Download Evolution -- History of Life

Evolution P.1
Evolution – Diversity vs Unity
O
ne of the most striking facts emerging from a study of natural history is the v_____ of life;
another, paradoxically, is its underlying u____.
Of the two million or more species of organisms alive today,
each can be grouped with others which it resembles. The
natural class______ of organisms according to their sim______
suggests that organisms are related.
The Creek naturalist and philosopher, Aristotle (384-322 B.C.) put
forward the view that there was a grad___ change in living forms
from plants to animals to man. This was the first
recorded hypothesis that biological ev______ had occurred, that is
that living organisms are descended from earl___, different
ones#.
# An alternative viewpoint is that each kind of organism alive today was created separately and has not
altered since its creation; that there is no relationship by descent between living forms. This hypothesis
is called 'Special Creation'. e.g. the Bible declared that each living thing was ‘brought forth after his kind’
and the usual interpretation of the statement was a literal one: that a ‘kind’ or species was unchangeable.
Theories of the Origin of Life (reference)
The major theories accounting for the origin of life on Earth are:
1. life was created by a supernatural being at a particular time (Special creation);
2. life arose from non-living matter on numerous occasions (Spontaneous generation);
3. life arrived on this planet from elsewhere (Cosmozoan);
4. life arose according to chemical and physical laws (Chemical / Biochemical evolution).
II)
The Idea / Theory of Evolution
The term 'evolution' has a special place in the study of life.
It has become the unifying concept of biology.
Evolution implies an overall grad___ development which
is both ord____ and seq______.
In terms of living organisms it may be defined as 'the
development of com______ organisms from
pre-ex_________ sim______ organisms over
the course of t_______'.
# The idea of evolution did not begin with Darwin. The concept
of continuity or gradual development of more complex species from
pre-existing simpler forms had occurred to several philosophers
and naturalists before the formal declaration of evolutionary hypotheses
in the 19th century.
Evolution P.2
II)
Evidence for Evolution
It is impossible to directly observe evolution that had taken place over millions of years, indirect
or circumstantial evidence is what we have to evaluate.
A)
Evidence from Geographical Distribution of Plants and Animals
Plants and animals are not evenly dist_______ around the world, identical conditions in different
parts of the world often have diff_____ characteristic animals and plants. This ‘discontinuous’
distribution is interpreted as the result of years of evol________ events and suggested the idea of
the common des_____ of species.
a)
Long-separated Continents
Organisms, living in the same or very
sim____ environmental conditions,
often acquired sim____ adap____
structures and lifestyles.
The tendency of un_______ organisms
to acquire sim___ adaptive features is
known as con_______ evolution.
Examples of convergence are
found in continents which have been
iso_____ for a long time.
Frequently different lines of evolution
appear to converge from divergent
starting points, so that similar niches
are occupied by unrelated organisms.
The best example comes from comparisons of the natural
history of Aus______ with that of other continents.
Australia has many species of pou____ mammals
(marsupials), but virtually no true (placental) mammals. Yet
Australia has a climate and habitats highly favourable to
pla______ mammals, for when these are introduced there they
have been very successful. eg. sheep & rabbit.
Thus Australia has mar_____ ‘moles’, ‘mice’, ‘anteater’,
and ‘flying squirrel’ occupying ni____ similar to those of
their placental counterparts in other continents. They are
NOT cl_____ related but come to look alike as a result
of c_________ evolution.
The explanation lies in the geographical iso______ of Australia, the result of continental drift.
Australia became isolated at about the time the mar_______ and pla______ mammals
diverged from a common anc______ (about 120 million years ago). In the absence of comp_____
from placental mammals, the marsupials have evolved into dif_____ forms that fill in all available
n_____ in Australia.
Evolution P.3
Q. What do you think are the marsupial counterparts for the following placental mammals ?
Placental mammals
Mole
Anteater
Antelope
Flying squirrel
Rabbit
b)
Australian Marsupials
Placental mammals
Prairie dog
Hyena
Marsupial mouse
Sloth
cat
Australian Marsupials
Volcanic / Oceanic Islands
When Darwin travelled on board the Beagle he studied the flora and fauna of islands, and observed that
islands often have unique species but are nevertheless seems closely related to those of the nearby
mainl____. The rational explanation for this 'discontinuous' distribution was that species originate
(evolve) in given areas and may subsequently disp_____ outwards from that point.
The range of form in finches that Darwin observed in the Galapagos Islands during the voyage of the
Beagle was explained in terms of adaptive r________. There are 14 species, the variations in b____
morphology reflect differences in feeding habits. Darwin suggested that these Galapagos finches
were all closely rel____, in spite of their differences. It was suggested that soon after the islands were
formed they were populated by a flock of finches from the mainland. These then evolved to fill the
av________ ni_____
It is nearly always possible to see evolutionary links between an is____ inhabitants and those of the
nearest cont_____ but, since sea provides an effective bar
for most of the time, the
island organisms evolve inde__________. Barriers, including the sea, mountain ranges, deserts or
simply an unfavourable temperature range are essential to the div___________ of life.
Thus from the geographical distribution of plants and animals, it can be concluded that:





