Elsevier Encyclopedia of Geology

Подождите немного. Документ загружается.

written record in order to separate historical trends.

Improvements in dating methods, and additional

information from geological, archaeological, bio-

logical, historical and other sources will help to

develop scenarios which might help the recognition

and response to future challenges.

Further Reading

Adams, J Europe during the last 150 000 years [online at

http://www.esd.ornl.gov/projects/qen/nercEurope.html]

Andersen BG and Borns HW (1994) The ice age world: an

introduction to Quaternary history and research with

emphasis on North America and Europe during the last

2.5 million years. Oslo: Scandinavian University Press.

Bjo

rck S (1995) A review of the history of the Baltic Sea,

13.0–8.0 ka BP. Quaternary International 27: 19–40.

Cunlifffe B (ed.) (1994) The Oxford Illustrated Prehistory

of Europe. Oxford – New York: Oxford University Press.

Donner J (1995) The Quaternary history of Scandinavia.

Cambridge: Cambridge University Press.

Emeis K-C and Dawson AG (2003) Holocene palaeoclimate

records over Europe and the North Atlantic: modelling

and field studies. The Holocene 13: 305–464.

Grove JM (1988) The Little Ice Age. London, New York:

Routledge.

Harff J, Frischbutter A, Lampe R, and Meyer M (2001)

Sea-level change in the Baltic Sea: interrelation of climatic

and geological processes. In: Gerhard LC, Harrison WE,

and Hanson BM (eds.) Geological perspectives of global

climate change. Tulsa, Oklahoma, American Association

of Petroleum Geologists in collaboration with the Kansas

Geological Survey and the AAPG Division of Environ-

mental Geosciences: 231–250.

Litt T, et al. (2003) Environmental response to climate and

human impact in central Europe during the last 15 000

years – a German contribution to PAGES-PEPIII. Quater-

nary Science Reviews 22: 1–124.

Pirazzoli PA (1991) World atlas of Holocene sea-level

changes. Elsevier Oceanography Series 58, Amsterdam,

London, New York, Tokyo: Elsevier Science Publishers

B.V.

Roberts N (1998) The Holocene – An environmental his-

tory. Oxford: Blackwell Publishers Ltd.

Scho

nwiese C (1995) Klimaa¨nderungen – Daten, Analysen,

Prognosen. Berlin: Springer Verlag.

a0005 EVOLUTION

S Rigby, University of Edinburgh, Edinburgh, UK

E M Harper, University of Cambridge, Cambridge, UK

Introduction

The theory of evolution by natural selection, put for-

ward by Darwin in 1859 (see Famous Geologists:

Darwin), is the greatest unifying theory of biology

and palaeontology. In this context, evolution is the

change that occurs between successive populations

of organisms, due to their modification in response

to selection pressures. The potential to change is pro-

vided by genetic variability within populations and by

genetic change through time (mutation). The pressure

for change to occur exists outside an organism and is

provided by interactions within the environment.

These interactions may be predominantly physical or

biological effects. Small-scale changes in populations,

giving rise to new species, are defined as microevolu-

tion. Larger-scale changes, such as the origin of new

higher taxa – which may have new body plans or new

organs – are defined as macroevolution. The study of

evolution also includes the study of patterns of diver-

sification and extinction. Macroevolution may be the

end result of microevolution working over a long

time-scale or it may be a suite of emergent properties

that require unique interpretations.

Historical Background

The presence of large numbers of species on the Earth

and the means by which they appeared were discussed

throughout the Enlightenment, though the use of the

word evolution did not become common until the

twentieth century. The possibility that one species

might change into another was of interest to Charles

Darwin’s grandfather, Erasmus Darwin, for example.

Studies that focused on the ways in which species

transform were begun by Jean-Baptiste Lamarck

and published in his Philosophie Zoologique in 1809.

