When particular expressions of a characteristic are naturally selected and those expressions are associated with particular gene variants, those gene variants will be more likely than others to be passed on to future generations. For example, let’s say that there exists a gene with two variants, T and t, and that these variants are associated with difference in the average height of plants from a particular (fictional) species, Pirasus arizonensis. From a baseline height of five inches, T increases the average height by 1/2 inch, whereas t decreases the average height by 1/2 inch.
If P. arizonensis seeds are transported by birds into an environment in which the fully grown plant is surrounded by plants from another (fictional) species, Torensi mojavensis, that has an average height of six inches, the taller species will limit the smaller species’ access to sunlight. This, of course, would be detrimental to the survival and reproductive success of P. arizonensis. Thus, any P. arizonensis plant that grows taller than the average height of five inches will tend to survive longer and reproduce more.
Let’s say that the following 300 P. arizonensis plants have grown in this new environment:
100 TT plants, which will have an average height of 6 inches; 100 Tt plants, which will have an average height of five inches; 100 tt plants, which will have an average height of four inches.
The plants that are smaller than the surrounding plants will be much more likely to die before reproducing. And of course, other environmental factors also will affect plant survival, but in a more random fashion. Let’s say that 80 of the tt plants die before producing seeds, 50 of the Tt plants die before producing seeds, and 10 TT plants die before producing seeds. The result: 72% of the seeds in the next generation will contain T but only 28% will include t. Thus, if 300 plants grow in the next generation, their numbers will be as follows:
156 TT plants; 120 Tt plants; 24 tt plants.
As you can see, there are 276 plants with at least one copy of the T variant, which is much larger than the number of plants with at least one t variant (144 plants).
The third requirement of evolution by natural selection — the increased reproductive success of individuals with particular expressions of a characteristic —must remain stable over generations. This means that the “selective pressure” on P. arizonensis plants with respect to their heights must not change. The six-inch tall (on average) T. mojavensis plants must continue to limit the amount of sunlight obtained by smaller P. arizonensis plants. In this case, the following change in the frequencies of the T and t gene variants in this species should occur:
As you can see, the frequency of T becomes almost 100% within five generations, which means that the population in this new environment now consists almost entirely of plants that are about six inches tall. Thus, over a very short period of time, natural selection can lead to a large change in the average expression of a characteristic in a population when individual differences in that characteristic are strongly associated with genetic differences.
Now, let’s return to the example from the previous post: the founding population of fruit flies on a tiny and isolated island buffeted by strong winds. Differences in the size of fruit-fly wings are strongly linked to differences in genes (Robertson & Reeve, 1952), to a degree similar to that described above for height differences in the fictional plant species. Thus, if the windy environment naturally select flies with smaller wings, gene variants correlated with smaller wings will increase in frequency over generations. This means that the population will evolve a smaller average wing size. This is shown in Figure 2 (the black bars represent the founding population and the white bars represent the population after a number of generations of natural selection for smaller wing sizes).
Although these two examples are fictional ones, there are many examples of natural selection and artificial selection (selection in which humans breed organisms that express particular characteristics). For introductions to and histories of the concepts of evolution and natural selection, see Colby (1996-1997), Endler (1986), and Zimmer (2001).
In its most general sense, biological evolution refers to changes over generations in a population—changes in features of the body, mind, or behavior.
The evolutionary approach attempts to explain mind and behavior in terms of biological structures and processes that have evolved over hundreds to thousands of generations. This approach assumes that species have evolved ways of responding (cognitively, emotionally, and behaviorally) to environmental events because these responses led to greater survival and reproductive success in ancestral populations.
To take one example, the human spinal cord develops in such a way that it can rapidly process sensory information related to the temperature of objects. When we touch an object that is very hot, the spinal cord immediately activates a reflexive response that rapidly pulls the finger away from the object. Because this response occurs automatically, we can’t explain it as the result of conscious choice. In fact, the hand typically is jerked away before the information reaches the cerebral cortex (activity in the cortex is necessary for the conscious perception of pain). The existence of this spinal-cord reflex may be explained as the product of evolution: individuals that quickly pulled a body part away from painful stimuli were more likely to survive and, hence, reproduce because this proto-reflex prevented severe bodily damage. This explanation asserts that evolutionary changes in spinal-cord reflexes were caused by natural selection.
Evolution By Natural Selection
Evolution refers specifically to changes in the frequencies of variants of a characteristic (biological, psychological, or behavioral) over generations. A characteristic is a feature of an individual, such as eye color, that can be distinguished from other features, such as hair color. Characteristics often have variants that involve observable individual differences. For example, eye color has many variants, such as shades of brown, green, gray, and blue. Hair color also has many variants, such as shades of black, brown, red, and blonde. We will refer to such variants as expressions of the characteristic. Evolution, therefore, is a change over generations in the frequencies of expressions of a characteristic within a population of organisms. An analogous way of saying this is evolution is a change over generations in the average expression of a characteristic within a population of organisms. For example, a population consisting of 99% brown-eyed individuals and 1% blue-eyed individuals may evolve over generations into a population consisting of 1% brown-eyed individuals and 99% blue-eyed individuals. The average expression of eye color in this population evolved from brown to blue.
