Natural selection driving adaptive changes is a powerful and intuitive explanation for the evolution of the living world around us. Evolution at the molecular level, however, is chiefly ruled by random genetic drift. The idea that an advantageous allele may be lost by chance in a natural population is rather difficult to explore in the classroom. Low-cost and hands-on educational resources are needed to make genetic drift more intuitive among students. In this exercise, we use colored beads to simulate drift and selection jointly affecting the fate of the genetic variants in an evolving population. Our aim is to teach students that natural selection does not determine but simply influences the fate of advantageous alleles because random genetic drift is always present.

Introduction

The evolutionary theory is the unifying principle in biology. Hence, the concept that evolution involves changes in the genetic composition of a population through time is extremely important. Charles Darwin’s brilliant insights on evolution by natural selection have not been overlooked. Today, natural selection is one of the most influential and pervasive concepts in the biological sciences (Besterman and Baggott 2007). Professional biologists are eager to dazzle students by explaining the wonderful adaptations that made a particular species possible, starting with the classics: the giraffe’s neck and Biston betularia (Lauer 2000; Scott 2004).

Many decades after the publication of The Origin of Species, however, Motoo Kimura sketched a different portrait of the pace of evolution at the molecular level (Kimura 1954, 1968). Random genetic drift was not only important; it was the chief force governing molecular evolutionary change. The intuitively appealing force of Darwinian selection was to be viewed as an exception at the molecular level (Kimura 1985; Ohta 1992).

Despite its importance, however, genetic drift is constantly neglected and often dismissed as an evolutionary force in general biology textbooks (Hammersmith and Mertens 1990; Linhart 1997) and even by professionals in the field (Staub 2002; Nelson 2007). There are many reasons for this recurring neglect. First, when addressing topics related to evolution, the teacher is bound to face some antagonism and hostility in the classroom (Alters and Nelson 2002; Johnson et al. 2012). Second, although many beautifully detailed examples are available to illustrate natural selection (Darwin 1859; Lauer 2000), genetic drift is abstract by nature and is not directly observed. Third, theoretical foundations that support neutralist expectations have a deep mathematical basis that makes genetic drift conceptually even more difficult to grasp (Lenormand et al. 2008). Finally, the idea that the genetic drift (i.e., chance) may overcome the directional force of natural selection is counterintuitive and is thus difficult to explore in the classroom.

Several educational resources have been developed to demonstrate the significance of genetic drift or natural selection (see McComas 1998; Staub 2002; Heim 2002; Young and Young 2003). Most of these hands-on exercises, though, focus on a single evolutionary force, neglecting the interaction between selection and drift that is usually explored in computer simulations (Populus, Alstad 2007; PopG, Felsenstein 2008). Computer simulations, however, still require an abstractly oriented mind since results are depicted in theoretical graphs. Furthermore, computer-based resources may not be available for students in most high schools or even in many undergraduate courses. Low-cost and hands-on resources are the key to make abstract concepts, such as genetic drift, more intuitive to the student (Colburn 1994; Brewer and Zabinski 1999).

This exercise uses the beads to simulate drift and selection simultaneously affecting the fate of the genetic variants in an evolving population. The conventional argument is that because an individual with an advantageous allele will have a higher probability of survival and reproduction, this higher probability will tend to persist in the offspring that bear the allele. Here, we aim to teach the concept that natural selection is the differential probability of the survival and the reproduction of variants and other related concepts. Natural selection does not determine but simply influences the fate of advantageous alleles because random genetic drift is always present (Gregory and Ellis 2009). We have been using this exercise successfully for over a decade to make genetic drift more intuitive for the biological sciences students at the Federal University of Rio de Janeiro.

In this exercise, different selective pressures will be simulated by the different probabilities associated with the beads. At the end of the exercise, we expect that the stochastic aspects that drive biological evolution will become clearer to students, who will tend to regard natural selection as the differential probabilities of survival and reproduction. This activity should be used in an evolutionary biology class or in an introductory biological sciences class with an evolutionary biology backbone.

