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AFTER MAN
A ZOOLOGY OF THE FUTURE
BY DOUGAL DIXON
INTRODUCTION BY DESMOND MORRIS
ST. MARTIN'S PRESS
NEW YORK
Published in the United States of America in 1981
by
St. Martin's Press
175
Fifth Avenue
New York
NY 10010
Library of Congress Number 81-56345
ISBN 0-312-01163-6
© Harrow House Editions Limited 1981
For information, write: St. Martin's Press
All rights reserved. No part of this work covered by the copyright hereon may be reproduced or used in any form by any means - graphic, electronic, or mechanical, including photocopying, recording, taping or information storage and retrieval systems - without written permission of the publisher.
Edited designed and produced by
Harrow House Editions Limited
7a Langley Street, Covent Garden, London, WC2H 9JA
Edited by James Somerville
Designed by David Fordham
Phototypeset by Tradespools Ltd., Frome, England
Illustrations originated by Gilchrist Bros, Ltd., Leeds, England
Printed and bound in the United States of America.
INTRODUCTION BY DESMOND MORRIS 9
AUTHOR'S INTRODUCTION 10
EVOLUTION 11
Cell Genetics : Natural Selection : Animal Behaviour :
Form and Development :
Food Chains
The Origins of Life : Early Living Forms : The Age of Reptiles
:
The Age of Mammals : The Age of Man
The World after Man
TEMPERATE WOODLANDS AND GRASSLANDS 36
The Rabbucks : The Predators : Creatures of the Undergrowth
:
The Tree Dwellers : Nocturnal Animals : The Wetlands
The Browsing Mammals : The Hunters and the Hunted : Tree Life
TUNDRA AND THE POLAR REGIONS 58
The Migrants : The Meaching and its Enemies : The Polar
Ocean :
The Southern Ocean : The Mountains
The Sand Dwellers : Large Desert Animals : The North American Deserts
The Grass-eaters : Giants of the Plains : The Meat-eaters
The Tree-top Canopy : Living in the Trees
: The Forest Floor :
Living with Water
: Australian
Forests :
The Australian Forest Undergrowth
ISLANDS AND ISLAND CONTINENTS 100
South American Forests : South American
Grasslands : The Island of Lemuria
:
The Islands of Batavia
: The Islands
of Pacaus
The Destiny of Life
Glossary : The Tree of Life : Index : Acknowledgements
FOR GAVIN
The author and publishers would like to express their thanks to the illustrators of this book. They are Diz Wallis (represented by Folio); John Butler and Brian McIntyre (represented by Ian Fleming and Associates Ltd); Philip Hood; Roy Woodard (represented by John Martin and Artists Ltd); and Gary Marsh
As soon as I saw this book, I wished I had written it myself.
It is a marvellous idea, beautifully presented. Many years ago, as a young
zoologist, I started
inventing imaginary creatures, drawing and painting them as an enjoyable contrast
to the demands of my scientific studies. Released from the restrictions of
evolution as it really is, I was able to follow my own, private evolutionary
whims. I could
make monsters and strange organisms, plant-growths and fabulous beasts of any
colour, shape and size I liked, letting them change and develop according to
my own rules, giving my imagination full rein. I called them my biomorphs and
they became as real to me as the animals and plants of the natural world.
Dougal Dixon's mind has obviously been working in a similar way, although the
creatures he has brought to life are very different from mine. Instead of inventing
a parallel evolution, as though it were taking place on another world, he has
given himself the intriguing task of contemplating a future evolution on our
own planet, closely based on species that exist at present. By waving a time-wand
and eliminating today's dominant species, including man, he has been able to
watch, through his mind's eye, the lesser animals gradually taking over as
the major occupants of the earth's surface.
Setting his scenario in the distant future, about 50 million years from now,
he has given the members of his new animal kingdom time to undergo dramatic
changes in structure and behaviour. But in doing this he has never allowed
himself to
become too outlandish in his invention. He has created his fauna of the future
so painstakingly that each kind of animal teaches us an important lesson about
the known processes of past evolution - about adaptation and specialization,
convergence and radiation. By introducing us to fictitious examples of these
factual processes,
his book is not only great fun to read but also has real scientific value.
The animals on these pages may be imaginary, but they illustrate vividly a
whole
range of important biological principles. It is this — the way in which he
has perfectly balanced his vivid dreamings with a strict scientific discipline
— that
makes his book so successful and his animals so convincing and, incidentally,
so superior to the often ridiculous monsters invented by the cheaper brands
of Science Fiction.
The only danger in reading this delightful volume is that some of you may reach
the point where you suddenly feel saddened by the thought that the animals
meticulously depicted in it do not exist now. It would be so fascinating to
be able to set
off on an expedition and watch them all through a pair of binoculars, moving
about on the surface of today's earth. Personally, I feel this very strongly
as I turn the pages and there is probably no greater praise that I can offer
the author than that....
