Download Non-random reproduction

Non-random reproduction ......................................
Natural selection ............................................
Sexual selection .............................................
Natural selection: random or non-random? .....................
Non-random selection .........................................
Non-random fecundity .........................................
Differential reproduction ....................................
Survival to reproduce ........................................
1
1
1
1
1
1
1
1
Environmental pressures ......................................
Biotic interactions ..........................................
The nature of natural selection ..............................
Gene as the selection unit ...................................
Group selection ..............................................
Species selection ............................................
Forms of selection ...........................................
Stabilization selection ......................................
2
2
2
2
2
2
2
2
Directional selection ........................................
Disruptive selection .........................................
Polymorphism .................................................
The nature of adaptation .....................................
Structural/anatomical adaptation .............................
Physiological adaptations ....................................
Behavioural adaptations ......................................
Adaptability .................................................
Lemarckism ...................................................
Neutral/non-adaptive characteristics .........................
Adaptive colouration .........................................
Cryptic colouration ..........................................
3
3
3
3
3
3
3
3
3
3
3
3
Warning coloration ...........................................
Aposematic coloration ........................................
Mimicry ......................................................
Batesian mimicry .............................................
Müllerian mimicry ............................................
Non-genetic variation ........................................
Interbreeding and viable young ...............................
Bergmann's rule ..............................................
Gloger's rule ................................................
Allen's rule .................................................
Gradual speciation ...........................................
Punctuated species ...........................................
Instant species ..............................................
4
4
4
4
4
4
4
4
4
4
4
4
4
Isolating mechanisms .........................................
Mechanisms of speciation .....................................
Allopatric speciation ........................................
Sympatric speciation .........................................
Parapatric speciation ........................................
Adaptive radiation ...........................................
Darwin's finches .............................................
Niche ........................................................
Classification of organisms ..................................
Systematics ..................................................
Classification ...............................................
Taxonomy .....................................................
Nomenclature .................................................
5
5
5
5
5
5
5
5
5
5
5
5
5
Interpretation of form and descent ...........................
Homology .....................................................
Homoplasy ....................................................
Analogous structures .........................................
Convergent evolution .........................................
Parallelism ..................................................
Divergence ...................................................
Cladistics ...................................................
6
6
6
6
6
6
6
6
Evolutionary systematics .....................................
The systematic hierarchy .....................................
Time and the planet earth ....................................
How do we age rocks? .........................................
Periods of the earth's history ...............................
7
7
7
7
7
Proterzoic ...................................................
Palaeozoic ...................................................
Mesozoic .....................................................
Cenozoic .....................................................
Cambrian .....................................................
Ordovician ...................................................
Silurian .....................................................
Devonian .....................................................
Carboniferous ................................................
Permian ......................................................
Triassic .....................................................
Jurassic .....................................................
Cretaceous ...................................................
8
8
8
8
8
8
8
8
8
8
8
8
8
Tertiary .....................................................
Palaeocene ...................................................
Eocene .......................................................
Oligocene ....................................................
Miocene ......................................................
Pliocene .....................................................
Pleistocene ..................................................
Recent .......................................................
Fossil record ................................................
Where are fossils found? .....................................
Kinds of fossils .............................................
How are fossils formed? ......................................
9
9
9
9
9
9
9
9
9
9
9
9
Fossils and earth history ................................... 10
Progression of prehistoric life ............................. 10
Fossils and evolution ....................................... 11
Fossils and ancient landscapes .............................. 11
Fossils and palaeogeography ................................. 12
Major evolutionary trends in fossil records ................. 12
11
EVOLUTION
DEFINITION:- The diversity among living things and the changes that
enhances their adaptations.
A population of animals may be considered from three different viewpoints.
1)Its ability to survive and reproduce in its immediate environment.
2)Its ability to change as its environment changes.
3)Its ability to change as it moves into other environments.
Evolutionary mechanisms are largely random that have no direct or causal link
with adaptations.
"They are as likely to impair adaptations as enhance them"
If ordinary natural selection led to an increase (or decrease) in male tail size
the male tail size would increase over generations, however, so would female
preference for longer or shorter tailed males. "Males with longer tails will tend
to be mated with females who will prefer males with longer tails. The offspring
of these mated pairs will not only inherit their father's long tails but also their
mother's preference genes for mating with long-tailed males. These causes
an increase in the frequency of those preference genes. Eventually natural
selection and sexual selection would come into equilibrium.
All the while there is an overall advantage to further tail development, there will
also be an overall advantage in favour of making it more preferable sexually.
This has been termed "RUNAWAY SELECTION".
In the absence of counter-selection the rate of tail increase (in males) and
preference for tail development (in females) should increase exponentially.
NATURAL SELECTION RANDOM OR NON-RANDOM?
NONRANDOM REPRODUCTION
With random selection the following consequences result:
Mechanisms that determine the relative frequencies of genotypes and the
amount of variation between genotypes in a population on which it can act.
NATURAL SELECTION
Is the process in which two separate biological phenomena: differential
survival and differential reproduction cause population allele frequency to
change from generation to generation.
SEXUAL SELECTION
1)A female must be as likely to mate with one male as with another male.
2)Any two gametes must be as likely to produce a viable zygote as any two
others regardless of the genotype.
3)Every individual zygote must have the same chance of developing into a
sexually mature and reproducing organism as any other.
Such conditions do not hold in reality, and therefore natural selection is usually
non-random.
How do traits in species evolve in only one sex e.g. Deer's Antlers or a
Peacock's Tail
NON-RANDOM SELECTION
Charles Darwin Proposed that such traits resulted from one of two selective
processes that favour mating success.
Mating customs within a species and patterns of courtship push powerfully in
non-random directions (e.g. sexual selection with the Peacock's tail).
1)Selection for defence structures (such required for combat between males
competing for females)
Selection will normally work against mutations causing deviations from the
ordinary pattern of courtship.
2)Selection for adornments with which males can attract females.
Patterns of courtship act as a conservative force, with deviations in normal
pattern having little chance of entering into the next generation.
This is termed sexual selection and presents certain problems that are not
completely resolved.
NONRANDOM FECUNDITY OR DIFFERENTIAL REPRODUCTION?
1)The traits that are favoured by females may work against the survival of the
male. Heavy antlers will reduce fitness other than when in combat; the
Irish Elk had antlers than spanned 3 metres across!
FECUNDITY depends on:-
2)A Peacock's tail feathers not only attract females for copulation, since they
will make the animal conspicuous they could also attract a potential
predator. Here we have a trade-off between being conspicuous to
attract a mate and being vulnerable to predation.
The number of gametes produced.
The proportion of gametes that are able to unite to form a viable zygote.
3)Why should a female choose to mate with a male whose endowments
(antlers or a long tail) expose them to the greatest risks and are
therefore least likely to be favoured by natural selection?
In order to overcome this problem (ambiguity) a model has been proposed
which accounts for male characteristics that impair survival being selected for
and maintained in a population being a consequence of FEMALE CHOICE.
The number of viable offspring (those capable of developing normally)
The production of many offspring will influence Natural Selection.
Theoretically the more offspring that are produced the greater the population
growth will be. (this would require that other factors would be constant).
Consequences of this would be that high fecundity species would be favoured.
In reality, a balance between fecundity and survival is maintained.
For example an offspring's chances of survival are low because of small egg
size, the selection may favour high fecundity.
What tends to happen is alternative solutions (strategies) are established.
12
High fecundity is associated with low survival (r strategy) and low fecundity is
associated with high survival (K strategy).
SURVIVAL TO REPRODUCE
In order to pass on your genetic material you need to develop until you are
sexually mature. It is the non-randomness of survival to reproductive age that
accounts for so many adaptations. The really important aspect of natural
selection is that you leave behind part of your genetic material which is
invested in your offspring.
