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.

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 (// email address is being protected from spambots. You need JavaScript enabled to view it.:7aYjctTe@12/18-3-Reconnection-and-Rates-of-Speciation#fig-ch18_03_01">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.


  Three different possible changes in the hybrid zone may occur over time. The first possible change, reinforcement, results when hybrids are less fit than either purebred species. Like a fork in the road, the species continue to diverge until hybridization no longer occurs. The second possible change, fusion, results when reproductive barriers weaken until two species become one. In this scenario species initially diverge, but then join together. In the third scenario, stability, fit hybrids continue to be produced at a steady rate.
Figure 18.24After speciation has occurred, the two separate but closely related species may continue to produce offspring in an area called the hybrid zone. Reinforcement, fusion, or stability may result, depending on reproductive barriers and the relative fitness of the hybrids.
What are three different pathways that species evolution may take in hybrid zones?
  1. stability, fusion, reinforcement
  2. allopatric speciation, sympatric speciation, fusion
  3. convergent evolution, divergent evolution, no evolution
  4. 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 (// email address is being protected from spambots. You need JavaScript enabled to view it.:7aYjctTe@12/18-3-Reconnection-and-Rates-of-Speciation#fig-ch18_03_02">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 (// email address is being protected from spambots. You need JavaScript enabled to view it.:7aYjctTe@12/18-3-Reconnection-and-Rates-of-Speciation#fig-ch18_03_02">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.


 In the gradual speciation example, a founder species of bird diverges into one species with a hooked beak, and another with strait beak. Over time, the hooked beak gets longer and thinner, and the straight beak gets shorter and fatter. In the punctuated equilibrium example, as in the graduated speciation example, the founder species diverges into one species with a hooked break and another with a straight beak. However, in this case the hooked and straight beaks gives rise immediately to long, thin and short, fat beaks.
Figure 18.25In (a) gradual speciation, species diverge at a slow, steady pace as traits change incrementally. In (b) punctuated equilibrium, species diverge quickly and then remain unchanged for long periods of time.
Describe a situation in which punctuated equilibrium is more likely to take place.
  1. There is a significant change in the environment over time, such as the breakup of a supercontinent due to tectonic activity.
  2. A species that has a competitor outcompetes it and drives it to extinction, freeing up more resources.
  3. There is a sudden and significant change in the environment, such as a volcanic eruption that divides a population that once shared a habitat.
  4. 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.