Evolution is a process of improvement. Hence, looking at
the animals and plants of today and their interactions - the delicate balance
between the flora, the
herbivores and the meat-eaters; the precise engineering of the load-bearing
structures of the giraffe's backbone; the delicate sculpting of the monkey's
foot, enabling
it to grasp objects as well as to climb trees; the subtle coloration of the
puff-adder's skin, hiding it completely among the dead leaves of the forest
floor - and trying
to project all of that into the future is a near impossibility. For how can
you improve upon perfection?
One trend that is foreseeable, however, is the ruinous effect that man is having
on the precise balance of nature. I have taken this not unjustifiably to an
extreme, with man having extinguished the species that are already on the decline
and
having wreaked terrible destruction on their natural habitats before dying
out himself and allowing evolution to get back to work, repairing his damage
and
filling in the gaps left behind. The raw materials for this reparation are
the kinds of animals that do well despite, or because of, man's presence and
which
will outlive him - those that man regards as pests and vermin. These are more
likely to survive than are the highly modified and interbred domestic animals
that he develops and encourages to suit his own needs. The result is a zoology
of the world set, arbitrarily, 50 million years in the future, which I have
used to expound some of the basic principles of evolution and ecology. The
result
is speculation built on fact. What I offer is not a firm prediction - more
an exploration of possibilities.
The future world is described as if by a time-traveller from today who has
voyaged the world of that time and has studied its fauna. Such a traveller
will have
some knowledge of today's animal life and so he can describe things with reference
to the types of animals that will be familiar to the reader. His report is
written in the present tense as if addressed to fellow time-travellers who
have voyaged
to the same period and wish to explore the world for themselves.
Sit back, fellow time-travellers, and enjoy the spectacle and drama of the
evolution of life on your planet.
Dougal Dixon
Wareham 1981
The sketches on this page are selected from the author's own working drawings and were used by the artists to prepare the plates and illustrations in After Man.
The biological cell, shown here in the process of replicating itself, is the fundamental building block that makes up all living things. The cell's capacity for infinite variation, when taking part in sexual reproduction, is at the root of evolutionary development.
The form and position of living things on earth can be attributed
to two things — evolution and environment. The study of evolution explores
how life originated,
how it diversified the way it did and how different creatures have
developed from others. The study of a creature's environment (ecology) shows
how
the various life forms interact with one another and how they interact with
the environment they inhabit.
In other words evolution can be thought of as showing a longitudinal section
through the life of our planet while ecology shows the same situation in
cross-section.
Each is inextricably entwined with the other and the two cannot be studied
totally independently.
Although both aspects deal with survival it should not be forgotten
that extinction is a very important factor. Without it there would be no
room for evolution to take place. There would be no new ecological situations
for
nature to fill by the evolution of new
animals and plants from older stocks. That evolution has taken place is apparent
both from the fossil record
and from the evidence contained within living plants and animals. Examination
of fossil remains reveals a general development from the simple to the more
complex and also the part played by the environment
in shaping an organism to prevailing conditions. In living creatures,
comparability in structure, embryonic development and chemistry are powerful
indications of similar
evolutionary history or of common ancestry.
Evolution is therefore not a process that has happened only in the past in
order
to establish the animals and plants of today's ecology, but is a constantly
continuing
process that we can study both from its results and from the fossil evidence
of the past. It has happened,
it is happening now and it will continue to happen as long as life remains
on this planet.
Most animal cells contain the same basic components. At the centre lies the nucleus (A), which contains the cell's genetic material. The mitochondrion (B), responsible for energy production, and the lysosome (C), which secretes chemical products, lie nearer the surface in the cytoplasm (D). The ribosomes (E), where the proteins are assembled, lie along a convoluted structure of membranes known as the endoplasmic reticulum (F). |
Animals, and indeed plants, are composed of microscopic bricks
called cells. The cells found in different organs and tissues of the same creature
are of
quite different sizes and shapes - bones are made from angular cells, kidneys
from
spherical cells, nerves from long, narrow cells - but all are made from similar
components. Round the outside of each cell is a skin, the cell membrane, enclosing
the gelatinous cytoplasm which carries a number of small structures called
organelles. The most important of these is the cell nucleus, which lies at
the centre of
the cell and carries the information from which the entire organism is built.
This
information is stored as a code, made up from a sequence of components contained
in a long molecule of a complex substance known as deoxyribonucleic
acid (DNA).
The DNA molecule is a little bit like a ladder that has been twisted throughout
its length. The shafts of the ladder are made up of sugar-phosphate molecules
and each rung consists of a pair of molecules known as nucleic-acid bases.