What causes evolution to occur in populations? For two decades beginning in 1836, Charles Darwin developed a credible naturalistic theory able to explain evolutionary changes — a theory that he began to develop when trying to interpret observations he had made during his five-year voyage on the H.M.S. Beagle (Darwin, 1839), as well as in research that he and others performed during the 23 years after Darwin returned from that voyage. This was the theory of evolution by natural selection. He published a detailed description of the theory in the first edition of the book, On the Origin of Species (Darwin, 1859; the sixth edition generally is considered to represent Darwin’s mature views on evolution and its causes.) No one before Darwin had so masterfully marshaled such an enormous amount of supporting evidence for the evolution of organisms. In addition, no one before Darwin had outlined such a compelling explanation of evolution: natural selection. Natural selection may be defined as the increased reproductive success of individuals with particular expressions of physical, mental, and/or behavioral characteristics. To put it most simply, Darwin argued that natural selection occurs when a subset of individuals in a population produce a greater number of offspring, on average, than others because they express a physical, mental, or behavioral variant that allows them to adapt better to their environments.
Let’s consider, for example, a fictional species of fruit fly that has just arrived on a windy and tiny island hundreds of miles from any other land. And let’s say that, in this founding population, there exists a a broad range of individual differences in wing size, as shown in the following graph.
As can be seen in the graph, some individuals have large wings, which are advantageous for flying speed and for the ability to stay airborne, whereas others have small wings, which result in slower flying speeds and greater difficulties with staying airborne. On this small and windy island, larger-winged flies probably would be more likely to get blown out to sea, whereas the smaller-winged flies would be less likely to suffer that fate. Thus, smaller-winged flies would be more likely to survive long enough to reproduce than larger-winged flies.
This fictional example illustrates well the simple idea behind natural selection: individuals differ in their reproductive success because they have variants of characteristics associated with the ability to adapt to local environmental conditions. Because individuals with particular variants adapt better relative to individuals with other variants, the former survive longer, on average, and, hence, have more opportunities to reproduce. In other words, the local environmental conditions consist of factors that impose biological, psychological, and behavioral demands on organisms. These factors “naturally select” those organisms best able to deal with the environmental demands: they survive longer and reproduce more than others in their local population.
Given the obvious fact that natural selection occurs, how does it produce evolutionary changes in populations of organisms? There are three requirements that must be met in order for evolution in the average expression of a characteristic to occur through natural selection:
There must be individual differences in the expression of the characteristic.
These individual differences must be associated with genetic differences.
The increased reproductive success of individuals with particular expressions of the characteristic must remain stable over generations.
The first requirement has already been discussed (see Figure 1). The second requirement involves the existence of genetic variants that affect the development of characteristics. A gene is the basic unit of biological heredity. Genes consist of sequences of chemical units (sections of DNA molecules) that are contained in chromosomes carried by the sperm of males and the ova (eggs) of females. In human reproductive cells (sperm and ova), there are 23 chromosomes, which together contain about 22,000 genes (Pertea & Salzberg, 2010). This means that, on average, each human chromosome contains about 1000 genes.
What Do Genes Do?
Genes influence the production of proteins and their use in developing and maintaining the body (for a history of the concept of the gene, see Rheinberger & Müller-Wille, 2010). For example, there are probably at least 16 genes that affect the development of eye color in humans (White & Rabago-Smith, 2011). But it seems that only two or three have major effects on individual differences in eye color. So, for purposes of explanation, let’s assume that there are only three genes that influence the development of eye color, which, for the sake of simplicity, we’ll refer to as Gene A, Gene B, and Gene C. As can be seen in the following table, babies receive one copy of each gene from their biological fathers (labelled as 1) and one copy of each gene from their mothers (labelled as 2):
Gene A has two variants: a brown variant and a nonbrown variant. If at least one brown variant is inherited from either parent, then, regardless of what is inherited at Gene B and Gene C, the person will develop brown eyes:
If, on the other hand, the nonbrown variant is inherited from each parent, then eye color is determined by what is inherited at Gene B and Gene C. Gene B has two variants: a brown variant and a blue variant. If at least one brown variant of Gene B is inherited from either parent, the person will develop brown eyes, regardless of what is inherited at Gene C:
If, on the other hand, the blue variant is inherited from each parent, the person will develop blue eyes depending on what is inherited at Gene C (which we will ignore for the moment):
Gene C has two variants: a green variant and a blue variant. If the blue variant of Gene B is inherited from each parent, then, if at least one green variant of Gene C is inherited from either parent, the person will develop green eyes:
If, on the other hand, the blue variant of Gene C is inherited from each parent, the person will develop blue eyes:
Thus, in our simplified example, eye color is determined by interactions among variants of three genes. The actual situation is much more complex: there are other genes as well as environmental factors that produce the many shades of eye color we see in real life.
Our example shows that gene variants, and interactions among them, contribute to the development of the physical characteristics of the body. In fact, you see evidence for this claim all around you: biological relatives often bear a strong resemblance to each other, as do conspecifics (members of the same species). Members of two closely related species typically don’t mate, and if they do, the mating typically doesn’t produce offspring. When interspecific matings are successful, however, the offspring generally express physical characteristics intermediate between the two species. For example, matings between male donkeys and females horses produce mules; and matings between male horses and female donkeys produce hinnies. Mules and hinnies have physical and behavioral characteristics that are intermediate between those of horses and donkeys. We’ll come back to this when we talk about matings between dogs and species that are closely related to them
The next post will look more closely at natural selection at the level of genes.