Once you have finished your reproductive life, there is little point to continues
survival, therefore, with the exception of mammals and social animals, survival
beyond your reproductive life is rare.
In social groups (Insect, Humans) non-reproducers may help to raise the next
generation and promote survival of the species. Natural selection places no
value in continuing post reproductive survival.
ENVIRONMENTAL PRESSURES
An individual's ability to reach sexual maturity depends on how competent it is
to withstand the rigours of the physical and biotic environment.
These are therefore determinants of natural selection. Called "biotic
environmental pressures".
For example take tropical ponds- their oxygen levels drop to zero when water
becomes stagnant, warmed and decaying organic matter becomes abundant.
This becomes problematic for a gill-breathing fish, however a fish that can
extract oxygen from air rather than water under these conditions will survive
whereas the other fish will not. The walking cat fish can survive a dried river
bed by breathing air through the mouth and exchange spent air from
underneath the gill cover (operculum).
From this example we can see that environment is a power biological element.
them with other compounds produced by the animal).
Passiflora has "responded" to the break in its defences by Heliconius by
evolving structures on its leaves that mimic the eggs of Heliconius. Heliconius
butterflies are deterred from laying eggs if eggs are already there, since these
eggs will hatch sooner and the caterpillars will consume all the leaf material.
One species Passiflora has even developed hooked hairs that immobilize the
caterpillars!
THE NATURE OF NATURAL SELECTION
DEFINITION OF NATURAL SELECTION: the consistent differential survival
and reproduction of the one group of individuals (particularly their genotypes)
compared with another group of organisms.
SELECTION: occurs when individuals of one genotype survive and reproduce
more successfully on average than those of a different genotype)
(Genotype= the genetic-components of an organism)
For an average superiority to exist and be expressed there must be several
copies of each gene, or several individuals of the same genotype.
Cannot have selection for whole chromosomes, since it can only last for a few
generations before it is broken-up by crossing over during meiosis.
Selection is most effective when it acts on a single gene which are faithfully
replicated and strongly inherited (dominant).
THE GENE IS THE MAJOR UNIT OF SELECTION
GROUP SELECTION: at the population level, this occurs when population of
one group can occur when the population of one kind emerge or die out at
different rates from populations of another kind.
BIOTIC INTERACTIONS
SPECIES SELECTION: species is the unit of selection.
There is the universal scramble to eat and not to be eaten, and this imposes
selective pressures on the members of any community.
An example of a biotic interaction with evolutionary significance and the
relationships herbivores and plants have.
FITNESS: The relationship between the organism's phenotype (= the
expressed characteristics of an organism, namely its form) and its
environment. The same genotype could have a different degree of fitness in
different environments.
It is generally disadvantageous for plants to be eaten, therefore plants evolve
protective devices such as thorns or toxic chemicals these are considered to
be defences.
The more fit an individual is the greater its genetic contribution to subsequent
generations. Clearly an individual can influence the frequency of alleles it
carries in future populations.
Herbivores which wish to eat these defended plants develop (evolve) counterdefences.
By promoting its own offspring we have individual selection.
THIS DEVELOPS INTO AN EVOLUTIONARY ARMS RACE
Passiflora plants (granadilla) possess toxic substances in the leaves that are
cyanide-based.
Such toxic substances are harmful to insects and the therefore they are best
avoided.
However caterpillars of the butterfly Heliconius and Acrea- have overcome
these defences. (generally toxic compounds are neutralized by combining
By promoting the survival of relatives with the same alleles because they have
descended from a common ancestor we have kin selection.
FORMS OF SELECTION
STABILIZING SELECTION: phenotypes in the middle range are favoured.
13
This system works against the extremes.
there is insufficient variation to permit the organism to adapt and therefore the
population will become extinct. Polymorphism is where distinct forms exist.
Example shells of the same species of land snail Cepaea species may have
different colour and coiling direction. Another example is the variation in
colour and patterning of the Asiatic Lady Beetle Harmonia axyridis.
Polymorphism also occurs in humans and includes factors such as blood
groups and skin pigmentation.
THE NATURE OF ADAPTATION
Favours systems that produce normal organisms, despite varying
environmental influences.
Process where members become better suited over generations to survive
and reproduce.
This is how selection acts on most populations most of the time.
STRUCTURAL AND ANATOMICAL ADAPTATIONS
Example After storms many seabirds are killed and washed up on beaches.
Mortality is highest amongst the largest and smallest individuals, but is lowest
among the average-sized birds.
This would include the morphological variation occurring in insect mouth parts,
birds feet and bills. They enhance the efficiency of the organism to function in
special environments. Other structural adaptations include animals shapes
and colours by which animals can conceal themselves such as butterflies
which appear like leaves and preying mantises which resemble flowers.
DIRECTIONAL SELECTION
PHYSIOLOGICAL ADAPTATIONS
Based on biochemical processes such as digestive enzymes which permit
utilization of specific foods (e.g. moths and wool) or a bony fish living in an
alkaline lake excreting urea instead of ammonia.
BEHAVIOURAL ADAPTATIONS
Include maternal behaviour, courtship, and homing.
Occurs when one phenotype e.g. large size, is favoured over another
phenotype. It results in a shift with the mean size changing in the direction of
largeness.
DISRUPTIVE SELECTION
Both extremes of a genotype are favoured
ADAPTABILITY
Organisms usually have an ability to adapt to gradual changes, but will die-out
if exposed to sudden changes. In stable environments there is less ability to
adapt to sudden changes, however, no environment is completely stable,
therefore will we always need to adapt in order to survive.
LEMARCKISM (theory now discounted)
Inheritance of acquired characteristics. Adaptations are genetically
transmitted to an individual's offspring. Example is the Giraffe and neck
stretching.
Theory: with stretching of the neck the Giraffe's neck will get longer.
The theory does not hold since it would require the following:-
By splitting the population into two or more type, disruptive selection has the
potential to produce polymorphic forms.
1)Requires mutation of cells in the body to give more stretch to the neck.
2)Adaptation caused by effort is improbable.
POLYMORPHISM
Variation has an adaptive value. Without variation, natural selection cannot
operate to improve the fitness of the organism.
3Non-adaptive harmful effects could as easily be transferred to the next
generation.
NEUTRAL/NON-ADAPTIVE CHARACTERISTICS
When environmental changes occur the population will be unable to adapt;
14
Example 1 the lotus flower has cyanides to defend itself against insect
herbivory, however, under cold conditions, the plant cell membrane is
disrupted and the toxic cyanide has not effect.
Example 2 insect larvae that produce silken threads which are used as
parachutes to disperse themselves are usually an advantage. However, under
some conditions (e.g. very windy conditions) the advantages outweigh the
disadvantages of a high mortality.
Example 3 the 3 metre horns of the giant elk, which were used in ritual
combat between males, became so large for the body that it lead to its
extinction.
Neutral features may arise as a result of pleiotrophy. This is where a gene
has more than one phenotypic effect. Natural selection may operate to
increase the frequency of a particular gene, while the unrelated effects that
accompany that gene are carried along.
Even though evolutionary changes may be adaptive; they may simply
represent alternative solutions. Example is the Indian Rhinoceros (Rhinoceros
unicornis) which has one horn and the African Back and White Rhinoceroses
(Diceros bicornis and Ceratotherium simun respectively) which has two horns.
for prey by means of a structure near the mouth that works like a lure.
In DEFENSIVE MIMICRY an animal resembles an inedible or dangerous
organism or object. A common feature of defensive mimicry is the eyespots
that may "fool", "startle" or disorient predators.
