Chapter 5: Natural selection
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)
Understanding natural selection is crucial to understanding wildlife ecology and practically any other topic in biology. This chapter gives you the basic concepts of natural selection so that you will be able to apply this important principle throughout this course, as well as in your day-to-day life.
WHAT IS NATURAL SELECTION?
Natural selection is the process by which the organisms in a population that are best adapted to the environment increase in frequency relative to less well-adapted forms, over a number of generations. The consequence of natural selection is that through time species (generally) develop characteristics that make them increasingly well-adapted to their environments, ultimately resulting in a world filled with a fascinating diversity of life forms. Natural selection is a simple but immensely powerful concept, and is a pillar of our understanding of biology at all organizational scales. This process commonly is summed up as “the survival of the fittest” in popular culture, although this simplification leaves out some of the important subtleties of natural selection, as described below.
WHAT ARE THE CONDITIONS NECESSARY FOR NATURAL SELECTION TO OCCUR?
Four general conditions necessary for natural selection to occur are:
- More organisms are born than can survive.
- Organisms vary in their characteristics, even within a species.
- Variation is inherited.
- Differences in reproduction and survival are due to variation among organisms.
If all four of these conditions occur, which they commonly do in both natural and human-influenced ecological systems, then natural selection will occur. If any of these is not true, then natural selection cannot occur. Let’s consider each of these in a bit more detail.
A. MORE ORGANISMS ARE BORN THAN CAN SURVIVE
If every individual organism born were to survive and reproduce to its maximum ability, we would expect explosive exponential growth in populations to occur regularly. The fact that this rarely happens (and never continues for long) indicates that there are many individuals born into the world with low probabilities of survival and reproduction. Let’s illustrate this concept with rabbits and assume they have an average litter size of 10 offspring (5 males, 5 females) and a generation time of approximately 4 months. Starting with a single female rabbit, and keeping track of just female organisms for the sake of simplicity, we will have 5 new female rabbits in 4 months. In four more months those new female rabbits will be reproducing, and we will have approximately 5×5=25 new female offspring. Those 25 rabbits will in turn produce 25×5=625 offspring in 4 months, and the process will continue so that after 10 generations, a mere 40 months, the number of new female rabbits born would be 50 billion! There is of course not enough space or food for this many rabbits in the real world and in a natural ecosystem forces such as competition, disease, predators, and harsh abiotic (physical) conditions prevent such exponential growth from occurring for long. Competition, disease, predation, and abiotic conditions are among the most important limiting factors in ecological systems, and they quite often shape the path of natural selection.
B. ORGANISMS VARY, EVEN WITHIN A SPECIES
Variation among individual organisms of the same species has been well documented, and you can illustrate this to yourself by thinking of the range of differences in physical appearance and aptitudes that exists among people you know. Of course, variation among individuals occurs in other species as well and this variation fuels the process of natural selection because it gives natural selection something to “act on.” That is, if a faster gazelle were better able to elude the claws of a cheetah, then this gazelle would be more likely to survive. If all of the organisms within a population were completely identical, it would not be possible for natural selection to occur; if all gazelles ran the same speed, then there would be no “faster” individuals to avoid the clutches of predators. There are biomechanical limits, of course, to just how fast a gazelle can run and eventually all healthy adult gazelles will be able to run at the same maximum speed. However, natural selection also acts on other factors, such as the ability to dodge back and forth or to detect the predator from further away. Meanwhile, natural selection also favors cheetahs that can overcome the speed and agility of gazelles. When you start to realize that each species varies in hundreds of different ways and has to constantly adapt to changing conditions (including behavior of its predators or of its prey), you begin to understand how the complex and often bizarre life on this planet has developed through time.
C. VARIATION IS INHERITED
In general the causes behind variation in organisms can be divided into two categories, environmental and heritable. Environmental variation is that which has no genetic basis, but is the result of the conditions under which an individual lives. For example, our generation tends to have a diet higher in protein than that of our grandparents and in general we grow taller than our grandparents did. This is due to the food we consume, not to any underlying genetic superiority. If you prefer a more “natural” example, imagine predators existing in sites rich vs. poor in prey species. If a predator is raised in a site rich in prey, it likely will eat more and grow larger. On the other hand, if raised in a place where prey are limiting, they would be more likely to remain smaller when fully mature, simply because they could not acquire sufficient food. Only when variation among organisms is inherited from the previous generation, i.e. it has a genetic basis, will natural selection be able to occur. Natural selection cannot act on variation that is due purely to environmental conditions. In reality, variation among organisms often is the result of a combination of environmental and heritable causes, as illustrated by the variation in height among humans. People may be short because they have short parents who possess genes for short stature that they have passed down to their offspring. However, people may also be short if they did not receive proper nutrition when they were in their growing years, even if they do possess genes for a tall stature. The first cause for shortness is heritable, whereas the second is environmental, but natural selection would only be able to act on height to the degree that it is in fact heritable.
D. DIFFERENCES IN REPRODUCTION AND SURVIVAL ARE DUE TO VARIATION AMONG ORGANISMS
It is widely observed that the probability of survival and reproduction often varies tremendously among organisms. Furthermore, differences in the traits of organisms will often be the cause of their differences in survival and reproduction. It is readily apparent that the organisms with the highest rates of survival and reproduction will be those that have their genes best represented in the next generation, and hence natural selection occurs whenever variation in survival and reproduction is at least partially caused by a heritable trait that varies among organisms.
Let’s look at the example of tule perch to demonstrate this. Tule perch are small (4-6 inch long) fish that occur only in the fresh waters of Central California. Each female tule perch gives birth to 15-40 young, which are essentially miniature adults (which swim away after being born). It turns out that the number and size of young produced by a female is an inherited trait and is an adaptation to the environment in which the perch live. Thus female tule perch that live in the Russian River produce 25-35 small young and typically become pregnant in their first year of life. In contrast, tule perch in Clear Lake typically produce 15-20 large young and wait until their second year to become pregnant. The reason for the striking difference in life histories of the two populations is the nature of the environments. The Russian River is a large, isolated coastal stream that fluctuates enormously in flow from year to year; in this harsh system each adult female has a relatively low probability of survival from year to year so natural selection has favored females that produce a lot of young quickly. Clear Lake, in contrast, is a relatively benign environment where each adult female has a fairly high probability of survival from year to year, provided they are large enough to escape predators. Thus natural selection favors females that produce large young and that devote all their energy in the first year to becoming even larger. If both forms were brought into laboratory aquaria and raised under identical conditions, the Russian River fish would still produce lots of small young and Clear Lake fish would still produce comparatively small numbers of large young.
Fig. 5.1. Tule perch and very recent young. Photo by P. B. Moyle.
WHAT ARE SOME EMPIRICAL EXAMPLES OF THESE CONDITIONS FOR NATURAL SELECTION?
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.
HOW DOES NATURAL SELECTION RELATE TO EVOLUTION?
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.
HOW IS NATURAL SELECTION RELEVANT TO WILDLIFE ECOLOGY AND MANAGEMENT?
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.