B)
Species ori________ in a particular area;
Species dis_______ outwards from that area;
Dispersal could only occur for those species where land masses were close enough together to permit
dis_____;
Physical b_______ such as the sea lead to separation of new varieties and their an______;
This genetic isolation leads to separate gene p____ and evolution of new sp_____.
Evidence from Fossil Record
Palaeontology is the study of plants and animals of the geol_____ past, as represented by their fossil
remains. Fossils are the dead re_____ of plants and animals, pres_____ in sedimentary rock, in
waterlogged peat or in the sticky gum that exudes from certain trees.
Palaeontologists are able to place layers
of sedimentary rocks in order of their
formation and they can estimate the a__
of each layer from the thickness of the
rock above and from carbon-dating
techniques. In this way, a geological
t____ s____ can be made.
Oldest fossil-bearing rocks contains
very f___ types of fossilized organisms
and they all have very si____
structures. Younger rocks contain a
greater v______ of fossils with
increasingly comp
structures.
Evolution P.4
Throughout the fossil record many species which app____ at an earlier period disapp_______ in the
younger rocks. This is interpreted in evolutionary terms as indicating the times at which species
org______ and became ex______.
The most convincing fossil evidence is found in cases where, in succ______ layers from one locality,
a series of fossils exhibits grad_____ change. Good examples of this are seen in the evolution of the
oyster and in a species of sea-urchin, where complete records of most intermediate forms are seen as
fossils in successive strata.
Intermediate forms of animal and plant organisation may be
interpreted as the l____ between related groups of species.
A possible example of an intermediate form, discovered in the fossil
record, is Archaeopteryx, a 'missing link' between reptiles and birds.
Fossil record tells us the seq______ and tim____ of the
appearance of the major phyla, it support a theory of prog____
increase in complexity of organisms and against the idea that
species cannot be ch
.
Q. One of the major criticism of using fossil as evidence is the lack of a continuous fossil record. Gaps in the
fossil record - Missing link’. How would you explain the incompleteness of the fossil record ?
C)
Evidence From Comparative Anatomy
Comparative anatomy is the study of the str________ similarities and differences between different
species in order to set up the evo_________ relationship between them.
a)
Homologous structures and Divergent evolution (adaptive radiation)
Organs of some animals are fundamentally the same in str_______ and anat_____ positions, but
they have adapted to dif______ functions. Such organs are said to be homologous.
The presence of homologous organs suggests a com
ancestry. Evolution have occurred in which
modifications of the fundamental structural plan to perform different functions in different
environment.
The evolution of div_____ forms from the s____ basic plan demonstrates adaptive radiation.
Adaptive radiation produces organisms capable of exploiting different e________ niches; this gradual
spreading out of the organisms is known as d________ evolution.
Q. Although the mammalian fore-limbs are
adapted for different functions, fundamentally their
structures are the same (the same number of bones,
muscles, nerves and blood vessels) and are just
modifications of the pentadactyl limb (limb with five
digits).
This suggests that organisms possessing them have
the same ancestry.
The figure show forelimbs of different mammals, match the
description and suggest the mode of life of the mammal
possessing it, name an example for such a mammal:
Special features
Mode of life / example
.
Forearms of different mammals
Evolution P.5
1. basic plan of pendadactyl
limb composing of: Upper
arm--humerus, forearm-radius & radius, wrist-carpels, and 5 digits with
phalanges
2. long and extending finger
bones, supporting a layer of
skin between the digits
3.
limb bones short and stout,
extra phalanges to form a
padde-like structure
4. radius and ulna fused for
better support, Digits
degenerated except one
(the 3rd one) which become
strong and extended,
adding extra length to the
limb. It is also hoofed so
that it become standing on
its nail’.
5. very stout and strong
forelimb, shaped like a
spade
6. long, flexible forelimb with
grasping digits
b) Analogous organs and Convergent evolution
Organs of some animals perform the s____ function though their structures are fund_________
different. Such organs are said to be analogous.
When different groups of animals adapt to the same habitat, they will develop characteristics that are
more or less similar to solve the same pro_____ (under similar sel_______ pressure ). This
phenomenon is known as c___________ evolution.
For example the w____ of a bird and the wings of an insect are analogous organs which are
adapted for flying but are fundamentally different. Similarly the fin of sharks and the forelimb of
whale are analogous organs which are adapted for swimming.
Evolution P.6
These are only dis______-related animals (though they are all vertebrates) which have
mastered movement in a marine environment from different starting points. As a result their bodies,
with a st__________ shape with fins or paddles, resemble each other in their outline and function,
but are derived from fundamentally different structures. Structures which fulfil the same roles but
have differing origins are called an
structures.
b) Vestigial organs
Some organisms possess non-fun_______ organs which, in other
organisms, have essential functions. Such organs are said to be
vestigial.
The greatly reduced c______ and ap_______ of man are useless
but they are very big and have important functions in the rabbit. It
is assumed that the caecum and appendix functioned normally in
man's ancestor, but since then degenerated as man
had changed from herb______ habit to omnivorous habit during
the course of evolution.
Whale and python have ve_______ hind limb girdles
embedded in the body. It suggested that they were evolved from
ancestors with functional hind limbs.
Likewise, slugs have vestigial sh____ embedded in their dorsal
surfaces.
D)
Comparative Anatomy of Embryos
More evidence of common ancestry comes from study
of the comparative anatomy of em_____.
Embryologists have noticed that vertebrate embryos in
the early stages of their development are difficult to tell
apart. Related organisms go through s______ stages
in their embryonic dev_________. The closer the
relationship of groups of animals and plant
through classification, the more do their embryos
res_____ each other.
Moreover, the arrangement of internal o____ such as
the arteries and the structure of the developing
h_____ in early vertebrate embryos also follow a very
si____ pattern. All these similarities strongly
suggest com
ancestries.
Evolution P.7
E)
Evidence from Comparative Physiology and Biochemistry
All living things have DNA as their genetic material, with a g______ code which is almost
uni_______. The processes of reading the code and protein synthesis, using RNA and ribosomes, are
very similar in prokaryotes and eukaryotes, too.
Physiological processes such as respiration involve the same types of step and similar or identical
biochemical reactions in all species. ATP is the universal e______ currency. Among autotrophic
organisms, the biochemistry of photo__________ is virtually identical, as well.
Thus biochemical analysis suggests a common o_____ for living things. The biochemical differences
between the living things of today are limited. This makes it possible to measure the relatedness of
different groups of organisms by the amount of the dif________ between specific molecules such as
D___, p_______ and en______ systems.
F)
'Genetic distance'- Differences between DNAs
The phylogenetic relationship between related species can be studied by comparing their ____ using
Rest______ Frag____ Length Polymorphisms (RFLPs).
The use of RFLPs was first reported by Botstein et al. (1980) during their construction of a genetic
linkage map in humans. RFLPs uses DNA re________ enzymes that recognize spe____ sequences in
DNA:



The restriction enzymes cleave the DNA at specific sites thereby creating frag____ of DNA.
The fragments are then electrophoresed on a gel,
hybridized and probed with a lab____ fragment of DNA.
Similarity between samples is based on the either the presence or absence of similar mobility
fragments (Botstein et al., 1980).
Samples that differ genetically create different quant___ of fragments as well as different fragments
of different l______. Genetic differences result in fragment differences as one or more base
differences could result in a loss of cleav___ site or the formation of a new s___.
Comparing DNA from a variety of living species can indicate quantitatively the degree of
relatedness of their genes. The degree of relatedness of various primates to humans estimated in this
way gives results as follows:
chimpanzee (an ape)
rhesus monkey
vervet monkey
galago (a prosimian)
97.6%
91.1%
84.2%
58.0%
The result of genetic studies agrees very well with other evidence about the evolution of the Primates.
b)
The Composition of Key Proteins
Another way of quantifying the degree to which species are related is to find the number of a_____
acid sequences they have in common in an essential protein such as cytochrome c or the protein of
haemo______ which occurs widely in vertebrates. Variations in amino acid sequences of proteins
arise from spontaneous m_______ in the genes that code for them.
The more d_______-related two organisms are the more d________ they will show in
protein composition, because more m_______ would have taken place over the geological time that
separates them.
Evolution P.8
It turns out that the sequences of amino acid residues of such proteins are similar in all living things
in which they occur. Between the human and the chimpanzee, our nearest Primate relative, we have no
differences in the amino acid sequence of our cytochromes and haemoglobin! When these results
were used to construct evo_________ tree, the results are remarkably similar to that from comparative
anatomy.
Difference in amino acid sequence in haemoglobin between Human and other animals:
Chinpanzee
0
Mouse
27
Gorilla
1
Kangaroo
38
Ape
2
Chicken
45
Rhesus monkey
8
Frog
67
Dog
15
Lamprey
125
Horse, cow
25
Sea slug
127
c)
Immunological Evidence
Immunology provides a method of detecting
differences in p
, and so quantifying
relatedness of species.
Human serum is used to induce the production of
anti______ in a rabbit, and the product,
Antihuman antibodies’ are obtained as a
suspension. Ser__ from other animals may be
tested against the Antihuman antidodies’.
The more closely related the animal is to humans,
the greater is the precipitation reaction that results.
Taking the precipitate produced by human serum
reacting with human antigens as 100 per cent, the
following values indicate the percentage degree of
evolutionary r__________:
Animal
chimpanzee, gorilla
gibbon
baboon
spider monkey
lemur
hedgehog
pig
Group
African ape
Asian ape
Old World monkey
New World monkey
Prosimian
Insectivorous mammal
Even-toed hoofed mammal
% degree of relatedness
97
92
75
58
37
17
8
F)
Evidence from Artificial Selection
By sel______ those members of a species which showed a fav________ variation, such as
increased size or improved flavour and artificially br_______ them by selective mating, selective
propagation or pollination, the des_____ characteristics were perpetuated. Continued selective
breeding by Man has produced the varieties of dom____ animals and plants of agricultural
importance seen today. There are vast differences between present-day varieties and their ancestors.
In terms of genetics. Man is preserving / s_________ those genes which are considered desirable
and el________ those which are undesirable for his purposes.
Evolution P.9
Whilst varieties of dogs. cats, birds, fish and flowers have been
produced for sporting or decorative purposes. A recently
developed form of artificial selection is the selection for res________
to antibiotics, pesticides and herbicides shown respectively by
pathogens, pests and weeds. A vicious circle is produced as new
strains of organisms become im_____ to the ever-increasing number
of chemical substances produced to contain and control them.
The principle of art______ selection is the same as nat_____ selection except that it is very much
qui_____ and the features selected may not be of sur_____ value in natural populations. It is really a
practical way of dem__________ selection in the evolution of species.
Q. Would you expect the artificially selected & bred species to be successful if let to live in the wild ? Explain.
G)
Evidence from Classification
If the species present on the earth were descended from a few simpler forms, then we could expect to
be able to classify them into ph____, cl_____, or____, fa______, gen____ and species just as is now
done. On the other hand, it is difficult to imagine how this would have been possible were the
organisms not related by des____, but each specially created according to individual plans.
The structural similarities between
organisms which forms the basis of a
nat_____system of
phylogenetic classification, suggest
the existence of an evolutionary
process.
These similarities and differences
between organisms may be
explained as the result of prog_______
adaptation by organisms within each
taxonomic group to particular
en____________ conditions over a
very long period of t_____.
IV)
The Mechanism of Evolution
Evolution by Natural Selection --- Darwin’s Theory
Darwin's theory can be summarised into three observable f____ plus two ded_______.
A)
a) Over-reproduction (Observation 1)
Organisms have enormous rep__________ pot_______, they often tend to increase in a
geometric ratio. Evidences for such enormous reproductive potential include:



