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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)

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.

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.

Four general conditions necessary for natural selection to occur are:

A. More organisms are born than can survive.
B. Organisms vary in their characteristics, even within a species.
C. Variation is inherited.
D. 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.

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.

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.

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.

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.


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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