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I
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And they
can be harmful, silent or beneficial
Gene pool total set of genes in a population at a
specific time
Microevolution slow change in the gene pool over time
due to natural selection
Gene flow alleles being passed between
populations by migration
Genetic drift change in allele frequency due to chance
and not natural selection
...
SPECIES

A biological species is a group of interbreeding (or potentially
interbreeding) individuals, reproductively isolated from other
such groups
Species are often composed of different populations (often in
different habitats) that are quite distinct
These are often called subspecies, races and varieties depending
on the degree of reproductive isolation
Limitations of the species concept:
Definition of a biological species does not apply in all situations:




The concept of a species being able to interbreed cannot
apply to extinct populations because this is unknown:
extinct forms must usually be classifies on morphological
grounds
Asexually reproducing organisms do not breed with each
other and so are assigned to species on the basis of
appearance of biochemical similarities

Darwin’s theory of evolution by natural selection
Darwin’s view of life was ‘descent with modification’:
descendants of ancestral forms adapted to different
environments over a long period of time
The mechanism for adaptation is called ‘natural selection’ and is
based on a number of principles
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Overproduction
o Species produce more young than will survive to
reproductive age
Variation
o Individuals vary from one another in many
characteristics, some variations are better suited
than others to the conditions of the time
Competition
o There is competition among the offspring for
resources
Survival of the fittest phenotype
o The individuals with the most favourable
combinations of characteristics will be most likely
to survive and pass on their genes to the next
generation
Favourable combinations increase
o Each new generation will contain more offspring
from individuals with favourable characteristics
than those with unfavourable ones

Types of Natural Selection
Stabilising

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


Most common trend in natural populations
Favours most common phenotype as the best adapted
Stabilising selection reduces variation by selecting
against the extremes at each end of the phenotypic range
The resulting bell shaped curve is narrower, but has
about the same mean

Directional





Most common during periods of environmental change
Favours the phenotypes at one extreme of the phenotypic
range
Directional selection reduces variation at one extreme of
the range while favouring variants at another end
The resulting bell shape curve shifts in the direction of
selection

Disruptive





May occur when environmental conditions are varied or
when the environmental range of an organism is large
Favours phenotypes at both extremes of a phenotypic
range over intermediate variants
This type of selection can lead to the formation of clines
or ecotypes and polymorphism
The resulting bell shaped curve acquires two peaks

III
...
The fossil record shows that the most primitive forms
are in the oldest strata
...
The
australopithecines were small-brained but walked on two legs
and in Homohabilis and Homoerectus we see the expansion of
the brain
...


Reasons why there are so few fossils:







Many kinds of organisms rarely become fossilized
because they are delicate or lack hard parts
Organisms may be in environments where decay is rapid,
such as in a humid forest
To be fossilized, organisms need to be buried but not
destroyed; they need to be covered in sediment
Sediment containing fossils must be solidified into rock,
and this must remain untouched for millions of years
without being metamorphosed (changed by intense heat
and pressure)
Fossils have to be found by someone who realizes what
they have found
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COMPARATIVE ANATOMY

Homologous structures have the same origins but have evolved
different functions e
...
The forelimb bones of frogs, crocodiles,
birds, humans, shales, bats and all other vertebrates are the same
and are arranged in a common pattern, although the limbs
perform very different functions in the different animals
...


In many vertebrates, the basic pentadactyl limb has been highly
modified to serve specialised locomotory functions
...


Analogous structures have different origins but have evolved
similar functions
...





wing of insects, birds and bats
eye structure in octopus and mammals
fins in fish and flippers in mammals

Embryology is the study of the embryological development, the
more similar, the less time has passed since the species diverged
...

Note: Although the early developmental sequences between all
vertebrates are similar, phylogeny is not retraced during
development

Vestigial organs are organs that have become redundant or
have lost their function
...






The basic principle of biogeography is that each plant and
animal species originated only once
...

The range of species can be very restricted or, as with
humans, almost the whole world
...


General principles for the dispersal and distribution of land
animals:




Closely related animals in different geographic areas
probably had no barrier to dispersal in the past
The most effective barrier to dispersal in land animals is
sea
The discontinuous distribution of modern species may be
explained by movement out of the area they originally
occupied, or by extinction

Oceanic islands often have species that are similar to, but distinct
from, those on neighbouring continents
The occurrence of these species suggests that they were island
colonisers that evolved in isolation differently to their ancestors
on the mainland
...

Good examples are found on islands offshore from large
continental land masses:




Galapagos Islands
Cape Verde Islands
Tristan da Cunha

Island Colonisers
Species may reach offshore islands in a number of ways:

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

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Flight, rafting across water, being passively blown by
winds
Very mobile species such as seabirds with the ability for
strong, prolonged flight, are not easily isolated in one
area and are often widespread in their distribution
Very mobile terrestrial species may be isolated by
changed in the environment caused by major
geographical events
Many species that become permanently isolated on
offshore islands have low mobility and weak powers of
dispersal

Biochemical and biotechnological evidence uses DNA and
protein similarities and differences to determine relatedness of
organisms
...
More
recent developments in molecular biology has allowed the
comparison of the amino acids in proteins, the DNA in
chromosomes, RNA and mitochondrial DNA
...