The very slow-breeding elephant: if it were to bring forth six young in a lifetime, and if their
descendants continued to breed at this rate, in 750 years there would be 19 million elephants.
Many fish are capable of laying millions of eggs, e.g. female cod lays about 2-3 million eggs per year.
Flowering plants generally produce many hundreds of seeds.
b) Relative constancy of the numbers of species (Observation 2)
Evolution P.10
Despite this tendency towards a geo_____ increase in numbers, the population of a given species
tended to remain c_______.
c) Struggle for existence (First deduction)
d) Variation among the offspring (Observation 3)
Among the offspring there are usually slight and perhaps hardly noticeable diff_______.
Generally, no two offspring were i________. That is var_______ exist among offspring.
e) Natural selection / survival of the fittest (Final deduction)
Like produces l____. Those which survive to breed are likely to produce offspring si______ to
themselves. The adv___________ characteristics which gave them the edge in the struggle for
existence are likely to be inh______ on to the next generation.
Individuals lacking favourable characteristics are less likely to survive long enough to breed.
Over many generations their numbers will dec____. The individuals with
fav________ characteristics will breed, with consequent in______ in their numbers.
The inheritance of one small variation will NOT, by itself, produce a new species. However, the
development and acc__________ of m____ sm____ diff__________ over many gen_________
might gradually lead to the appearance of a n___ species – one that has become so diff_____
that it cannot b_____ with the original stock of organisms.
B)
Darwin’s dilemma
Darwin's chief difficulty was that he did not understand how organisms gen
variations on
which natural selection could act. It was generally accepted at that time that inheritance is blending,
that is, some substance is passed from each parent to the offspring in which the two contributions mix
and so the progeny would tend to be inter_______ in all characters (the so called paint-pot
inheritance).
Thus a red flower, crossed with a yellow flower, would sometimes produce orange offspring. This
would lead to increasing uniformity within the species and not to the range of different characters
required for natural selection to work upon. It was difficult to see how favourable
variations could acc_________ if their effects were diluted constantly in this way.
In an attempt to answer this problem Darwin proposed that envir_______ change could have brought
about a spont______ production of new variations that could then be inh______--which is in effect
another version of Inheritance of acq____ character’.
Evolution P.11
Ironically, the solution to the apparent problem of replenishing variation was discovered in Darwin's
lifetime although he was never aware of it; Mendel’s work which stated that the basic genetic factors
are ‘part______, do not b____ and can reappear in subsequent generations’ could have account
for the nature of var______ necessary for natural selection to occur.
C)
Neo-Darwinism
This refers to a new interpretation of Darwin’s theory of natural selection in the light of modern
g_____. It can be defined as: the theory of organic evolution by the natural selection of gen________
determined characteristics.
Some definitions in population genetics :
Population
A population is a group of sexually reproducing organisms of the s____ species
occupying a particular a____, the individuals in it share a com___ gene pool.
Gene pool
It is the sum total of all the genes within the reproductive cells of all the members of a
pop________.
Alleles
frequency
Natural
selection
It is the relative prop______ of the alleles of a gene present in a population.
Evolutionary
change
A population whose gene pool shows consistent ch_____ from generation to generation
is undergoing evolutionary changes.
A process that determine allele frequencies in population through une_____ rates of
reproduction of different phenotypes. The selection is done by the environment, thus all
environmental factors act together to select which alleles are more adv_________
and adaptive in this particular environment
The essential features of NeoDarwinism are
:
1. Genes are responsible for determining our phenotypes. Phenotypic variation in the absence of g______
bases will not produce evolution. For variations to have significance in evolution, they must be her______
variation.
2. Source of genetic variation:
 Mut_____ is the ultimate s______ of all variation: it presents n__ possibilities to the organism.
 Sexual Recom_______ of alleles during meiosis (crossing over & independent segregation)
and random fertilisation provides the immediate source of variation in sexual organisms. Mutant
genes can be brought into the company of different alleles with which they will produce different
phenotypic effects as a result of gene resh________.
3. Natural selection 'chooses' the b____-adapted organisms (that is, those with the most
favourable collection of phenotypic characters) in a population to survive and breed more successfully
than their less well-endowed competitors.
4. Individuals do NOT evolve, but p___________ do.
5. As a result of persistent natural s_________ forces acting on the g____ pool of a p__________, the gene
fre_______ in the population might change. A population whose gene pool shows a consistent /
directional change from generation to generation is undergoing evolutionary change.
6. Mechanism of speciation: Change in allele frequency within populations, as a result of effect of natural
selection on the range of phenotypes produced by mutation and sexual reproduction, lead to the
formation of r___ and sub_______.
If genetic iso_____ persists over prolonged period of time, the subspecies might evolve in such different
directions that they do not inter______ successfully again when they come together. Speciation is said to
occur and the subspecies are then considered as separated sp______.
Evolution P.12
D)
Selection
Selection is the process by which those individuals who appear phy_____, phy__________
and beh_________ better ad_____ to the environment sur____ and re_____; those organisms not so
well adapted either fail to reproduce or die- i.e.be el________.
The former organisms pass on their suc________ characteristics to the next generation, whereas the
latter do not. Therefore selection can be seen to operate through the processes of differential
mort____ and diff_______ reproduction.
Selection depends upon the existence of phenotypic v________ within the pop_______.
When a population increases in size, certain environmental factors become lim_____, such as
food availability in animals and light in the case of plants. This produces com________ for resources
between members of the population. Those organisms exhibiting characteristics which give them
a competitive ad________ will obtain the resource, survive and reproduce.
Both env___________ factors and pop_______ size operate together to produce a selection
pres____. Which operate in a pop_________, increasing or decreasing the fre______ of an allele
within the gene p____ of the population.
Changes in al___ frequency in a l____ run can lead to evol_______ changes.
Three Types of selection (Reference)
There are three types of selection process occurring in natural and artificial populations and they are
stabilising, directional and disruptive selection.
They may be best explained in terms of the normal distribution curve associated with the continuous
phenotypic variation found in natural populations.
Evolution P.13
1)
Stabilising selection
This operates when phenotypic features coincide with st____ optimal
environmental conditions and competition is not severe. The selection
force tends to eliminate ext_____ from the population.
Stabilising selection pressures do not promote evol________ change
but tend to maintain phen_____ stab____ within the population from
generation to generation.
This is probably the most common way in which natural selection
operates. Organisms which have lived for millions of years in very st____
environments closely resemble their ancient relatives. The oyster
and the coelacanth fish are examples of creatures which have
retained an almost constant form, presumably by the operation of
stabilising selection.
Q. Where do you suppose these living fossil’ would likely to be found? Search for information about one such organism
and be ready to present your findings in the class (2 minute presentation).
2)
Directional selection
This form of selection operates in response to gr____ changes in
environmental conditions. It exerts selection pressure which moves the
m___ phenotype towards one phenotypic ext____. Once the mean
phenotype coincides with the new optimum environmental conditions
s_________ selection will take over.
This kind of selection brings about evolutionary ch____ by producing a
selection pressure which favours the increase in frequency of new alleles
within the population.
Directional selection forms the basis of art______ selection where the
selective breeding of phenotypes showing desirable traits increases the
fre_______ of those ph________ within the population.
3)
Disruptive selection
This
is
probably
the
rarest
form
of
selection.
Fluct______ conditions within an environment, say associated with
season and climate, may favour the presence of more than one
ph________ within a population. Selection pressures acting from
within the population as a result of increased competition may push
the phenotypes away from the population mean towards the
ext______ of the population.
This can spl__ a population into two subpopulations. If gene flow
between the subpopulations is prevented, each population may give
rise to a new species. In some cases this form of selection can give
rise to the appearance of different phenotypes within a population,
a phenomenon known as poly__________.
Intensity of selection pressure
External factors may include an increase in numbers of p______ or p_______ or competition from
other species -i________ c_____________. Changes in c________ conditions etc.
Internal factors such as a rapid increase in the size of the p_________ can result in
increased competition for environmental resources - i___________ competition.
Evolution P.14
One possible result of increased selection pressure is that it may cause organisms to become more
sp________ to particular modes of life or narrower environmental conditions. This may be a
disadvantage for the future of that species. Increased uniformity and dependency by a species
increases the likelihood of that species becoming ex____ should environmental conditions change. The
fossil record contains many extinct organisms that were bizarre and oversp________.
E)
Recent Examples of Evolution (by Natural Selection) in action
There is no way we can prove natural selection was responsible for the evolutionary process that has
taken place in the past. But examples of n_______ s________ in action are abundant all around us.
a)
Industrial melanism in moths
The moth's wild-type colour is speckled grey (mottled form) which offers it good cam______ on a
lichen-covered tree-trunk. Its chief pre______ are birds which hunt it by sight. An occasional
m________ occurs which makes the moth very dark and conspicuous against a normal tree trunk.
At the time of the Industrial Revolution, smoke-laden air
began to kill l____ and deposit black films on city tree-trunks.
Melanic moths show up less against sooty tree-trunks than do
wild-type ones. The first recorded specimen of melanic Biston
betularia occurred in 1848 in Manchester and, by 1895, about 98
per cent of the moths in that area were dark.
Such an increase implies a powerful selective adv______ to the
melanic form and illustrates how rapidly gene-fre_________ in
a population can change under such circumstances.
Predatory bird act as agent of n______ s________, favouring the survival of the best-cam________
moths. Where surfaces are blackened by soot the melanic form is better camouflaged and predation
is largely restricted to the mottled form. In areas not polluted by soot the reverse is true and the
mottled form predominates. There are more than 150 species of moths known to exhibit industrial
melanism.
Q. What kind of selection is shown in the case of the melanism in moth? (hint: think about the polymorphic form of moths
that resulted)
b)
Metal tolerance in plants
One of the most convincing examples of natural selection is provided by the evolution in certain
grasses of tolerance to heavy metals, such as copper, zinc and lead.
Various physiological mechanisms* have evolved to allow these plants to grow and reproduce in
soils, e.g. spoil from mines, where heavy metals are present at concentrations that kill normal plants.
Indeed, the plants have become so well adapted to living in such conditions that on unpolluted soil
they do not fare so well and are out-comp____ by normal plants.
Evolution P.15
*In some species the toxic metals bind to organic molecules in the cell walls where they remain trapped, unable
to harm the cells. In other species the metals are stored in the vacuole. Other species trap the metals at special
membrane sites. Some plants simply export the metals from their cells to the external environment.
c)
Insecticides
The wide______ use of insecticides has provided a selection p________ which has resulted in
adaptive changes within a very short time. It has taken only a few years, and in some cases only
months, to produce strains of insects which have become completely re______ to many insecticides.
The widespread use of DDT to control insect pests has lead to the evolution of re________ to
this chemical by many species. One Nigerian survey of mosquitoes in the species Anopheles found that
in unsprayed villages almost no flies were resistant to DDT. However in villages which had been
subject to regular spraying, approximately 90 per cent of the flies were homozygous for resistant
alleles.
In house flies, several mechanisms are known which permit resistance. Four genes are involved and each has a
distinct function. Two of the DDT-resistant genes enable the poison to be detoxified, each by a different route.
Another gene reduces the rate at which DDT penetrates the cuticle. The fourth gene acts by an unknown route.
d)
Antibiotics
The resistance of bacteria to antibiotics has arisen by the sel______ of resistant mutants. As with
types of insecticide resistance, some resistance to antibiotics has evolved by 'switching on' genes
to control the production of anti______-splitting enzymes.
Resistance to antibiotics has already destroyed the usefulness of several drugs. The first signs of
staphylococcal resistance appeared soon after penicillin became extensively used in hospitals.
Resistance has now appeared in certain staphylococci to all major antibiotics, often as a triple
resistance to penicillin, tetracycline, and streptomycin.
Nowadays doctors tend to be more
restrained than they were about how many different antibiotics they prescribe. That way, a few really
effective ones can be kept in reserve to be used only if a bacterium is resistant to all the more common
antibiotics.
Q. Name three features of bacteria that enable them to evolve so quickly in response to selection pressure.
Q. Antibiotic resistance is becoming a hot issue in HK. There are malpractices in both the medical practitioners
and the patients that should be blamed. What are these malpractices?
e)
Adaptive immunity in mammals
Micro-organisms and insects rep________ at such an alarming rate that it is relatively easy to
visualize how they can adapt faster than man can combat them. Prolifically breeding mammals such
as the brown rat and the rabbit have also evolved resistant strains capable of withstanding warfarin
and myxomatosis virus, respectively.
Warfarin was developed as a rat poison. It interferes with clotting in the blood. In 1960, strains were
first recognized which were immune to warfarin. The incidence of resistant rats in parts of Wales
had risen to 50% by 1970 because of the intensive selective pressure of the widespread use of
warfarin.
Myxomatosis-resistance in rabbits became widespread in the mid 1950s. Prior to this, the virus was
responsible for up to 90% mortality of rabbits in certain areas. The genetic response to this selective
pressure was the production of mutants which spent a greater proportion of time above the ground,
like hares. Normal rabbits live in crowded warrens underground where the vector of the virus, the
rabbit flea, can spread rapidly throughout a population. Those that spent most time above ground were
favoured, whereas previously they were selected against because of predation. Even though the rabbits
Evolution P.16
were not physiologically resistant, their altered behaviour offered them protection. In the Australian
population, however, a genetically resistant strain emerged after the initial epidemics. Most British
rabbits are now genetically resistant to myxomatosis.
f)
Sickle-cell anaemia
This is caused by a mutation which resulted in the incorporation of an incorrect a____ acid at one
point in the protein chains of the h__________ molecule. The mutated gene is known as Haemoglobin
S or HbS and is recessive. Those persons homo_____ for the gene suffer acutely from the disease.
The disease is an often fatal form of anaemia which is relatively common in West A____. Whenever
the blood cells of a victim encounter a low level of oxygen, as in the venous blood of tissues, they are
liable to collapse to a s____ shape and may form blockages and other complications in blood vessels.
Sufferers have only a 20% chance of surviving to maturity as compared with normal people.
Het_________ individuals suffer also, even though the symptom is milder. Given this information,
one would NOT expect the mutation to be favoured. Indeed, such harmful mutations are frequently
elim________. This happens rapidly where it is a dominant gene, more sl_____ where it is a recessive,
as selection acts only when it appears in the ph_______.
However, there are still large areas of Africa and Asia where the gene occurs, usually in the
hetero_________ form in 15-20% of the population. There are even communities within these regions
with frequencies of 40%. Clearly, the gene is not being removed from the population.
The answer lies in resistance to mal___. Children with HbS have been shown to have a 25%
better chance of surviving malarial attacks than those with normal genes. Selection is NOT acting to
remove the genes causing sickle cell anaemia. They are removed when hom_______ but this
selection is outweighed by their selective advantage when het_________, due to better resistance on
the part of heterozygotes to malaria.
Examples of natural selection in action are abundant. It is widely believed that over time natural
selection has been the prime mechanism dri_____ evolution and is still oper_____ today.
V)
A)
Speciation and Isolating Mechanisms
The definition for a species
The term