MÜLLERIAN MIMICRY: different unpalatable species with aposematic
colorations converge on particular colour/pattern combination (often
yellow/black/red).
Example three types of mimicry in two species can be exhibited. The
Monarch butterfly Danaus plexippus and the Viceroy Limenitis archippus.
Most Monarch butterflies become unpalatable from feeding on Milkweed
plants. Some Viceroy butterflies are palatable, but since they resemble the
Monarch we called the mimicry Batesian, however, some Viceroy butterflies
are unpalatable and therefore there resemblance to the Monarch is called
Müllerian mimicry. Monarch butterflies that have not feed on milkweed are
palatable, but since they are identical with Monarchs that have feed on the
Milkweed plants we can say that the palatable Monarchs resemble
unpalatable Monarchs and demonstrate AUTOMIMICRY.
SPECIATION
NATURAL SELECTION MAY NOT INEVITABLY LEAD TO ADAPTATIONS.
Heredity produces a range of reactions or capabilities.
ADAPTIVE COLORATION
The differences that count are genetic
CRYPTIC COLORATION: Example: peppered moths Biston betularia. Like
many insects, the peppered moths of England have genes that produce the
dark pigment melanin. If such darkening is more common in areas where soot
covers the vegetation on which the insects rest during the day, it is called
industrial melanism. Prior to the mid 19th century, industrial melanism was
rare in the peppered moths, and almost all of the population possessed lightgrey wings, that were well camouflaged against lichen-covered tree trunks.
With the industrial revolution , large amounts of coal were burnt, which caused
soot to be deposited on the trunks of the trees making them dark and the first
melanic (dark forms) were reported. With increasing pollution, the proportion
of melanic moths increased to the point where 98% of the moths were the
dark forms. This phenomena was explained by the effects of predatory birds
which removed the light coloured moths, which were far more visible than the
melanic forms against dark, soot-covered tree trunk. With tightening-up of
pollution levels, there is less soot in the air and the tree trunks have regained
their lichen-covered appearance and melanic moths have become more
conspicuous and subjected to higher predation. There is now a reversal of
fortune with light-coloured moth becoming more dominant. This represent one
of the clearest cases of natural selection promoting adaptive coloration.
NON-GENETIC VARIATION
WARNING COLORATION (APOSEMATIC): many insects taste bad, and are
disagreeable to predators. If insects advertise their unpalatability, they can
"teach" the predators to avoid eating them. By using very bright colours and
combinations of colours (e.g. Yellow, Red and Black) an animals such as
many insects and amphibians advertise their distastefulness.
Effects of season (moult into winter fur)
Generational variation, dry season insects vary in colour from wet season
insects.
Social variation - caste system with workers and queens.
Effects of disease, accidents and parasites.
SPECIES groups of similar organism that can breed with each other and
produce fertile offspring.
INTERBREEDING AND VIABLE YOUNG
Often interbreeding of adjacent population leads to a gradual change in
population over changing environmental conditions.
A sequence of poorly separable populations with gradual change from one
area to another is called a CLINE
Such environmental gradients are common in Warm-Cold,
Wet-Dry, North-South
Sometimes groups of different species respond in similar way.
MIMICRY: you can always mimic an unpalatable species, even if you are not
palatable yourself. A species that is palatable but has the coloration of an
unpalatable species is termed a mimic. This animal uses a model that is
unpalatable with which it mimics. This type of mimicry is called BATESIAN
MIMICRY.
We also have AGGRESSIVE MIMICRY where an animal or part of an animal
resembles an object that attracts prey. The Angler fish, for example, "fishes"
BERGMANN'S RULE
Average species: their size is smaller in warmer climates and larger in cold
climates.
Surface to volume ratio and conservation of heat.
15
ALLEN'S RULE
This serves to maintain, the integrity of species.
Protruding body parts, ears, bill, tails shorter/smaller in colder climates.
GLOGER'S RULE
Colours tend to be darker in warmer climates and lighter in cool climates.
SPECIES ARE UNITS OF EVOLUTION
Species are groups of organisms are so similar in structure and heredity that
populations intergrade.
CREATION OF SPECIES
GRADUAL OR PUNCTUATED?
Sudden evolutionary leaps product of mutation.
A new species is a new reproductively isolated population of organisms and a
single mutation rarely if ever leads to such a population. Most workers used to
view speciation as a gradual process (e.g. Darwin).
They argued that big changes occur by the accumulation of many small
changes.
Dynamic changes in genetic material.
Evidence of non-coding intervening sequences of genes (INTRONS), selfish
or junk DNA, (repetitive sequences) and jumping genes suggest a previously
un suspected potential for large abrupt changes in chromosomes architecture
and its function on structural genes.
Prevents contamination by genes of closely related species: especially if the
other species is still evolving from the first species.
Any biological or physical factors that decrease interbreeding between
different species is called an isolating mechanism or barrier. Most species are
protected by one isolating mechanism.
This prevents populations belonging to different species from interbreeding.
Mechanisms that prevent interspecific crosses (prezygotic or premating
mechanisms)
Potential mates do not meet
(Ecological isolation)
Potential mates have different breeding seasons
(Temporal isolation)
Potential mates meet but do not mate
(Behavioral isolation)
Functional or metabolic factors prevent fertilization
(Physiological isolation)
Copulation is attempted but is not secured
(Mechanical isolation)
Mechanisms that impair success of interspecific
crosses (postzygotic or postmating mechanisms)
INSTANT SPECIES
Product of hybridization.
Hybridization that results in polyploidy will give rise to individuals which cannot
interbreed with parental material, but these polyploid species can breed
amongst themselves.
Occurs in plants
Sperm transfer takes place but egg is not fertilized
(Gamete mortality)
Egg is fertilized but zygote fails to survive
(Zygote mortality)
Zygote produces an F1 hybrid, but its viability is decreased (Hybrid unfitness or
breakdown)
Occurs in lizard called the Gila Monster (Heloderma suspectum).
The tetraploid hybrids of these are similar looking, but are reproductively
isolated, they resulted from crossing of two species of Gila monster.
An alternative theory to gradualism is punctuated equilibrium. As a new
species undergoes most of their phenotypic modifications as they first branch
off from the parent species and then change little after that.
The three arguments for this theory are:
1) Fossil record which reveals long periods of little change (stasis)
2) Example are the 21 spp. of snails , with 16 species evolving over a long
period and five species evolved rapid changes after the period of stasis.
An F1 hybrid zygote is produced and is fully viable but it is partly or completely
sterile, or it produces a defective F2 generation.
(Hybrid infertility or sterility)
MECHANISMS OF SPECIATION
Key processes in speciation
Genetic change within the population
Splitting of population by sudden or gradual decreases in interbreeding
between population groups
ALLOPATRIC SPECIATION
Geographically isolated
HOW QUICK IS RAPID?
ISOLATING MECHANISMS
SYMPATRIC SPECIATION
16
These make them easily recognizable in any biological classification.
Subdivided in the same general area: biological isolation.
The linnaean concept is the binomial classification. Each animal is given a
genus and a species name.
PARAPATRIC SPECIATION
Speciation in the same area, isolation created by different ecological niches.
ALLOPATRIC SPECIATION
Most effective form of isolation.
ADAPTIVE RADIATION
Development of many species from a single ancestor. This species increases
to occupy different ecological niches.
A NICHE is defined as having to do with the way an organism makes its living,
including the way of life, habitats, food and food sources.
For example Homo sapiens
genus species ....Note Capital H
The convention is to underline or write in italics, this is because it is a foreign
language, usually Latin or Greek.
example Homo habilis
We can also abbreviate the names. If we use the names Homo habilis and
Homo sapiens we can then use the following abbreviations H. habilis and H.
sapiens respectively. However, if we bring in another species of animal, for
example the striped hyaena we have to refer to it by the full name Hyaena
hyaena, because if we just wrote H. hyaena it would mean Homo hyaena.
