Chapter 18
Summary of Speciation
a) (1) The concept of the species as a natural unit which is reproductively
isolated.
A species is a group of actually or potentially interbreeding nature
populations which are reproductively isolated from other such groups.
Sometimes, crosses can be made between two closely related
populations but the progeny are infertile and cannot themselves produce
offspring. For example, a horse and a donkey can mate but the mule
produced is infertile. The horse and the donkey must therefore be
considered as separate species. The horse and donkey are said to be
reproductively isolated from one another.
(2) The extinction of species
During the course of evolution, over millions of years many species
have come and gone as environmental conditions have altered. Those
species that have been able to adapt to change have survived and in turn
have given rise to new species that were even better adapted to the new
conditions. The populations of those species unable to adapt, and
therefore less successful at competing for their essential needs, have
decreased in number to the point of extinction.
Since the Coming of Man, the rate of environmental change has
dramatically increased resulting in a rapid micro-evolution. To that
extent it is arguable that Man’s activities have enriched, rather than
impoverished the world’s flora, and to a lesser degree, fauna. Much of
the richness and beauty of the wild flowers is associated, not with
natural vegetation, but with secondary communities, which under the
influence of grazing, burning and primitive agriculture, have largely
taken their place. Nature organisations are increasingly aware of the
difficulty in conserving the flora and fauna of any ‘developed’ region of
the world, namely that in most cases the ‘rare’ species are the very ones
that do not have an obvious niche in today’s dominant and stable
vegetation-types, regard-less of Man’s activities.
(ref. Evolution of Wheat)
On a world scale, however, the position is very different. Using modern
technology. Man is now able to destroy the natural climax vegetation
swiftly and ruthlessly, and vast areas of Tropical forest together with
the whole of their complex, interrelated plant and animal communities
are disappearing very rapidly. once extinct, they cannot be recreated.
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Of course, some other vegetation, more capable of resisting Man’s
onslaught will survive and new species will evolve; but, since the
evolutionary timescale is so different, we destroy in a decade what took
millions of years to arise. The inevitable result is a drastic reduction in
the variety of flora and fauna worldwide. Much of our effort in nature
conservation is necessarily a rescue operation, at the eleventh hour, of
plant and animal species and communities which cannot possibly
survive the onslaught of Modern Man.
b)
The importance of isolating mechanisms, including barriers to gene
exchange leading to the conservation of some species for example
marsupials.
Each individual in a population contains a certain combination of genes.
Other individuals in the population have different forms and
combinations of these genes. The total of all the genes in a population is
known as the gene pool.
The evolution of new species is dependant upon changes in the gene
pool of a population. These changes are brought about in a number of
ways.
a) New alleles appear by Mutation, and are reshuffled by random
assortment of chromosomes and crossing over during meiosis.
b) The process of Natural Selection results in certain alleles being
favoured and their frequency increases.
c) When individuals, or groups, join one population from another,
(Gene Migration) new alleles are introduced.
d) When a small group of organisms become isolated from the rest of
the population, (Genetic Drift) the small group often does not possess
the complete range of genes typical of that species. After several
generations, this isolated group becomes distinctive since the frequency
of certain genes have changed.
Physical isolation can result from the following barriers to gene exchange:
1.
Geographical isolation:
a) Continental drift – This has resulted in the formation and destruction
of land bridges between different land masses at various times in the
history of the Earth. Where land masses have collided, mountain ranges
have been created. Where they have moved apart, deep ocean-filled
trenches have appeared. These are both examples of physical barriers
which have isolated various groups of flora and fauna at one time or
another.
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(ref. Evolution of Marsupials.)
Continental Drift or ‘Plate Tectonics’ also involved considerable volcanic
activity which, occasionally, led to the creation of new islands. These
later became colonised by a variety of organisms which rapidly adapted
to take advantage of the new-found habitats and niches. (ref. Darwin’
Finches)
b)
Climatic changes – These have resulted in the creation of a variety of
physical barriers such as:
- changes in sea level which have isolated islands. (ref. The Long-tailed
Field Mouse, The ‘Bastard’ Mountain Ash)
- changes in temperature resulting in deserts, glaciers etc. (ref. The
Scottish Primrose)
2.
Ecological isolation
There is such a range of ecosystems, and within each habitat a variation
in ecological conditions that it is hardly surprising that many groups or
organisms have shown Adaptive Radiation and have evolved the ability
to modify their shape, physiology and behaviour in order to adapt to the
particular situation they find themselves in. (ref. Buttercup Diversity)
3.
Reproductive isolation
Plants may be reproductively isolated in a variety of ways:
a) Seasonal – Plants may flower at different times of the year.
b) Diurnal – Plants may be adapted for pollination by day or night flying
insects.
c) Flower Structure – Differences may make cross-pollination
impossible.
d) Pollinator – The pollinating insect may react differently to different
flowers.
e) Cross Incompatibility – The pollen may not grow down through the
carpel.
f) Seed Inviability – The seed may fail to develop.
g) Defective Hybrids – These may show defective development.
h) Hybrid sterility – The hybrid may be infertile.
Animals may also be reproductively isolated in several ways:
a) Behavioural – courtship behaviour ensures mating between members
of the same species.
b) Seasonal – mating seasons do not overlap.
c) Habitat – Habitat preferences keep members of different species apart.
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d) Mechanical – Physical non-correspondence of genital organs prevents
successful transfer of gametes.
e) Prevention of fertilisation – Failure of gametes to fuse.
f) Hybrid Inviability – Abortion of inferior zygote.
g) Hybrid Sterility – The offspring may be infertile.
h) Hybrid Breakdown – First generation hybrids are viable and fertile but
later generations are not.
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Adaptive radiation
This term applies to any situation wherein a species evolves into two or
more divergent forms adapted to distinct modes of life. (Ref. Darwin’s
Finches, Evolution of Marsupials, Buttercup Diversity, etc.)
Over great periods of time related organisms have diverged into a number
of different groups depending on ecological conditions. (Ref. Buttercup
Diversity)
The evolution and origin of new species.
A species is a group of actually or potentially interbreeding natural populations
which are reproductively isolated from other such groups.
The evolution of a new species is dependent upon changes in the gene pool of a
population. These changes are brought about in a series of stages.
Stages in Speciation
If a small population moves to a new environment (such as in the case of
Darwin’s Finches) it may find itself with Reduced Selection Pressures, e.g.
free from competition with other species, predators, parasites etc. There then
will follow a Population Explosion resulting in Increased Variation, since
alleles, previously selected against, then have a chance of being expressed.
Natural Selection pressures will then act on the population forcing some
members to migrate to new environments or, through adaptive radiation, having
to occupy a separate ecological niche within the same environment. Once
Physically Isolated, either geographically, ecologically or reproductively,
barriers will exist to prevent gene exchange. The two gene pools will then
evolve independently and as a result of a series of Mutations there will come a
time when the two groups can no longer interbreed, even if they were brought
together again. They would be Genetically Isolated and thus regard as separate
species.
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Chapter 18
Summary Of Stages In Speciation
A.
Reduced Selection Pressure
- due to lack of competition
B.
Population Explosion
- to occupy the newly available niches.
C.
Increased Variation
- as more genes are expressed.
D.
Natural Selection
- favours certain alleles
E.
Physical Isolation
- by geographical, ecological or reproductive barriers
F.
Mutation
- producing new alleles.
G.
Genetic Isolation
- resulting in formation of new species.
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