Lamarck argued that an ‘internal force’ caused

offspring to differ slightly from their parents and

also that acquired characters could be passed on to

the next generation. One of his examples was that of

giraffes, whose long necks were assumed to be a

160 EVOLUTION

product of successive generations reaching for higher

and higher leaves. He suggested that each giraffe

lengthened its neck slightly by this activity and, in

turn, passed on to its descendents the capacity to

grow longer necks. He visualized species as forming

a chain of being, from simplest to most complicated,

with each species being capable of transforming

into the next in line, and all existing indefinitely. In

Britain this work was disseminated by both Richard

Owen, who was generally supportive of the theory,

and Charles Lyell (see Famous Geologists: Lyell), who

was critical of it.

Charles Darwin encountered work by both of these

scholars and also explored huge tracts of the natural

world during his 5 years study on the Beagle

(1831–1836). His work on a number of organisms,

notably finches collected from the Galapagos Islands

in the Pacific, persuaded him that organisms were

adapted to their particular niche and that species

were capable of change. The process by which this

change could occur was a preoccupation of Darwin’s

in the succeeding years. As early as 1838, he had read

the seminal work of Malthus on populations, but he

was still working on the scope and implications of his

theory when he was forced to publish by correspond-

ence from Alfred Russel Wallace. A joint paper pre-

sented to the Linnaean Society in 1858 was followed

the next year by his classic work On the Origin of

Species.

Darwin’s theory of species originating through nat-

ural selection can be set out in a small number of

propositions. First, organisms produce more offspring

than are able to survive and reproduce. Second, suc-

cessful organisms – those that survive long enough to

breed themselves – are usually those that are best

adapted to the environment in which they live. Third,

the characters of these parents appear in their off-

spring. Fourth, the repetition of this process over a

long time-scale and many generations will produce

new species from older ones.

The consequences of this theory are enormous. Not

least, they caused scientists at the time to reconsider

their assumption of a chain of life. Evolution by nat-

ural selection is a response to the local environment

and is not predetermined on a grand scale. Organisms

do not necessarily evolve into more complicated

species over time. Amongst the general public, the

theory was seen as being in conflict with a literal

reading of the Bible, a view that persists amongst a

religiously conservative minority.

In the years after publication, the most significant

weakness of Darwin’s theory was perceived to be its

failure to supply a plausible mechanism for the inher-

itance of characters. However, this mechanism

was supplied when Gregor Mendel’s (1865) work on

heredity was rediscovered in the early twentieth cen-

tury. Mendel observed that characters were passed

from parent to child in a predictable fashion

depending on the relative dominance of the traits

carried by each sexual partner. Characters did not

‘blend’ in the offspring, which is what Darwin had

suggested and which astute critics had pointed out

would actually have prevented evolution from occur-

ring. These observations opened the door to the

modern study of genetics. After some decades of

debate, a modern consensus was reached in the

1940s, which is the basis for our current understand-

ing of Darwin’s ideas.

Evolution and Genetics: The

Living Record

Evolution is possible because the genetic transmission

of information from parent to offspring works as it

does, in a Mendelian fashion. Subsequent work on

genetics has elucidated the exact means by which this

occurs and has shown how variation can be developed

and sustained in a population.

The information that can be passed from one gen-

eration to the next in a population is contained on

strands of DNA (deoxyribonucleic acid), or occasion-

ally RNA (ribonucleic acid), within each cell. A DNA

molecule forms from a series of nucleotides, which

are joined up like beads on a string. Each nucleotide

has, as one of its elements, a base. The four types of

base DNA are adenine, thymine, guanine, and cyto-

sine (usually abbreviated to A, T, G, and C). Two

strings of nucleotides join via base pairs to make the

double-helix shape of DNA. A always joins to T, and

C always joins to G. Sequences of bases are the code

that stores the information needed to produce an

organism. This includes information about making

the various parts of the cell or set of cells and also

information about the rates at which different pro-

cesses should occur and their relative timings. Each

piece of information that the DNA holds is called a

gene. Genes can be sequences of DNA or can be little

pieces of DNA separated by other sets of bases. Most

of the DNA appears to have no purpose and is called

non-coding DNA. A human is produced from about

30 000 genes that use about 5% of the nucleotides of

our DNA (Figure 1).

