4-1-4. Reconnection and Rates of Speciation

Learning Objectives

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 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 1). 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.

Art Connection:

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?

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 2). 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 2). 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.

Art Connection:

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?

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

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.

Link to Learning

Visit this website to continue the speciation story of the snails.

Section Summary

Speciation is not a precise division: overlap between closely related species can occur in areas called hybrid zones. Organisms reproduce with other similar organisms. The fitness of these hybrid offspring can affect the evolutionary path of the two species. Scientists propose two models for the rate of speciation: one model illustrates how a species can change slowly over time; the other model demonstrates how change can occur quickly from a parent generation to a new species. Both models continue to follow the patterns of natural selection.

Art Connections

Exercise 1

Figure 1. 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?

Show/Hide Solution

Figure 1. Fusion is most likely to occur because the two species will interact more and similar traits in food acquisition will be selected.

Exercise 2

Figure 2. Which of the following statements is false?

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

Show/Hide Solution

Figure 2. Answer B

Review Questions

Exercise 3

Which term is used to describe the continued divergence of species based on the low fitness of hybrid offspring?

reinforcement
fusion
stability
punctuated equilibrium

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Exercise 4

Which components of speciation would be least likely to be a part of punctuated equilibrium?

a division of populations
a change in environmental conditions
ongoing gene flow among all individuals
a large number of mutations taking place at once

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Free Response

Exercise 5

What do both rate of speciation models have in common?

Show/Hide Solution

Both models continue to conform to the rules of natural selection, and the influences of gene flow, genetic drift, and mutation.

Exercise 6

Describe a situation where hybrid reproduction would cause two species to fuse into one.

Show/Hide Solution

If the hybrid offspring are as fit or more fit than the parents, reproduction would likely continue between both species and the hybrids, eventually bringing all organisms under the umbrella of one species.

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