Transcript Image PowerPoint

Chapter 9
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geological times is a current edition of the textbook, whose
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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.
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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.
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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.
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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.
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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.
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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.
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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)
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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)
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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.
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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.
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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.
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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)
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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).
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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.