DEPARTMENT OF BIOLOGY
The Basics of Evolution
The Fundamental Process
Biological evolution occurs when individual organisms with particular characteristics replace other individuals with other characteristics. It is not that the characteristics of any single individual change, or evolve, during its lifetime. Rather, each individual lives its life in its own way, producing offspring if and when it can.
The key to evolution is the degree of success with which each individual produces offspring. Any genetically-heritable characteristics can be passed on to offspring. Consequently, those individuals that have the most offspring contribute the most to future generations. Those individuals that have the least offspring contribute the least to future generations. Over the course of many generations, some genetically-heritable characteristics are lost from the population overall, while others become common.
Evolution can occur only if there are different genetically-heritable characteristics in a population of organisms. There must be genetic diversity. In nearly all populations of nearly all species, there is genetic diversity. Partly, this is because the genetic information is re-assorted at each generation. It is also because mutations occasionally occur, causing changes in genetic information. Once a mutation occurs, and is passed on to the offspring of the first individual to carry that mutation, then the mutation becomes a part of the genetic diversity of the population.
Evolution can occur rapidly or very slowly. It occurs slowly, if at all, when the particular characteristics of a population make it well-suited to the environment in which it lives. Under such conditions, most mutational changes make individuals less suited to the environment. As a result, the more common characteristics remain the norm.
Rapid evolution can occur during times of environmental change. A population's environment can change if the individuals in that population migrate to a new location. Or, environmental conditions can change for numerous reasons, including long-term climate change, the introduction of a new species, or the loss of a previously-common species. In a new environment, genetic variations that were previously uncommon may be advantageous. Individuals with these variations may now out-compete their fellows, and their genetic variations may become the new norm for that population.
The fundamental principles of evolution are these:
1. Evolution depends upon genetic variation.
2. Evolution occurs as some genetic variations become common in populations, and others become rare.
3. The source of genetic variation is mutation. Genetic variation can be augmented by genetic reassortment during the production of offspring.
4. Individual organisms do not change or evolve. Evolution is the replacement, over the course of numerous generations, of some genetic variations by other genetic variations.
5. It is not possible to mutate in anticipation of environmental change, or to direct mutations to specific characteristics. Mutation is not a conscious process.
Below, we explore some of the details:
DNA, Genes, Mutations, and the Characteristics of Organisms
Genetic inheritance depends upon genes, which are segments of DNA, the fundamental chemical of chromosomes. DNA carries the "information" that determines how organisms grow and develop, and that determines many of their characteristics. It does not dictate all of an individual's characteristics, because many aspects of most species are shaped by the peculiarities of the environment in which they live.
Every individual of every species begins life as a single cell. In the case of humans, that single cell is the fertilized egg, which contains one set of chromosomes contributed by the mother, and one set of chromosomes contributed by the father. As the fertilized egg divides and the cells differentiate to become all of the different cell types of a human, the DNA molecules of the fertilized egg must be duplicated over and over, so that each of our trillions of cells contains an exact copy of the DNA contained in the fertilized egg. DNA replication must be tremendously accurate to ensure that every cell contains the information that it needs.
Although DNA replication is tremendously accurate, it is not 100% accurate. Occasionally, mistakes are made. If mistakes occur in the DNA of genes, then those genes are altered.
DNA is a chemical. Therefore, it follows the laws of chemistry. Consequently, DNA molecules can be damaged--chemically altered--by radiation, chemicals, cosmic rays, oxygen radicals, etc. Although DNA damage can often be repaired, it is not always repaired, and repair may be imperfect. Damage and/or imperfect repair can also alter genes.
These changes to DNA are mutations. Because they occur by normal, chemical mechanisms, it is impossible to prevent them from occurring. It is also impossible to cause them to occur in specific genes . They occur at random.
"Random mutation" does not mean "un-caused mutation." It means that the mechanisms that cause mutations cannot choose which part of a DNA molecule to affect. This is illustrated in the figure on the right, which shows an oxygen radical (a common mutagen) inside the nucleus of a cell. It is surrounded by DNA, from many different genes, but all of the DNA is chemically the same. The oxygen radical has an equal likelihood of reacting with any nucleotide in any DNA molecule near it. The probability that a base will be modified, and thus cause a mutation, is statistically random.