pecies’ can be defined in different ways:
In terms of classification, it is one of the
smallest
/
lowest
u___
of classification commonly used;
members of the same species shared the
greatest
resem______
structurally
and functionally.

In terms of breeding & genetics, members of the
same species are capable of inter_______
and produce fer___ offspring.
Evolution P.17
B)
Speciation
This is the process by which one or more species ar___ from previously ex____ species.
If speci_____ occurs whilst the populations are sep_____ it is termed allopatric speciation. If the
process occurs whilst the populations are occupying the s____ geographical area it
is called sympatric speciation.
Allopatric speciation is characterised by the occurrence, at some stage, of spatial separation or
geo________ sep_______. This inability of organisms or their gametes to meet leads to rep_______
isol______, which will in turn produce a bar____ to g___ flow and thus population become
gen_______ isol____.
i)
Allopatric speciation
Organisms belonging to a given species rarely exist naturally as a single large population. It is usual
for a species to exist as small interbreeding populations, each with its own g___ pool.
Spatial separation by geo_______
barriers such as mountain ranges, seas or rivers means that
sp_____ separated pop_______ of the same species may encounter a vari___ of
environmental conditions and different selection p_______.
As a result of geo_______ barriers, gene f___
between populations might become interrupted.
As a result of this each subpopulation becomes
genetically is
.
The effects of natural sel_______ on the range
of ph______ produced by mut_____ and sexual
recom________ will result in changes in allele
and genotype fre________ within the
populations, this lead to the formation of r___
and subsp____.
If the genetic isolation persists over a
prolonged period of t____ and the subspecies
then come together to occupy the same area they
may or may not inter_____. If the breeding is
successful they may still be considered to belong
to the same species. If the breeding is
unsuccessful, then sp________ has
occurred and the subspecies may now
be considered to be sep______ species.
Q. An initial factor in the process of speciation may be the reduction in the intensity of selection pressure within the
population. This may lead to increased variability. These new phenotypes may enable the population to increase its
geographical range if the phenotypes show adaptations to environmental conditions.
a. Under what circumstances would there be ‘reduction in the intensity of selection pressure in the environment ‘?
b. What is the genetic basis of the origin of the ‘increased variability’ ?
Evolution P.18
Sympatric speciation
i)
In complete contrast to allopatric speciation, species undergoing sym_____ speciation are NOT
geographically isolated by, for example, a mountain or a river. The speciating populations share the
same ter________.
Sympatric spe_______ events are most common in p____ when they double or triple the number
of chromosomes, resulting in a condition called p________. In such cases there might also be
intraspecific speciation (auto_____________) or inter-specific speciation (allo____________).
In the latter case it is also called interspecific hyb_________. Polyploidy results in an ‘Instant
species’ in contrast to allopatric speciation which requires a much long period of t____.
Sympatric spe_______ can also explained by disr______ selection which favours individuals at both
ext_____ of a range of phenotypes over those in the middle. Thus the gene pool may become sp__
into two distinct gene p____. This may be a way in which n___ species are formed.
An example of how disruptive selection might lead to sympatric speciation:
Tolerance to heavy metal contaminated soil
The residues of mines often contain high concentrations of toxic heavy metals (e.g., copper, lead) that
most plants are unable to grow on them. However, some hardy species (e.g. certain grasses) are able
to spread from the surrounding uncontaminated soil onto such wasteland.
These plants develop resistance to the toxic metals while their ability to grow on uncontaminated soil
decreases. Because grasses are wind pollinated, cross-breeding between the resistant
and nonresistant populations goes on. But evidently, disruptive selection is at work. Higher death
rates of both

