Chapter 9 The images on this CD have been lifted directly, without change or modification, from...

Chapter 9

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.

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Speciation Four steps lead to speciation. First, a single species is an interbreeding reproductive

community. Second, a barrier develops, dividing the species. Third, separated into different habitats, the divided populations become differentiated through the accumulation of differences. Fourth, so different have the separate populations become, that is when the barrier disappears and they overlap again. Interbreeding does not occur.


Reproductive isolating mechanisms (RIMs) Different mechanisms prevent reproduction between individuals of different species. These

may occur premating or postmating, as illustrated here with two species of salamander.


Ring species—salamanders The ensatina salamander (Ensatina eschscholtzii) occurs from Canada to Southern

California with interbreeding between adjacent populations through this range. The Central Valley—a dry, hot lowland area—is divided into a coastal arm and inland arm. However, where these two arms of the species meet again in Southern California, interbreeding does not occur.


Convergent evolution—mammals Australian marsupials resemble placental mammals in the rest of the world. Within the

relative isolation of Australia, the marsupials entered similar habitats as counterparts among the placentals elsewhere. Under similar selective pressures, similar features and ecological lifestyles evolved, but upon a marsupial theme.


Allopatric speciation The populations of Tamarin monkeys are separated on the sides of the Amazon River.

Where the river tributary is wide and individuals on opposite banks do not interbreed, the populations are diverging toward separate species. Where the river tributary is narrow, the individuals still interbreed.


FIGURE 9.1 Defining Species (a) Morphospecies. Viewed today, at one moment in time, species A, C, and E are clearly

distinct, demarcated by current natural discontinuities between them. (b) Paleospecies (chronospecies). Viewed historically, through time, discovered fossil intermediates (B and D) fill in the missing gaps above, giving us a more or less continuous series with no obvious discontinuities between them.


FIGURE 9.4 Clinal Variation In the leopard frog (Rana pipiens), tadpoles exhibit a range of temperature tolerances,

generally enduring colder temperatures in higher (northern) latitudes and warm temperatures at lower (southern) latitudes. (From J. Moore)


FIGURE 9.5 Reproductive Success In the leopard frog (Rana pipiens), eggs from females in the north were fertilized with

sperm from males progressively farther to the south. The degree of embryo or tadpole abnormalities was scored, from A (normal young) through progressively more abnormalities to F (high death rate). This study was done by J. Moore in 1949. Today, this study and others prompt biologists to actually divide leopard frogs into subspecies or even different species. (From J. Moore)


FIGURE 9.6 Leopard Frogs Today-Zygotic Isolation The four groups of leopard frogs resemble one another closely in their external appearance. But

early tests of interbreeding produced defective embryos in some combinations, leading biologists to suspect that these might be different subspecies or even different species. Research on males’ mating calls indicates that the various groups differ substantially, and that such prezygotic behavior separates and reproductively isolates members of each group, producing four species: (1) Rana pipiens; (2) Rana blairi; (3) Rana utricularia; (4) Rana berlandieri.


FIGURE 9.7a Yarrow, a Member of the Sunflower Family (a) A transect, cross section of the yarrow’s distribution from sea shore to high mountains

is shown across California. Note the clinal variation in height.


FIGURE 9.7b Yarrow, a Member of the Sunflower Family (b) Three garden plots were selected at three different locations along the transect- sea level (Stanford), 4,600

feet (Mather), and at 10,000 feet (Timberline). Yarrow seeds collected from five locations along this transect-San Gregorio, Knight’s Ferry, Aspen Valley, Tenaya Lake, Big Horn Lake-were planted, grown into young plants, and then divided into equivalent tufts, clones, planted at the three garden sites-Stanford, Mather, Timberline. The resulting germination and growth of these five collected clones planted at these three garden sites is graphically indicated. Note especially that sea-level clones (from San Gregorio) at high elevations did poorly (died), and high-elevation clones (from Big Horn Lake) at low elevations still did not grow to large heights. After Clausen, Keck, Hiesey.


FIGURE 9.8 Ring Species - Herring Gulls As glaciers retreated, herring gulls (Larus argentatus) were released out of a north Pacific refugia spreading one

way across North America and into western Europe; and spreading in the other direction across Alaska into Siberia. From Siberia, as the herring gull now extended its range further across Asia, it tended to differentiate, producing a subspecies (or species by some ornithologists) such as the vega gull (Larus vegae) and farther west the lesser blackbacked gull (Larus fuscus). Eventually its current circumpolar distribution became established (dashed lines). Adjacent subspecies interbreed (solid arrows), but where the ends of the circular range of the herring gull meet and overlap in Europe, there is very little interbreeding. (Simplified originally from Mayr, 1963)


FIGURE 9.11 Convergent Evolution-Cacti Different families of desert plants have evolved similar adaptations to the desert’s dry, hot

conditions - namely, succulent shoots with spines. Two such plant species are shown from Africa. The third, from the New World, is the endemic member of the true cactus family (Cactaceae).


FIGURE 9.12 Latitudinal Gradients-Mammals The numbers of mammal species, from high latitudes (north) to low latitudes (south), are

shown along the lines. Note the general increase in the number of species from north to south across the various latitudes.

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