Chapter 7 The images on this CD have been lifted directly, without change or modification, from textbooks and image libraries owned by the publisher, especially from publications intended for college majors in the discipline. Consequently, they are often more richly labeled than required for our purposes. Further, dates for geological intervals may vary between images, and between images and the textbook. Such dates are regularly revised as better corroborated times are established. Your best source for current geological times is a current edition of the textbook, whose dates should be used when differences arise.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Diversity of dogs  Artificial selection has produced numerous breeds of dogs, which diverged from wolves about 15,000 years ago.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Corn  Artificial selection through the centuries evolved the modern male tassel and female ears of corn from wild grass.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Corn varieties  From left to right: popcorn, sweet corn, flint corn, dent corn, and pod corn.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Tulips  Tulips come in a variety of colors and stripes, but never in a deep color of completely black. This variety does not exist, at least so far, within the species.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Natural selection on snails  Birds, such as this song thrush, hunt snails and break their shells open against “anvil rocks” where debris collects. The snail (Cepaea normalis) has several distinct color morphs, which are camouflaged against different natural backgrounds.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Range of water snakes  The northern water snake (Nerodia) occurs throughout eastern North America. It is darkly patterned, except on some islands in Lake Erie where many are light colored.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Galápagos finches—natural selection on beak shape  Finch beaks are their tools for opening seeds. Different seeds open best with different beak size and features, which are inherited characteristics. When all seed sizes were easily available in normal seasons, ground finches (Geospiza fortis), shown here, with large and small beaks survived about equally well. But in dry seasons, small, easy-to-open seeds were scarce. Large, hard-to-open seeds were more plentiful. Predominantly, finches with large, strong beaks could open the large seeds and survive (red), but those with small beaks did less well (green).

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Clover fields—natural selection for shortness  On the left, tall clover is grazed by cattle, leaving short plants to survive and reproduce. On the right, in ungrazed fields the clover grows tall, reproduces, and these plants pass along their taller characteristics.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Types of natural selection  Occurrence of a range of traits is represented by a bell-shaped curve. Natural selection acts on this curve at different points (arrow) to produce different outcomes. a) Directional selection. Selection acts against one extreme, here the light colored morph, so that in subsequent generations it becomes less and less infrequent moving the moths in the directions of the now more frequent dark morph. b) Disruptive selection. Individuals with intermediate features are selected against, eventually producing over the subsequent generations a divided result with two color morphs at the extremes with their own bell-shaped distributions. c) Stabilizing selection. The extremes are selected against producing over subsequent generations a population with less variation, reflected in the tighter bell-shaped curve. The example shown here is birth weights in human infants, which tend to predominate around the middle of the extremes.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Secondary sexual characteristics—kudu  This male kudu, a resident of Africa, sports spiraled horns which are part of its social display during the reproductive season.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Sexual dimorphism  The large, male California sea lion, is distinctive from the surrounding, smaller females.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Red winged blackbird  This male red winged blackbird illustrates the colorful shoulder feathers, which are displayed during courtship and territorial defense.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Peacock  The peacock has a luxuriant tail and bright body used to attract the attentions of the female, the peahen.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Sexual dimorphism in lions  a) Young male, with mane b) Female

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display FIGURE 7.1 Diversity of Dogs  Artificial selection has produced numerous breeds of dogs, which descended from wolves, Canis lupus.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display FIGURE 7.2 Diversity of Tomatoes and Roses  Through selective breeding, tomatoes with different shapes have been produced; within the rose family, different colors and flower structures have been artificially selected.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display FIGURE 7.4 Begonias  Artificial selection of begonias for flower shape, size, and color has produced distinct varieties.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display FIGURE 7.5 Peppered Moth  The moth occurs in two color phases, peppered and melanic. (a) Both phases are displayed against an unpolluted, lichen-covered tree. (b) Both phases are displayed against a dark tree, on which the lichen were killed by pollution.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display FIGURE 7.6 Snail Selection  The shell of the snail Cepaea occurs in three color phases: brown, pink, and green. In different habitats-beech woodlands, meadows-different-colored shells are common or rare. In deciduous woodlands, the frequency of the color phases changes from spring to summer.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display FIGURE 7.7 Water Snake Differential Survival  (a) Scored color phases A (light) to D (banded). In young born on the islands, most are C or D (dark and banded). But by the time the snakes reach adult stage, most are A or B (light colored). (b) On the islands, predaceous gulls feed on young snakes, usually spotting and eating the more conspicuous banded snakes, producing differential survival of mostly unbanded snakes. Occasional immigrants from the mainland return some of the genes for banded color.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display FIGURE 7.8 Bell-Shaped Curve  Arranged by height, these cadets show variation from short to tall, with most falling somewhere in between. This variation, common to most traits, is shaped like a bell; hence the name.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display FIGURE 7.9 Types of Selection-Stabilizing, Directional, and Disruptive  The bell-shaped curve represents the distribution of a character in a population. The shading indicates where in that variation selection acts to eliminate individuals. (a) In stabilizing selection, extremes are eliminated, leading to a narrowing of the variation. (b) In directional selection, one extreme is eliminated, shifting the curve. (c) In disruptive selection, individuals with intermediate variation are eliminated, producing two bell-shaped curves at the extremes.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display FIGURE 7.11 Red-Winged Blackbird, Male  Note the epaulets-colored feather patches on its shoulders-that it uses in territorial and mating displays.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display FIGURE 7.12 Barn Swallows  To test the effects of male tail length, four groups of males received four treatments. “Shortened” tailed males were produced by cutting out the middle of the tail and regluing the tip to the base. “Elongated” tailed males were produced by cutting and gluing the removed section (from shortened tailed) to the middle of the tail. “Control I” males had tails cut but then reglued, thereby exposing them to the basic treatment but without changing tail length. “Control II” males received no tail clipping or gluing. The solid boxes represent the average number of young each group of males produced during a summer; the vertical lines express the range of variation of the results (standard deviation). The control groups did not differ significantly from each other, but both differed from shortened males, which produced significantly fewer offspring, and from elongated males, which produced significantly more offspring. (After M Ø ller, 1988)