less resistant plants growing on contaminated soil and

more resistant plants growing on uncontaminated soil
leads to increasing divergence of the populations into two subpopulations with the extreme
manifestations of this trait.
Evolution P.19
In all cases, either allopatric or sympatric speciation will eventually
lead to certain forms of rep_______ isolation and hence g_____ i______ of
the subpopulations, in time increasing genetic diver______ of the g____ p___
of the sub_________ might lead to production of separate species.
Reproductive Isolating mechanisms
Speciation will only occur as a result of the formation of barriers which lead to reproductive isolation.
This can be brought about by mechanisms acting before or after fertilisation:
Prezygotic
mechanisms
(barriers to the formation of hybrids)
Seasonal
isolation


Occurs in animals where differences in genitalia prevent
successful copulation and in plants where related species of flowers
are pollinated by different animals
Ecological
isolation


Occurs where two species mate or flower at different times of the
year.
Behavioural
isolation


Occurs where two species inhabit similar regions but have different
habitat preferences; for example Viola arvensis grows on calcareous
soils whereas Viola tricolor prefers acid soils
Mechanical
isolation


Occurs where animals exhibit courtship patterns, mating only results
if the courtship display by one sex results in acceptance by the other
sex; for example certain fish, bird and insect species
Postzygotic
mechanisms
(barriers affecting hybrids)
Hybrid inviabilit
y


Hybrids fail to produce functional gametes; for example the mule
(2n = 63) results from the cross between the horse (Equus equus, 2n =
60) and the ass (Equus hemionus, 2n = 66)
Hybrid sterility


Hybrids are produced but fail to develop to maturity; for example
hybrids formed between northern and southern races of the
leopard frog (Rana pipiens) in North America
One way in which allopolyploidy can produce an Instant’ species:
Diploid 2n1 chrom
osomes
meiosis
Diploid 2n2 chrom
osomes
meiosis
Gamete
n1 chromosome
s
Gamete
n2 chromosome
s
Diploid hybrid
n1 +
n2 chromosom
es
Non-disjunction
during asexual
reproduction
chromosomes cannot
pair, therefore
meiosis fails
no Viable
Gametes
(sterile)
Tetraploid hybrid
2n1 +
2n2 chromosomes
chromosomes
all
find homologous
partners
Viable Gametes
With
n1 + n2
chromosomes
(self-fertile)
Evolution P.20
Why is polyploidy important?
Polyploidy has long been recognized as a prominent force shaping the evolution of plants. Many
important crop plants, such as alfalfa, cotton, potato, and wheat polyploids. Based on fossil records, 70% of
angiosperms were estimated to have had a polyploidization event in their species’ history.
Many polyploids are more vigorous than their diploid progenitors and both autoand allo-polyploids have built in mechanisms for maintaining high levels of
heterozygosity.
Thus polyploids may exhibit a phenomenon similar to hy_____ vigor, or heterosis.
Polyploids also are known to exhibit new ph______ variation that can arise shortly
after polyploid formation.
Hybrid vigor, also known as het_____, is an agriculturally important phenomenon
describing the observation that the hybrid offspring of two inbred genetically
different varieties produces higher y_____ than either one of the two parental
lines.
Evolution P.21
G)
Concept Map: Evolution and the proposed mechanisms of Evolution
Key words :
changes, generations, genotypes, gradual process,
successive, population of organisms,
Evolution
is a
in which
undergo
in
over
Reference reading I: Lamarck's theory of Evolution
Lamarck published his theory of evolution in1809. His theory is
developed from a series of postulates.

The formation of a new organ in the body, is the result of a new
need which has arisen and continues to be felt by the animal.

The extent of development of organs and their power of action
is proportional to their use.

All changes in or acquisition to the bodily organization of an
individual during its life are transmitted to its descendants by
the process of reproduction.
There is no evidence of inheritance of characters acquired during
the life of an individual. The theory of Lamarck fails to give an
adequate explanation of evolution because the postulates on which it
is based cannot be accepted in the light of present knowledge.
Evolution P.22
Reference reading II: Darwin, Wallace and the theory of Natural Selection
A)
Darwin, Wallace & Natural Selection
Charles Darwin was born in 1809, the son of a doctor, and like many great people he had an
undistinguished academic career. In 1831 he accepted an unpaid post as naturalist on the survey ship H.M.S.
Beagle, which spent the next 5 years at sea along Coasts of S. America.
The Beagle returned to Falmouth in October 1836 via
the coast of Chile, the Galapagos Islands, Tahiti, New
Zealand, Tasmania and South Africa.
For most of this time Darwin was concerned with
studying geology, but during a five-week stay on the
Galapagos Islands he was struck by the similarities
shown by the flora and fauna of the islands
and mainland.
In particular he was intrigued by the characteristic distributions of species of tortoises and finches.
He collected a great deal of biological data concerned with variation between organisms which convinced him
that species were not immutable. On his return home his work on the selective breeding of pigeons
and other domestic animals gave him a clue to the concept of artificial selection.
An earlier Essay on the Principles of population by the Reverend Thomas Malthus, published in 1778,
had highlighted the consequences of the reproductive potential of Man. Darwin applied this to other
organisms and saw that despite this the numbers within populations remained relatively constant.
Having collated a vast amount of information he began to realise that under the intense competition of
numbers in a population, any variations which favoured survival in a particular environment would increase
that individual's ability to reproduce and leave fertile offspring. Less favourable variations would be at a
disadvantage and organisms possessing them would therefore have their chances of successful reproduction
decreased. These data provided Darwin with the framework to formulate, by 1839, a theory of evolution by
natural selection, but he did not Publish his findings at that time. Indeed Darwin's greatest contribution to
science was not so much to show that evolution occurs but how it might occur.
In the meantime, Alfred Wallace, who had travelled widely in S. America, Malaya and the archipelago, ,
had come to the same conclusions as Darwin regarding natural selection. In 1858, Wallace wrote a 20-page
essay outlining his theory and sent it to Darwin. This stimulated and encouraged Darwin and in July 1858,
Darwin and Wallace presented papers on their ideas at a meeting of the Linnean Society in London. Over a year
later, in November 1859, Darwin published On the Origin of species by Means of Natural Selection . All 1250
printed copies were sold on the day of publication and it is said that this book has been second only to the Bible
in its impact on Man's thinking.
During his
voyages on the
Beagle, Darwin
visited the
Galapagos
Islands which
are about 1000
km off the
northwest coas
t of South
America.
Darwin
realised that the
birds on the
Galapagos
Islands were
different from
those of the
mainland.
He counted 13
species of finch,
and noticed that
each had a
slightly
different
shaped beak .
Evolution P.23
a. In figure above, which beak is adapted to which food?
b. Natural selection only partly explains why this adaptive
radiation took place in the finches of the Galapagos. The
geographical isolation mechanisms operating in the islands
also played an important part in the evolution of the finch
species. What main factors have contributed to the evolution
of so many finch species in the Galapagos?
c. The following statements relate to the diagrams in figure
below. Write out the statements in the correct order and,
beside each, put the letter of the diagram which corresponds
to it.
Corresponding
diagram
Event

The finches increased in numbers and, under the influence of natural
selection, gradually became adapted to the local environment.

