Answer

In a nutshell, accumulated microevolution of isolated populations can produce new species (macroevolution), as illustrated in the image below (top). The image at right illustrates various ways that populations become isolated.

As you’ll recall, microevolution involves a positive feedback between gene regulation—the embryonic formation and continuous reformation of the bodies in a population—and natural selection, which selects for reproductively successful attributes in a population. Along the way, genetic innovations can cause the development of (harmful and beneficial) characteristics—through copying errors, duplication, gene transfer, mutation, etc. Thus, generation after generation, the environment selects for the characteristics of a population that best allow it to successfully persist. And when conditions change, the environment selects for new characteristics. Microevolution is the reason why body form and function are typically so finely tuned to lifestyle—because, generation after generation, the bodies of populations of organisms are selected for their ability to perform the functions associated with the lifestyle.

Recall that natural selection acts on the attributes of organisms living in a population. Often, an attribute molded by natural selection for one purpose is, when selection pressures change, later coopted to another purpose. For example, in the next unit we’ll discuss the origin of upright posture, which fossil evidence indicates developed in trees as natural selection acted, generation upon generation, on activities that require free hands such as gathering fruit and holding infants. Later, when climate cooling and drying caused grasslands to expand and forests to become isolated, natural selection acted on these organisms with upright posture and ‘free hands’ to mold a body that could walk between forests and use their hands to find roots and tubers and to scavenge meat. The story goes on, but the point is, that natural selection continuously molds the bodies of populations for success. If the body form is already well suited for a specific lifestyle, then natural selection acts to maintain that body form. In contrast, if the body form is poorly suited for a new environment, natural selection acts across generations to mold a body that can succeed in that environment. If selection-induced changes can keep up with change, the positive feedback between gene regulation (development) & innovation and natural selection can produce a new species. If natural selection cannot keep up with the changes, the species goes extinct. Thus, both extinction and speciation can result from environmental change.

As shown above, microevolution and population isolation typically produce macroevolution (speciation or extinction). Population isolation can result from geographical barriers such as mountain ranges, canyons, & seas—either as these features develop or as subpopulations move to isolated areas. Once isolated, these populations can experience different selection pressures and experience distinct genetic innovations. Eventually, as microevolutionary shifts in the attributes of isolated populations shift in different directions through time, new species are born—or, if they are unable to adapt, become extinct.

In addition to geographical barriers, behaviors can isolate populations; that is, populations can become isolated in the same area if a subpopulation changes the mechanism, style, or timing of feeding, lifestyle, mating, …. For example, when a subpopulation begins specializing in a new food course (occupying a new niche), natural selection can mold that subpopulation differently than the parent population despite both occupying the same geographical area. In the same way, altered mating seasons or modified mating behaviors such as calls, color patterns, or dances can isolate populations that occupy the same area.

Answer

A geographic barrier formed as shown below (left). The volcanic formation of Central America at ~3.5 Mya connected North and South America and separated the Caribbean from the Pacific. This event isolated an originally interbreeding population of porkfish that have since become separate species. The porkfish shown below is from the Caribbean.

Explanations such as this are testable. For example, the explanation of porkfish speciation suggest that many species would have become isolated by this event. In addition, with a land bridge between N & S America, we should see a mixing of those populations. Observations confirm both predictions. In fact, the land bridge caused many S American marsupial mammals to go extinct, when they were outcompeted by placental mammals arriving from N America.

Answer

These species became isolated as their behaviors changed; specifically, they began occupying different niches. This explanation predicts that these species would show specific genetic similarities. Genetic studies confirm this prediction.