Other abbreviations used include Homo sp. which would mean a single animal
species belonging to the genus Homo. Homo spp. refers to more than one
species belonging to the genus Homo.
Each species in an area ordinarily has its own niche.
Adaptive radiation into multiple niches is most vividly demonstrated on islands.
INTERPRETATION FORM AND DESCENT
Begins with the arrival of pioneers, an example are Darwin's finches. A small
ancestral finch arrives on these pacific islands from South America.
HOMOLOGY: correspondence between structures in different organisms
arising from the fact that these structures were inherited from a common
ancestry. For example
Human's arm |
Dog's foreleg | these are homologous
Bird's wing
| structures
Bat's wing
|
This finch has evolved into 14 distinct species. Of the six ground species,
three feed on seed, two on cactus plants and one on seed and cactus plants.
Of the six tree species the majority eat insects. Their speciation is reflected by
beak structure, changes from small and light, small and long to large and
heavy. One species is like as woodpecker and it also uses a cactus spine to
probe for insects in crevices.
Fish's pectoral fin | these are still
Whale's flipper | homologous
Since these species only occur on this island and no where else we can say
that they are endemic.
Homology is based on anatomical or molecular evidence showing degrees of
relationships among organisms.
These 14 species can be roughly grouped into two categories. Six species
are tree dwelling, six species are ground dwelling.
HOMOPLASY: anatomical features in different organisms that resemble each
other but DO NOT share a common ancestry. For example
The other two are "warbler-like" and eat insects.
CLASSIFICATION OF ORGANISMS
Need to systemize knowledge, to communicate and to understand how they
arose and discuss their evolutionary relationships.
SYSTEMATICS: diversity of organisms and their evolutionary relationships.
Determine similarities and differences.
CLASSIFICATION: the ordering of living organisms into groups on the basis of
association (namely similarity).
TAXONOMY: theoretical study of classification including principles,
procedures and rules.
NOMENCLATURE
The application of distinctive names to each group of organisms or taxons.
Legs of insects
Limbs of man
Where structures are not homologous but share the same function =
ANALOGOUS STRUCTURES.
Often structures that are homoplastic are also analogous. However,
structures may be analogous without being homoplastic. For example
Gills of fish | have different structure
Lungs of man | but a similar function
However, in the fish the homolog of the human lung is the swim bladder,
which has the same origin but now serves a different function.
CONVERGENT EVOLUTION
When features of two unrelated organisms come to resemble each other in the
course of time. Example 1: marsupial animals of Australia and placental
animals elsewhere in the World. There are flying forms, fossorial forms (live
17
underground), herbivore forms, carnivore forms in each. Example 2: fishlike
forms in mammals (Whales) and dinosaurs (Ichthyosaurs).
The five-toed condition is regarded as a shared primitive or a
PLESIOMORPHIC CHARACTER.
PARALLELISM
Where the ancestry of the two forms are not very different and that they have
a similar function. For example social behaviour among bees, wasps and
ants.
DIVERGENCE
Two forms becoming progressively dissimilar.
Because cladistics defines each branching point by an evolutionary point such
as a derived or APOMORPHIC CHARACTER. It is therefore necessary to
seek a novel character to determine the branching point separating the lizard
from the other four vertebrates. Hair is one such derived character and is
shared by all animals except the lizard. We can call "hair a
SYNAPOMORPHIC
character for mammals.
In order to identify the sequence of subsequent branches we need to examine
the anatomy further. The cat, dog, seal all share a kind of cheek teeth called
involuted teeth, but the cow does not possess them. The next branching point
in the tree separates the seal from the cat and dog on the emergence of
carnassial teeth (specialized for cutting and shearing) in the cat and dog.
Finally cats branched off from dogs because of a newly developed character
the retractable claws.
EVOLUTIONARY SYSTEMATICS
A classification that reflects the dual nature of evolutionary change.
 The splitting of phyletic lineages, that is the
branching in the phylogentic tree.
 The invasion of new niches and major new
environments.
CLADISTICS
Classifies according to sequential order in which branches arise from a
phylogenetic tree. It however, ignores the degree of their divergence. This
tree of relationships is represented by a series of dichotomous branches and
is called a cladogram. Branches arise at each branching point and are defined
by the new homologies that are unique to the various species on those
branches.
Let's analyze a cladogram for five vertebrates, a lizard, a cow, a seal, a dog
and a cat.
The one feature that is common to all is that they have five toes.
Therefore five toes cannot be used to determine any branching points.
In sharp contrast to cladistics, evolutionary systematics do weigh the derived
characters (APOMORPHIES) that determine the branching points in the tree.
An evolutionary innovation, for example, is one that signifies the successful
invasion and exploitation of a major new environment is given more weight
than a new character with a minor functional role.
In assigning weights to different new or novel characters, evolutionary
systematics do engage in subjective judgements.
Emphasis on the sequential order in which branches arise from the
phylogenetic tree without regard to the degree of divergence has lead to novel
hypotheses of evolutionary relationships. For example the relationship
between the cow, lungfish and trout, are expressed in the cladograms below.
18
When the cladistic method is applied, we find that lungfishes are more closely
related to cows than the trout, because like cows, they share a novel feature:
the internal nares in the roof of their mouth cavity. Thus based on the
branching sequence, lungfish and cows are sister groups because they share
a more recent common ancestor than the trout.
express the fact that cows came to differ and lungfish remained similar to their
common ancestor? A cladist would argue that it should not, because it is the
sequence of branches that truly reflect relationships in the form of sister
groups that share the most recent common ancestor in the genealogical tree.
IN EVOLUTIONARY SYSTEMATICS major events in adaptive evolution are
reflected in the classification. By weighing evolutionary novelties in
constructing phylogenetic trees, evolutionary systematics intend their
classification to say something about how rapidly a new branch diverged, how
it changed in relation to its sister group, how many additional characters it
acquired, and in which new environments it/they invaded.
THE SYSTEMATIC HIERARCHY
Originally proposed by Linnaeus.
Organization
An evolutionary classification sees things differently, and gives weight to the
fact that a lungfish and a trout are both fishes and a cow is a mammal.
KINGDOM
PHYLUM
CLASS
SUBCLASS
SUPERORDER
ORDER
SUBORDER
FAMILY
GENUS
SPECIES
We will do more on classification in the next lectures.
TIME AND THE PLANET EARTH
We do not know how old the planet is, however, it has been estimated that the
solar system is more than 3 billion years old.
HOW DO WE AGE ROCKS?
When a mineral containing a radioactive element is first crystallized it includes
no radio-active products. With time there are changes in elements, and radioactive products accumulate in the crystal at a constant rate of disintegration.
These products remain in the along with what is left of the original material.
The ration of radioactive products and remaining minerals give us a gauge as
to the age of the crystal.
URANIUM was one of the first elements used. ARGON and POTASSIUM are
now more usually used.
REMEMBER: cladistics ignore the degree of divergence, that fact that the
lineage leading to cows diverged profoundly, passing through the structural
grades of amphibian and reptile, while the lineage of lungfishes diverged far
less is not relevant in the cladogram. Even though lungfish and trout both
have fins and gills and live in water their resemblances are based on primitive
features. Based on the emergence of an evolutionary innovation, the internal
nares, there is little doubt that lungfish are more closely related to one another
than to the trout.
THE EARTH
If one were to emphasize the degree of morphological divergence, one would
consider the trout and the lungfish more closely related to each other than
either is to the cow.
