On the Origin of Species | Study Guide

Charles Darwin

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On the Origin of Species | Chapter 9 : Hybridism | Summary

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Summary

In this chapter Darwin explains when two "pure" species interbreed, they may produce few offspring, and when these offspring—hybrids—attempt to reproduce, they are unable to do so. Darwin recognizes fertility and sterility represent two ends of a spectrum with innumerable gradations and the scenarios he outlines initially do not represent the sum total of possible variability. For instance, he notes in the fertilization of two distinct species—Crinum capense with C. revolutum—fertile offspring are produced, despite the fact it is not in many other species. Thus, Darwin states the "degree of sterility" in hybrids cannot be considered "absolutely universal."

Even so, Darwin lays out general rules that govern hybridity. Hybrids created from two species are generally sterile. Equally, hybrids and crosses are affected more by "unfavorable conditions." Darwin concludes the laws governing these processes are "simply incidental or dependent on unknown differences in their reproductive systems." He is aware there are laws governing these processes but is unable to come to a conclusion regarding their nature.

Regarding the evolutionary cause of sterility and hybridization, Darwin attempts to place the process within the context of natural selection but is unable to reach a satisfactory conclusion. He moves to a discussion of the physical mechanisms by which reproduction is prevented—such as the inability of the "male element" to "reach the female element." He ultimately admits he can find no adaptive benefit to sterility and hybridization.

Analysis

Darwin's discussion of sterile species crosses and hybrids speaks to the larger long-term processes of natural selection and species formation. He attempts to attribute an adaptive function to the tendency for sterility in these organisms. However, these tendencies are perhaps best understood today as the results of speciation. While not every instance of interspecific breeding results in sterile offspring, in many cases they do. Organisms with minimal genetic differences between them are able to interbreed, but distinct species are often unable to. This is partially attributed to the larger degree of difference resulting from genetic distance due to space or time.

Prior to the widespread adoption of genetic-based species distinctions, species were largely defined in part by their ability or inability to reproduce together. This concept is referred to as "reproductive isolation." Many scientists consider reproductive isolation to be a result of speciation. Reproductive isolation helps preserve distinct species, thereby preventing innumerable gradations and "transitional" forms. This can occur due to genetic differences, physical barriers, or any number of other physical factors.

While Darwin was correct in his characterization of species crossings and hybridization, he did not fully characterize why sterility would occur because he failed to understand the genetic basis for these variations. Equally, while he characterized some of the physical incompatibilities that could occur in the reproductive systems of crossed species or hybrids, he was not aware of the mechanism. The mechanism would only be realized when Mendel's genetic work was rediscovered. It is now known when species fail to produce viable offspring—or produce infertile offspring—it is often the case that their chromosomes are incompatible and thus genetically incapable of reproducing.

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