There are only four of these bases and the sequence in which they are found
along the
twisted ladder gives the coded instructions from which the whole organism is
formed. Although repeated in its entirety in the nucleus of each eel! of the
organism, only certain parts of the code are needed to build up particular
organs.
   |
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| FERTILIZATION: 1. Sperm 2. Ovum 3. Ovum nucleus 4. Chromatid |
  |
The sperm
penetrates the ovum (A) and comes to lie alongside the ovum nucleus
(B). The chromosomes of both sperm and ovum divide into separate
strands known as chromatids. Corresponding chromatids move to opposite
ends of the ovum (C), where they are surrounded by nuclear membranes
(D). The structure then splits into separate cells (E).
|
|
The peculiar thing about the DNA molecule is its ability to
reproduce itself. The molecule splits along its length and unwinds so that
each half of the ladder
consists of a shaft and a series of half-rungs. The missing ladder halves are
built from the pool of sugar-phosphate bases, which is supplied by the creature's
food and is present in each cell nucleus. As each of the four types of nucleic
acid base in the strand attracts only a specific kind of nucleic acid base
to itself, when two new complete strands of DNA are formed they are absolutely
identical
to each other in the sequence of their components. This is the most important
process involved in cell multiplication and underlies the growth of all organisms.
However,
to grow, organisms also require proteins in the form of either structural elements
such as collagen, in the case of the packing
tissue between organs,
or as enzymes which aid specific biological processes. Although the production
of proteins is carried on outside the cell nucleus it is controlled by the
DNA and is produced in a way analogous to DNA replication. The messenger that
transmits
the DNA's instructions to the protein production centre, the ribosome, is a
molecule known as RNA. It is formed along partly "unzipped" sections
of DNA and differs only subtly from it. The messenger RNA travels to the ribosome,
where
it links up with another form of RNA, transfer RNA, which bears amino acids.
It is from these amino acids that the proteins are formed. The RNA molecules
are merely code carriers and ensure that the amino acids link together in the
correct sequence to form the protein type required. In this way DNA controls
the workings of the whole cell and hence of the whole organism.
| During cell division, when new cells are being formed, the DNA (A) contained within the dividing cell unzips and forms new molecules of DNA along its free ends (B) from the nucleic-acid bases and sugar phosphates contained in the cell nucleus. To produce messenger RNA, the DNA comes apart partially (C) and links with broadly similar material; the sugar phosphate backbone is slightly different chemically and one of the nucleic acids is substituted. The messenger RNA moves to the ribosomes, where it links up with transfer RNA, which carries amino acids (D). The messenger RNA contains the code that ensures that the transfer RNA is linked together in the correct sequence to produce the chain of amino acids that form the desired protein. |
 |
The DNA molecules
in the cell nucleus are aggregated into structures called chromosomes, and
specific groupings of nucleic-acid base sequences on the DNA
give rise to
specific traits in the organism. These groupings are called genes. Half the
chromosomes in a creature's cells, and hence half its genes, come from its
mother and half
from its father. This is reflected in the alignment of the chromosomes during
cell division. The chromosomes then are arranged in pairs, mother-donated ones
aligned with identical father-donated ones so that comparable genes are side
by side. Even though each gene in a pair contributes to the determination of
a particular characteristic, one gene often masks the effect of another.
As part
of the reproduction process special cells known as gametes - that is sperms
or eggs - containing only half the number of chromosomes found in ordinary
cells, are formed in the sex organs. Although one chromosome from each pair
is present in each gamete, none is identical to any of the chromosomes received
from either the mother or the father, but contains a mixture of material from
both parents. This characteristic of gamete chromosomes is primarily responsible
for the variation between individuals of the same species that is seen in nature.
During fertilization, the gametes unite with others from a second individual
to produce a complete cell, with the full number of chromosomes, which in turn
divides and builds up a completely new organism with genetic characteristics
derived from both parents.
This, briefly, is the sophisticated mechanism that enables plants and animals
to reproduce and pass on their distinctive traits from one generation to the
next. It is small changes, or mutations, in the genes involved in this process
that allow evolution to take place. A mutation results in a variation in the
characteristics of the adult organism growing from the cell containing the
gene. In most cases the change that takes place is harmful and gives the organism
a
disadvantage in the competitive world outside. The organism perishes and the
mutant gene perishes with it. Occasionally however the mutant gene produces
a trait that gives the organism a distinct advantage in its fight for survival.
The variation in genetic make-up that sexual reproduction makes possible produces
the range of characteristics that are found throughout individuals of a single
species. Natural selection, which may be thought of as the directional impetus
of evolution, acts on this variability, favouring certain characteristics and
rejecting others according to their survival merit.
Natural selection, resulting from the environmental conditions in which an organism lives, can have one of three different influences on a population. It can be stabilizing, directional or diversifying. The stabilizing influence can be seen where conditions have remained unchanged over a long period of time. The resultant environment consequently supports a well-balanced population of animals and plants in which evolutionary development is disadvantageous. Under such circumstances any change occurring in a plant or animal will bring it out of the environment's neat, efficient, time-honoured survival pattern and put the creature at a disadvantage, eventually resulting in its extinction. Its more conservative contemporaries on the other hand will survive. Animals that have been subjected to stabilizing selection for a long period of time may seem quite unspecialized and primitive compared with those of similar ancestry that have experienced a more eventful evolutionary history. Often they are characterized by passive survival mechanisms such as heavy armour, or high fecundity to offset losses through predation.
