4 the evolutionary history of life
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602 Chapter 13 NEL
13.313.3Pathways of EvolutionLarge-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 PathwaysOnce a new species forms, its evolutionary pathway may diverge from that of the orig-
inal species. Disruptive selection may continue long after speciation has occurred,
resulting in a pattern ofdivergent evolution. Species with significantly different mor-
phological and behavioural traits may arise, as shown in the various modifications of ver-
tebrate 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 ofconvergent
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 traitsamong 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).
divergent evolution occurs when
two or more species evolve increas-
ingly 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
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 andfew 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 compe-
tition 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
Figure 1
Although the shark(a) and the
dolphin (b) are similar morpho-
logically, their genetic history
differs. The shark is a fish whilethe dolphin is a mammal.
adaptive radiation process in
which divergent evolution occurs in
rapid succession, or simultaneously,
among a number of groups to pro-
duce three or more species or
higher taxa
(a) (b)
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The Evolutionary History of Life 60NEL
Section 13
Niche Placental mammals Australian marsupials
Mouse
Burrower
Glider
Climber
Anteater
Cat
Wolf
mole marsupialmole
anteater
numbat(anteater)
mouse marsupialmouse
lemur
spottedcuscus
flyingsquirrel
flying phalanger
bobcatTasmaniantiger cat
wolf Tasmanianwolf
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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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,how-
ever, is that of the 800 species of fruit flies of the genera Drosophilaand 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.
604 Chapter 13 NEL
Kona finchextinct
Laysanfinch
Akiapolaau
Maui parrotbill
Kauai akialaoa
Amakihi
Liwi
Apapane
founder species
fruit and seed eaters insect and nectar eaters
Figure 3
The Hawaiian honeycreepers
provide an excellent example of
adaptive radiation. Selective pres-
sures on a single founding species
have produced many species with
bills of different shapes and sizes.
Island Extinctions are CommonRemote islands are fragile environ-
ments 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 preda-
tors. 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.
DID YOUKNOW??
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