It is tremendously important to recognize that mutations are changes in DNA. A person cannot mutate. A turtle cannot mutate (and turn into a ninja). Why not? Because a chemical mistake in the DNA of one cell affects only that cell, and is not spread throughout the body to all of the cells. A mutation in the DNA of an adult human will not change that person [unless the mutation occurs in a gene that controls cell division, in which case the mutated cell may begin to divide uncontrollably, and become a cancer].
To change the characteristics of a whole organism, a mutation must occur in a cell in the gonads, destined to become an egg or sperm, and become incorporated into a fertilized egg, and develop into a complete individual. Only then can a new mutation, a new DNA change, become a part of every cell in an individual organism. Only then can a new mutation change the characteristics of the organism. In other words, if an individual is exposed to mutation-causing chemicals or radiation, that individual will not mutate . However, that individual's offspring may carry mutations. Once the offspring reproduce, and pass DNA changes to the next generation, then the mutations become part of the genetic diversity of that species.
If that mutation gives an individual an advantage, so that the individual is more likely to produce healthy offspring, then the numbers of individuals with that particular genetic variation will increase with each generation that passes. If a mutation gives an individual a disadvantage, so that the individual reproduces less successfully (or dies), then that particular genetic variation will be lost from the gene pool of that species.
Genes, Proteins, and Cellular Micromachines
To understand how mutations can change the characteristics of organisms, it is necessary to understand how genes work. In general , genes are segments of DNA that carry the information for proteins. Genes do no more than this. Inside cells, genes just sit there, waiting for their information to be used. "Using" the information means following the chemical processes by which cells produce proteins.
We will ignore, for now, the process that cells use to produce proteins. It is enough to say that the information in each gene dictates the production of a single type of protein. Human DNA is estimated to contain information for around 30,000 different proteins. Each of these proteins is a specific kind of cellular "micromachine" that has a specific function.
Proteins are produced by assembling "building blocks" called amino acids. There are 20 different amino acids that are used in cells. From these, an infinite number of different proteins can be built, depending on how many of these building blocks are strung together, and the order in which they are assembled. The differences in functions of proteins depend on the differences in amino acid sequence of the proteins.
A mutation in a gene--a change in the DNA--has the likely consequence of changing the amino acid sequence of the protein whose information that gene carries. This, in turn, can change the way that the protein micromachine works. To see the types of effects that this can have on the characteristics of an entire organism, it may be best to discuss some specific examples:
1. Eye color
Eye color in humans is determined by several different genes that produce different kinds of proteins. One gene, called EYCL3 , carries the code for an enzyme (a protein that catalyzes a chemical reaction) that produces a brown pigment. This gene is used, or "turned on" in the cells of the iris. An individual who inherits a functional gene for this enzyme from either parent will be able to produce the brown pigment in her irises, and will have brown eyes. Mutations in this gene can cause the protein not to work. An individual who inherits non-functional genes for this enzyme from both parents will be unable to produce the brown pigment. As a result, the individual will have green or blue eyes. (The green pigment also depends on a gene that produces an enzyme; the blue color is a result of the way that iris cells reflect light, and is not based upon a blue pigment).
This particular gene need not be either functional or non-functional. As with any gene, there are many, many different variations possible. One variation may produce a protein that is a very active enzyme; individuals with this version of the gene will have very dark brown eyes. Another variation produces an enzyme that works, but not very well. This enzyme cannot produce as much brown pigment. Individuals with this version of the gene will have light brown eyes.
The gene is the set of instructions for the enzyme; the enzyme produces the pigment. Variations in the gene sequence, resulting from mutations, create enzymes with varying degrees of activity. This shows up in the human population as variation in the intensity of brown color in the irises of our eyes.
2. Hair color and skin color
The story for hair color and skin color is similar to that for eye color. The genes that determine the color carry the information for enzymes that produce pigments. If we produce the enzyme, we make the pigment (brown hair, or brown skin). If we do not produce the enzyme, we do not make the pigment (blonde hair, or light skin). Different variations of the genes result in different variations in the individual's characteristics.
3. Alcohol tolerance
Some individuals cannot tolerate alcohol. This is the result of carrying a particular version of the gene, ALDH2 . It is thought that the version of the gene that produces this characteristic first arose in Asia, since the inability to tolerate alcohol is most common in Asian populations.
Alcohol is metabolized by two enzymes. The first (ADH) converts alcohol to acetaldehyde. The second (ALDH) converts acetaldehyde to acetic acid. We can then use the acetic acid in our energy-metabolism pathways.