When sexual reproduction occurs, one copy of the

DNA (carried on chromosomes) of each parent is

passed to the children. The offspring therefore have

two sets of instructions within their DNA. The pair of

genes that share a common function are called alleles,

and the combination of alleles controls the effect on

the bearer. However, this effect will not be passed to

EVOLUTION 161

Figure 1 A diagrammatic representation of DNA, genes, and chromosomes. (A) The molecular structure of a double strand of DNA. Each strand is made up of a chain of sugars (yellow)

and phosphates (purple), linked together by a set of four bases: thymine (orange), adenine (green), guanine (blue), and cytosine (red). The shapes of these bases cause adenine to bond to

thymine and guanine to bond to cytosine. This makes each strand of DNA a mirror-image of the other. (B) A piece of DNA carries information in the form of sets of bases, in this example

GGTCTGAAC. (C) A gene is a set of useful bits of DNA, which code for a particular protein or carry out a particular instruction. Genes may be formed in pieces separated by long intervals.

(D) A chromosome is a folded up cluster of DNA found in the nuclei of eukaryotic cells.

162 EVOLUTION

the next generation, but rather one of the alleles will.

This will then combine with another allele to generate

another product. Although the results of allele com-

bination can have a complicated range of expressions

in a cell or a body, the alleles don’t mix, so variation is

maintained.

A wide range of errors can occur when the DNA

strand is replicated during reproduction. These can

affect the non-coding DNA or the genes and can

produce mutations of varying effect depending on

whether it is genes that control the production of the

body or the timing or duration of elements of this

production process that are affected.

Time and Narrative: The Fossil Record

Biologists have explored theories of evolution in

tandem with palaeontologists, who can retrieve nar-

ratives of evolutionary change from the fossil record.

The ability to study change over millions of years is a

great advantage of using fossils. Theoretically, it

should be possible to study aspects of the morpholo-

gies of fossils collected bed-by-bed throughout a rock

sequence in order to elucidate patterns of evolution.

However, the preservation of individual fossils is

often poor; most depositional events produce signifi-

cant time-averaging, and the fidelity of long records

of sedimentary sequences is often questionable. At

some scale, all deposition is intermittent, and this

means that there are gaps of some scale in all narra-

tives retrieved from the fossil record. Fossils preserved

in lakes or deep-sea cores may be less affected by this

problem than fossils from more dynamic environ-

ments, and research has generally concentrated on

these locations.

Microevolution

The set of potentially interbreeding members of a

population forms a species, which contains a range of

variation in its appearance (the phenotype) and in its

genetic codes (the genotype). In practice, most living

species are defined on the basis of phenotypic charac-

teristics rather than genetic information or reproduct-

ive potential. In fossil studies of evolution, only the

phenotypes are available, and the definition of a

species must be based on clusters of phenotypic char-

acters, which are taken as proxies for the potential to

interbreed.

Natural selection acts on a set of individuals, so

that the physical characteristics of the group and the

underlying genotypes change over time. This process

eventually gives rise to new species and is known

as microevolution. Biologists class only gene shifts

within populations as microevolution and define

anything larger, including the appearance of new

species, as macroevolution. To palaeontologists, the

distinction is usually between speciation and anything

higher, such as the emergence of new genera or of new

organs.

Sometimes a species gradually changes through time

until the point comes where the fossil representatives

of successive populations are recognized as a different

species. However, a parent species often splits into

more than one offspring species or evolves into an

offspring species that coexists with the parent species

for some time. In this case, the original population

must split into two or more subsets that cannot inter-

breed with one another. The two best-known methods

of achieving this are called allopatric speciation and

sympatric speciation.