The horse's earliest known ancestor, Hyracotherium, a small long-toed creature no bigger than a dog, inhabited the extensive forest areas found on the earth between 50 and 60 million years before the Age of Man. As conditions became drier at the end of the Tertiary and the woodland receded, the creature became progressively better adapted to life on the plains. Its feet changed radically; the outer toes disappeared, leaving a single horny hoof. Its legs became longer as it evolved into a fully fledged running animal and its dentition and digestive system changed from that of a browser to that of a grazer as its diet altered from leaves to grass. The most important structural changes occurred about the time of Merychippus, which appeared about 25 million years ago. |
The directional influence of natural selection is more evident when the environment itself changes. Under these circumstances evolutionary changes occur such as to give the impression that the organism is evolving along a set path with a particular goal in view. This is quite erroneous and arises from the fact that in the context of its environment the most recent member of an evolutionary series always appears much better adapted than the earlier intermediate stages which, where they are known, look half-formed and incomplete by comparison, even though they were equally well adapted to the environment's own earlier intermediate stages. An example of this is the evolution of the horse, which developed from a small forest-living browser into a large, long-legged running grazer as its environment altered from forest to open grassy plain. The small changes that enabled it to deal most effectively with its changing environment were continually selected for throughout its history and in this way the horse evolved.
(1) British lesser black-backed gull, Larus fuscus graellsii, (2) Scandinavian lesser black-backed gull, Larus fuscus fuscus, (3) Siberian vega gull, Larus argentatus vegae, (4) American herring gull, Larus argentatus smithsonianus, (5) British herring gull, Larus argentatus argentatus |
From the original finch that arrived at the Galapagos Islands from South
America, around fifteen separate species evolved to fill the island's vacant
ecological niches - each species with specialized characteristics suited
to its own individual diet. The finches fall broadly into three distinct
groups according to habitat - cactus, tree and ground dwellers-and differ
mainly in the shape of the bill. It is thought that to begin with birds
were scarce on the island allowing the finches to evolve forms suitable
for all the environmental slots available. |
The diversifying influence of natural selection takes effect
when a new environment is established offering a fresh range of food resources
and living spaces.
An animal species entering this environment may well evolve different forms
that are specifically adapted to
each of these
living spaces, or ecological niches. In the absence of competing animals these
different
forms will eventually develop into completely new species. This is the kind
of thing that happens when an island, or a group of islands, is thrown up by
volcanic
activity in the open ocean. The unpopulated island is slowly colonized by animals
which gradually diversify into different species to exploit the whole area
effectively. The classic example of evolutionary diversification is seen in
the Galapagos
Islands of the Pacific Ocean. Early in their history a small finch arrived
that subsequently evolved into tree-living, insect-eating forms, seed-eating
forms with heavy bills and a form that ate burrowing grubs winkled out with
cactus spines. The large number of resulting species reflected the large
number of ecological niches available on the islands.
Birds, with their power of flight, are usually the first vertebrates to reach
a new island and consequently far-flung islands can usually be counted upon
to produce an interesting bird fauna. Typical are the heavy flightless birds,
such
as the moa, Dinornis, of New Zealand, the dodo, Raphus, of Mauritius and the
elephant bird, Aepyornis, of Malagasy, all of which evolved in the absence
of ground-living predators. The intervening sea was an effective barrier preventing
interbreeding between the far-travelled individuals that reached the island
and the original stock back home. Such barriers to interbreeding are necessary
in the evolution of new species.
Races or sub-species often co-exist in the same area, exploiting slightly different
environments or food resources but retaining the ability to interbreed. They
may even exist as a chain of sub-species reaching from one region to another,
each sub-species able to interbreed with the next one to it. When the species
at the ends of the chain are quite different the chain is called a cline. Occasionally
a cline may form a ring, for example round a mountain range, where the two
end members, although next to one another and related, are so different that
no interbreeding
is possible and are, technically speaking, different species. This poses problems
in taxonomy since, as interbreeding is possible elsewhere throughout the ring,
the members must strictly be considered as sub-species of the same species.
Once a group becomes isolated from its original population it may develop on
its own to such an extent that, if the isolating barrier later disappears and
the two populations once more intermingle, interbreeding is no longer possible.
They are now, by definition, two different species. The differences are accentuated
if the new location the isolated group finds itself in is basically unsuitable.
The group will very quickly disappear except for maybe a few individuals at
the extremes of the species range that show some slight affinity for the environment.
The species that then develops will be descended from those few individuals
that
were genetically different from the main population in the first place and
contained by chance genetic traits which made them innately more likely to
survive.
Because organisms are capable of infinite variability and have an inherent
tendency to change when set in an unstable environment, new species appear
more rapidly
when the environment is changing quickly. Evolution is so efficient that no
ecological niche is left vacant for long. Something will always develop to
fill it.