PALAEOZOIC ERA "Ancient Life" covers the first 350 million years of the
Phanerozoic time, a span from 600 million to 250 million years ago. It is
subdivided into six periods, which correspond to critical geological events.
The first of its periods is the Cambrian. Marked by expansions and extinctions
of early archaic plants and animals, the Palaeozoic era was terminated by the
Current debate focuses on the question: Should or should not classification
PERIODS OF THE EARTH
Before 600 million years ago is a group of ERAS collectively known as the
PRECAMBRIAN EON (ARCHEAN and PROTERZOIC ERAS). The EON that
we live in is called the PHANEROZOIC and is divided into the PALAEOZOIC,
MESOZOIC, CENOZOIC ERAS.
19
Permian mass extinction.
MESOZOIC ERA "Middle Life" covers the time from 250 million to 65 million
years ago. It has three periods and is often called the Age of Reptiles.
CENOZOIC ERA "Recent Life" extends from 65 million years ago to the
present. Its two periods are divided into seven epochs. The cenozoic era is
known as the Age of Mammals. We are now living in the Holocene epoch.
Within the ERA are PERIODS and within periods are EPOCHS.
The PALAEOZOIC has the following periods:CAMBRIAN 570-500 million years
Abundant marine invertebrates
Primitive marine algae
Explosive evolution of Phyla
Brachiopods (hinged shells, fixed to the seabed by a foot
Arthropods ("shelled" creatures such as trilobites, relatives of modern
insects, spiders, lobsters and crabs).
Echinoderms (spiny-skinned animals) including sea-urchins, starfish and
sea lilies (Crinoids).
The earliest animals with a notochord and myomeres (muscle segments)
called Pikaia
The first major extinctions occurred of the stromatolites (communities of
cynobacteria)
ORDOVICIAN 500-435 million years
Life remained in the sea
Diversification of the mollusc, particularly sea snails (Gastropods) and
nautiloids).
 Coelenterates, some built large reefs.
Graptolites, a now extinct hemichordate, the precursor to the chordate
animals?
The earliest vertebrates, which were jawless, fish-like creatures possessing
a bony armour and were called ostracoderms.
The Trilobites reached their climax, as did euryptids which were marine
predators related to modern spiders and scorpions.
At the end of the Ordovician there was an extinction which reduced the
numbers of coral, sea lilies and trilobites.
SILURIAN 435-395 million years
Plants colonized the land. The first plants occurred in marshes at the edges
of the sea.
 Corals became most abundant.
The first jawed fish-like animals called Placoderms, some reaching
enormous sizes such as Dinichthys.
Giant arthropods (sea scorpions) may have been the first animals to move
onto the sea-shore.
DEVONIAN 395-345 million years
Also known as the "Age of the Fish". With fish developing internal bony
skeletons, swim bladders (the earliest form of vertebrate lung). One
group of fish developed paired fins, which eventually enabled them to
crawl out of the water and move about on land.
Such crawling fish evolved into the first amphibians.
The cartilaginous fish (sharks and their allies became very dominant.
Trees and forests colonized the earth.
Terrestrial invertebrates, particularly insects, dominated the land.
Another large extinction of fish and marine invertebrates (about 70% of
species) ended the Devonian period.
CARBONIFEROUS 345-280 million years
The climate became drier, and gymnosperm plants (without flowers)
developed. The carboniferous forests were extensive, the remains of
which have became fossilized into coal. The first conifer plants evolved.
Insect life proliferated in the carboniferous forests.
The first reptiles evolved from the amphibians. They laid hard-shelled eggs
and had dry, scaly skin to retain their body moisture. Therefore they did
not need to stay near water to survive.
In the sea Ammonites developed from the nautiloids (cephalopods).
PERMIAN PERIOD 280-230 million years
Swampy, primitive forests with fern trees being replaced by conifer and
ginkgo trees.
Many deserts were established.
Amphibians developed into large sized animals.
Insects continued to radiate.
Reptile radiation began their radiation, with the sail-back dinosaurs, or
synapsids which
eventually lead
to the evolution
of mammals.
At the end of the Permian was the greatest extinction in the evolution of
animals with over 96% of all species disappearing, and was the first
extinction ti directly effect terrestrial life. Groups of animals particularly
badly affected were the sea lilies, corals and ammonoids.
THIS MARKED THE END OF THE PALAEOZOIC AND THE BEGINNING OF
THE MESOZOIC
TRIASSIC PERIOD 230-195 million years
Known as the "Age of the Reptiles
Conifers, such as firs, pines and cedars expanded and horsetails and seed
ferns declined in numbers. Palm trees started to evolve.
Ammonites recovered from their near extinction to co-dominate the seas
with bony fish.
Reptiles also returned to the sea with evolution of the turtles (anapsids),
Ichthyosaurs (dolphin-like animals) and Plesiosaurs.
Ancestors to the snakes and lizards evolved.
The beginning of the radiation of the dinosaurs.
At the end of the Triassic up to 75% of marine invertebrate species and
some land dwellers vanish, perhaps in an impact-related catastrophe,
however, the first mammals (or mammal-like reptiles) and the dinosaurs
make it through.
JURASSIC PERIOD 195-140 million years
Climate was warm almost everywhere, even the poles had no ice.
The sealevel was very high and extremely shallow seas covered much of
the land.
Cone-bearing plants dominated the landscape.
The first small mammals appeared. These warm-blooded animals were
most active at night and feed mainly on insects.
First fossil evidence of "feathered birds".
Reptiles had radiated into the air (pterosaurs), on land dinosaurs and in the
water.
CRETACEOUS PERIOD 140-65 million years
The continents had split during the Jurassic (in the Triassic they were joined
as one supercontinent) and now in the Cretaceous Period the continents
were moving away from each and the world started to resemble what it
looks like today.
20
The first Angiosperm (flower plants) appeared, which provided a new source
of food.
The mammals developed further, with three forms; the monotremes (egglaying), marsupials (pouched) and placentals.
At the end of the Cretaceous another extinction occurred, with all of the
dinosaurs becoming extinct, all of the ammonites and most of the
microscopic plant life in the seas.
Many scientists believe the Cretaceous-Tertiary (K-T) extinction was caused
by a meteorite impact.
TERTIARY PERIOD 65-2.5 million years ago
Divided into following EPOCHS:PALAEOCENE: 65 to 55 million years ago. Flowering plants became
abundant. Mammals already divided into early insectivores
and primates.
EOCENE: 55 to 38 million years ago. Fossil evidence of a small horse.
Grasslands and forest present. Many small mammals and
larger mammals, such as primitive whales, rhinoceros, and
monkeys.
OLIGOCENE: 38 to 25 million. Fossil evidence of primitive apes. Elephants,
camels, and horses develop. Climate generally mild.
MIOCENE: 25 to 14 million. Many grazing animals. Flowering plants
and trees similar to modern.
PLIOCENE: 14 to 11.5 million. Fossil evidence of ancient human beings
near the end of the epoch. Birds, mammals, and sea life
similar to modern types. Climate cools.
QUATERNARY PERIOD 2.5 million years ago to the present.
Divided into following EPOCHS:PLEISTOCENE: 2.5 millions to 10 000 years ago. "THE ICE AGE". Modern
human beings present. Mammoths and other animals become
extinct.
RECENT: Civilization spreads. Human beings are the major form
of life!
THE FOSSIL RECORD
DEFINITION
Fossils are remains of prehistoric organisms. Preserved by burial under
countless layers of sedimentary material, they are a record of the history of life
beginning approximately 3.5 billion years ago, the study of which is called
PALAEONTOLOGY.
WHERE ARE THEY FOUND?
