Chapter 13 : Evolutionary Mechanisms

Figure 13.1

Evolutionary time frame. The timescale indicates some of the major events in the evolution of living organisms.

Figure 13.2

Artificial selection. A single, short-legged species known as Tomarctus gave rise to various species of jackals, coyotes, and wolves. The wolf (Canis lupus) is the immediate ancestor of the domesticated dog (Canis familiaris). The original ancestor of wolves and dogs contained a great deal of hidden genetic variation that, under selective pressures imposed by humans, has been molded into more than 100 breeds recognized by the American Kennel Club. (Photographs of dogs 1 to 4 courtesy of the American Kennel Club. Dog 5, Rosie the basset hound, is a close friend of the author.)

Figure 13.3

Normal frequency distribution. Most characteristics of organisms in nature have a continuous frequency distribution, which is sometimes a normal frequency distribution that can be represented by a bell-shaped curve. In a normal frequency distribution, most, but not all, organisms have a certain characteristic, which statisticians refer to as the mean or average. Before Darwin and Wallace, biologists thought of the mean as the “correct” form of the characteristic, and organisms lacking that form of the trait were considered deviant. Darwin and Wallace shifted attention from the ideal form to the variation in form, or in statistical terms, from the mean to the variance.

Figure 13.4

Darwin's finches. On the Galapagos Islands, Darwin observed at least 14 different species of finch with different beak shapes, only 8 of which are shown here. He hypothesized that the 14 species evolved from a single ancestor. Competition for food provided the selective pressure that led to the different beak shapes.

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Figure 13.5

Macromutations. In these diagrams of four types of chromosomal aberrations, the letters represent genetic loci.

Figure 13.6

Gene deletion and duplication. The letters indicate genetic loci, and the lowercase and uppercase letters represent different alleles for the same trait. In meiosis, if homologous chromosomes are not aligned perfectly when crossing over occurs, certain genes, indicated by the letters, will be lost from some chromosomes and added to others. As a result, some gametes have gene duplications and others have gene deletions.

Figure 13.7

Disruptive meiotic pairing in chromosomes with translocations or inversions. (A) Translocation mutants. The organism has inherited one set of normal nonhomologous chromosomes 1 and 2 from one parent (solid-colored chromosomes).Its other parent had a reciprocal translocation between chromosomes 1 and 2 (two-colored chromosomes). Under these conditions, pairing of homologous chromosomes in meiosis involves all four chromosomes instead of two members of a homologous pair. Four of the six possible gamete types are inviable. (B) Inversion mutation. When a normal chromosome pairs with its homolog, if one member of a pair has an inversion mutation (highlighted), crossing over can lead to gametes with gene deletions and gene duplications. Many of these will not be viable; viable gametes contain either a normal or an inverted chromosome. The chromatids with inversions that do not cross over survive in the population, and those that do cross over are selected against. Therefore, the genes in the inverted segment tend to be inherited as a unit.

Figure 13.8

Transposable genetic elements. (A) Transposable genetic elements that move by replicative transposition are duplicated in the process of jumping: a copy-and-paste mechanism. (B) Nonreplicative transposable genetic elements move in a cut-and-paste manner. No copy is made.

Figure 13.9

Sexual reproduction, genetic variation, and recombination summary. The simplified schematic shows the three ways genetic variation is created during sexual reproduction. (1) A single pair of homologous chromosomes is shown. Gametes produced in meiosis differ from the parental genotype because they contain half the amount of DNA and a random assortment of nonhomologous chromosomes (not shown); some had been inherited from the mother and some from the father of the gamete producer. (2) During fertilization, genetic material from two sources is combined, creating an offspring that differs genetically from both parents. (3) Finally, during gamete production in that individual, crossing over between its maternal and paternal (homologous) chromosomes occurs, creating within-chromosome genetic variation. Although crossing over (3) and independent assortment (1) are discussed sequentially, both occur in each round of gamete production.

Figure 13.10

Asexual reproduction. Offspring are genetically identical to the parent and, as such, are clones of the parent and each other. Any genetic variation in a clonal population is due to mutation.

Figure 13.11

Bacterial conjugation. Genetic material is exchanged between F+ and F- cells, creating two F+ cells.

Figure 13.12

Transformation. A cell takes up free DNA from its environment, integrates it into the chromosome, and expresses the encoded products.

Figure 13.13

Transduction. A virus serves as a conveyer of genetic material from one organism to another. In this example, the source and recipient of the transferred DNA are both bacterial cells, and the virus is a bacteriophage.

Figure 13.14

Natural selection. Selective forces exert different types of pressure on phenotypic traits in nature. In all of the cases shown, the selective force (arrows) is acting on external coloration, a trait that exhibits continuous variation in this population. (A) Stabilizing selection selects for the mean color pattern and against those that deviate from the mean, decreasing the variation in this trait. (B) Directional selection eliminates one of the extreme types but not the other and shifts the mean to a different phenotype. (C) Disruptive selective forces select for the two extremes and select against the mean, leading to a new type of frequency distribution known as a bimodal distribution because it has two peaks.

Figure 13.15

Sexual selection. In most species, males compete with each other for access to reproductive females and use a wide variety of strategies to entice females to mate with them. (A) Visual displays. In Anolis lizards, males extend a brightly colored flap under the chin. When this catches the attention of a female, they then do a series of pushups. (Photograph courtesy of David Crews.) (B) Chemical induction of receptivity. In the aquatic animal the red-spotted newt (Notopthalamus viridescens), males capture females with their hind legs and rub the female's snout with cheek glandular secretions for as long as 4 to 5 hours. (C) Male competition occurs in ways that are much more subtle than physical combat. In this walking stick population in Costa Rica, because males outnumber females two to one, competition for reproductive access to females is intense. Once a male inseminates a female, the pair stays in copulo for 3 to 4 days. By riding piggy-back on the female, he prevents other males from mating with her. (D) Male walking sticks (length indicated by blue ×s) are much smaller than females (length indicated by yellow ×s). In many insects, the number of eggs a female produces is proportional to her size, so reproductive success increases with female size. Therefore, competition for access to the largest females is especially intense. This population displayed positive assortative mating; larger females mated with larger males.

Figure 13.16

Coevolution. Coevolution can lead to highly complex interactions that often increase not only the fitness of both species but also the dependence of one species on another. In Costa Rica, Acacia plants provide three species of Pseudomyrmex ants with food and housing. (A) Acacia cornigera thorns. Ants excavate new, green acacia thorns, creating a safe haven when the thorns mature. The dark circles visible on the tips of some thorns are entrances to hollowed-out thorns. (B) Nectaries and Beltian bodies. The ants feed on sugar water secreted by specialized stem structures (nectaries) and on the yellow, protein-rich Beltian bodies on the leaf tips. The plant produces these foods solely for the ants. What do the ants offer the plant in return? (C) Pseudomyrmex ferruginea ants. The ants defend the plant. In response to the slightest disturbance, ants rush to the site to protect the plant from any animal attempting to eat it. (D) Cleared vegetation surrounding acacias. The ants remove competitors. The three shrubs in the center of the photo are acacias. You can see that the ants have removed vegetation from the surrounding area.