Evolution does not involve the conscious will of the organism.
Nor does it happen through any adaptation that is forced on it by its surroundings,
or
any strategy
learned by the organism during its lifetime being passed on by it to its offspring.
It happens, simply, because certain characteristics in an organism's genetic
make-up are either selected for or selected against by the particular characteristics
of environment in which it finds itself. The environment, in this context,
is the physical surroundings of the organism, such as the topography, the temperature
or the rainfall, and the other organisms that coexist with it, both those
that
it feeds on and those that feed on it.
The rate of evolution has little to do with the rate at which genetic mutation
occurs - the important factor is the environment's rate of change; the speed
at which new pathways open up into which new forms may evolve and develop.
As well as being responsible for structural and morphological traits in an
animal the genetic make-up of a cell also gives rise to behavioural traits
that allow
an animal to interact with its neighbours and with its environment in a way
that ensures its survival.
It can be argued that the function of an organism is merely to pass on its
genes to the next generation. Evidence to support this view can be drawn from
patterns
of behaviour seen in animals. Behaviour is, simply, an animal's active response
to its environment, and along with growth and reproduction is one of the factors
that defines a living thing.
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The visual courtship
display of birds is an important part of a behavioural pattern that
also includes song; male bird song is designed to attract
females as well as to deter rival males. Visual display may take place
independently with the intention of attracting a mate. An individual,
usually a male, postures and signals until it has secured the attentions
of a potential
mate. The pair then display in concert, each responding to the other's
gestures with the object of discovering the other's willingness or readiness
to mate. Many species rely on
resplendent plumage for display. In mast cases the males are ostentatiously
feathered, whereas the females are drab by comparison. The movements
and gestures in courtship display are usually those associated with
aggression
or appeasement. In some species preening and mock sleep are all part
of display. (A) Gannet, Morus bassanus, (B) Sage grouse, Centrocercus urophasianus, (C) Cormorant, Phalacrocorax carbo, (D) Brolga crane, Grus rubicunda, (E) Great crested grebe, Podiceps cristatus, (F) Adelie penguins, Pygoscelis adeliae |
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Investigations into the song of the chaffinch, Fringilla coelebs, have provided a fascinating insight into the role of learning in behaviour. It was found, as indicated on the sound spectograph shown opposite, that young chaffinches reared in isolation were capable of only a rudimentary song and that to produce the fully developed form they had first to hear the song of others in the wild. |
Male fiddler crabs, Uca spp., attract mates by waving their large fiddle claws. The gestures they make both in the shape of the movement and in its speed varies between species living in the same area and ensure that only females of the correct species are attracted. As only matings between individuals of the same species are likely to produce fertile offspring, those males not possessing the genes that produce the correct waving patterns are likely to disappear. |
Birds crowd together when a hawk appears, thus making it more
difficult for the hawk to seize an individual. Running herbivores dodge about
to escape a
swifter
predator so that it becomes exhausted before catching any of them. Young birds
stay close to their mothers until they are mature enough to fend for themselves.
These, like all aspects of behaviour, have evolved to aid survival. A gene
that introduces
a behaviour pattern that does not contribute to the survival of the species
is soon eliminated.
Courtship rituals are a very complex aspect of behaviour. The exact motion
of a bird in a display dance or the movement of a lizard's head as it approaches
a prospective mate indicates to its future partner that it is in breeding condition
and that it is a member of the correct species. The latter point is important,
for although mating between two related but separate species may produce offspring,
they will almost certainly be sterile. Such matings are a total waste of time
and effort from the point of view of evolution, as they do not successfully
propagate
the creature's genes and are therefore to be avoided.
These activities are all instinctive hereditary behaviour patterns. Other behaviour
patterns are learned and are also ultimately derived from the animal's genetic
make-up. The ability to establish the appropriate action by trial and error,
or by the example of others around it, is an ability conferred on an animal
by its genes.
Aggression is an element of behaviour that is perhaps more complicated than
it first appears. One might ask why, if the object of aggression is to remove
one's
competitors, do not animals fight to the death each time there is a conflict?
Apart from the obvious risk involved, the answer is probably that, as an animal
has no chance of killing all its potential rivals, by killing an isolated one
it is just as likely to assist its competitors as to benefit itself. In most
cases combat in the animal world takes the form of mock battles and aggressive
displays which do little physical damage to the creatures involved, but do
establish the dominance of one or other of the participants. Thus the animal
that wins
a contest achieves what it has set out to do, that is to gain or retain the
resource in dispute without suffering injury itself. The loser also derives
benefit in
that he escapes serious injury and retains the possibility of contesting future
issues, where he may eventually be successful. It is difficult to see how this
strategy could be learned and it is more likely that it is the product of evolutionary
development; those animals adopting the strategy are more likely to reproduce
and therefore the genes responsible for the behaviour are passed on in preference
to others that result in less successful behavioural patterns.
