isolation grows, what was once one species becomes two. A number of different mechanisms ranging from the behavioral to the structural and the molecular are involved in generating reproductive isolation. Behaviors may not be “attractive,” genitalia may not fit together,^[160] gametes may not recognize and fuse with one another, or embryos might not be viable - there are many possibilities.

Ring species: Ring species demonstrate a version of allopatric speciation. Imagine populations of the species A. Over its geographic range there exist a number of subpopulations. Subpopulations (A₁ to A₅) and (A_a to A_e) have limited regions of overlap with one another and where they overlap they interbreed successfully (→). But populations A₅ and A_e may no longer interbreed successfully – are these populations separate species? In this case, there is no unambiguous answer (and we have to get used to the idea of ambiguity). In part this ambiguity is a basic biological trait. Populations are continuous over time, but individuals within a population vary, and this variation leads to evolutionary change. In the real world, "intact" ring species are unlikely; over time the links between the various subpopulations will be broken and one or more species may arise. Consider the black bear Ursus americanus. Originally distributed across all of North America, its distribution is now much more fragmented. Isolated populations are free to adapt to their own particular environments and migration between populations is limited. Clearly the environment in Florida is different from that in Mexico, Alaska, or Newfoundland. Different environments will favor different adaptations. If, over time, these populations come back into contact with one another, they might or might not be able to interbreed successfully - reproductive isolation may have occurred and one species may become many.

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