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Chapter 5: Natural selection

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Edited by Peter Moyle & Douglas Kelt
By Mary Orland, Douglas A Kelt, and Peter B. Moyle September 2004

"Nothing in biology makes sense except in light of evolution"

- Theodosius Dobzhansky (1973)

Differences in rates of survival and reproduction are the driving force behind natural selection, and they usually vary greatly among organisms in the natural world. The Atlantic cod (Gadus morhua) is a dramatic illustration of this. The average female cod produces 2 million eggs in a single spawning, which would clearly lead to billions of cod very quickly if each and every one survived. However, 99% of these eggs die in their first month, and 90% of those remaining after the first month will die before their first year. This means that only about 200 of the original 2 million cod survive to age one, and by the time the cod get to their reproductive age of 2-4 years on average only 2 of the original 2 million are still alive. It is quite clear that those two cod would have their genes represented disproportionately in the next generation, and therefore any heritable traits that allowed those two fish to survive and reproduce would have strong selective pressure.

Large mammals in the wild also generally display considerable variation in survival and reproduction, although perhaps not quite as dramatic as those seen in fish. Red deer are found in Europe and are closely related to the American elk. The reproductive success of females in a red deer population were carefully tracked over the entire lifetime of the animals (Fig. 5.2). As you can see in the figure, the most common outcome is that a female red deer does not manage to produce even a single offspring that lives to its first birthday. Among those red deer that do produce offspring that live to one year, the median number of offspring is only 4.5 per lifetime, while a small percentage of females manage to produce a dozen or more offspring that live to their first birthdays. As with the cod, it is readily apparent from these data that a relatively small number of organisms are passing down a disproportionate amount of the genetic material that comprises the subsequent generations.

Fig. 5.2. Most red deer in Britain never contribute to next generation. This figure shows that over 35% of female deer never produce any young that survive 1 or more years. Of those that do, most (about 22%) produce 8 young that survive at least one year. (Data from Clutton-Brock et al. 1982. Red deer. Univ. Chicago Press.)

While measuring variation in survival and reproduction strongly suggests that natural selection is in effect, fully verifying the conditions necessary for natural selection also requires linking these differences in survival and reproduction to a heritable trait. This was done in a classic 1950s study with a land snail, Cepea nemoralis, in Oxford England. The shells of these snails are highly variable, with some snails being solid in color and others have strong stripes or bands. Predation by birds is a major source of mortality for the snails. Censuses found that while 47% of the snail population was banded, 56% of the snails eaten by birds were banded. Clearly birds were preferentially eating the banded snails, perhaps because the unbanded snails were harder to see. This variation in survival would clearly cause selection for the spread of the genes for unbanded snails. The fact that there still remains so many banded snails in the population suggests that there are also other processes at work in the ecosystem that favor banded snails over the unbanded snails.

Evolution is defined as a change in the gene frequencies of a population through time. Natural selection can lead to evolutionary changes because it tends to raise the frequency of genes that increase the relative probability of survival and reproduction for the organisms that possess them in a population. Hence, natural selection causes changes in the frequency of genes through time and thereby causes evolution. It should be noted, however, that there are other causes of evolution as well. Migration, genetic drift1, genetic mutation, and non-random mating can all change the frequency of genes in a population. However, natural selection is the only cause of evolution that results in greater adaptation. An adaptation is a feature of an organism enabling it to survive and reproduce in its natural environment better than if it lacked that feature. For this reason, natural selection is an especially important process in evolution.

As scientists we would like to point out that there is very little scientific controversy regarding evolution. The theory of evolution is supported throughout the field of biology by thousands of examples and studies with no convincing evidence to refute it. There are some creationists who seek to discount the certainty that scientists have in evolution by saying that it is "just a theory." This in part stems from the fact that to scientists the term theory means something quite different than it does in the culture at large. To scientists a theory is a clearly defined set of general principles that have been mathematically described and repeatedly validated with experiments and field data. For example, physicists describe gravity and electricity with "the theory of gravity" and "the theory of electromagnetism" not because they are uncertain about the existence of gravity and electricity, but precisely because they are highly certain about the nature of these phenomena. By the same token, biologists call evolution a theory because it is a clear, powerful idea that is well supported by evidence. Few ideas in science ever have the importance, clarity and validation to be called theories. Evolution happens continuously everywhere there is life. To deny it exists is to deny our ability to learn how the world works through observation.

Natural selection is at work to some degree in all ecological systems at all times. For that reason it is inevitable that it will have an influence on the interactions between humans and wildlife. One example of such an interaction is a phenomenon observed in many fisheries; after humans begin to harvest a population, the size of the average fish declines. Harvesting by humans reduces the lifespan of the average fish in a population. This means that a fish is better off starting to reproduce when it is younger and smaller, because if it waits until it is older and larger it may get harvested first and not reproduce at all. A tradeoff generally exists for organisms between putting energy into their own growth and into reproduction, as indicated in the tule perch example. Heavy harvesting by humans selects for those individual fish that reproduce younger and put more energy into reproduction rather than growth, and hence results in the average fish becoming smaller in the harvested population.

An area in which natural selection bears directly on human affairs is in the evolution of resistance to pharmaceutical drugs and pesticides by microbes and insects. Malaria was nearly eradicated in the mid 20th century because the mosquito species that carries the Plasmodium parasites were highly susceptible to the pesticide DDT, and drugs were discovered that attacked the parasites in the human bloodstream. However, natural selection favored those few individual mosquitoes that happened to be resistant to DDT and other pesticides, so that now many mosquitoes are resistant to our pesticides, and consequently malaria is increasingly difficult to control. The evolved resistance of mosquitoes to pesticides has combined with the evolved resistance of the parasite itself to antibiotic drugs, helping to make malaria a widespread disease again; it is currently a major cause of death and illness in many tropical countries. There is some speculation that malaria may become more widespread in temperate regions such as North America with climate change, because the warm conditions necessary for the mosquitoes that carry malaria may begin to occur at higher latitudes. Thus, the consequences of natural selection have very real implications for you, your family, and your lifestyle.

Additional reading. A number of books have been published in recent years that attempt to convey the majesty and wonder of natural selection and evolution to non-hard-core scientists.

Two very good books that come to mind are:
Garrett, L. 1994. The coming plague: newly emerging diseases in a world out of balance. Farrar, Straus and Giroux.
Weiner, J. 1994. The beak of the finch: a story of evolution in our time. Knopf.

1 Genetic drift is a complicated concept but perhaps most clearly explained with reference to a bag of jelly beans. Imagine you had 100 jelly beans in a bag—ten beans of each of ten colors—and you pulled out 10 beans without looking first. It is likely that you would not fully represent the diversity of jelly beans in sample of 10 beans; that is, you likely would not pull a single bean of each color. It is more likely that you would accidentally pull two or three beans of some colors, and none of other colors. If jelly beans could breed and make more jelly beans (and if jelly bean color was genetically inherited), then the next generation of beans would not represent the original 100 beans. Instead, they would look more like the jelly beans in the sample. We could say that by sampling a small part of the population the representation has drifted slightly. Thus, genetic drift is a phenomenon that may occur when populations become small or fragmented; the consequences are that we lose some degree of genetic variability.


Table of Contents

1. Roots of the modern environmental dilemma: A brief history of the relationship between humans and wildlife
2. A history of wildlife in North America
3. Climatic determinants of global patterns of biodiversity
4. Biodiversity
5. Natural selection
6. Principles of ecology
7. Niche and habitat
8. Conservation biology
9. Conservation in the USA: legislative milestones
10. Alien invaders
11. Wildlife and Pollution
12. What you can do to save wildlife

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