14.14. Isolating Mechanisms

Prezygotic Isolating Mechanisms

Geographical Isolation. This mechanism is perhaps the easiest to understand. Species that exist in different areas are not able to interbreed. The two populations of flowers in the first panel of table 14.1 are separated by a mountain range and so would not be capable of interbreeding.

Ecological Isolation. Even if two species occur in the same area, they may utilize different portions of the environment and thus not hybridize because they do not encounter each other, like the lizards in the second panel of table 14.1. One lives on the ground and the other in the trees. Another example in nature is the ranges of lions and tigers in India. Their ranges overlapped until about 150 years ago. Even when they did overlap, however, there were no records of natural hybrids. Lions stayed mainly in the open grassland and hunted in groups called prides; tigers tended to be solitary creatures of the forest. Because of their ecological and behavioral differences, lions and tigers rarely came into direct contact with each other, even though their ranges overlapped thousands of square kilometers. Figure 14.32 shows that hybrids are possible; the tigon shown in figure 14.32c is a hybrid of a lion and tiger. These matings do not occur in the wild but can happen in artificial environments such as zoos.

Figure 14.32. Lions and tigers are ecologically isolated.

The ranges of lions and tigers used to overlap in India. However, lions and tigers do not hybridize in the wild because they utilize different portions of the habitat.

(a) Tigers are solitary animals that live in the forest, whereas (b) lions live in open grassland. (c) Hybrids, such as this tigon, have been successfully produced in captivity, but hybridization does not occur in the wild.

Temporal Isolation. Lactuca graminifolia and L. canadensis, two species of wild lettuce, grow together along roadsides throughout the southeastern United States. Hybrids between these two species are easily made experimentally and are completely fertile. But such hybrids are rare in nature because L. graminifolia flowers in early spring and L. canadensis flowers in summer. This is called temporal isolation and is shown in the third panel in table 14.1. When the blooming periods of these two species overlap, as they do occasionally, the two species do form hybrids, which may become locally abundant.

Behavioral Isolation. In chapter 37, we will consider the often elaborate courtship and mating rituals of some groups of animals, which tend to keep these species distinct in nature even if they inhabit the same places. This behavioral isolation is discussed in the fourth panel of table 14.1. For example, mallard and pintail ducks are perhaps the two most common freshwater ducks in North America. In captivity, they produce completely fertile offspring, but in nature they nest side-byside and rarely hybridize.

Mechanical Isolation. Structural differences that prevent mating between related species of animals and plants is called mechanical isolation and is shown in panel five of table 14.1. Flowers of related species of plants often differ significantly in their proportions and structures. Some of these differences limit the transfer of pollen from one plant species to another. For example, bees may pick up the pollen of one species on a certain place on their bodies; if this area does not come into contact with the receptive structures of the flowers of another plant species, the pollen is not transferred.

Prevention of Gamete Fusion In animals that shed their gametes directly into water, eggs and sperm derived from different species may not attract one another. Many land animals may not hybridize successfully because the sperm of one species may function so poorly within the reproductive tract of another that fertilization never takes place. In plants, the growth of pollen tubes may be impeded in hybrids between different species. In both plants and animals, the operation of such isolating mechanisms prevents the union of gametes even following successful mating. The sixth panel in table 14.1 discusses this isolating mechanism.

Postzygotic Isolating Mechanisms

All of the factors we have discussed up to this point tend to prevent hybridization. If hybrid matings do occur, and zygotes are produced, many factors may still prevent those zygotes from developing into normally functioning, fertile individuals. Development in any species is a complex process. In hybrids, the genetic complements of two species may be so different that they cannot function together normally in embryonic development. For example, hybridization between sheep and goats usually produces embryos that die in the earliest developmental stages.

Figure 14.33 shows four species of leopard frogs (genus Rana) and their ranges throughout North America. It was assumed for a long time that they constituted a single species. However, careful examination revealed that although the frogs appear similar, successful mating between them is rare because of problems that occur as the fertilized eggs develop. Many of the hybrid combinations cannot be produced even in the laboratory. Examples of this kind, in which similar species have been recognized only as a result of hybridization experiments, are common in plants.

Figure 14.33. Postzygotic isolation in leopard frogs.

Numbers indicate the following species in the geographic ranges shown: (1) Rana pipiens; (2) Rana blairi; (3) Rana sphenocephala; (4) Rana berlandieri. These four species resemble one another closely in their external features. Their status as separate species was first suspected when hybrids between them were found to produce defective embryos in the laboratory. Subsequent research revealed that the mating calls of the four species differ substantially, indicating that the species have both pre- and postzygotic isolating mechanisms.

Even if hybrids survive the embryo stage, however, they may not develop normally. If the hybrids are weaker than their parents, they will almost certainly be eliminated in nature. Even if they are vigorous and strong, as in the case of the mule, a hybrid between a female horse and a male donkey, they may still be sterile and thus incapable of contributing to succeeding generations. Sterility may result in hybrids because the development of sex organs may be abnormal, because the chromosomes derived from the respective parents may not pair properly, or from a variety of other causes.

Key Learning Outcome 14.14. Prezygotic isolating mechanisms lead to reproductive isolation by preventing the formation of hybrid zygotes. Postzygotic mechanisms lead to the failure of hybrid zygotes to develop normally, or they prevent hybrids from becoming established in nature.