Some fossils are abundant in the strata of the Earth's crust. The chalk cliffs of
Dover, England and the Niobrara Chalk of Kansas in the USA are composed
of complex platelets of algae, so small that millions fill a cubic millimetre.
Shells of invertebrate marine animals, such as brachiopods, bryozoans,
clams, snails, corals, and echinoderms, are preserved in many beds of
limestone, and the bones and teeth of vertebrates are sometimes so
numerous that they form deposits called "bone beds." In other
SEDIMENTARY ROCKS, such as the majority of the world's RED BEDS,
shells and bones are rarely found, although tracks and burrows may be
abundant. Fossils are rarely found in sediments of PRECAMBRIAN TIME,
more than 600 million years old, although younger sediments yield some fossil
remains.
KINDS OF FOSSILS
Entire or partial bodies of organisms, whether plant or animal, whether
preserved in original form as replacements (see following section) or only as
moulds or imprints, are called body fossils. In contrast, marks left in rock by
the activities of organisms are called trace fossils. These include artifacts,
burrows, faeces, tracks, and trails. Fossils so small as to be measured in
micrometers or millimetres are called microfossils, and those in the centimetre
and meter range are called megafossils. Microfossils are studied in the field of
MICROPALAEONTOLOGY. A related study of microscopic spores, pollen
grains, and cysts extracted from sediment by hydrofluoric-acid treatment is
called palynology. Invertebrate paleontologists, vertebrate paleontologists, and
paleobotanists (see PALAEOBOTANY) study megafossils. Chemical
paleontologists study the invisible fossil record of organic macromolecules, the
presence of which in rocks can reveal the existence of certain groups of
organisms in the distant past.
HOW AND WHERE ARE FOSSILS FORMED?
According to the ancient Greek philosopher ARISTOTLE, living organisms
could arise by spontaneous generation from mud. Fossils were thought to be
the abortive results of this process. Some early philosophers, however,
recognized them as remains of once-living organisms, as did Leonardo da
Vinci and other Renaissance scholars.
The Earth is teeming with life, all of it searching for food; organic compounds
of hydrogen and carbon. As a result, very little digestible organic matter
escapes destruction, and indigestible skeletal material, such as shells, bones,
and teeth, has a much better chance of burial and preservation. Shell material
is typically composed of calcium carbonate, as are mollusc shells; teeth and
bones are composed of calcium phosphate; sponge spicules, diatom frustules,
and radiolarian tests are composed of opaline silica. The highly indigestible
organic jackets of spores and pollen grains also commonly escape
destruction. Such materials form most of the body fossils common in layered
rocks.
Much rarer are accumulations of sediment in settings from which scavengers
are excluded and in which the bodies of plants and animals, carried in from
outside, may retain their general form. Although few such occurrences are
preserved and discovered, they are of greatest value to the paleontologist
because they give the most comprehensive view of past life. Certain rock
formations are well known for this reason. One is the Precambrian Gunflint
Chert on the north shore of Lake Superior, Canada. It contains well-preserved
bacteria and blue-green algae approximately 2 billion years old. Another, the
Burgess Shale of British Columbia, Canada, contains carbonaceous films of
soft-bodied marine worms and crustaceans of the Cambrian Period. In Illinois,
USA, at the Carboniferous Mazon Creek locality, both land plants and marine
invertebrates are preserved. The Holzmaden oil shale (see SHALE, OIL) of
southern Germany, of Early Jurassic age, is well known for its many fish and
CRINOIDS, as well as for its ichthyosaurs and other marine reptiles that have
been found there and displayed in museums throughout the world. Even
better known is the Late Jurassic Solnhofen limestone of southern Germany,
where the quarries, worked for lithographic stone, have yielded not only large
numbers of well-preserved jellyfish and horseshoe crabs, but also flying
reptiles (pterodactyls) and the world's earliest known bird,
ARCHAEOPTERYX.
Some of the most spectacular fossils of the Cretaceous Period, including fish,
marine reptiles, pterodactyls, and birds, have come from the Smoky Hill Chalk,
near Hays, Kansas, USA. A remarkable Eocene lake fauna has been
21
uncovered in the Green River Formation, near the town of Fossil, Wyoming,
USA. Along the shore of the Baltic Sea, insects, spiders, and the like are
found preserved in AMBER, fossilized resin exuded from trees.
The most information on Pleistocene faunas, from vultures to bisons, wolves
to sabre-toothed cats to beetles, has come from tar seeps, such as the LA
BREA TAR PIT in Los Angeles, California, USA where prehistoric animals got
mired and embalmed in the tar, as they do in modern tar pits. Most
extraordinary are the few finds, in Alaska and Siberia, of prehistoric but
presumably post-Pleistocene MAMMOTHS, frozen into the arctic
PERMAFROST and preserved by natural refrigeration.
Important as these unusual localities are, most of the fossil record is
composed of skeletons that can be recovered from ordinary sediment, in a cliff
or quarry or roadcut, or from wells drilled deep into the ground. These
skeletons and skeletal fragments may be preserved as the original material,
as hollows (moulds) formed by dissolving the original matter to leave only an
imprint, or as replacement material, with a mineral such as quartz or pyrite
having replaced the original bone or shell.
FOSSILS AND EARTH HISTORY
Toward the end of the 17th century, the naturalist Robert HOOKE turned his
attention to spectacular marine fossils found in his native England.
Determining that these must be the remains of once-living animals, he noted
that they did not resemble any living species then known, causing him to
believe that life might have changed at some time in the past and that fossils
might be a chronological guide to geologic history. Hooke also noted that
these fossils looked more like tropical shells than species then living on British
shores and wondered whether Britain's geographic latitude had also changed
since the time these animals lived. The first suggestion was verified a century
later, and the second three centuries later with the discovery of
CONTINENTAL DRIFT.
scientists to calibrate Earth history in actual years.
THE PROGRESSION OF PREHISTORIC LIFE
The earliest megafossils, some 1 to 3.5 billion years old, are
STROMATOLITES, calcareous masses resembling heads of cabbage,
comparable to structures now being formed in tropical intertidal and shoalwater settings by blue-green algae. Microfossils from CHERTS this old include
a variety of bacteria and colonial filaments, as well as single cells that closely
resemble modern blue-green algae in form. By approximately 1 billion years
ago, a wider variety of such microscopic cells had appeared, including some
that may have had cell nuclei. In the deposits of approximately 600 million
years ago, imprints of soft-bodied invertebrate animals are found; and in
sediments 570 million years old (the beginning of the CAMBRIAN PERIOD),
the first faunas of skeletal invertebrates appear: mollusc, brachiopods, and
trilobites. The appearance of these invertebrates, in rocks deposited at the
beginning of the PalaeoZOIC ERA, coincides with the first widespread signs of
burrowing. In the ORDOVICIAN PERIOD (500-425 million years ago) marine
faunas became much more diverse. Most of the modern classes of
invertebrates were now represented, as well as OSTRACODERMS, the first
fishlike organisms. In the SILURIAN PERIOD (425-400 million years ago)
landmasses were colonized by rapidly evolving higher plants, whose
supporting structures and water-conducting vessels now made life on dry land
possible. In the DEVONIAN PERIOD (400-345 million years ago) the main
groups of FISH: COELACANTHS, LUNGFISH, SHARKS, bony fish, and the
extinct ARTHRODIRES: were differentiated. Forests and the first primitive
INSECTS appeared on land, as did AMPHIBIANS. The CARBONIFEROUS
PERIOD (345-280 million years ago), known for its great coal deposits,
witnessed the development of REPTILES, the first animals having an amniote
egg, which enables the embryo to develop on dry land.
