13.3
Pathways of Evolution
Large-scale evolutionary patterns help to outline the probable evolutionary history of life
on Earth. Patterns that occur on a more local scale can demonstrate how processes of change
among species may have contributed to and been influenced by these large-scale events.
Divergent and Convergent Pathways
divergent evolution occurs when
two or more species evolve increasingly different traits, resulting from
differing selective pressures or
genetic drift
convergent evolution occurs
when two or more species become
increasingly similar in phenotype
in response to similar selective
pressures
homoplasies similar traits found
in two or more different species,
resulting from convergent evolution
or from reversals, not from common
descent; also called analogous
features
(a)
Figure 1
Although the shark (a) and the
dolphin (b) are similar morphologically, their genetic history
differs. The shark is a fish while
the dolphin is a mammal.
adaptive radiation process in
which divergent evolution occurs in
rapid succession, or simultaneously,
among a number of groups to produce three or more species or
higher taxa
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Once a new species forms, its evolutionary pathway may diverge from that of the original species. Disruptive selection may continue long after speciation has occurred,
resulting in a pattern of divergent evolution. Species with significantly different morphological and behavioural traits may arise, as shown in the various modifications of vertebrate limbs and the activities that would accompany them (Chapter 11, Figure 2 in
section 11.5). Natural selection can also operate to produce striking similarities among
distantly related species (Figure 1). An excellent example of this pattern of convergent
evolution can be seen among mammals. For about 50 million years, marsupial mammals
in Australia have evolved in isolation from placental mammals throughout the rest of the
world, yet natural selection has favoured the evolution of species with similar traits
among mammals of both groups (Figure 2). Convergent evolution is not restricted to
organisms that are geographically isolated. Sharks and dolphins, for example, share wide
overlapping geographic distributions and have both evolved very similar streamlined
bodies well suited for their high-speed carnivorous behaviour. Traits that are similar in
appearance but that have different evolutionary origins are referred to as homoplasies
(also known as analogous features).
(b)
Sometimes divergent evolution occurs in rapid succession, or simultaneously, among
a number of populations. This process, known as adaptive radiation, results in one
species giving rise to three or more species. The best-documented examples are found
on remote archipelagos where the first organisms to arrive have a choice of resources and
few or no competitors. Consider, for example, the evolutionary path thought to have
been followed by the ground finches that migrated from South America to the Galapagos
Islands millions of years ago. While living in South America, these finches would have
become ideally adapted for eating medium-size seeds. Finches born with an unusually
small bill might have tried to feed on smaller seeds but would have faced fierce competition from other birds that fed on small seeds. Similarly, larger-billed ground finches would
have had to compete with large-seed feeders. For the few finches that first reached the
Galapagos Islands, the competition would have been eliminated. Assuming that the
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Section 13.3
Niche
Placental mammals
Australian marsupials
mouse
marsupial
mouse
mole
marsupial
mole
Mouse
Burrower
flying
squirrel
Glider
flying phalanger
lemur
spotted
cuscus
Climber
anteater
Anteater
numbat
(anteater)
bobcat
Cat
wolf
Tasmanian
“tiger cat”
Tasmanian
wolf
Wolf
Figure 2
The marsupials of Australia and the placental mammals in other parts of the world have
undergone convergent evolution resulting in species that appear similar occupying similar
ecological niches.
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The Evolutionary History of Life
603
DID YOU
KNOW
?
Island Extinctions are Common
Remote islands are fragile environments for indigenous populations,
where the lack of competitors and
predators render them vulnerable to
intruders and environmental change.
Many bird species on the Hawaiian
islands were flightless and exhibited
no avoidance behaviour for predators. Unfortunately, the introduction
of rats, cats, pigs, and dogs to the
islands has resulted in the extinction
of 60 species of birds. Similarly, all
the New Zealand species of Moa,
one of which weighed 250 kg,
became extinct shortly after the
arrival of humans to the islands.
Galapagos at the time bore various plants bearing different-size seeds, the founding
finch population would have been very successful. Individuals born with different-size
bills would have been able to find a rich supply of food. But seed size would not have been
the only environmental variable directing the evolution of the finches. Although the
founding population was composed of ground feeders, the Galapagos provided empty
niches for birds to feed from trees, on insects, from cactus, and from other specialized
sources. In fact, these niches were eventually filled by 13 species descended from the
founding ground-finch population.
The Hawaiian islands are the location of many excellent examples of adaptive radiation.
For example, numerous species of honeycreepers, a group of birds with a wide array of
bill shapes and sizes, are also thought to have evolved from a single ancestral species
(Figure 3). Also, 30 species of silverswords, a group of herbaceous plants, are thought to
have evolved from a single North American ancestor. The most dramatic example, however, is that of the 800 species of fruit flies of the genera Drosophila and Scaptomyza. They
are so similar that scientists believe they may all have evolved from a common ancestor.