Although alcohol itself interacts with our brain cells to make us feel giddy (among other things), acetaldehyde is toxic, and makes us feel sick (or worse). Therefore, the ability to tolerate alcohol depends, in part, on how rapidly we can convert acetaldehyde to acetic acid. The version of the ALDH2 gene that results in inability to tolerate alcohol produces a protein that prevents acetaldehyde conversion. Individuals with this genetic variation metabolize alcohol to acetaldehyde, but are unable to get rid of the acetaldehyde. As a result, they become sick very quickly, and rapidly learn to avoid alcohol. Although this may sound unfortunate, it turns out that this particular genetic characteristic provides virtually 100% protection against becoming an alcoholic.
Genes and Morphology
The several genes and proteins described above provide some examples of how mutations in DNA can cause changes in the characteristics of individuals. In each case, however, we have discussed enzymes that affect pigments or metabolism, and not genes that affect morphology--the shapes of organisms. Most people, when they think of evolution, envision images of animals that look very different from each other, such as fish and mammals, or dinosaurs and birds. These differences are very dramatic, and may seem unlikely to occur by mechanisms similar to those that can influence characteristics as simple as hair color. And yet, the same rules apply.
For any organism, or organ within that organism, the component parts are in a specific, characteristic relationship to each other. This applies even at the level of single cells. Skin cells are on the outside, intestinal cells on the inside. How do these cells "know" what to do?
Every cell type adopts its particular "identity" by "turning on" a specific set of genes. Of the 30,000 or so genes in human chromosomes, each cell type expresses only a subset. It is the combination of genes that are used in a cell that gives it its particular characteristics. This means that cells don't "know" what to do; rather, the set of genes that they express makes them what they are. But, this just pushes the Difficult Question one step farther: what determines which genes are expressed in any particular cell?
There is more to a gene than just the DNA segment that provides the information for a protein. There are also DNA segments that provide information for when and where that gene should be used to produce protein. These DNA segments that regulate the use of genes are called, not surprisingly, regulatorysequences, or sometimes, control elements. Every gene has one or more control elements; some genes have many control elements, allowing them to have very complex patterns of regulation.
Control elements, by themselves, do little. To function, they must have specific proteins bound to them. We might think of control elements as a kind of "parking spaces" on the DNA, where different types of proteins can park. When one of these specific types of proteins has parked on the DNA, it signals to the cellular machinery that the nearby gene is to be used. Thus, what makes a cell become a particular type of cell is the gene-control proteins that it contains. These proteins control which genes are used in that cell.
Great. Now we know the mechanism by which cells produce different proteins. But, it doesn't answer the Difficult Question yet. Again, it pushes it one step further: what determines the production of these gene-control proteins?
To answer this question, we need to discuss some embryology. A fertilized egg is not exactly homogeneous. Rather, it contains a variety of chemicals, some of which are proteins, whose amounts vary within the egg. This is illustrated in the figure below, in which each color represents a different chemical. This asymmetric distribution of chemicals enables the egg to have a definite "top" and "bottom," "head" and "tail," and "left" and "right," even before any obvious embryological development has occurred.
When the fertilized egg starts to develop, it first divides into two smaller cells, then these divide to form four even-smaller cells, then these divide to form 8 cells, and so on. For a "generic" vertebrate embryo, we see something like that shown in the figure below. On the right-hand side of the top row, we see the stage called the blastula, which is a hollow ball of very small, "normal-sized" cells. In cross section, colored to match the figure above, this hollow ball would look somewhat as shown on the right. Cells in different regions of the embryo would have different amounts of the different proteins or other chemicals that were distributed asymmetrically in the original fertilized egg. Cells on the top contain different molecules than cells on the bottom.
It appears to be sufficient to specify head vs tail, and dorsal (back) vs ventral (belly); this automatically establishes left and right.
The molecules we have illustrated here in different colors control gene expression. They can be either gene-control proteins, or chemicals that activate gene-control proteins. As a result of the uneven distribution of these proteins, each cell in the embryo acquires its own combination of these proteins, in specific concentrations. The result is that cells in different parts of the embryo "turn on," or begin to use, different genes.
