Natural Selection and the Concept of Fitness

Notes on the Web - Unit Six - Part 4
Natural Selection, the Concept of Fitness, Adaptation, and Imperfections

Bruce G. Stewart

General Objectives and Study Guide

Your objectives for these Notes on the Web and associated readings and exercises are:

To list and explain each component of natural selection theory;
To state natural selection theory in from the gene viewpoint;
To define fitness and explain how there can be multiple fitness strategies in individuals of the same species;
To recognize conditions that would favor one reproductive strategy over another in terms of fitness;
To give examples of a variety of fitness strategies;
To explain the function of light in the reproductive strategies of lightning beetles;
To describe alternative behaviors of lightning beetles that enhance fitness;
To propose a logical explanations of potential fitness for any example of animal behavior or structure;
To know common terms and facts associated with the topic of natural selection and fitness.

Related Textbook Readings:

For General Biology (BIO 1114) Class only: Johnson and Losos (2008) Chapter 17 Sections 17.5, 17.6, 17.7
For General Zoology (ZOO 1114) Class only: Hickman, et al. (2006) (to be assigned in class)

Great Grasshoppers, Great Literature and Great Science

Nick leaned back against a stump and slipped out of the pack harness. Ahead of him, as far as he could see, was the pine plain. The burned country stopped off at the left with the range of hills. ...

Nick sat down against the charred stump and smoked a cigarette. ...

As he smoked, his legs stretched out in front of him, he noticed a grasshopper walk along the ground and up onto his woolen sock. The grasshopper was black. As he had walked along the road, climbing, he had started many grasshoppers from the dust. They were all black. They were not the big grasshoppers with yellow and black or red and black wings whirring out from their black wing sheathing as they fly up. These were just ordinary hoppers, but all a sooty black in color. Nick had wondered about them as he walked, without really thinking about them. Now, as he watched the black hopper that was nibbling at the wool of his sock with its fourway lip, he realized that they had all turned black from living in the burned -over land. He realized that the fire must have come the year before, but the grasshoppers were all black now. He wondered how long they would stay that way.

Carefully he reached his hand down and took hold of the hopper by the wings. He turned him up, all his legs walking in the air, and looked at his jointed belly. Yes, it was black too, iridescent where the back and head were dusty.

"Go on, hopper," Nick said, speaking out loud for the first time. "Fly away somewhere."

He tossed the grasshopper up into the air and watched him sail away to a charcoal stump across the the road.

Excerpts from Big-hearted River: Part I by Ernest Hemingway (1938)

Natural Selection Basics

In the story above, Nick asked some good questions. The answer, of course, is that natural selection can account for his interesting observations. Do you see why? It is a wonderful feeling to get to know some of the workings of the world around us. Go next summer to "Ten Acre Rock" near Reagan, Oklahoma and look at the patterns of the grasshoppers that live on those huge granite exposures (an ancient "batholith" as it is called in geology). You may be surprised!

To get started on our own understanding of natural selection, let us begin by stating the primary components of Darwin's great theory:

Individuals within a species differ in their morphology, physiology, and behavior (that is there is variation among members of a species), and
Some of the variation among individuals of a species is genetically-controlled and therefore is heritable (that is these variations can be passed on to offspring), and
Organisms have high reproductive potential and tend to produce more offspring than can survive to reproduce because there is competition for limited resources, and
Some individuals, by virtue of having characteristics that serve them better in survival and reproduction, will produce more offspring than others; that is, they will be naturally-selected because of particular environmental conditions to which they happen to be better adapted than their competitors, thus
Evolutionary change will occur as a consequence of natural selection, and this change will tend to produce species whose members have traits that are adapted to their environments.

Darwin and others in his day were unaware of the molecular nature of the gene. In fact, even Mendel's studies of inheritance patterns were unknown to scientists over 40 years after the publication of The Origin of Species! Yet Darwin's creative and perceptive mind was able to see a wide range of other evidences that the process of natural selection goes on in living systems.

In modern genetic terms, the gene is known to be the unit which causes the expression of traits that can be naturally selected. This emphasis can be illustrated as follows:

All organisms have genes which code for protein synthesis and which are heritable; these proteins regulate the development of the nervous system, muscles, and other aspects of the structure of the organism and even determine in large degree many aspects of its behavior, and
Within a population of organisms, many genes are present in two or more forms (alleles) which code for slightly different forms of the same protein; these modified proteins cause differences in development, physiology, structure, and/or behavior of the organisms; thus, there is variation, and
There will be competition between alleles of a gene for loci on chromosomes, and some alleles will be naturally selected as a result of the effects they have on the reproductive success of the organism that carries them.
Any allele that can make more surviving copies of itself than its alternatives will replace the alternatives in the gene pool.

