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The dog was the first domesticated animal. Domestication is an “evolutionary process [that] has been influenced by humans to meet their needs” (Secretariat, 1992, p. 3). In other words, domestication of a species causes biological changes over generations through selection by humans for favorable traits (i.e., traits that are useful, valuable, aesthetically pleasing, etc.).

Domestication led to extraordinarily large changes in the behavioral characteristics of domestic dogs, as well as in their physical characteristics (which is obvious when one compares the many breeds of dogs). Scientific research on the evolution of dog behavior began in the mid-1800s, most notably in the work of Charles Darwin (Darwin, 1872). In the middle of the twentieth century, a deeper understanding of the evolution of dog behavior was gained by combining behavioral analyses with classical genetic analyses of dog breeds (for a review, see Scott & Fuller, 1965).

Figure 1. An extreme example of the effects of domestication on the physical characteristics of a subspecies of the gray wolf.

Over the past 20 years, archaeological discoveries in combination with the results of highly sophisticated genetic analyses have shed a great deal of light on the evolution of domestic dogs (for a review, see Larson, Karlsson, Perri, et al., 2012). For example, there now is little doubt that domestic dogs evolved from the gray wolf, which is found in many parts of Europe and Asia (Honeycutt, 2010; Wayne & Ostrander, 2007). In fact, domestic dogs (Canis lupus familiaris) are considered to be a subspecies of the gray wolf (Canis lupus lupus). This means that, although dogs and wolves have physical features that often are very different, they can mate and produce fertile offspring.

Nevertheless, there still is much controversy about when and where domestic dogs originated. These disagreements are focused on the answers to two questions: when did domestic dogs “split” from gray wolves and where did this happen? These questions have proved difficult to answer because the results of genetic and archaeological research are complex and, hence, very difficult to interpret.

Genetic research on differences in DNA sequences have led to a wide range of estimates about when dogs and wolves first diverged: sometime between 20,000 to 100,000 years ago. One reason for the wide variation in these estimates is that dogs and wolves probably continued to interbreed, not only over long periods of time but also in many locations (Vilà, Savolainen, Maldonado, et al., 1997).

Archaeological researchers find no clear evidence for the existence of domestic dogs until about 15,000-30,000 years ago (Germonpré, Sablin, Stevens, 2009; Ovodov, Crockford, Kuzmin, et al., 2011). A major difficulty with interpreting archaeological findings, however, is that physical characteristics typically used to distinguish domestic dogs from wolves (e.g., the size and position of the teeth, the size and shape of the skull, etc.) probably varied much more in ancient dog populations than they do today (Larson, Karlsson, Perri, et al., 2012). In addition, nothing is known about the variation of these traits in populations of ancient wolves. In other words, the physical characteristics used to distinguish modern dogs and wolves probably overlapped to a relatively large extent in ancient dogs and wolves, thereby making it very difficult for archaeologists to know if they are looking at the bones and teeth of a wolf or a dog.

Figure 2. The 33,000-year-old skull of a wolf-like animal reputed to be a dog (Ovodov, Crockford, Kuzmin, et al., 2011)

Figure 2. The 33,000-year-old skull of a wolf-like animal reputed to be a dog (Ovodov, Crockford, Kuzmin, et al., 2011)

As of now, both the archeological and genetic evidence allow us to conclude with certainty that domestic dogs existed at least 15,000 years ago (Larson, Karlsson, Perri, et al., 2012). It is still an open question, however, if ancient dog populations leading to modern domestic dogs first diverged from wolves earlier than that.

In the next post, I’ll review what recent research seems to tell us about the genetic differences linked to the many physical and behavioral differences between (a) modern domestic dogs and their ancestral species, and (b) the various breeds of modern domestic dogs.

You may contact me at drjeffryricker@gmail.com

References

Darwin, C. (1872). The expression of the emotions in man and animals. London: John Murray. Retrieved January 21, 2013, at http://darwin-online.org.uk/content/frameset?itemID=F1142&viewtype=text&pageseq=1

Germonpré M, Sablin MV, Stevens RE, Hedges REM, Hofreier M, Stiller M, et al. (2009). Fossil dogs and wolves from Palaeolithic sites in Belgium, the Ukraine and Russia: Osteometry, ancient DNA and stable isotopes. Journal of Archaeological Science, 36, 473-490. doi: 10.1016/j.jas.2008.09.033

Honeycutt, R. L. (2010). Unraveling the mysteries of dog evolution. BMC Biology, 8(20). doi:10.1186/1741-7007-8-20

Larson, G., Karlsson, E. K., Perri, A., Webster, M. T., Ho, S. Y. W., Peters, J., et al. (2012). Rethinking dog domestication by integrating genetics, archeology, and biogeography. Proceedings of the National Academy of Sciences, 109, 8878–8883. doi: 10.1073/pnas.1203005109

Ovodov, N. D., Crockford, S. J., Kuzmin, Y. V., Higham, T. F. G., Hodgins, G. W. L., & van der Plicht, J. (2011). A 33,000-year-old incipient dog from the Altai Mountains of Siberia: Evidence of the earliest domestication disrupted by the Last Glacial Maximum. PLoS ONE 6(7): e22821. doi:10.1371/journal.pone.0022821

Scott, J. P., & Fuller, J. L. (1965). Genetics and the social behavior of the dog. Chicago: University of Chicago Press.