As cells begin to use different genes, they begin to do different things. Among the things they do is move. The first set of movements, called gastrulation, brings cells from different parts of the embryo into contact. The contact between cells acts as a signal to activate new sets of genes, which causes new cell movements. Through the repeated sequence of cell movements, cell contact, and gene activation, the business of embryo development is controlled--and the numbers of different kinds of cells increases.
The morphology of the embryo, and of the animal that finally is formed, is established by this series of cell movements and changes in gene expression. The overall controls are the proteins that are responsible for communication between cells, the gene-control proteins, and the cellular information-relay system that enables cell-cell communication to change the patterns of gene activation. This basic description applies to all of the developing portions of the embryo.
An important feature of this kind of embryo developmental control system is that it works on a very small scale. When the embryo is only 2 cells, or 4 cells, or 8 cells, then the entire embryo can communicate. As the embryo grows larger, however, and as the number of cells increases, then the control systems function only on small portions of the embryo. As the eye begins to form, these kinds of gene-control systems operate within the "field" of eye-forming cells. As the ear begins to form, these kinds of gene-control systems operate within the "field" of ear-forming cells. As the liver begins to form, or as the pancreas begins to form, there are similar kinds of controls.
One organ-development system for which a great deal is known (but not yet everything) is the limb. We will discuss it as an example, recognizing that other organ systems undoubtedly develop through similar kinds of genetic control mechanisms.
Vertebrate Limb Development--an Example of the Development of Body Parts
At a relatively early stage in development (in the figure above, the last image) vertebrate embryos develop a series of bumps along their backs, and two small nubbins on each side. The two nubbins are the limb buds. There are two limb buds on the right (forelimb and hindlimb), and the corresponding two on the left. These begin to form when the embryo reaches a developmental stage at which each body segment begins to differentiate from the others.
The limb buds are very small -- small enough that cell contact and diffusible molecules can serve as cell-cell communication mechanisms to establish cellular identities. A variety of genes are activated in limb buds. Some produce diffusible short-range hormones, some produce gene-control proteins. In early stages of limb bud development, an abbreviated pattern of gene expression is something like that shown in the diagram shown above (see Tickle, 2000, www.ijdb.ehu.es/fullaccess/fulltext.feb00/Tickle.pdf). These different genes establish cellular identities in the limb bud, and consequently establish the "pattern" of limb development.
If we use a human arm to provide terminology, we can describe it this way: The expression of Shh (red in the diagram), determines the "thumb-to-pinkie" pattern. The pinkie forms on the side closest to the cells that activate the Shh gene. The different Hox genes ensure that the digit next to the pinkie is the 4th finger, and the one next to that is the 3rd finger, and the one next to that is the index finger, and the farthest is the thumb.
As the limb grows outward from the flank of the embryo, additional genes and diffusible molecules (such as retinoic acid, a form of Vitamin A) establish the positions of shoulder, upper arm, forearm, wrist, and fingers. This same basic pattern has been seen in the developing limbs of the vertebrates that have been studied.
Because these developmental events all occur in a very small piece of tissue, encompassing a relatively small number of cells, any changes in the cell-cell communication systems or the diffusible molecules can have dramatic effects on the overall pattern that eventually forms. A chicken wing differs from a human hand only in being smaller overall, having the outer digits fail to develop, and having the cells between the digits fail to die. All of this is controlled by the relative positions of cells that activate the genes for certain gene-control proteins, under the influence of diffusible small molecules, and cell-cell contact.
As we said above, we can use the limb as an example of the kinds of controls that govern the development of body parts. Whether limbs, eyes, livers, on pancreases, all body parts start out as very small groups of cells in which cell-cell contact and diffusible molecules can set up a pattern of which cells activate which gene-control proteins. As the gene-control proteins activate additional genes, the developing organ acquires the various protein micromachines that build the structures.
Internal Controls in Developing Systems
An important discovery is that biological systems are capable of regulating their development to ensure that all of the parts are in the correct relative positions. An excellent example is seen in an experiment performed by Barry Sinervo (Sinervo, B. and Huey, R.B. 1990. Allometric engineering: testing the causes of interpopulation differences in performance. Science 248:1106-1109.): he removed material from one lizard egg and injected it into another lizard egg, thereby producing one egg that was much smaller than normal, and one that was much larger than normal. The lizards that hatched from these eggs were normally-formed lizards, but smaller or larger than normal. This kind of experiment (manipulating eggs or embryos) has been performed with many different species, and leads to the very clear conclusion that developing systems can control their relative proportions. That is, removing half of the material in an egg does not result in a lizard with half of its body parts. It produces a normal-looking, but small, lizard.
