Microevolution is defined as the change in the genetic makeup of a population over time. Over the course of a few generations, selective breeding can rapidly drive genetic change in a desired direction (see figure 14.1). In the natural world genetic change is driven by natural selection. Using our knowledge of genetics, we will discover that all populations are in a constant state of genetic change over time - evolution.

Figure 14.5 on page 226 of your text uses an example of a population of butterflies that is in genetic equilibrium - a population that does not change genetically over time. This example is important for two reasons. First, it shows how alleles are inherited in successive generations of a population (as opposed to the movement of alleles from individual parents to offspring). Second, it helps us think about the conditions that must be present in order for a population to remain genetically stable (at equilibrium). The example in your text uses the alleles A and a which determine wing color (AA - Dark Blue; Aa - Medium Blue; aa - White). In this population of 1000 individuals (490 AA, 420 Aa, and 90 aa), 70% of the alleles are A and 30% of the alleles are a. These percentages translate into allele frequencies of 0.7 and 0.3, respectively. In the Hardy-Weinberg equation, p and q are used to represent the frequencies of the A and a alleles in the population, so for this population, p=0.7 and q=0.3. Now imagine that this generation reproduces. Each individual mates exactly once and mate choice is random. If these two conditions are true, then the allele frequencies of the offspring can be determined using a punnett square. Allele frequencies of the gametes are multiplied to give the frequency of each genotype resulting from fertilization. In this example, the offspring generation (F1) has the same distribution of alleles and phenotypes as the parental (P) generation. Note that in order for this to occur, we had to assume that mating was random, and that all individuals had equal reproductive success. In real populations, these criteria are never met. The normal situation in a real population starts with more individuals than can survive and reproduce. Not all of the individuals that are born will end up reproducing, so the condition of equal reproductive success is always violated. In addition, mate choice is not random. In any population, some individuals will be preferred mates and others will be less successful at mating (sexual selection, see section 14.9) The fact that mating is not random and that reproductive success is not equal in a population mean that that population is evolving - changing genetically over time.

In addition to the random mating and equal reproductive success criteria for genetic equilibrium, three other conditions must be met: no mutation, large population, and isolated population. Mutations - heritable changes in the DNA sequence - give rise to new alleles, changing the distribution of alleles in a population, and therefore leading to genetic change over time. Populations at equilibrium must be large so that the random nature of reproduction is not biased by small sample size (see section 14.11). Finally, populations at equilibrium must be isolated, meaning that there are no changes in the frequency of alleles due to immigration of emigration.

A closer look at the tendency for populations to have unequal reproductive success (sections 14.6, 14.7, and 14.8) shows that this tendency arises from the fact that population growth, a universal characteristic of populations, cannot proceed indefinitely. Eventually, individuals in any population will end up competing for resources. The reproductive success of some individuals will be higher that of others, and this unequal success will be driven by natural selection - one of the best-documented processes in biology. Selection can be directional, as in the case of the moths in figure 14.11. In this case, reproductive success of individuals on one tail of the distribution is limited, driving the distribution toward the other tail. This type of selection the common response of populations to pesticides or antibiotics. Selection can also be stabilizing (figure 14.12) where the individuals having extreme values of a train have decreased reproductive success, or disruptive (figure 14.14) where individuals having intermediate values for a trait are disfavored.

You now have a good understanding of the basics of genetic change in populations. Since the conditions required for genetic equilibrium are not met in any population, this means that all populations are constantly evolving. In the coming lectures, we will look at how this genetic change can give rise to new species and to the history of speciation over geologic time.