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Chapter 12
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geological times is a current edition of the textbook, whose
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Cheetahs
Cheetahs are one threat to monkeys, which produce a distinctive alarm call when a
cheetah is spotted.
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Patterns of macroevolution
Phyletic evolution (anagenesis) envisions gradual divergence of a lineage as the bellshaped mean of successive populations changes, until a new species if formed. Punctuated
equilibrium (cladogenesis) envisions long periods of more or less unchanging species
persistence, suddenly interrupted by speciation, producing a new species.
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Pace of macroevolution
For some biologists, each pattern implies a different rate of new species appearance.
Punctuated equilibrium produces new species relatively rapidly. Phyletic evolution produces
new species more gradually and sometimes termed “Gradualism.”
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Mode and tempo of evolution
Gradual and rapid appearance of new species occurs in the fossil record.
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Genetic drift—bottleneck effect, in theory
In a large collection of individuals, here the blue and yellow marbles, approximately equal
numbers of both are present. However, when just a few persist to start the next
generation, chance alone may yield mostly blue. Because most are blue, the next
generation, even if large numbers are produced, are now mostly blue.
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Genetic drift—bottleneck effect, in practice
The original population of cheetahs contained many alleles of a particular gene. Habitat
loss and excessive predatory control brought their numbers down, by chance leaving only a
few to breed with much less genetic diversity. Even as their numbers were partially
restored, the limited genetic diversity in the reduced cheetah population did not also bring
recovery of the lost genetic diversity. Without such diversity, cheetahs have been
susceptible to breeding deficiencies.
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FIGURE 12.1 Alarm Calls
(a) Vervet monkeys fall prey to leopards, as well as to snakes and eagles. Each-snakes,
leopards, and eagles—practice different predatory styles and arrive from different
locations. Depending upon the predator, the monkeys emit different alarm calls that elicit
different adaptive escape responses. (b) Shown are the abrupt chirp call (leopard) and
staccato grunts of the threat-alarm bark (eagle).
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FIGURE 12.2 Parental Genetic Investment
When diploid organisms (P) produce offspring, each offspring (F1) receives half of its
genotype from each parent. In turn, these offspring (only the female is shown) produce
progeny (F2) carrying a quarter of the genotype of the original parents. The genetic
contribution of the female is followed, although the same outcome applies to the male.
Each individual is diploid, represented by the divided half; the amount of the original
female’s genetic contribution (cross-hatch) is followed here.
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FIGURE 12.3 Parental Care
(a) The female parent defends and saves her three offspring, but she dies in the effort.
Nevertheless, because half of her genotype is carried in the offspring (F1), a total of 11/2
of her genotype survives. (b) No parental defense and the female survives, but all offspring
(F1) perish, leaving a total of only one (1) of her genotype surviving (the same applies to
males).
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FIGURE 12.4 Brood Parasite
The large young cuckoo (right) has evicted the smaller young of the host meadow pipit
(left) and begs for food.
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FIGURE 12.5 Macroevolution and Microevolution
The overall pattern of tetrapod evolution (macroevolution) can be examined in closer
detail. The cross section in time through Sauropod evolution reveals the multiple species
lineages of which it is composed. At the bottom, one lineage is enlarged, showing the
populations comprising the species (microevolution).
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FIGURE 12.6 Quantum Evolution
Within taeniodonts, a group of extinct placental mammals, two lineages evolved. One was the original
group of taeniodonts, the conoryctines that survived into the late Paleocene; the other lineage was the
stylinodonts, which evolved rapidly (quantum evolution) across a transition to a new adaptive zone
(lifestyle). Compared to the beaver-sized conoryctines, the bear-sized stylinodonts evolved specialized
dentition especially suited to rough and highly abrasive foods, well-developed claws, and strong muscles
suggesting a digging foraging style. (After Simpson 1953.)
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FIGURE 12.8 Cladogenesis, Details
Where sudden changes occur, they can be represented with an angular, branching phylogenetic tree (cf.
phyletic evolution, figure 12.7 or slide 4). Each independent lineage produced is a clade, shown here as
Clade 1 and Clade 2. Vertical sections represent more or less unchanging persistence of a species; branch
points represent the time of speciation where populations diverge and become two distinct species. Time
runs upward; species divergence is indicated along the horizontal scale. The balloons show details of the
phylogeny in a species before speciation (light shading), at a branching point of speciation wherein two
species form (light and dark shading), and the subsequent fate of each species thereafter.
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FIGURE 12.9 Peripheral Isolates
The geographic range of a species may include major populations fragmented into several
smaller, isolated populations. These isolated populations can be peripheral to the main
populations, or they may occur within a main population that has contracted around it.
(After Bock 1979.)
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FIGURE 12.11 Fossils and Phylogeny
Time passes vertically, and geographic distribution is shown horizontally. (a) The “balloons” indicate
temporal and geographic distribution of species A-H. Gaps between species, as they might appear in the
fossil record, are indicated by short vertical brackets at the left. Location 1 indicates a fossil excavation
site that preserves only part of the actual history of this evolving group. (b) The pattern of evolution
expressed as a phylogenetic tree. Note that most new species originate in a peripheral, isolated part of the
distribution (e.g., species B) or from a central, isolated population (e.g., species E).
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FIGURE 12.12 Lizards-From Limbed to Limbless
Cross sections through the posterior end of the lizard embryo are depicted. (a) Legged
lizard. The somite, an embryonic population of formative cells, grows downward to meet
mesenchymal cells, which together stimulate the sprouting of the limb bud capped by an
apical epidermal ridge. (b) Legless lizard. The somites fail to grow downward, thereby
failing to initiate the limb bud, which regresses, producing an embryo that is limbless.
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FIGURE 12.13 Hox Genes and Rapid Evolution-Lizards to Snakes
Hox genes associated with the chest region in lizards (a), expand their influence, leading to loss of forelimbs (b). By
other changes in embryology, more vertebrae are added to the vertebral column, producing an elongated body (c).
Either by a shift in influence of other Hox genes and/or by changes in limb bud growth (for example, see figure
12.12), hindlimbs are lost and an essentially modern snake body is produced (d). These steps may have occurred in
a different order. Certainly, other changes accompanied these three basic steps to consolidate and integrate them.
But apparently the major steps from lizard to snake are built upon only a few gene or embryonic modifications.
Different Hox genes (Hox) are indicated at locations wherein mutations in them are hypothesized to produce a
change in body design.