A more recent finding shows the importance of this kind of "internal control," as illustrated in the figure below. A genetic change that alters the length of a fish jaw does not just produce fish with jaws that are too short in an otherwise normal head. Instead, the entire head changes shape to accommodate the shorter jaw. This is somewhat surprising, since the jaw and the other structural elements of the head develop from different groups of cells. Apparently, the changes in the head result from the same kind of developmental plasticity as was seen with the too-large and too-small lizards.
This is important because it illustrates that a mutation that alters one piece of a very complex structure need not cause the structure to fail, because developmental controls can often accommodate the change .
There is a general misconception, or perhaps "worry," that a mutation that changes the size of one body part relative to others would disrupt the entire organism. The pieces wouldn't fit. We now know from experimental manipulations and from observations of naturally-occurring mutations, that the pieces do fit. Developmental controls don't just make the pieces; they make the pieces and the connections among the pieces, so that the overall organism works.
The Evolution of Morphology
As we said above, evolution occurs when individuals carrying some particular genetic variation have more offspring than others, and over the course of many generations, out-compete their fellows. We also said that the source of genetic variation is mutation--changes in DNA. The three sections, "Genes and Morphology," "Limb Development," and "Internal Controls in Developing Systems," provide a brief summary of how genes can control the shapes of organs, limbs, and the whole organism. By linking these different concepts, we can understand how evolution of morphology can occur.
A genetic mutation--a change in DNA--can alter the function of a protein if it occurs within the part of the gene that is the code for the protein. A mutation can also change the time or place that the gene is activated, if it occurs in a gene's control elements. Any of these kinds of mutations can alter the morphology (shape) of an organism. The internal controls of development ensure that an altered body part still integrates properly with the rest of the organism.
This means that evolution of morphology can occur through the mutation of single genes. It is not necessary for all of the genes to mutate at once.
Thus, it requires only minor alterations in the developmental controls in early limb development to cause differences in overall limb morphology that we see as seemingly quite dramatic, such as those illustrated below.
These considerations of the limbs of existing vertebrates provides us with insights into the evolution of limbs. Fossil evidence suggests that land animals are the descendents of lobe-finned fish similar to the coelacanth. Unlike most fish today, these fish (both the living ones and the fossilized ones) display bones in their fins that are remarkably similar to the bones in modern animal limbs. We can understand, based on the molecular and developmental biology of limb development, how mutations in the genes that control fin (limb) development could, over the course of millions of years, result in limbs such as we would find on an amphibian.
As described by Carl Zimmer in At the Water's Edge--how life came ashore and then went back to sea again , fossils of animals from Greenland give us insights into this transition. Plant fossils associated with the fossils of these animals indicate that they lived in swampy environments, somewhat similar to Mangrove swamps today. In this environment, large fish would have difficulty swimming among the tangled vegetation. However, pushing with their fins would be very effective. Pushing provides the selective pressure, which would enable the occasional individual with slightly stronger fins to catch prey more easily, and escape predators more readily. Over the course of numerous generations, stronger fins would become the norm in the population.
This example of limb evolution, under-pinned by an understanding of limb development, provides insight into several principles of evolution.
- Evolution results from the increase in frequency of particular genetic variants in a population, at the expense of other genetic variants.
- Genetic variation results from mutation of DNA.
- Mutations in DNA affect the activity or expression pattern of proteins--micromachines that carry out the mechanics of life.
- Evolution usually occurs by modification of pre-existing structures, rather than the appearance of altogether new structures. In the case of limb evolution, the pre-existing structures were the fins of lobe-finned fish.
- Changes to one part of a complex structure, resulting from mutation, can often be accommodated by compensatory changes in the rest of the complex structure, not by additional mutation, but through internal control mechanisms that operate during embryo development.
Natural Selection – If Mutation is Random, Why Does Evolution Occur at All?
Every time that scientists have examined the process of mutation, seeking to learn if there are recognizable patterns, the answer seems to be that mutation is essentially random. If we think of the causes of mutation, such as chemical mistakes in DNA replication or repair, or physical damage due to cosmic rays or other radiation, we see that there is no reason to expect mutation to be anything except random. And yet, evolution has produced highly-complex life forms, with a great many specialized adaptations that make them appear as if they were specifically designed to live where and how they do. How can an apparently random process result in apparently directed evolution?