Some finches from the second island managed to fly back to the first
island, but reproductive isolation had occurred between them and the
existing population.

Originally, there were no finches on the islands. Some finches from the
mainland managed to fly across to them.

Some of the finches managed to fly to a second island where the
environment was different.

This process was repeated over and over again as the
finches colonised more and more of the islands.

Adaptation to the conditions on the second island gradually took place.
Correct
order
Reference reading III:
A)
Misconceptions about Darwin’s theory of Evolution
1. Darwin was the first to propose that Evolution occurred
The idea of evolution can be traced to a thousand years before Darwin back to that of Aristotle time. The
important contribution of Darwin is to propose a mechanism that explain how evolution might take place---namely by natural selection.
2. Darwin tried to explain how life originated on Earth.
Darwin made no attempt to describe how life originated on Earth: his concern was how new species might
arise from pre-existing species.
3. According to Darwin: “Man is descended from apes”.
The statement implies the idea of ‘a linear progression of evolutionary event leading from apes to human
being’ which is simply wrong. The correct interpretation of the evolutionary relationship between Man
and apes is that man and apes probably have a common ancestry, just as we say we Chinese all
originated from some common ancestry around the Huang river a very long time ago.
Evolution P.24
B)
Arguments against the theory of Natural Selection
Opponents of the theory of evolution by natural selection (though not of the fact of evolution) make a number
of criticisms:
1. Mutations are usually harmful, so it is impossible to conceive that random mutations could accumulate to
the benefit of a species.
2. Whilst complex organs, such as the eye, function effectively, it is difficult to imagine how a halfevolved eye would be useful. (The theory of natural selection demands that each 'stage' in the evolution
ofan organ must be beneficial in itself, without the need to propose a directive force which 'works towards' a
distant end.)
3. The theory does not account for the evolution of the higher taxonomic categories (genus, family,
order, class, phylum).
These criticisms can be met as follows:
1.
Since mutation is a rare event, organisms are heterozygous for a mutant gene when it first appears.
Harmful effects can be suppressed if they are recessive, and natural selection therefore favours those
organisms in which the mutant gene's phenotypic effects are minimal, until total recessiveness is achieved.
All organisms carry a reservoir of potentially harmful mutant genes whose expression is limited or
suppressed. The majority of these genes will have minor effects, since those with profoundly harmful
attributes are likely to kill the organism before natural selection has the opportunity to modify their effects.
Polygenic inheritance is particularly suited to subtle evolutionary change. Since a particular character is under
the control of many genes, none of which has a very great effect, one mutation will not cause much
phenotypic change. More likely, that character will merely appear a shade more extreme so that a gradual shift
occurs as mutations accumulate. Such a process is much less likely to cause dramatic, harmful effects than
the mutation of a 'major', non-polygenic gene.
Suppressed mutant genes which are kept 'in reserve' may have effects which are beneficial
under changed environmental circumstances. The genes conferring lead-tolerance to Agrostis tenuis
appeared to 'served no useful purpose' until the grass encountered lead in its surroundings.
2.
Complex organs develop their attributes gradually. A simple light-dark detector performs less well than a
vertebrate compound eye, yet its possessor may survive better than a competitor without it. Improvements can
be added gradually.
The eye did not appear suddenly in the offspring of an eyeless creature; it is the result of an evolutionary
development that took many millions of years. Amphioxus, a primitive fish-like form which has no eyes but
light-sensitive pigment cells in the brain may have been the starting point. From this rudimentary beginning
the eye has passed through innumberable stages, each slight change useful to its possessor,
and therefore adjusted to the demands of the environment by natural selection.
3.
The evolution of the higher taxonomic categories appears to require a different process from that of
speciation. The discontinuities between the groups is wide; there are no organisms half-way between ferns
and conifers or between butterflies and beetles.
The problem lessens when we realise that existing plants and animals are a mere fraction of those types
which have existed. The extinction of a group's relatives leaves it distinct and becoming more distinctive as it
diversifies and specialises. Fossils fill some gaps, although even these represent a biased sample of past life.
Most organisms were not suitable for fossilisation and the earliest ones have been destroyed by crushing
and heat in the rocks. If every organism that has lived could be resurrected, the collection would be much more
of a continuum than a collection of all living species.
Life has evolved over the last several million years. In such a time-span, events become inevitable which
would be highly improbable in the short-term.
Evolutionists believe that evolution is a creative response of the living matter to the challenges of the
environment. The role of the environment is to provide opportunities for the biological inventions. Evolution is
due neither to chance nor to design; its is due to a natural creative process.’
Reference : http://www.talkorigins.org/origins/faqs-evolution.html
http://evolution.berkeley.edu/evosite/misconceps/index.shtml
Evolution P.25
C)
Can Evolution be controlled by man
The answer is 'Yes’ in certain circumstances.' The achievements of such control are most obvious in
those species, such as cultivated c___ and dom_______ animals, which man has directly utilized for
his own benefit and convenience; the possibilities are present in many other species; while the
attempts are most uncertain and most controversial in man himself:
Artificial Selection--Plant and animal breeding
Man has been practising domestication of animals and plants since early human civilisation. The basis of
artificial selection is the isolation of natural populations and the selective b_______ of organisms
showing characteristics or traits which have some usefulness to Man.
The selective forces operating might have been uncon____ effects such as protection offered by man from
natural predators which will have relaxed selection pressures towards evasion mechanisms; or con_____
selection will have occurred towards such aims as increased grain yield in wheat or faster hunters among
dogs.
Over thousands of years these processes have produced large changes from the original forms
and enormous div_____ among contemporary forms, as can be readily seen among dogs.
In artificial selection Man is exerting a dir______ selection pressure which leads to changes in allele
and genotype frequencies within the population. This is an evolutionary mechanism which gives rise to new
breeds, strains, varieties, races and subspecies. There are two forms of artificial selection called inbreeding
and outbreeding.
1)
Inbreeding
This involves selective reproduction between cl____ related organisms, for example between offspring
produced by the same parents, in order to propagate particularly des_____ characteristics. The extreme
form of inbreeding is s___-fertilization. Inbreeding is a particularly common practice in the breeding
of 'show' animals such as cats and dogs. It was used by livestock breeders to produce cattle, pigs, poultry
and sheep with high yields of milk, meat, eggs and wool respectively, but for reasons stated below inbreeding
is not now widely practised .