Thus we can see that selection will favor those genes that promote survival and reproduction by the organism that carries them. We shall now turn to the organismal level to see what kinds of traits can be selected for and thereby be manifested in the form of particular structures, functions, and behaviors of individual organisms. The crucial concept that we must now explore is that of fitness in an evolutionary sense.

The Concept of Fitness and its Relationship to Diversity of Structure, Function and Behavior

Fitness is the term used to describe how successful an organism is at leaving offspring compared to other members of its species. Common sense and logic indicate that gene forms must promote the successful reproduction by organisms in order to continue to exist in the gene pool of the following generation. Furthermore, those that do this best, will be found in increasingly higher proportions. Thus, the phenotypic affects (structures, functions, behaviors) of those genes can be observed to see how they function in aiding the fitness of the organism.

Evolutionary strategies found in the diversity of living species illustrate a wide range of contrasting forms of fitness. Often times, different behavioral strategies are required to take advantage of different environmental resources. Or sometimes what is fit in one situation is not as fit in another. Or sometimes there are more that one possible successful way to be fit even within the same species! A quote from Ecclesiastes, Chapter 9 Verse 11 (KJV) illustrates this point that there is "more than one way to skin a cat" (pardon the cliché!):

I returned and saw under the sun, that the race is not always to the swift, nor the battle to the strong, neither yet bread to the wise, nor riches to men of understanding, nor yet favour to men of skill; but time and chance happeneth to them all.

With living things, there is no way a prior (ahead of time) to state that this or that trait will always be successful in increasing fitness. It depends on the interactions between the organism and its overall environment (including other organisms).

Reproductive strategies are especially important in fitness since the way an organism reproduces has direct affects on its fitness. To illustrate we will consider some examples of contrasting reproductive strategies that are known in nature, and you should use your critical thinking skills to explain why each method could be selected for under certain conditions.

Reproductive Strategies Have Direct Impacts on Fitness

There are different strategies for number of mates.

Monagamy. In monogamy individuals have only one mate (or one mate in a breeding season). This is typical of many birds including our nation's symbol, the Bald Eagle (Haliaeetus leucocephalus).

What do you think would cause the "natural selection" of this behavior? Take some notes from our class discussion of this.

Polygamy. Polygamy describes the reproductive behavior of individuals having more than one mate. This is the exception in birds, but there are some interesting examples. For example, the Brown-headed Cowbird (Molothrus ater) has males who mate with multiple females. The female also has the interesting behavior of being a nest parasite! That is, she lays her eyes in the nests of other species (the hosts) and never cares for them at all.

What do you think would cause the "natural selection" of polygamous behavior in the Brown-headed Cowbird? Take additional note on this in class.

Mate Choice Affects Fitness.

There are different strategies by both males and females for choosing or obtaining mates with "good genes" or simply getting the maximum number of mates possible. For example, females of some species select males who demonstrate particular characteristics of "good genes." Lek mating systems, such as in Prairie Chickens, are good examples of where females choose particular males from a group at a traditional male gathering site (such as the "booming ground" of prairie chickens). Take some notes on our lecture discussion of lek mating systems.

Males of some species select the females. This is an exception to the usual pattern found in nature where females tend to be more choosy. Females of the Giant Water Bug lay their eggs on the backs of males who carry them and care for them until they hatch; this is such a great investment on the part of the male that they are "choosy" for females who gain the right to mate with them.

Males of some species defend territories or harems which allow them access to one or more females. Countless examples of territoriality in animals are known; for example, male songbirds often defend a territory by using song, aggression, etc. Males of some species (e.g. the Red Deer of Europe) defend harems of females and gain the most matings in that manner.

Note on the human species, Homo sapiens: R esearch in sociology demonstrates that natural human societies were: 44% - polygynous, 39% monogamous with occasional polygyny, and the rest monogamous. How could you explain this natural variation in a single species?

Number of Young Produced Affects Fitness

Fitness sometimes is increased by having many young, sometime few:

mass numbers of young - such as in salmon, frogs, insects, bobwhite, etc.
few young - such as in humans, chimpanzees, elephants, some birds, etc.
Be able to give examples of when each case would be an advantage and would therefore be selected for. You can take some class notes on this.