Heat can be used to separate the hybridised strands
...

o This is done by finding the temperature at which
it unzips into single strands again

DNA sequencing
Recent advanced techniques have enabled the sequence of DNA
in different species to be determined
...
Distantly related
species have had more time for difference to accumulate:


The greater the time elapsed since common ancestry, the
greater the time for mutations to occur
...


Selective breeding/artificial selection mimics natural
selection
...


V
...
Gene flow
occurs when individuals move from one population to another
...
Local natural selection
pressures mean that the phenotypic features of the members of
one deme may differ from those of another
A cline is a pattern of variation between individuals where there
is a continuous increase or decrease in some phenotypic
characteristics between adjacent populations
...

A ring species occurs when a cline has formed a circle and where
the ends of the cline meet the organisms are sufficiently distinct
that two separate species are seen
...
e
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The birds’ behaviour and genetic characteristics change
gradually around the ring, which is broken in Siberia where two
forms of the warbler live but do not interbreed, because their
mating calls are so different
...
It becomes significant in
small populations as there is a higher probability of alleles being
lost by chance than in large populations
...
Alleles
present and their frequency may not be representative of the
original population and the founder population is much more
subject to the effects of random genetic drift
...

The bottleneck effect occurs when a large population is suddenly
reduced in size, the result of either a catastrophic environmental
event or human impact
...

When small, the population is subject to genetic drift which may
further reduce genetic diversity
...


VI
...

Speciation occurs when gene flow has stopped or is prevented
between populations where it previously existed
...
These isolating mechanisms may be
prezygotic, or they may be postzygotic
...
They are
barriers to gene flow
...

Geographical barriers prevent species interbreeding but are not
considered to be RIMs because they are not operating through
the organisms themselves
...
Some plants have flowers that can only be fertilized by
certain insects
Gametic isolation
Mating can take place but the sperm doesn’t survive in the
reproductive tract of the female or gametes fail to fuse due to the
lack of recognition molecules on the surface of the gametes

Postzygotic Isolation
Hybrid inviability
A zygote is formed but doesn’t develop properly and die at some
point early in their development
Hybrid infertility
A hybrid forms but is sterile and unable to reproduce
Hybrid break down
The hybrid offspring are fertile but produce many infertile or
inviable offspring (F2 generation and later have low fertility)

Taking a very simple view, speciation can happen in one of two
way



Splitting
Budding

There are two forms of speciation:



Allopatric
Sympatric

Allopatric speciation
Allopatric speciation occurs during an initial period of
geographical isolation caused by some geographical barrier such
as on an island, by mountains, river, canyon, ocean etc
...
e
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Sympatric speciation
Sympatric speciation occurs without geographical isolation
...

Over time with no gene flow between the populations changes in
the gene pool build up due to natural selection, mutations,
genetic drift etc
...


In plants, instant speciation can also occur by polyploidy where
every chromosome is represented 3 or more times
...
The resulting hybrid,
with chromosomes from each parent species, may be sterile
...
Self-fertilisation may then produce a viable, fertile
hybrid
...


Autopolyploidy
Occurs within the same species
...
It occurs when chromosomes fail
to separate during meiosis or from the failure of the cell to divide
after the chromatids have separated
...


Stages in species formation; different types of isolating
mechanisms operate and different amounts of gene flow take
place as two populations diverse to form new species
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RATES OF CHANGE

There are two extreme hypotheses about the pattern of
evolution
...
Small
variations that fit an organism slightly better to its environment
are selected for: a few more individuals with more of the helpful
traits survive, and a few more with less of the helpful trait die
...

Change is slow, constant and consistent as show by transitional
forms in the fossil records
...


Punctuated Equilibrium
Punctuated Equilibrium is a pattern of evolution in which long
stable periods with very little evolutionary change (Stasis), is
interrupted by brief periods of rapid change
...

Moving into a new habitat with the availability of new
niches
...


The species changes very rapidly over a few generations, then
settles down to a period of little change
...

A species can evolve by just one of these or by both
...


Divergent evolution
Diversification from an ancestral group into one or more new
species
...


Adaptive radiation
Adaptive radiation is a form of divergent evolution
...
It is the result of
a sudden availability of a diverse range of niches caused by…




The evolution of some new feature that enables a group
to occupy new niches
...


Typical steps in the process of adaptive radiation


Members of an ancestral species move into new habitats
...
They may
become geographically isolated (e
...
because of ice,
mountain building, rise of sea level, volcanic activity)
and/or there may be environmental change
...


Convergent Evolution
Two or more types evolve towards greater resemblance to each
other in response to similar niche requirements
...
Structures
that are similar in function but have evolved independently from
different origins are called analogous structures or homoplasies
...

Convergence is purely phenotypic, being confined only to certain
bodily features linked to adaptation to a particular niche
...

Sometimes, whole organisms with evolutionary different origins
appear very similar because they occupy the same or very similar
niches
...


Parallel Evolution
Parallel evolution occurs when two or more related groups
evolve features independently of each other
...
Generally, similarity arising in
closely related lineages is regarded as parallel evolution, whereas
similarity arising in more distantly related taxa is convergent
evolution

Coevolution
The reciprocal evolution of two or more species with an
ecological relationship
...

Coevolution is a likely consequence when different species have
close ecological interactions with one another including…

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Predator prey relationships
Parasite-host relationships
Mutualistic relationships i
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plants and their pollinators
Competition

Examples:


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Unpalatable tropical butterflies and their mimics
Rough skinned newt and the common garter snake
Hummingbirds and flowers



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