In this section, you will explore the following questions:
- What are the pathways of species evolution in hybrid zones?
- What are the two major theories on rates of speciation?
Connection for AP® Courses
Speciation can both occur gradually over time in small steps or in bursts of change known as punctuated equilibrium. With punctuated equilibrium, a species may remain unchanged for long periods of time. The primary influencing factor on changes in speciation rate is environmental change.
Information presented and the examples highlighted in the section support concepts outlined in Big Idea 1 of the AP® Biology Curriculum Framework. The AP® Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® exam questions. A learning objective merges required content with one or more of the seven science practices.
|Big Idea 1||The process of evolution drives the diversity and unity of life.|
|Enduring Understanding 1.C||Life continues to evolve within a changing environment.|
|Essential Knowledge||1.C.1 Speciation and extinction have occurred throughout Earth’s history.|
|Science Practice||5.1 The student can analyze data to identify patterns or relationships.|
|Learning Objective||1.20 The student is able to analyze data related to questions of speciation and extinction throughout the Earth’s history.|
Hybrid zones provide scientists with spaces from which to research the factors that cause reproductive isolation, and thus, speciation. The spatial patterns of hybrid zones reveal much about these factors and allow for inferences to be made about the number and degree of obstacles to gene flow as well as the number and types of contacts between species.
You may wish to identify for students examples of hybrid zones, or encourage them to research zones for themselves. Pose questions to students about why hybrid zones might be viewed a “natural experiments” in which to study the process of speciation. Encourage them to compare and contrast the information they find about different hybrid zones. Have them consider what questions they might ask about the species in the zones and what information they might expect to gain from asking them.
Speciation occurs over a span of evolutionary time, so when a new species arises, there is a transition period during which the closely related species continue to interact.
After speciation, two species may continue interacting indefinitely or even recombine. Individual organisms will mate with any nearby individual who they are capable of breeding with. An area where two closely related species continue to interact and reproduce, forming hybrids, is called a hybrid zone. Over time, the hybrid zone may change depending on the fitness of the hybrids and the reproductive barriers (Figure 18.24). 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 occurs and the two species become one. Barriers remain the same if hybrids are fit and reproductive: stability may occur and hybridization continues.
stability, fusion, reinforcement
allopatric speciation, sympatric speciation, fusion
convergent evolution, divergent evolution, no evolution
natural selection, genetic drift, gene flow
Hybrids can be either less fit than the parents, more fit, or about the same. Usually hybrids tend to be less fit; therefore, such reproduction diminishes over time, nudging the two species to diverge further in a process called reinforcement. This term is used because the low success of the hybrids reinforces the original speciation. If the hybrids are as fit or more fit than the parents, the two species may fuse back into one species (Figure 18.25). Scientists have also observed that sometimes two species will remain separate but also continue to interact to produce some hybrid individuals; this is classified as stability because no real net change is taking place.
Varying Rates of Speciation
Scientists around the world study speciation, documenting observations both of living organisms and those 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 rates of speciation. In terms of how quickly speciation occurs, two patterns are currently observed: gradual speciation model and punctuated equilibrium model.
In the gradual speciation model, species diverge gradually over time in small steps. In the punctuated equilibrium model, a new species undergoes changes quickly from the parent species, and then remains largely unchanged for long periods of time afterward (Figure 18.25). This early change model is called 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.
There is a significant change in the environment over time, such as the breakup of a supercontinent due to tectonic activity.
A species that has a competitor outcompetes it and drives it to extinction, freeing up more resources.
There is a sudden and significant change in the environment, such as a volcanic eruption that divides a population that once shared a habitat.
There is a stable and unchanging environment in which a species can flourish.
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. 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. Because its gene pool quickly became so small, any variation that surfaces and that aids in surviving the new conditions becomes the predominant form.
Visit this website to continue the speciation story of the snails.