Inquiry & Analysis

Does Natural Selection Act on Enzyme Polymorphism?

The essence of Darwin's theory of evolution is that, in nature, selection favors some gene alternatives over others. Many studies of natural selection have focused on genes encoding enzymes because populations in nature tend to possess many alternative alleles of their enzymes (a phenomenon called enzyme polymorphism). Often investigators have looked to see if weather influences which alleles are more common in natural populations. A particularly nice example of such a study was carried out on a fish, the mummichog (Fundulus heteroclitus), which ranges along the East Coast of North America. Researchers studied allele frequencies of the gene encoding the enzyme lactate dehydrogenase, which catalyzes the conversion of pyruvate to lactate. As you learned in chapter 7, this reaction is a key step in energy metabolism, particularly when oxygen is in short supply. There are two common alleles of lactate dehydrogenase in these fish populations, with allele a being a better catalyst at lower temperatures than allele b.

In an experiment, investigators sampled the frequency of allele a in 41 fish populations located over 14 degrees of latitude, from Jacksonville, Florida (31° North), to Bar Harbor, Maine (44° North). Annual mean water temperatures change 1° C per degree change in latitude. The survey is designed to test a prediction of the hypothesis that natural selection acts on this enzyme polymorphism. If it does, then you would expect that allele a, producing a better "low- temperature” enzyme, would be more common in the colder waters of the more northern latitudes. The graph on the right presents the results of this survey. The points on the graph are derived from pie chart data such as shown for 20 populations in the map (a pie chart diagram assigns a slice of the pie to each variable; the size of the slice is proportional to the contribution made by that variable to the total). The blue line on the graph is the line that best fits the data (a "best-fit" line, also called a regression line, is determined statistically by a process called regression analysis).

1. Applying Concepts

a. Variable. In the graph, what is the dependent variable?

b. Reading pie charts. In the fish population located at 35° N latitude, what is the frequency of the a allele? Locate this point on the graph.

c. Analyzing a continuous variable. Compare the frequency of allele a among fish captured in waters at 44° N latitude with the frequency among fish captured at 31° N latitude. Is there a pattern? Describe it.

2. Interpreting Data At what latitude do fish populations exhibit the greatest variability in allele a frequency?

3. Making Inferences

a. Are fish populations in cold waters at 44° N latitude more or less likely to contain heterozygous individuals than fish populations in warm waters at 31° N latitude? Why this difference, or lack of it?

b. Where along this latitudinal gradient in the frequency of allele a would you expect to find the highest frequency of heterozygous individuals? Why?

4. Drawing Conclusions Are the differences in population frequencies of allele a consistent with the hypothesis that natural selection is acting on the alleles encoding this enzyme? Explain.

5. Further Analysis If you were to release fish captured at 32° N into populations located at 44° N, so that the local population now had equal frequencies of the two alleles, what would you expect to happen in future generations? How might you test this prediction?

Test Your Understanding

1. Darwin was greatly influenced by Thomas Malthus, who pointed out that

a. food supplies increase geometrically.

b. populations increase arithmetically.

c. populations are capable of geometric increase, yet remain at constant levels.

d. the food supply usually increases faster than the population that depends on it.

2. Darwin proposed that individuals with traits that help them live in their immediate environment are more likely to survive and reproduce than individuals without those traits. He called this

a. natural selection.      

b. arithmetic progression.

c. the theory of evolution.     

d. geometric progression.

3. A great deal of research has been done on Darwin’s finches over the last 70 years. The research

a. seems to often contradict Darwin’s original ideas.

b. seems to agree with Darwin’s original ideas.

c. does not show any clear patterns that support or refute Darwin’s original ideas.

d. suggests a different explanation for the evolution of finches.

4. One of the major sources of evidence for evolution is in the comparative anatomy of organisms. Features that look different but have similar structural origin are called

a. homologous structures.     

b. analogous structures.

c. vestigial structures.

d. equivalent structures.

5. A large group of organisms lives in a large, stable ecosystem. There is no competition for resources. Individuals show no mate preferences. All organisms appear to be identical except for a few individuals in the most recent generation of offspring that exhibit a different fur coat color and pattern. The ecosystem and population are geographically isolated from other populations of the same organism. Which Hardy-Weinberg assumption seems to have been violated?

a. large population size

b. random mating within the population

c. no mutation within the population

d. no input of new alleles from outside or loss of alleles

6. A population of 1,000 individuals has 200 individuals who show a homozygous recessive phenotype and 800 individuals who express the dominant phenotype. What is the frequency of homozygous recessive individuals in this population?

a. 0.20                 

b. 0.30

c. 0.45                 

d. 0.55

7. A chance event occurs that causes a population to lose some individuals (they died)—hence, a loss of alleles in the population results from

a. mutation.          

b. migration.

c. selection.          

d. genetic drift.

8. Selection that causes one extreme phenotype to be more frequent in a population is an example of

a. disruptive selection.  

b. stabilizing selection.  

c. directional selection.

d. equivalent selection.

9. A key element of Ernst Mayr’s biological species concept is

a. homologous isolation.

b. divergent isolation.   

c. convergent isolation.

d. reproductive isolation.

10. Which of the following is not a prezygotic isolating mechanism?

a. behavioral isolation

b. ecological isolation

c. hybrid infertility

d. None of the above.