In allopatric-speciation events a single original

population is split into two geographically isolated

elements. This is a common phenomenon over geo-

logical time as continents fragment, mountains rise,

or sea-levels change. Each geographically separated

fragment of the initial population contains only a

fraction of the original genetic variation, so it may

tend towards difference from the original population

without any active selection, although this is now

regarded as a minor component in the formation of

new species. More importantly, different geographical

regions will tend to produce different environmental

stresses from those that were experienced before sep-

aration, leading to the selection of different successful

characters in the separated populations. This eventu-

ally leads to significant changes of form in the isolated

populations, which may finally produce new species.

An example of allopatric speciation has been re-

covered from the fossil record of Plio-Pleistocene

(3–0.4 Ma) radiolarians, which are siliceous plank-

tonic protists, collected in the North Pacific.

A divergence in the forms of two sister species of the

genus Eucyrtidium was found to have occurred at

around 1.9 Ma, following a short period when the

populations had been separated from one another.

During sympatric speciation the emerging species

share a geographical range but may become separated

over time by differences in behaviour or in resource

exploitation. Adaptive pressures act differently on

these populations, and different characteristics will

be favoured, leading to a progressive change of form

and eventually to reproductive isolation. At this point

a new species will have appeared. There is some

doubt about the mechanism by which species first

begin to diverge without becoming geographically

isolated, although the generation of new species

in this way has been demonstrated for a number of

types of animal. Studies on cichlid fishes in African

lakes show that the most closely related species of fish

EVOLUTION 163

often live in the same lake, rather than in adjacent

lakes, as might be expected if allopatric speciation

had occurred (Figure 2). Although it seems intuitively

obvious that populations that become physically

dissimilar will eventually be unable to produce

offspring, the genetic basis for this change can be

demonstrated in the laboratory but not yet fully

explained.

The fossil record can be used as a tool to help in the

understanding of evolution and the formation of new

species. It may be that evolution progresses gradually

for most of the time, an idea known as phyletic grad-

ualism. The classic fossil example of this slow con-

tinuous process of morphological change is the study

by Peter Sheldon of Ordovician trilobites recovered

from deep-water shales in central Wales. Eight differ-

ent genera of trilobite, including well-known forms

such as Ogygiocarella, were found to exhibit incre-

mental changes in rib number through the duration of

one graptolite zone, which probably represents sig-

nificantly less than 1 Ma. Gradual change is generally

difficult to observe in the imperfect fossil record. It

could be argued that in a less continuous sedimentary

record (or one sampled less finely) this sequence of

events would appear as a series of abrupt changes.

Commonly, what is preserved is a long period where

little or no change is observed followed by the abrupt

appearance of a new form.

The theory of punctuated equilibrium attempts to

explain this phenomenon not as the product of an

imperfect fossil record but as a common pattern of

evolutionary change. This is done by applying the

concept of allopatric speciation to the problem.

Eldridge and Gould, who developed the idea of punc-

tuated equilibrium, argue that most species probably

arise in small, geographically isolated areas and that

they arise rapidly as they encounter new selection

pressures. At some later time the evolved offspring

species may move back into areas where it encounters

its parent species and may out-compete this form. In

most areas where this happens, the geological record

will show one species – the parent – abruptly replaced

by another – the offspring – with no intermediate

steps. The chance of the isolated population being

represented in the fossil record during the short

period of its evolution into a new species is very slim

(Figure 3). It may be that Williamson, in a study of

molluscs in Plio-Pleistocene sediments from Lake Tur-

kana, found one such rare fossil example of punctu-

ated equilibrium. Species of gastropod and bivalve

both appeared to remain static in shape for long

periods of time, punctuated by brief periods when

their shape changed abruptly.

Although some studies seem to show a punctuated-

equilibrium style of evolution, others appear to show

that evolution has progressed via phyletic gradualism,

and a consensus has yet to emerge regarding

these theories. In practice, most evolution is probably

the result of a mixture of punctuated and gradual

periods of change, partly depending on the scale of

Figure 2 The difference between allopatric speciation and

sympatric speciation, using the example of fishes living in

lakes. (A) Sympatric speciation occurs due to changes in behav-

iour or mode of life, in this case by a partitioning of the original

population into limnetic and benthic groups. Here, descendent

species are most closely related to species living in the same

lake. (B) Allopatric speciation occurs following geographical sep-

aration of the populations, in this case caused by a fall in lake

level. Descendent species are most closely related to fishes

living in adjacent lakes.