Secretariat, C. B. D. (1992). The Convention on Biological Diversity. Retrieved December 30, 2012, from http://www.cbd.int/doc/legal/cbd-en.pdf

Vilà, C., Savolainen, P., Maldonado, J. E., Amoim, I. R., Rice, J. E., Honeycutt, R.L., et al. (1997). Multiple and ancient origins of the domestic dog. Science, 276, 1687-1689.

Wayne, R. K., & Ostrander, E. A. (2007). Lessons learned from the dog genome. Trends in Genetics, 23, 557–567. doi: 10.1016/j.tig.2007.08.013

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When particular expressions of a characteristic are naturally selected and those expressions are associated with particular gene variants, those gene variants will be more likely than others to be passed on to future generations. For example, let’s say that there exists a gene with two variants, T and t, and that these variants are associated with difference in the average height of plants from a particular (fictional) species, Pirasus arizonensis. From a baseline height of five inches, T increases the average height by 1/2 inch, whereas t decreases the average height by 1/2 inch.

If P. arizonensis seeds are transported by birds into an environment in which the fully grown plant is surrounded by plants from another (fictional) species, Torensi mojavensis, that has an average height of six inches, the taller species will limit the smaller species’ access to sunlight. This, of course, would be detrimental to the survival and reproductive success of P. arizonensis. Thus, any P. arizonensis plant that grows taller than the average height of five inches will tend to survive longer and reproduce more.

Let’s say that the following 300 P. arizonensis plants have grown in this new environment:

100 TT plants, which will have an average height of 6 inches;
100 Tt plants, which will have an average height of five inches;
100 tt plants, which will have an average height of four inches.

The plants that are smaller than the surrounding plants will be much more likely to die before reproducing. And of course, other environmental factors also will affect plant survival, but in a more random fashion. Let’s say that 80 of the tt plants die before producing seeds, 50 of the Tt plants die before producing seeds, and 10 TT plants die before producing seeds. The result: 72% of the seeds in the next generation will contain T but only 28% will include t. Thus, if 300 plants grow in the next generation, their numbers will be as follows:

156 TT plants;
120 Tt plants;
24 tt plants.

As you can see, there are 276 plants with at least one copy of the T variant, which is much larger than the number of plants with at least one t variant (144 plants).

The third requirement of evolution by natural selection — the increased reproductive success of individuals with particular expressions of a characteristic —must remain stable over generations. This means that the “selective pressure” on P. arizonensis plants with respect to their heights must not change. The six-inch tall (on average) T. mojavensis plants must continue to limit the amount of sunlight obtained by smaller P. arizonensis plants. In this case, the following change in the frequencies of the T and t gene variants in this species should occur:

Figure 1. Changes in the Frequencies of T and t Over Five Generations.

As you can see, the frequency of T becomes almost 100% within five generations, which means that the population in this new environment now consists almost entirely of plants that are about six inches tall. Thus, over a very short period of time, natural selection can lead to a large change in the average expression of a characteristic in a population when individual differences in that characteristic are strongly associated with genetic differences.

Now, let’s return to the example from the previous post: the founding population of fruit flies on a tiny and isolated island buffeted by strong winds. Differences in the size of fruit-fly wings are strongly linked to differences in genes (Robertson & Reeve, 1952), to a degree similar to that described above for height differences in the fictional plant species. Thus, if the windy environment naturally select flies with smaller wings, gene variants correlated with smaller wings will increase in frequency over generations. This means that the population will evolve a smaller average wing size. This is shown in Figure 2 (the black bars represent the founding population and the white bars represent the population after a number of generations of natural selection for smaller wing sizes).

Figure 2. Evolution of a Population of Fruit Flies Undergoing Natural Selection for Smaller Wing Size

Although these two examples are fictional ones, there are many examples of natural selection and artificial selection (selection in which humans breed organisms that express particular characteristics). For introductions to and histories of the concepts of evolution and natural selection, see Colby (1996-1997), Endler (1986), and Zimmer (2001).

You may contact me at drjeffryricker@gmail.com

References

Colby, C (1996-1997). Introduction to evolutionary biology (Version 2). Retrieved November 14, 2012, from http://www.talkorigins.org/faqs/faq-intro-to-biology.html

Endler, J. A. (1986). Natural selection in the wild. Princeton, NJ: Princeton University Press.

Robertson, F. W., & Reeve, E. C. R. (1952). Studies in quantitative inheritance. I. The effects of selection for wing and thorax length in Drosophila melanogaster. Journal of Genetics, 50, 416-448.

Zimmer, C. (2001). Evolution: The triumph of an idea. New York: HarperCollins.

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