Prolonged inbreeding lead to a reduction in fert____ -- a particular problem in the breeding of livestock.

Intensive inbreeding reduces the vari______ of the genome (the sum of all the alleles of the species) by
increasing the number of homo______ genotypes at the expense of heterozygosity
Implication of inbreeding in evolution

Meiosis is ineffectual in generating variability
Recombination is meaningless in an individual which is completely homozygous. The progeny are
genetically un_____ and identical to the parent from which they derive.

Variability is exposed
Since there are no heterzygotes in the population, recessive alleles cannot be hid___ in the heterzygous
form; They are exposed as homozygotes and acted upon by n______ selection immediately.
Unfavourable ones are rem____ from the population.
There is immediate fitness of the at the expense of flexibility, i.e it does not allow variation.
All the inbreeding pure lines breed true and each produces progeny which are identical to one another and to
their parents. If the parents are well adapted to their environment then all of the progeny are also well
adapted and there is im______ fitness. There is no wastage due to variable genotypes which are not well
adapted.

In the short term, inbreeding may be adv_________, particularly where conditions are of an extreme kind.
The disadvantage of inbreeding is that it lacks long term flex______ and makes no provision for the
fut____. When the environment changes there is only a limited pool of var________ from which to
draw, and if none of lines are viable in the new environment then they will all die.
Without the regular release of variation in each generation, inbreeders have no capacity to adapt, and in
evolutionary terms they have no future.
Evolution P.26
2)
Outbreeding
i) Hybrid vigour resulting from outbreeding
It involves crossing individuals from genetically distinct populations. Outbreeding usually takes place between
members of different varieties or strains, and in certain plants between closely related species. The
progeny are known as hy____, and have phenotypes showing characteristics which are sup
to either of
the parental stocks. This phenomenon is known as hybrid vi
or heterosis.
Hybrids produced from crossing homozygous parental stocks from different populations are called F1 hybrids
and show advantages such as increased fr___ size and s___ number, increased re________ to disease
and earlier maturity.

Increased vigour results from the increased hetero
-- which arises from gene mixing. e.g, whilst
each homozygous parent may possess some, but not all, of the dominant alleles for vigorous growth, the
heterozygote produced will carry all the dominant allele.

Increased vigour can also result from new phen______
particular combinations of alleles in the heterozygote.
due
to
interaction
between
ii) Implication of outbreeding for evolution

Maintain heterozygosity and variabilty
In natural populations of outbred species, there is a wide range of variations present among the
individuals. Only some of them are well adapted to the prevailing environmental conditions. If there is
a change in the environment, some of the variants may be better suited to the altered conditions.

Future adaptability and long term flexibility
Genetic systems based on outbreeding are therefore more flexible than those based on
inbreeding,
and make more provision for the future. Outbreeders are more adaptable and their capacity for change
also means that they are able to survive under a greater variety of conditions
iii) Example of animal breeding : Artificial insemination
In farm animals, it is frequently used as a rapid means of spreading the beneficial characteristics of a
particular male throughout the stock.
It involves the collection of semen, its dilution and, if required, storage ror long period at very low
temperatures. When required the Semen can then be injected into fenales to cause pregnancy. Using this
technique, one male can be used to fertilize many more females than would be possible using natural mating;
the semen can be stored even after the animal has died; and it can be sent over long distances so that the
genetic material can be used as widely as possible.
Artificial insemination is most widely used with cattle, particularly cows for milk production, but the technique is
now becoming more common in pig breeding and can be used with almost any mammal.
Q. the semen from a single bull can be used to sire as many as 90 000 cattle a year. With so large a number,
why do you think it is extremely important for farmers to maintain accurate breeding records?
iv)
Reference Reading : Artificial selection in Human?
Could the kind of artificial selection we have been discussing be imposed on humans? To most people this idea
seems repugnant, conjuring up visions of selective breeding and compulsory birth control and eugenics. In
fact it already happens to a slight degree as when, for example, a couple with a history of abnormality in
their children or close relatives decide, perhaps on the basis of information given them by a genetic counsellor,
not to have any more children.
Recently, though, advances in molecular genetics, it is already possible to detect carriers for a recessive
allele. Further, if in vitro fertilisation is accompanied by suitable biochemical tests carried out at the 8-16 cell
stage, doctors may be able to determine whether the embryo will develop into a healthy baby or show any
genetic abnormalities. This information could then be made available to the woman before a decision has to
be taken as to whether or not to implant the embryo into her womb. To what extent genetic testing, particularly
prenatal testing, be available to parents?
Evolution P.27
VI) Evolution : A mixed-up
A He believed species changed by the use or disuse of parts of the body.
B this can be caused by mutation, meiosis or environmental influences.
C Evolution, from a primitive type of organism to several divergent forms
adapted to distinct modes of life.
D The type of bird Darwin found many species of.
E Selective breeding by man. A demonstration of evolution under the
direction of human influence.
F Geographical barrier to interbreeding.
G The oldest fossils of vertebrates are these.
H The birds on the island Darwin made famous showed many different
varieties of this body part.
I This prevents varieties of the same species breeding because they are in
season at different times.
J Factors preventing interbreeding.
K A lizard-like fossil which has feathers.
L A famous variety of moth which changed from pale to dark when their
trees got covered in soot.
M Grouping of different organisms.
N The islands which Darwin made famous.
O A famous naturalist spent five years on this boat.
P These survive.
Q Organisms' remains that are found in certain types of rock.
R The type of rock in which ancient organisms can be found preserved.
S When populations become this it can be because they feed and lay their
eggs on different plants or breed at different times of year.
T A group of organisms which can interbreed successfully.
U The change in a population of a species over a large no. of generations.
V The limbs of vertebrates have 5 fingers in common.
W Can be caused by ultraviolet light or X-rays.
X The process the moths (Clue L) went through when they changed colour.
Y Adult vertebrae might be different but these remain similar in all
vertebrates.
Z Survival tests set by the environment.
a Parts with the same purpose but different fundamendal structure.
Organisms possession of these structures do not imply a close evolutionary
relationship. It merely indicates adaptations to similar conditions.
b Birds evolved from these animals.
c Man who believed species changed genetically.
d They believe in the origin of the species as described in the book of
Genesis.
ADAPTIVE
RADIATION
ANALOGOUS
ARTIFICIAL
SELECTION
BEAGLE
BEAK
ARCHAEOPTERYX
PEPPERED MOTH
BREEDING
BARRIERS
CLASSIFICATION
EMBRYOS
EVOLUTION
FINCHES
FISH
FITTEST
FOSSILS
FUNDAMENTALIS
TS
GALAPAGOS
REPROODUCTIVEL
Y ISOLATED
LAMARCK
MUTATION
INDUSTRIAL
MELANISM
CHARLES DARWIN
NATURAL
SELECTION
PENTADACTYL
REPTILE
SEDIMENTARY
SPATIALLY
ISOLATED
SPECIES
ECOLOGICAL
BARRIER
VARIATIONS
The end !