This question is one of the "logical" problems that many people have with evolution. It is simply counter-intuitive that a random process can give rise to highly-ordered structures, and to adaptation to specific environments. Perhaps the best way to address this issue is to provide some examples in which we model the process.
Example 1: using colors to represent individuals
In this example, we consider two populations of 10 individuals. Each individual can reproduce, but because of ecological constraints (food supply, for example), the environment maintains the population at a maximum of 10 individuals. Using random choice (throwing dice, for example), we have assigned different colors to the 10 individuals--and we use the same colors for each population. The two populations look like this:
The left-hand population is in a cool environment, then individuals colored blue, green, and purple have a competitive advantage over the other colors, with purple being most successful. The right-hand population is in a warm environment, with red, orange, and yellow having a competitive advantage. Red is most successful. In the next generation, we have this:
There isn't really much difference between this generation and the previous generation. Most of the different colors (genetic variants) are present in each population. Some of the variants have increased in frequency, while others have decreased in frequency. Compared to the prior generation, each one looks pretty much like the parental generation, except that some of the genetic variations are a bit more common.
In the next generation, we have this:
And then this:
And then this:
Each population changes slowly with time, from generation to generation, as some individuals have more offspring than others. The two populations began with identical genetic diversity, based on random "mutation." However, the environmental conditions were different, so selection was different. The cool environment selected for the cool colors, and against the warm colors. The warm environment selected for the warm colors, and against the cool colors. Mutation was random, but selection provided a direction to the evolution.
Example 2: leaf shape
In this example, a species of shrub has spread across a valley, and up into the mountains on either side of the valley. As the climate warms, the shrubs lower down (in the valley) die out, and the shrubs higher up (in the mountains) survive. But, on the north side of the valley, the shrubs are on mountain slopes that receive full sun; rain water dries rapidly. On the south side of the valley, the shrubs are on mountain slopes that receive little sun; rain water dries slowly. Thus, one population is in a dry environment, and the other is in a wet environment.
Both populations start with identical genetic diversity, resulting from random mutations that affect leaf shape. Some leaves are wide, some are narrow, some are in-between. They look like this:
Wet environment Dry Environment
Of course, the shape of a leaf is not a trivial matter, if you are a plant. A broad leaf can capture more sunlight, perform more photosynthesis, and thus provide more food for the whole plant. A narrow leaf is much less effective. However, the more broad a leaf is, the more stomata it has--openings that allow CO2 to enter and O2 to escape. The more stomata a leaf has, the more water it loses by evaporation. Therefore, a plant with broad leaves requires more water than a plant with narrow leaves; it is much more likely to wilt on a hot, dry day. From these considerations, we can see that in a wet environment, where water loss is not a serious problem, wide leaves would be advantageous. However, in a dry environment, where water loss is a problem, wide leaves would be a liability.
After a few generations, the distributions of leaves in the two populations look like this:
After a few more generations, they look like this:
And, after a few more generations, they look like this:
Wider leaves lose more water during the day, so in the dry environment, wide leaves are selected against. Narrow-leaved plants produce more seeds than wide-leaved plants. However, wide leaves can carry out more photosynthesis than narrow leaves can, so in the wet environment, wide leaves are selected for . Wide-leaved plants produce more seeds than narrow-leaved plants.
The genetic variation in leaf shape was determined by random mutation. However, the environmental conditions determined which variations were more successful, and provided a kind of direction to evolution.
From these examples, it should be evident that the random nature of mutation does not cause evolution to be random. Random mutation simply provides an array of genetic variants for selection to choose among. If there are genetic variants that are successful in the particular environment, then those genetic variants prosper. They produce more offspring. Eventually, they become the norm.
What if there are no genetic variants that are particularly successful? What if the environment changes rapidly, and there just don't happen to be any mutations in the population that enable any individuals to do well? Then, as has happened over and over during the history of life on earth, the population will die out. The species may go extinct.
Mutating in Order to Survive
We often develop the idea that evolution occurs because a species of plants or animals mutates "in order to survive." It seems as if this must have happened, because the plants and animals that are now alive are well adapted to their environments, and appear to be much better adapted than their ancestors that are recorded in the fossil record. Indeed, the ancestral species died out, which seems to indicate that they were not "good enough" to make it, while the ones that could make themselves "better" are with us today.