The Earth's sedimentary strata are initially layers of muds and sands, each
covering an older stratum and being covered, in turn, by a younger one (see
STRATIGRAPHY). They form, in this manner, a historical sequence, and the
fossils that they contain can be arranged in time, by what has come to be
known as the law of superposition. Early in the 1800s, William SMITH, in
England, noted that fossils were distinctive of individual beds or groups of
beds in such sequences of strata, and that distinctive assemblages of fossils
could be traced cross-country. Geologists soon discovered that the sequences
of fossils in England could be matched with similar sequences elsewhere in
the world. Any given area contains a stratigraphic record of only some part of
Earth history. By combining information from many different areas, geologists
can determine global Earth history. Nearly two centuries of such efforts,
including the description of fossils in monographs and journals, have resulted
in ever more detailed classification of the more fossiliferous part of Earth
history: the last 600 million years.
The MESOZOIC ERA (225-65 million years ago), which includes the
TRIASSIC, Jurassic, and Cretaceous periods, is known particularly for the
evolution of gigantic reptiles, both in the sea (ICHTHYOSAURS,
PLESIOSAURS, and MOSASAURS) and on land (DINOSAURS). Flying
reptiles (PTERODACTYLS) and BIRDS, as well as MAMMALS, appeared
during the JURASSIC PERIOD (190-135 million years ago). On land, forests
of CONIFERS and CYCADS had largely replaced the lycopod- and seed-ferndominated forests of the Palaeozoic Era. In the sea tiny calcite-armoured,
photosynthetic unicells called coccolithophorids appeared, and massive
calcium carbonate (chalk) deposition began in the deeper oceans (see OOZE,
DEEP-SEA). The CRETACEOUS PERIOD (135-65 million years ago) is the
time of origin of two great groups of plants:- the flowering plants
(ANGIOSPERMS) on land and the DIATOMS in water. Flowering plants
changed the face of the Earth in many ways and triggered a great wave of
evolution among the insects. At the end of Cretaceous time, the extinction of
dinosaurs resulted in the spectacular evolution of terrestrial mammals, and
giant sharks and marine mammals replaced large reptiles in the sea.
The smallest units of this classification, generally characterized by certain
species or combinations of species, are called zones. These may be
recognizable only locally, but many have been traced worldwide.
Combinations of zones are the chief criteria for the recognition of worldwide
units of geologic time, called stages, generally having a duration of
approximately 10 million years. These in turn include the larger time periods,
called systems and eras.
The group of mammals known as the PRIMATES:- now represented by
lemurs, monkeys, apes, and humans dates back to the beginning of the
CENOZOIC ERA (65 million years ago to the present), but human-like
creatures (see PREHISTORIC HUMANS) are known only from the last few
million years, the Pliocene and PLEISTOCENE epochs. Homo sapiens,
modern humans, appeared in the Old World during the Pleistocene but did not
reach the Americas until its latter part.
In terms of time, the fossil record yields a relative chronology rather than an
absolute one (see GEOLOGIC TIME), but the development of RADIOMETRIC
AGE-DATING methods, using the decay of radioactive isotopes, has enabled
Fossils record the progressive evolutionary diversification of living things, the
progressive colonization of habitats, and the development of increasingly
complex organic communities. The development of new species and such
22
larger groups of species as genera and families has gone on throughout time,
but so also has the loss of species by extinction. The rate of extinction at
some times in Earth history greatly exceeded the rate of speciation, and the
faunas and floras of the world became reduced. Notable biotic crises occurred
in Cambrian time, near the end of the Devonian time, at the close (PERMIAN
PERIOD) of the Palaeozoic Era, and at the end of the Cretaceous Period.
The causes have not been determined.
addition, evolution does not normally occur throughout a species, but in
interbreeding populations, occupying some small part of the geographic range
of the species as a whole. It is such populations and adjacent populations,
linked by exchange of genes, that deviate from the ancestors and from the
species as a whole, to form races or subspecies, and, eventually, species. At
any one time, a species is a combination of such groups, diverging
episodically from each other. As geography and habitat change, these groups
shift about, either blending when they meet, abruptly displacing each other, or
coexisting side by side in different ways of life.
FOSSILS AND EVOLUTION
The fossil record corresponds to the general theory of organic evolution, and
any group of plants or animals can be seen to change through the record of
strata. The smallest general unit of classification is the species, recognized in
the fossil record by great similarity of form. Most species are seen to have an
existence that is short in terms of Earth history, usually one to several million
years, more rarely tens of million years, and very rarely hundreds of million
years. Most genera (groups of related species) can be traced through time
spans of tens of millions of years; larger units of this classification system, or
taxonomy, tend to persist through longer time spans. The majority of the
classes and phyla of invertebrate animals, for example, have records
beginning in the Early Palaeozoic Era, spanning essentially all of recorded
animal history.
All of the larger groups of animals are seen to have evolved a wide range of
life forms. Reptiles, for example, having evolved in late Palaeozoic time out of
small or medium-sized predatory amphibians, developed into a great diversity
of animals that included herbivores and carnivores, dwarves and giants,
creepers, runners, climbers, burrowers, swimmers, and flyers, all in
competition with other groups of animals living in the same ways. They have
continued to occupy some of these functions (niches, in ECOLOGY)
throughout their history and have vacated others. Reptiles are still among the
more effective small creeping and running predators on land (lizards, the
tuatara), replaced by birds as aerial dwellers and by mammals as large
terrestrial herbivores and predators. A succession of animals have been
marine "superpredators." In the Triassic and early Jurassic periods, this was
the domain of ichthyosaurs; in late Jurassic and early Cretaceous time, these
were replaced by plesiosaurs; in the Cretaceous, by mosasaurs; in the
Eocene, by the zeuglodont whales (mammals); and since the Miocene, by the
great carcharodont sharks. The fossil record suggests that evolution has
resulted mainly from the tendency of all species to experiment with new ways
of living, to thereby exploit new opportunities as they arose in an everchanging world, gaining a new foothold in one place and losing one in another.
With Charles DARWIN's theory of evolution in the mid-1800s came the
expectation that the fossil record would provide unbroken evolutionary
sequences, in which species after species would be seen to emerge gradually
from its ancestors and pass, equally gradually, into its descendants. Most
species, however, are seen to appear abruptly, to maintain their typical form
for most of their history, and to vanish as suddenly as they appeared. This
failure to trace coherent lineages of ancestors and descendants does not
prevent recognition of changes in larger groups of animals: horses are seen to
have developed through Cenozoic time from small to large, from five-toed to
one-toed, and from short- to long-toothed, but complete records of the
transition from one species to another have not been found. This caused
some palaeontologists to doubt Darwin's belief that evolution proceeds by the
gradual accumulation of small changes. It has now been determined that
evolution does not proceed steadily, in response to some mysterious internal
force, but in response to new opportunities. In a stable, unchanging
environment, a well-adapted species is not likely to change, whereas in a
changing one it may find better opportunities by changing its way of life. In
One of the most difficult problems in evolutionary palaeontology has been the
almost abrupt appearance of the major animal groups:- classes and phyla in a
fully-fledged form, in the Cambrian and Ordovician periods. This must reflect
a sudden acquisition of skeletons by the various groups, in itself a problem.
The existence of soft-bodied ancestors is now documented by the late
Precambrian fauna of southern Australia, but the lack of well-documented
animal remains in older rocks indicates that differentiation of the major groups
of animals occurred more rapidly than did their subsequent evolution.
FOSSILS AND ANCIENT LANDSCAPES
The fossil record contains a history of the evolution of life on Earth and
provides geologists with a chronology far more detailed and widely applicable
than that of GEOCHEMISTRY. It also contains much information about the
geographical-ecological changes that have occurred in the course of geologic
time. This interpretation of the fossil record predates the other, in that some of
the early Greek philosophers and Renaissance naturalists recognized certain
strata as marine and as evidence of former higher sea levels, on the basis of
the enclosed fossils, long before the evolutionary nature of fossils was known.