Natural selection acts at the level of individuals. It is the individual organism that lives or dies, reproduces or fails to reproduce. Populations evolve; that is, the characteristics of the group of organisms change over time. In general, the physical characteristics of an individual are fixed throughout its lifetime. An individual cannot change in its lifetime and then pass new characteristics to their offspring. Because some individuals live and some die, the average characteristics of populations change over time.

Changes in the heritable characteristics of the individuals in a population over time is one useful definition of evolution (there are others). Evolution by natural selection, as first proposed by Charles Darwin, includes four conditions:

1. Variation: There are significant differences between the individuals in populations. Furthermore, it is generally assumed that these variations are random (i.e. not purposeful). In this simulation, different colored beads model random variation. For the purposes of this simulation, these beads are assumed to be different forms of individuals of the same species, for instance a population of butterflies.
2. Inheritance: The variations that exist within the population must be inheritable from parents to offspring; that is, they can be passed on in genes. Darwin clearly recognized that this was the case, although he did not know about genes or DNA and did not originally propose a genetic method by which this could occur. In this simulation, inheritance is "perfect" – that is, offspring inherit the exact form of their parents, for instance red butterflies only reproduce red butterflies.
3. Overpopulation: As a consequence of reading Malthus' Essay on the Principle of Population, Darwin realized that in natural populations more offspring are born than can possibly live to reproduce. In this simulation, overpopulation is modeled by having only part of each generation's offspring survive to be able to reproduce.
4. Differential Survival and Reproduction: Given the three conditions described above, certain individuals will survive and reproduce more often than others, and such individuals (and their offspring) will therefore become proportionally more common over time. This, in a nutshell, is evolution by natural selection. In natural environments, one of the most noticeable forms of natural selection is predation. Predators eat other organisms, while prey are eaten by them. In the 1950s, H.B.D. Kettlewell and his colleagues carried out one of the most important investigations into the theory of evolution by natural selection. Kettlewell studied the effects of bird predation and air pollution on the genetic and morphological traits of the peppered moth (Biston betularia) populations in southern England.

In our natural selection game (actually a real-time simulation), we will study a closely related phenomenon — the evolution of protective coloration. Many animals, especially insects, are very well camouflaged against visual detection by predators, especially birds. In some cases the insects mimic some part of their habitat, such as a leaf.

Types of Natural Selection:

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Natural selection can cause changes in the mean and variation of a population. Your text does an excellent job of presenting possible results of natural selection. Review this material before coming to lab.

These graphs show the distribution for a trait in a population before and after stabilizing selection has occurred. The most common (average) value for a particular trait is the one best suited for the environment. The high frequency results from selection against the extreme values (below or above the average).

In this example, bright blue wings are the most common color in this population of butterflies. The red arrows indicate selection against lighter colors to the left in this graph, and against darker colors to the right.

Variation decreases each generation as more individuals with the average value (bright blue wings) reproduce, and fewer individuals with more extreme values (pale blue and purple wings) reproduce.

Thus the distribution of color variation becomes a more narrow bell-shaped curve.

This is the type of selection observed in a stable environment. If the environment changes, then the pressure exerted by natural selection might also change.

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These graphs show the distribution for a trait in a population before and after directional selection has occurred. In this scenario the amount of variation might not change, but the average value changes; the location of the mean moves.

This occurs when the environment changes and consequently a different set of individuals in the population have greater reproductive success. There might be a change in foliage that makes the lighter colored butterflies easier to spot by predators.

In this scenario, the average wing color has shifted to the right as individuals with darker colored wings experience greater reproductive success.

Directional selection could have occurred in the opposite direction if individuals with lighter-colored wings had greater reproductive success.

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These graphs show the distribution for a trait in a population before and after disruptive selection has occurred. This occurs when the environment shifts in such a way that now two sets of individuals enjoy greater reproductive success. This could happen with the introduction of new plants (different camouflage options), new animals (different predators), or more likely a dramatic shift in the geography (introduction of a new road or river).

Here individuals with the average wing color no longer enjoy a reproductive advantage and thus the number of individuals with bright blue wings decrease.

Meanwhile, individuals with the extreme wing colors have a reproductive advantage and thus the number of individuals with pale blue or white colors increase and individuals with purple wing colors increase.

This distribution is described as a bimodal distribution because it has two wing colors that are both common.