Reproductive Behavior is Related to Fitness Benefits

The Amazing Case of the Lighting Beetle Family: Behaviors that Explained in Terms of Natural Selection and Fitness

Let's now explore one striking example of complex behaviors in a group of familiar species, the lightning beetles (or fireflies) of the family Lampyridae. The examples come from over twenty years of detailed study of these organisms by Dr. James E. Lloyd and his graduate students at the University of Florida. The studies have been published in many peer-reviewed journals, including the prestigious journal, Science. Some information is provided below but the details will be given in lecture.

Communication in this group of species is by chemoluminescent flashes; the species are active at twilight or night and the color of the flashes (yellow or green hues) of a given species is related to visibility during their active period. Mate location is the primary function of the flashing behavior and physiology (though we will see a striking example of another function):

Males fly about emitting species-specific flash patterns which can be distinguished by:

flash rate (e.g. number of sequences per minute)
flash number per sequence
flash duration (such as in Morse Code!)
flight pattern during flash sequence

Females of most species are flightless, worm-like creatures (though some species have wings) and give a species-specific response flash from the ground to signal their male that they are reproductively ready to mate. The female flashes can be distinguished by:

flash length
delay time for response (time after male last flashed)

Females are a scarce resource: Since each female is in effect no longer available for breeding after mating once (at least for a while), there are far fewer available females than males. This results in extreme selection for traits that help males obtain mates. Consider these observations:

Males may flash a few 1000 times over periods as long as eight days to get a single response!
Females need to flash only a few times over only about six minutes to get a male!

Detailed examples of the following species will be discussed in class: Photinus tanytoxus, Photinus macdermotti, Photuris versicolor, and Photuris cinctipennis. The following illustration shows the flash patterns of the males and females of Photinus macdermotti. Notice how specific these are. For example, the male's pulse of light lasts 0.12-0.16 seconds, and is giving 2 seconds apart with a frequency of one sequence every 8-10 seconds. However, notice that there is a "mimic" that gives an identical response flash as that of the Photinus macdermotti female.

What do you think the function of the mimic flash might be?

In class we will discuss the patterns and functions of the flashes of several lightning beetle species. Take careful notes and make sure you understand and can explain the fitness benefits and reasons for the existence of deceptive and "abnormal" behaviors that occur in some of these species (including rape!). Natural selection gives us the answer to why behaviors exist that the natural theologians could not even have imaged existed!

Natural Selection Has Produced an Impressive Range of Adaptation in Living Things

Aside from direct fitness benefits imparted by the various examples reproductive strategies and behaviors described above, all aspects (structures, functions, behaviors, etc.) of individuals of living species potentially provide advantages in survival and therefore can increase reproductive success (i.e. fitness) indirectly. Anything aspect of a species that has impacts on fitness is subject to the evolutionary influence of natural selection. This has produced a wide and fascinating range of adaptations. The examples are endless, and all you need to do to see this is look around at the living world and ask yourself what might be the survival benefits of some aspect of any critter you happen to observe.

Cryptic Coloration that Helps Species Avoid Predation or Helps them Catch Prey

Example: Color-changing Bioluminescent Squid. Earlier in the semester in Unit One, I presented a link with an example of use of light flashes by a fish to blind its predators. Here is another link with a more detailed discussion of the use of use of color change by a bioluminescent squid to avoid being detected by predators. Read this example and be able to explain the adaptation. The Color-changing Squid - from The Bioluminescent Web Page

Note: This Section on Adaptation and a Separate Section on Imperfection "Design" is Actively Under Construction. There is a great deal of material that is not yet converted into a form for your Notes on the Web. This material will be presented in lectures/labs and you will need to take careful notes. You will be shown a large, illustrated PowerPoint presentation in that will address these topics in a way that most students seem to find quite interesting.

Reminder about Textbook Study

As with other topics, your textbooks have excellent presentations of the materials on the evolution. including generous excellent illustrations. Check the general objectives above to make sure that you have covered all of the topics in the textbook readings. In particular, be sure to study the textbook readings for speciation, adaptive radiation, types of selection and other related topics.

The "Self Test" and other questions will be helpful for general biology students, although many more detailed questions will be included in the lecture exam. Similarly, study questions in the zoology textbook will be helpful review for general zoology students, but again, they are not comprehensive.

As with all materials throughout the semester, you will have opportunities to ask questions or ask that any relevant material from your assignments be discussed in class and/or in threaded discussions on Internet.

To Sexual Selection and the Explanation of Bizarre Animal Behaviors

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Literature Cited

Hemingway, Ernest. 1938. Big Two-Hearted River: Part I, pages 209-218 IN The Short Stories of Ernest Hemingway. Charles Scribner's Sons. New York. 499 pp.