Learning Goals

In some cases, after two populations have been separated and evolved in different ways possibly even becoming different species, the two populations may come into contact again. It is possible that if they are still similar enough that they may recombine or just interact and remain separate indefinitely. In some instances, individual organisms within a population can and will mate with any nearby individual with whom they are capable of breeding. The area where two closely related species continue to interact and reproduce, forming hybrids is referred to as a hybrid zone. Over time, the hybrid zone may change depending on the fitness of the hybrids and the presence/absence of any reproductive barriers.

A final possibility that has been observed is the eventual development of the hybrids as a species of their own. In this case, the hybrids are fit but only in relation to other hybrids, meaning that they will be most likely to reproduce with other hybrids. Over time this can result in reproductive barriers between the hybrids and the original parental species and lead to the formation of a separate species. The ability to analyze DNA has shown that hybrid speciation is a fairly common phenomenon, particularly in plants.

Scientists around the world study speciation, documenting observations of living organisms, and ancestral organisms found in the fossil record. As their ideas take shape and as research reveals new details about how life evolves, they develop models to help explain speciation rates. In terms of how quickly speciation occurs, we can observe two current patterns: the gradual speciation model and the punctuated equilibrium model.

In the gradual speciation model, species diverge gradually over time in small steps. In this model, a species evolves by accumulating small variations over a long period of time. In the punctuated equilibrium model, a new species forms when a small group undergoes changes quickly from the parent species and then remains largely unchanged for long periods of time afterward. We call this early change model punctuated equilibrium because it begins with a punctuated or periodic change and then remains in balance afterward. While punctuated equilibrium suggests a faster tempo, it does not necessarily exclude gradualism (Fig. 2).

The primary influencing factor on changes in speciation rate is environmental conditions. Under some conditions, selection occurs quickly or radically. Consider a species of snails that had been living with the same basic form for many thousands of years. Thus they are morphologically stable and layers of their fossils would appear similar for a long time. When a change in the environment takes place—such as a drop in the water level—a small number of organisms are separated from the rest in a brief period of time, essentially forming one large and one tiny population. The tiny population faces new environmental conditions that differ from those in the larger population. Because its gene pool quickly became so small, any variation that surfaces and that aids in surviving the new conditions becomes the predominant form.

Punctuated equilibrium does not:

• Suggest that Darwin’s theory of evolution by natural selection is wrong.
• Mean that the central conclusion of evolutionary theory, that life is old and organisms share a common ancestor, no longer holds.
• Negate previous work on how evolution by natural selection works.
• Imply that evolution only happens in rapid bursts.

Punctuated equilibrium predicts that a lot of evolutionary change takes place in short periods of time tied to speciation events.²

In some instances speciation isn’t always a precise division. Hybrid zones, areas in which closely related species both occur can form. In these zones some organisms may reproduce with other similar organisms of the other species. The fitness of the resulting hybrid offspring can affect the two species’ evolutionary path, reinforcement, fusion or stability. If the hybrids are less fit than the parents, reinforcement of speciation occurs, and the species continue to diverge until they can no longer mate and produce viable offspring. If reproductive barriers weaken, fusion can occur and over time the two species may become one. If the hybrids are equally fit and thus reproductive: stability may occur and hybridization may continue.

Speciation rates can be influenced by many factors but one of the key ones is environmental changes. While natural selection patterns are consistent, there are two models for the rate of speciation. In the gradual changes are slow over a long period of time and in the punctated there is a rapid change followed by a long period without change. The fossil record can be used to study the changes but not all changes are reflected in the record.