Figure 3 The differences between phyletic-gradualism and

punctuated-equilibrium models of speciation. (A) In phyletic

gradualism the shape change is gradual and populations are

seen to move across morphospace continuously. Periods of spe-

ciation are relatively long and can be recorded in the fossil

record. (B) In punctuated equilibrium the shape change is inter-

mittant, rapid, and related to geographical separation of a part of

the population. For most of the time the form of the population

is static. In most areas no speciation event is seen, and the

fossil record shows abrupt changes of morphology with no

intermediate stages.

164 EVOLUTION

observation. Work by Johnson on Jurassic oysters

(Gryphaea) from across western Europe provides a

good example of this aggregate pattern. Change over

approximately 6 Ma was generally slow, but rapid

periods of change in isolated populations were also

observed. One unfortunate result has been the sugges-

tion that punctuated equilibrium is antithetical to

Darwinian evolution. In this usage it is not, as even

the rapid bursts of evolution implied by the theory

would take place via a series of gradual (i.e. small-

scale) changes in the form of the organism concerned.

Macroevolution

Macroevolution is the study of all evolutionary events

or effects larger than the appearance of a new species.

This includes studies of long-term change in the geo-

logical record and of the emergence of new higher

taxa, for example new phyla. Linked to both of these

topics is the difficult issue of how significant new

structures or organs can evolve. Palaeontology is cen-

tral to this study, as it provides a measure of time and

can identify the most likely dates of appearance of

new characters or taxa.

The single biggest and most important argument

about macroevolution is whether it is a scaled-up

version of microevolution or something different. If

it is different, then those differences may be a reflec-

tion of the emergent properties of this complicated

system and hence still reliant on microevolutionary

processes occurring. More controversially, it has been

argued that macroevolution includes rapid and large-

scale changes of form that necessitate steps that might

initially produce organisms that are less successful

than their ancestors. This is completely counter to

Darwinian ideas of evolution. An example of these

issues can be presented via a consideration of the

evolution of major groups of tetrapods.

All living vertebrates with pentadactyl limbs (that is

mammals, reptiles, amphibians, and birds) evolved

from an ancestral fish, with the process beginning in

freshwater lakes and rivers in the Devonian (Figure 4).

Since then a wide variety of adaptations have

appeared in these higher groups, such as feathers,

fur, and wings. It can be convincingly demonstrated

that some lineages, or evolving lines, acquired these

characters gradually, by microevolutionary processes.

The classic example of this is the origin of mammals

from reptile ancestors through the Triassic and Early

Jurassic. Character change occurred at a relatively

constant rate throughout this 100 Ma period, and

intermediate forms are well known in the fossil

record. However, the process by which these new

characters appeared may have controls that are not

seen in microevolution and which are hinted at by

the suggestion that, during this event, significant

evolution tended to occur in small-carnivore groups.

More controversially it has been argued that some

new characters, for example wings, could not have

evolved by gradual steps as they would have been

useless in their early stages of development. Compli-

cated counter arguments that invoke the possible uses

of wings without flight, for example, have not cleared

up the controversy.

Looking at the history of life, it is clear that there

have been periods of major increases in diversity and

periods of major innovation. Significant increases

in diversity tend to happen after mass extinctions

and are called evolutionary radiations (see Biological

Radiations and Speciation). Empty niches created by

the extinction event are quickly filled as organisms

radiate to form new species that are able to exploit

the available resources. Evolutionary radiations may

Figure 4 A simplified evolutionary tree for tetrapods, those

vertebrates with pentadactyl limbs. This group of organisms

evolved from lobe-finned fishes during the Devonian. Whilst the

evolution of mammals from cynodonts was a gradual process in

which macroevolution appears to conform to microevolutionary

expectations, the origin of wings in pterosaurs and birds is more

difficult to explain without evoking some new process unique to

evolution at this scale.