Throughout the animal kingdom behaviour patterns are designed to ensure the
survival of the individual's genes rather than the survival of the individual.
Loyalty
is shown to the closest relatives, since the closer the relative the larger
the number of similar genes in its make-up.
The protective instinct which causes a mother bird to put itself in danger
or even sacrifice its life in order to save its brood is a behavioural trait
calculated to promote the survival of its own genes. As the genes of the mother
bird are present in the brood and the several members of
the brood have a better chance of reproducing and spreading their genes than
has the single parent bird, it is to the advantage of her own genes to preserve
the lives of her chicks even at the expense of her own. Less obvious is the
gene-survival behaviour of social insects, such as bees and ants. A member
of such a group
will fight to the death indiscriminately to ensure the survival of the colony.
In this case members of the colony are much more closely related to one another
in genetic make-up than are other animals within a single breeding population.
The survival of the colony therefore ensures the survival of the individual's
genes despite the death of the individual.
Many mating ploys, particularly those seen in birds, may seem actually to reduce
the individual's chance of survival rather than to increase them. The breeding
plumage of many male birds, as well as being attractive to a mate, makes them
visible to predators. Birds possessing particularly long and spectacular tail
feathers must find them a great disadvantage when escaping from a predator.
It is possible such handicaps to survival may be devices to show just how successful
the male is - if it can survive with all that working against it, then it must
be good! Hence the female is instinctively attracted to the male that puts
on
the most extravagant display.
Natural selection lays down rules about precisely which form of life is most suitable for colonizing a particular environment. This evolutionary feature can give rise to a large number of different animals with the same superficial appearance. When the animals concerned have evolved from the same ancestor and have developed independently along similar evolutionary lines, they are said to have evolved in parallel. When the ancestors are different and the animals have evolved along quite different lines to produce the same final shape, their evolution is called convergent. An example of parallel evolution can be seen in the development of Equus, the horse, which appeared at the end of the Tertiary in North America, and Thoatherium, a remarkably similar ungulate which evolved at the same time in the then isolated continent of South America. The two forms developed independently along similar lines from similar ungulate ancestors in response to the same set of environmental conditions. An example of convergent evolution is found in the development of the shark, Carcharodon, the fish-lizard, Ichthyosaurus, and the dolphin, Delphinus — three animals from totally different classes but having adopted the same streamlined shape, swimming fins and tail in order to exploit the same niche in the same environment, that of active fish-eaters in the sea.
Of the shark, fish-lizard and dolphin, only the shark has evolved from a marine creature. The fish-lizard and dolphin were evolved from a land-living reptile and mammal respectively. Despite their radically different ancestry they have all adopted the same streamlined form to suit their aquatic mode of life, and together form a striking example of convergent evolution. |
One consequence of particular animal shapes fitting particular ecological niches
is that widely separated places with the same climatic and environmental conditions
may support very similar faunas even though they have evolved from different
stocks. The tropical grasslands of South America, Africa and Australia all
at one time supported animals with similar physical characteristics - long-legged,
running grazers, swift carnivores, burrowing insectivores and slow-moving heavy
browsers. In Australia they were marsupial, in Africa placental and in South
America of both types. Despite their differing ancestry many of these creatures
were outwardly similar. Such situations arise not only in different places
at
the same time, but also in different places at different times.
The influence of latitude on animal shape and form has two oddly contrasting
effects. One known as Bergman's rule predicts that, within related groups,
animals living nearer the poles will be larger. The other, Allen's rule, states
that,
again in related groups, those living nearer the poles will have smaller extremities.
Both effects are heat-conservation measures designed on the one hand to preserve
body temperature and on the other to prevent frostbite.
Genetic changes may be minor and quite imperceptible or they may result in
changes that alter the species dramatically. The land snail, Cepaea nemoralis,
lives
in a variety of habitats in the temperate woodland and can have any of several
different shell markings. Where the ground is open and grassy a plain yellow
coloration disguises the snail best and snails with other markings are easily
seen by predators and quickly devoured. Where the ground is covered by leaf
litter, brown striped forms are better camouflaged and other forms are selected
against.
This gives rise to populations of predominantly yellow snails in open grassy
areas and brown striped snails in woodland. A similar effect was observed in
the peppered moth, Biston betularia, during the early days of man's industrial
revolution. Up until then the species had consisted largely of grey and white
speckled individuals which were perfectly camouflaged against the lichen-covered
tree trunks where they lived. A black form also found in the population was
easily seen and eaten by birds and was therefore uncommon. With the arrival
of heavy
industry the trees became caked with soot and turned black, affording a perfect
camouflage background for the black form. The white form was then selected
against by predators and the moth population became predominantly black. Later,
with
the coming of clean air laws, the atmosphere and the tree trunks became less
soot-laden and the moth population swung back to give a bias towards white
and grey individuals once more. These changes involved only varieties within
the
same breeding population and there was at all times a constant exchange of
genetic material as they took place. If, however, the environmental changes
had been
permanent and the
different varieties had become isolated from one another, they would have in
time become different species.