Fruit flies are likely to continue to evolve rapidly within the Hawaiian archipelago. As
each new volcanic island forms, it can be invaded by organisms from older adjacent
islands. The new founding populations will evolve to form additional new species.
The term adaptive radiation is also applied to the evolution of entire groups of species.
For example, the mammals, which all share a single common ancestor, have undergone
adaptive radiation, filling many different feeding niches represented by such major taxa
as rodents, carnivores, whales, bats, primates, and ungulates.
Kona finch
extinct
Kauai akialaoa
Laysan
finch
Amakihi
Akiapolaau
Liwi
Maui parrotbill
Figure 3
The Hawaiian honeycreepers
provide an excellent example of
adaptive radiation. Selective pressures on a single founding species
have produced many species with
bills of different shapes and sizes.
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Apapane
fruit and seed eaters
insect and nectar eaters
founder species
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Section 13.3
Coevolution
When two species are completely dependent on one another for survival, their evolutionary pathways become linked. This fascinating pattern, called coevolution, can be
found, for example, in figs that are dependent on a specific wasp for pollination. Without
the wasp, the fig cannot reproduce; in turn, the fig wasp can only reproduce within specially modified sterile fig flowers. Highly specialized flower structures have coevolved in
the fig alongside unusual behaviour among the wasp pollinators (Figure 4). In a similar
way, leaf-cutter ants have coevolved with a fungus that is their sole food source. The ants
harvest leaves, which they do not eat but bring to underground chambers, where the
leaves nourish the growth of the fungi. The ants cannot survive in the absence of their symbiotic fungi; in turn, the fungi are found nowhere else on Earth and, therefore, have
become dependent on cultivation by the ants for their survival. Coevolution is widespread among flowering plants and their pollinators, and among parasites and their hosts.
SUMMARY
coevolution process, sometimes
referred to as reciprocal adaptation,
in which one species evolves in
response to the evolution of
another species
Local Patterns of Evolution
•
Depending on the selective forces at work, the evolution of two different species
can result in either a divergence or a convergence of traits.
•
Adaptive radiation, which occurs when the environment favours a variety of
divergent traits, is prevalent in diverse environments when competitive pressures
from other species are reduced or absent.
•
Species coevolve when they are dependent on one another for survival.
Figure 4
Each of the roughly 900 species of
fig is pollinated by its own unique
fig wasp species.
Section 13.3 Questions
Understanding Concepts
1. Ongoing disruptive selection results in what evolutionary
pattern(s)?
2. Compare and contrast divergent and convergent evolution.
Provide an example to support each point you make.
3. Provide an explanation for the vulnerability of populations
8. In 1862, after examining the star orchid of Madagascar
Island, Darwin predicted that a pollinating hawkmoth
with a tongue of just less than 30 cm in length would
be discovered. Forty years later, a pollinating hawkmoth
with a 25-cm-long tongue was found. What specific
understanding of evolution would Darwin have used
to make his prediction?
indigenous to a remote island.
4. Leaf-cutter ants obtain food through the leaves they carry
to their subterrestrial habitat, but do not eat the leaves.
How does the pattern of coevolution offer an explanation?
Applying Inquiry Skills
5. Many species of fish and aquatic birds exhibit counter
shading—their upper dorsal surfaces are dark, while their
lower ventral surfaces are much lighter.
(a) What pattern of selective pressure is most likely at work?
(b) What environmental factors might be causing this
selection?
6. Before the arrival of humans, almost every remote island on
the planet had one or more species of flightless bird. How
does evolution account for this?
7. Evolution predicts that each flightless bird species evolved
on a single island. Therefore, no two islands should be
home to the same species of flightless bird. How could
biologists test this prediction? Research this topic to find
out whether this prediction has been tested.
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Making Connections
9. Brazil nuts are harvested for human consumption and for
the production of Brazil nut oil, a very valuable oil that is
often used in soaps and shampoos. The trees that produce
Brazil nuts are indigenous to tropical rain forests of South
America. However, when these trees were planted in huge
monoculture plantations, they failed to become pollinated
or produce many nuts. How might evolutionary history
account for this observation?
10. The use of an insecticide or a fungicide could potentially
harm an entire ecosystem, rather than just target specific
organisms. Discuss this statement, using the leaf-cutter
ants as a model.
11. Many populations of indigenous peoples, following their
initial contact with nonindigenous peoples, have suffered
devastating losses as a result of previously unknown diseases. How might evolutionary biology account for their
low resistance to these diseases?
The Evolutionary History of Life
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