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
Transcript
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.
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).
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.
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.
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.
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.
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 MØller, 1988)
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