This notion is also satisfying from a human-centered view of the world. We are here now, so we must be the "best" species ever to evolve. Ancient species were much more primitive, so evolution would seem to work as some kind of drive to become better -- and eventually become human.
But does this make sense? It is even possible?
If we think about the evolution of plants, in which they acquire narrow leaves in dry climates, or broad leaves in wet climates, we must wonder how they can do this. As far as we know, plants can't think. As far as we know, they cannot predict what the world will be like in a few hundred generations. And, as we know for certain (see above), mutations occur randomly. There is no way to prevent mutations from happening, and no way to cause them to occur in specific genes.
If it is not possible to make mutations happen on cue, it simply cannot possible to mutate in order to adapt to new conditions.
And yet, so many species seem so well-suited to their environments. How can this have happened if they didn't know what they were doing? The examples above should help resolve this conundrum. Random mutation is sufficient to account for the observation, if there is enough genetic diversity in the population that some individuals have characteristics that are advantageous, and enable them to survive changing environmental conditions. As long as environmental conditions don't change too fast, the continued occurrence of mutations is capable of producing genetic variations -- some of which may happen to be useful. Fortunately, the rate of mutation is high enough that useful mutations have occurred sufficiently often to accommodate many of the environmental changes that have occurred.But not always. If useful mutations don't happen to occur, or if the environment changes too fast (as might occur if a giant asteroid strikes the earth), then species die out. They become extinct. It is instructive to consider the fact that most of the earth's species have gone extinct. Those of us alive now are the lucky remnants of various genetic lineages in which advantageous mutations did occur.
An excellent example of how selection operates on individuals of a population, and how the individuals do not "plan" their mutations is offered by the following story, which is taken from Shadows of Forgotten Ancestors by Sagan and Druyan.
Instincts are inborn behaviors that animals display without necessarily thinking logically about it. We recognize a great many instincts in different breeds of dogs--such as chasing and retrieving sticks (retrievers), digging (beagles, selected for hunting rabbits), and pulling (huskies, selected to work in dogsled teams). We don't usually think of our own instincts, but we have them.
One human instinct that is variable, but present in a significant fraction of the population is fear of the dark. Many of us recall being afraid of the dark, or of "monsters" when we were children. Most of us recall our parents telling us "there's nothing to be afraid of; go back to sleep." If Mom and Dad were telling us not to be afraid, then where did we get our fear of the dark? It cannot be a learned behavior, but must be an instinct.
There are good reasons to have an instinctive fear of the dark. In our history, before civilization, the world was a scary place. There were many predators that hunted at night. In a very real sense, there were monsters out there. The world in which our ancestors lived was perilous.
So, picture the following: Our ancestors are sitting around the fire at night--or maybe they are perched on the limbs of trees.* Most of them have a vague unease, and are afraid to venture out into the night. But, this genetically-coded uneasiness, like any genetically-coded trait, is variable. Some individuals are more afraid, some are less afraid.
So here is everyone sitting around being afraid, and one guy says, "You guys are wimps. I'm going for a walk." He goes out into the night, and is eaten by lions.
Whose genes did we inherit?
This simple scenario offers several important insights into the nature of evolution and natural selection. They are:
1. Selection operates on individuals. Individuals can be eaten, or they can live until morning. The entire tribe did not get eaten at once, nor did the entire tribe live until morning.
2. "Survival of the fittest" really is a poor phrase to describe this. We don't think of "the wimps" as being "fit," but they are the ones who passed on their genes. In evolutionary terms, "fitness" refers to the production of offspring, and not to anything else. In this case, being fearful increased an individual's fitness in this particular lion-filled environment.
3. Selection operates on traits that already exist. The individuals who were afraid survived; the individuals who were not afraid were likely to be eaten. No one was eaten by lions, and then developed the fear-of-the-dark trait.
4. There is no planning in evolution, no "mutating in order to survive." Our ancestors did not sit around the fire thinking, "I bet that in a few hundred thousand years, our descendents will be better off if they are afraid of the dark--so I'm going to mutate, and become afraid." Instead, each individual lived his or her life normally. The ones who happened to have genetically-coded traits that were advantageous in that environment passed on their genes to more offspring.
last updated:Jan. 15, 2009