Evolution 14.4

Objectives Explain the significance of gene pools in understanding evolution. Tell how genetic drift, gene flow, mutation, and natural selection contribute to changes in a gene pool. Explain what is meant by the term fitness. Describe recent evidence for microevolution on the Galápagos Islands.

The gene pool consists of all the alleles (alternative forms of genes) in all the individuals that make up a population.

Changes in Gene Pools The processes that lead to genetic variation—mutations and sexual recombination—are random. However, natural selection (and thus evolution) is not random. The environment favors genetic combinations that contribute to survival and reproductive success. There is a change in the frequency of alleles—how often certain alleles occur in the gene pool.

                                                                                                                         Each plant in this hypothetical population of wildflowers has 2 alleles for flower color. In all, there are 14 red-flower alleles (R) and 6 white-flower alleles (r). The frequency of each allele is calculated as a ratio based on the total of 20.

Merging Mendel's and Darwin's theories led to a way of looking at evolution based on genetic changes in populations. Microevolution is evolution on the smallest scale—a generation-to-generation change in the frequencies of alleles within a population.

In contrast to microevolution, populations that do not undergo change to their gene pools are not presently evolving. This condition is known as the Hardy-Weinberg equilibrium Such equilibrium of a gene pool means that the frequency of alleles in that gene pool are constant over time. In fact, populations rarely remain in Hardy-Weinberg equilibrium for long in nature. Importance- it is useful because it provides a "no change" baseline that makes it possible to recognize when a gene pool is changing.

What mechanisms can change a gene pool What mechanisms can change a gene pool? The two main factors are genetic drift and natural selection Genetic Drift A change in the gene pool of a population due to chance is called genetic drift.

Example, the first generation of the small wildflower population illustrated in Figure 14-25 consists of nine plants with red flowers (RR and Rr) and one plant with white flowers (rr). It is partly chance that affects which plants reproduce. By the third generation, no plants carry the allele for white flowers. The result is a change in allele frequencies in this population .                                                                                                                                                                                                                                                                                               

Figure 14-25 Only the alleles of organisms that successfully reproduce in one generation appear in the gene pool of the next generation. In this population of ten plants, the frequency of white-flower alleles was reduced to zero due to genetic drift. All populations are subject to some genetic drift. However, the smaller the population is, the more impact genetic drift has on that population.

The Bottleneck Effect Disasters such as earthquakes, floods, droughts, and fires may drastically reduce the size of a population. Reducing the size of the population also reduces the size of its gene pool (Figure 14-26). By chance, certain alleles may then be represented more frequently than others among the survivors. Some alleles may be eliminated altogether. Such genetic drift, called the bottleneck effect, decreases genetic variation in a population.

Gene Flow and Mutation Although genetic drift and natural selection are the main causes of changes in gene pools from one generation to the next, other mechanisms also have a role. These mechanisms include gene flow and mutation.

                                                                                                                                                               Figure 14-26 Marbles falling through the narrow neck of a bottle serve as an analogy for the bottleneck effect. Compared to the original population (in the bottle) the new population has less variation.

The loss of variation due to a bottleneck effect could reduce the ability of a population to adapt to environmental change. Example, the cheetah very little variation among the species.

Founder Effect Genetic drift is also likely when a few individuals colonize an isolated island, lake, or some other new habitat. The smaller the colony, the less its genetic makeup will represent the gene pool of the larger population from which the colonists came. Chance reduces genetic variation. Genetic drift in a new colony is known as the founder effect because the change relates to the genetic makeup of the founders of the colony. The founder effect likely contributed to changes in the gene pools of the finches and other South American organisms that arrived as strays on the Galápagos Islands.

Gene Flow The exchange of genes with another population is referred to as gene flow. Gene flow occurs when fertile individuals or their gametes (sex cells) migrate between populations. Example, suppose a population that neighbors the wildflowers pictured in Figure 14-25 consists entirely of white-flowered individuals. A windstorm may blow pollen from these neighbors to the mostly red-flowered population. Interbreeding would increase the frequency of the white-flower allele in the original population. Gene flow tends to reduce genetic differences between populations. Gene flow can eventually mix neighboring populations into a single population with a common gene pool.

Mutation a mutation is a change in an organism's DNA. If this mutation is carried by a gamete, the mutation enters the population's gene pool.

Over the long term, mutation plays a key role in evolution as the original source of the genetic variation that is the raw material for natural selection. Mutations are especially important as a source of variation in asexually reproducing organisms that clone themselves rapidly, such as bacteria. Example, a new mutation that is favorable can rapidly increase in frequency in a bacteria population due to natural selection.

Genetic drift, gene flow, and mutation can cause microevolution, or changes in allele frequencies. But they do not necessarily lead to adaptation Of all causes of microevolution, only natural selection usually leads to adaptation.

fitness—the contribution that an individual makes to the gene pool of the next generation compared to the contributions of other individuals. Survival to reproductive maturity, is necessary for reproductive success. But even the biggest, fastest, toughest frog in the pond has a fitness of zero if it is sterile. Production of healthy, fertile offspring is all that counts in natural selection.

Figure 14-31 The Grants documented changes in beak size among medium ground finches over many years. Their data relate this microevolution of beaks to environmental change.