 Changes in the environment due
to the industrial revolution gave the black mutant forms (B and C)
of the peppered moth, Biston betularia, an advantage
in urban areas, where they largely replaced the previously predominant
grey and white speckled form (A). Because of the low level of atmospheric
pollution, the population in rural areas was for the most part unaffected. |
SOUTH AMERICA |
AFRICA |
AUSTRALIA |
|
HEAVY BROWSERS |
 |
 |
 |
Astrapotherium |
Rhinoceros |
Diprotodon |
|
| GRAZERS |  |
 |
 |
Thoatherium |
Zebra |
Kangaroo |
|
| CARNIVORES |  |
 |
 |
Prothylacynus |
Hunting dog |
Tasmanian wolf |
|
| SMALL OMNIVORES |  |
 |
 |
Neoreomys |
Dormouse |
Bandicoot |
|
| ANT EATERS |  |
 |
 |
Stegotherium |
Aardwark |
Numbat |
|
| BURROWING INSECT EATERS |  |
 |
 |
Necrolestes |
Golden mole |
Marsupial mole |
|
| FLIGHTLESS BIRDS |  |
 |
 |
Phororacos |
Ostrich |
Emu |
In looking at the life on the grasslands of Africa and Australia around the time of man and comparing it with the life that existed on the plains of South America some time earlier, during the middle Tertiary, we can see that animals with similar life styles appear to evolve similar shapes and sizes in corresponding environments. It makes no difference whether these environments are separated by time, space or both, they are by far the most important single evolutionary factor governing the shape and form of living creatures. Large herbivorous animals, very similar in appearance to the rhinoceros, and long-legged, swift-running grazing animals appeared in all three environments. Carnivores, insectivores and omnivores all superficially similar to one another evolved. The most strikingly similar groups were the burrowing insect-eaters and the flightless birds, which because of their highly specialized modes of life developed along broadly the same lines.
Mimicry is a separate imitative phenomenon in which a creature,
usually for reasons of defence, takes on the physical appearance of another
animal or of
a plant
or indeed of a totally inanimate object like a bird dropping. In the case of
animals mimicking other animals there are two important forms. The first, known
as Mullerian mimicry, occurs when a number of dangerous or unpalatable species
evolve the same coloration or patterning to gain protection by association.
Animals exhibiting this form tend to have vivid colours which make them stand
out against
the background and act as a warning, The second form, Batesian mimicry, involves
totally harmless creatures adopting the coloration or appearance of inedible
or dangerous species in order to take advantage of their warning coloration
and so escape predation. Other forms of mimicry exist that enable predators
to approach
prey which they themselves mimic. The insects and in particular the butterflies
with their striking wing patterns are the masters of mimicry, but it is also
found among the vertebrates and among the plants.
As we have seen the rate of evolution is largely dependent on the rate of change
of the environment rather than on any trait possessed by the animal itself.
Even so it seems that the higher a creature is situated on the evolutionary
ladder
the more rapidly it evolves. For example, bivalve genera exist for, on average,
about 80 million years, fish genera for 30 million years and ungulate and carnivore
genera for six to eight million years. The shorter the life-span of a genus
the more quickly another evolves to take its place. This results in a larger
turnover
of genera in land-based habitats, where life on the whole is more highly evolved
than in the sea.
Plants tend to evolve much more slowly than animals and the flora existing
during the Age of Man consisted mainly of plants that evolved at the beginning
of the Cretaceous period while the dinosaurs were still the dominant form of
land animal.
The food chain is a fundamental concept in ecology and is
the sequence in which organisms eat one another. It is more appropri¬ate to
regard the process as
a pyramid rather than a chain, since in any environment there are many more
animals
lying at the lower ends of the chains than at the top.
The base members of the pyramid are the plants, the primary producers, which
use the sun's energy to synthesize raw food from carbon dioxide in the air
and minerals in the soil. From the myriad members of this broad base all food
chains
weave upwards towards the carnivorous animals at the apex. For example, in
the far north, during the Age of Man, the plants that grew in the brief summers
were
fed upon by insects, which were eaten by small birds, which were in turn eaten
by small carnivores such as foxes, which were ultimately eaten by large carnivores
such as polar bears. Similarly the microscopic plant plankton existing in the
sea at that time lay at
the base of a food chain which extended upwards through fish and seals and
again to the polar bear. Nothing hunted live polar bear, although once dead,
scavengers
and micro-organisms from lower down the food chain fed on the carcase, reducing
it ultimately to the inorganic substances on which the plants at the base of
the pyramid feed. Except in the world of parasites, where the number of organisms
supported at each stage increases rather than decreases, food pyramids like
these can be constructed for every type of habitat on earth, with in each case
a single
predator or small group of predators lying at the top.
