29 Reconnection and Speciation Rates

By the end of this section, you will be able to do the following:

  • Describe pathways of species evolution in hybrid zones
  • Explain the two major theories on rates of speciation

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.

Reconnection

After speciation, two species may recombine or even continue interacting indefinitely. Individual organisms will mate with any nearby individual with whom they are capable of breeding. We call an area where two closely related species continue to interact and reproduce, forming hybrids a hybrid zone. Over time, the hybrid zone may change depending on the fitness of the hybrids and the reproductive barriers (Figure 1). If the hybrids are less fit than the parents, speciation reinforcement 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.

VISUAL CONNECTION


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 1: After 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.

If two species eat a different diet but one of the food sources is eliminated and both species are forced to eat the same foods, what change in the hybrid zone is most likely to occur?

Answer:
Fusion is most likely to occur because the two species will interact more and similar traits in food acquisition will be selected.

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 we call reinforcement. Scientists use this term because the hybrids’ low success 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 2). Scientists have also observed that sometimes two species will remain separate but also continue to interact to produce some individuals. Scientists classify this 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 speciation rates. In terms of how quickly speciation occurs, we can observe two current patterns: 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 2). 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.

VISUAL CONNECTION


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 2: In (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.

Which of the following statements is false?

  1. Punctuated equilibrium is most likely to occur in a small population that experiences a rapid change in its environment.
  2. Punctuated equilibrium is most likely to occur in a large population that lives in a stable climate.
  3. Gradual speciation is most likely to occur in species that live in a stable climate.
  4. Gradual speciation and punctuated equilibrium both result in the divergence of species.


Answer:
Statement b is false.

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.

Glossary

gradual speciation model
model that shows how species diverge gradually over time in small steps
hybrid zone
area where two closely related species continue to interact and reproduce, forming hybrids
punctuated equilibrium
model for rapid speciation that can occur when an event causes a small portion of a population to be cut off from the rest of the population
reinforcement
continued speciation divergence between two related species due to low fitness of hybrids between them

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Introductory Biology: Evolutionary and Ecological Perspectives by Various Authors - See Each Chapter Attribution is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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