EVOLUTION 165

also be facilitated by the appearance of major innov-

ations, such as the evolution of hard parts by a variety

of different taxa close to the Precambrian–Cambrian

boundary. The two are not necessarily coupled. For

example, eukaryotes, the complicated internally

divided cells with which we are most familiar, evolved

more than 2 Ga ago (possibly much more), but they

did not become widespread or common until much

later at around 1 Ga ago. They could not radiate until

there was adequate oxygen present in the Earth’s

atmosphere and oceans, as they depended on this

molecule for respiration.

It seems likely that evolutionary selection can work

at the species level as well as at the level of an individ-

ual within a population. Species-level selection

favours species that have lower extinction rates and

higher origination rates than their ‘competitors’.In

the long-term, these species become more abundant

at the expense of their less-successful competitors.

Distinguishing between the levels of selection is ex-

tremely difficult in practice, but the theory helps to

demonstrate the emergent properties of species.

Some types of extinction, or reductions in diversity,

may also be explained by macroevolution and, in

turn, throw light on the mechanisms of evolution. It

is clear that the evolution of a new species will in-

crease competition for resources and may force an-

other species to become extinct if it is unable to

compete successfully. The pattern of species extinc-

tion would be expected to be one of increasing chance

of extinction with species age, but in some cases this

does not seem to be so. Instead, species age does not

appear to correlate with the likelihood of extinction.

Van Valen has used this observation (which is itself

somewhat contentious) to suggest a novel hypothesis

for macroevolutionary patterns. He suggested that

competition for resources produces a dynamic equi-

librium between species, in which each will continue

to evolve in order to survive. This is the core of the

Red Queen hypothesis, which suggests that organ-

isms evolve to keep their biological place or, to para-

phrase the quotation from Alice in Alice Through the

Looking Glass, they ‘run to keep still’.

The characters that help organisms to survive at

times of low extinction rate may be different from

those that make survival of mass extinctions more

likely. In other words, the criteria by which species

are selected may vary with extinction rate. Specialist

species tend to have greater survival potential at times

when extinction rates are low and reduced survival

potential when extinction rates are high. In addition,

it has been suggested that small species have a greater

chance of surviving mass extinctions than larger

species, though the overall trend in evolution is

clearly not towards smaller species.

The level of understanding of genetics is now so

great that it is possible to explore macroevolution in

this way. In traditional views of macroevolution, a set

of ways in which different forms could be produced

with small changes in the genome was known as

heterochrony. The idea was that different parts of

the body grew at different rates. In some examples,

this might be a difference in the rate at which sexual

maturity was reached relative to the rate at which the

rest of the organism (the somatic portion) developed.

If sexual reproduction became possible at an earlier

stage in body development, this was known as pedo-

morphosis. The classic living example of this is the

axolotl. This resembles a juvenile salamander, com-

plete with external gills, but reproduces at this stage

of development. If it is injected with extract from the

thyroid gland, an axolotl will develop into an adult

salamander. A genetic view of this kind of evolution is

that there has been a change in the regulatory genes

that switch on and off the protein-coding gene se-

quences within cells. If these genes start to operate

at new rates, then the phenotype will change shape, in

some cases dramatically.

It is now known that some genes, especially a group

known as Hox genes, control development by in-

structing the different parts of the growing embryo

on which part of the body should be built. It is known

that these genes are more common in vertebrates than

in other groups of animals and that there was a single

period when these genes duplicated (or rather, dupli-

cated twice), so that vertebrates carry four times as

many of these genes as do invertebrates. This multi-

plication occurred between the evolution of the

cephalochordates and proper vertebrates, probably

during the Cambrian period. It is tempting to assume

that this evolutionary event facilitated the increase in

complexity needed to produce vertebrates and may

have made them more ‘evolvable’ since. Whether or

not cause and effect can be proved in this example, it

points to a growing understanding of the relationship

between genes and macroevolution.

Elsevier Encyclopedia of Geology - vol I A-E

Подождите немного. Документ загружается.