At the top of every food pyramid sit the carnivores, the last link in
an energy transfer chain that begins with the plants - the initial food
synthesizers. This food in the form of leaves and fruit is passed on to
the herbivores, lying higher up the pyramid, and ultimately through them
to the carnivores. Similar pyramids exist throughout the world in all environments
from the tropics to the poles. |
The general layers in the pyramids are the primary producers already mentioned, the herbivores and the carnivores. Throughout the pyramid both scavengers and microscopic decomposers operate. If one of the key members of a food pyramid layer were removed by disease or environmental change the structure would become unstable. The species lying below the vacant slot would increase unchecked to a point where they outran their food supply and their numbers would be controlled by starvation. In reality this seldom happens and another predator soon appears capable of filling the unoccupied niche.
In a field of clover (A), it is interesting to speculate what might happen if one tier of the food pyramid were removed. If the voles were largely wiped out by disease (B), the owls would be deprived of prey and soon leave (C), causing the insect population to expand uncontrollably (D). This situation is unlikely to last and the vacant niche would rapidly become reoccupied in one of three ways; a totally new insect-eating creature such as a bird would arrive, bringing with it its own predator (E), or another species of vole would invade, bringing back the owls (F), of a remnant of the original vole population, resistant to the disease, would reinstate itself (G). |
As a rough rule of thumb a predator requiring one unit of energy for subsistence needs to take in ten equivalent units of energy from the herbivores on which it preys. Similarly each herbivore needs to receive ten units from the vegetation. The vegetation's energy is derived solely from the sun, and again, of ten units absorbed by a plant, no more than one unit is used effectively. |
Plants can only use so much of the energy they absorb from
the sun. It is difficult to measure, but certainly no more than an eighth of
one per cent of the sunlight
falling on a plant can be stored by being converted into sugar. The chemical
energy in the sugar is used by the plant to build up the complicated organic
compounds that go to form its structure. It is this sugar, and the energy contained
in it, that a herbivorous animal obtains when it eats grass. However, it cannot
convert all the plant's stored energy into its own requirements - the maximum
efficiency of an animal is about ten per cent. This ten per cent factor is
present at all stages of the food chain and means that in any environment a
hundred herbivores
can support only ten carnivores, and these ten carnivores can in turn support
only one "second-stage" carnivore. These figures are oversimplified
and refer to animals of the same size. The important factor is the weight of
the animals rather than the numbers of individuals. The ten per cent factor
holds true for every stage in the complicated pattern of food chains and is
an important
factor leading to the stable shape of the food pyramid.
The dependency of feeding efficiency on sunlight is the reason why different
parts of the earth support quite different numbers of organisms. In the tropics,
where the sunlight is intense, much more solar energy is available to be absorbed
by plants. Hence, where other factors such as rainfall allow, there is more
vegetation per unit area than in temperate or polar climates. This large amount
of plant
material is able to support a large number of herbivorous animals, which in
turn support a large number of carnivores. In the Arctic, on the other hand,
the low
level of solar energy produces a much more sparse vegetation, and hence there
are fewer herbivores and even fewer predators.
The variety of species at each level in the pyramid depends on the variety
of plants at the base. On tropical grassland, for example, where there are
short
grasses, herbs, tall grasses, bushes and trees, each of the large number of
indigenous animal species eats a different collection of plants. Therefore
the animal that
eats roots does not compete with the animal that eats the low herbs or the
animal that eats the tall grasses. Even those that do have a broadly similar
diet are
sufficiently different in some way so as not to compete directly - for example,
one may eat in the daytime and the other at night. In this way the ecological
niches are multiplied and the processes of evolution ensure they are all filled.
The principle that “nature abhors a vacuum” is as true in biology as it is
in physics. An ecological niche is never left vacant for long and something
will
evolve to occupy it as soon as one appears. Within each species, however, competition
is strong and each particular niche will support only so many individuals.
Struggles between members of the same species are usually formalized into stylized
displays
in which little real damage is done. Territory is preserved and mates are chosen
without recourse to any actual
combat. This appears to be the strategy that leads to greatest success in maintaining
a creature's position in the ecosystem.
The predations of carnivores do little to upset the balance of the food pyramid.
By preying only upon the weak, sick and elderly - a practice forced upon them
by the fact that a healthy adult can usually outrun or fight off an attack
- it ensures that only the fittest survive. If the fit, healthy adults of a
species
cannot outrun or fight off an assault their species will swiftly become extinct
and the niche will be taken by another creature. In this respect predators
can be thought of as no more than impatient scavengers.
 |
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 |
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|
THE PANOPLY OF EVOLUTION The chart shows a selection of animals typical of particular periods in the earth's geological past. The periods are dated in millions of years before the Age of Man.
|
||||||
At
the time of man, a chain of sub-species, or cline, existed around
the North Pole with the British lesser black-backed gull, Larus
fuscus graellsii, and the British herring gull, Larus argentatus
argentatus, as end members. All neighbourring species of the cline
could interbreed with one another excepting the end members, which,
by the time the chain ivas complete, were too distantly related
to mate with one another successfully.