The best example of this is the recognition of ancient seas and land masses.
The deposit of LOESS containing grass seeds and land-snail shells can be
quite easily recognized as the windblown accumulation of dust in an ancient
grassland or prairie. The accumulation of PEAT or coal, containing abundant
woody material along with spores or pollen, and possibly skeletons of land
animals, is evidence of an ancient peat bog or swamp. A bed of limestone
containing a wide variety of clams and snails belonging to marine families, as
well as the remains of sea urchins or other ECHINODERMS (a phylum that
seems always to have been restricted to the sea), is evidently of marine
derivation.
The fossil record can be used to reconstruct ancient environments. For
example, strata of TERTIARY age in the oil-bearing "transverse basins" of
California, such as the Los Angeles and Ventura basins, contain many
microfossils of the protozoan group called FORAMINIFERA. These were
studied because they provided a means of tracing strata from one oil well to
another, in accumulations of sediment thousands of meters thick. This
sedimentary sequence began with stream deposits in the Oligocene epoch,
passed through a long marine phase in the Miocene and Pliocene, and
reverted to mammal-bearing alluvial deposits in the PLEISTOCENE EPOCH
(see also QUATERNARY PERIOD). In order to learn something about the
conditions under which the oil-bearing marine portion was deposited,
palaeontologists compared the fossil assemblages with the depth range of the
same species or the most closely related species living today off the California
coast. Interpreted in this manner, environmental change from continental,
through shallow near-shore, into deep-water (more than 1,500 m), back
through the shallow water, and into alluvial was revealed. This history has
been confirmed by the study of fossil fish scales. Although sardine scales are
found throughout the marine parts, angler fish and other species of the
BATHYAL ZONE are found only in association with deep-water foraminifera.
23
This is one example of the field of study that is called PALAEOECOLOGY.
Palynologists studying pollen grain assemblages from lake beds and peat
bogs have determined the shifting of forests and grasslands that occurred in
the later stages of the Pleistocene ICE AGE and in the interval since then and
are establishing a much-needed history of climates, which can be related
directly to the historical record (see POLLEN STRATIGRAPHY). Study of the
foraminifera in marine cores enables scientists to chart the distribution of
OCEAN CURRENTS and water masses during the height of the last
glaciation, 18,000 years ago, in an effort to understand the Ice Age and its
cause.
Fossil chemistry is another area of research for palaeoecologists. Many
elements occur in two or more atomic forms that differ from each other in
weight isotopes. Thus, oxygen occurs as O16 and O18, O16 being by far the
most abundant. When an organism such as a foraminifera builds a skeleton
of calcium carbonate, it incorporates O16 and O18 in a proportion that depends
on the ratio in which they occur in the ambient water, and on the temperature
of the water. The higher the temperature, the less O18 is put into the skeleton.
If the ratio of the isotopes has remained constant in the oceans, then the
ratios of such isotopes in a foraminiferal shell are a direct indication of
temperature. The isotopic composition of the seas, however, has not remained
constant, and the ratios in the shells can be disturbed by chemical changes
after burial, but palaeoecologists have demonstrated, for example, that the
thermal structure of the oceans in Cretaceous times, was much different from
what it is today, with much warmer midwater masses.
FOSSILS AND PALAEOGEOGRAPHY
The distribution of plant and animal species in the world today reflects the
interplay between the physical environment, which, for example, restricts polar
bears to the high latitudes, and geographical barriers to migration, which, for
example, have kept polar bears from invading the Antarctic and keep the
marine snakes of the Pacific side of Panama from invading the Caribbean
Sea. Such barriers divide the world into biogeographic provinces. The fossil
record enables palaeontologists to reconstruct such provinces for the past and
to study their history. South America, for example, was joined to Africa during
most of the Mesozoic Era, as its Triassic and Jurassic reptile faunas
demonstrate. When it broke away, in mid-Cretaceous time, with the opening of
the South Atlantic Ocean, the dinosaurs of South America succumbed to the
Cretaceous crisis and the mammals took their place as the dominant land
animals, forming a very different kind of mammalian fauna that evolved
independently of the Eurasian-American fauna. Australia underwent similar
changes except that it remained isolated, and its indigenous mammalian fauna
(a marsupial one) has evolved to its modern state. South America became
linked with North America during the Pliocene Epoch, and the two very
different mammalian faunas invaded each other's territories, pitting species
against species for existence. Eventually, North America assimilated some
South American mammals (ground sloths, armadillos, opossums, and
porcupines), but the majority of South American species became extinct.
The fossil record also shows that the Caribbean and eastern Pacific marine
faunas were essentially identical in Miocene times, diverging only since the
Panamanian land bridge closed in Pliocene time. The question of whether to
build a Panamanian sea-level canal has focused attention on what would
happen to present faunas if communications were to be reestablished. Would
the Pacific sea snakes, for example, invade the Caribbean to the detriment of
fisheries there?
MAJOR EVOLUTIONARY TRENDS AND THE FOSSIL RECORDS
The significance of the taxonomic categories in studies of the history of life,
with illustrations from the animal kingdom.
PHYLUM: the fundamental division of the animal kingdom, distinguished by
unique aspects of the basic "body plan". Most arose in the late Precambrian
from small soft-bodied ancestors. Hence, there is no fossil record of
intermediate forms. Evolutionary relationships are determined largely from the
embryology and comparative anatomy of living representatives.
CLASS: major division within the phylum, distinguished by basic modifications
of the body plan. Most marine classes with fossil records appear early in the
Phanerozoic. Fossil record contains intermediates between classes in some
phyla (e.g. Mollusca).
ORDER: higher taxon, new orders appearing throughout the Phanerozoic.
Marine fossil record shows nearly constant numbers of animal orders from the
Ordovician to present.
FAMILY: lower taxonomic category for which comprehensive, accurate data is
available for all marine and continental animals. Data on 5000 or so fossil
families show such features of the history of life as major evolutionary
radiations and mass extinctions. Many fluctuations in species, such as minor
mass extinctions, are not reflected in families.
GENUS: smaller taxonomic unit than family showing more evolutionary detail.
Accurate data available for only a few groups (e.g. nautiloid cephalopods).
Between 25 000 and 40 000 fossil genera known.
SPECIES: basic unit of evolution, with more than 250 000 fossil animal
species described in the last 250 years, but not accurate listings available.
GENERALIZATIONS
The total number of living organisms has increased. The divergence between
animal groups has continued to increase. The are more difference among
contemporary organisms that in the past (e.g. horses). Increasing diversity
has not be maintained at constant rates, rather there were great burst of
evolutionary diversity).
This sudden increase in species has posed problems for biologists and
geologists, since gradual increase in species diversity (GRADUALISM) was
accepted until fairly recently. This was based on the numerous instances in
the fossil record in which fossils appear not to have changed during all the
time that a stratum was being deposited , but the fossils immediately above
and below it are quite different. Because of the acceptance of gradualism,
such gaps in the fossil record, were ascribed to kinds of geological
disturbances. However, more and more evidence was being found that there
were periods characterized by faster evolution of species. This idea has been
formalized as a theory called punctuated equilibrium. Punctuated equilibrium
describes evolution as generally characterized by long periods of equilibrium
punctuated by brief periods of change (To a palaeontologists "brief" means
many thousands of years). There are in fact some fossil sequences that show
such sudden punctuations that cannot be explained by geological
disturbances. There are also many undisturbed fossil sequences that show
gradual evolution. In fact even in a single series of fossils, some features
24
change rapidly while others change gradually.