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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