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Environmental and biotic change
Synthesis: patterns, processes and scales (see slides for diagrams)
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Biotic patterns
Environmental processes
Spatial scales

Evolution and the influence of the environment
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Ecosystems vary in response to changes in the physical environment, both in space and
through time
...
g
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Interbreeding can occur within a species, but not between species
...

Example of genetic isolation – between horse and donkeys
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Capable of mating and producing offspring as a mule
Mules are infertile
Cannot reproduce their mixed genes will not be passed on to further generations
Test for whether organisms belong to the same species is if they can produce fertile
offspring

Individuals: phenotype and genotype
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Phenotype – physical appearance of an organism – environment can effect phenotype
Genotype – genetic constitution of an organism – can effect phenotype
For changes to be passed on and be of evolutionary significance they have to be the
genotype

Genes and DNA
Key questions:
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What does genetic material consist of
How can the genetic makeup of an individual influence its physical appearance
How does genetic material get passed on
How do variations in the genetic composition of a group arise through time

A gene is a segment of a molecule called deoxyribonucleic acid (DNA) that serves as a unit of
hereditary information
A gene is a sequence of DNA that codes for a particular protein
Genes reside in tread shapes bodies found in the nucleus of every plant or plant cells called
chromosomes

What does genetic material consist of?
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Deoxyribonucleic acid (DNA) structure
o 5 carbon sugar
o Phosphoric acid
o Nitrogenous base
The phosphate and sugar are strongly (covalently) bonded
Four bases
o Adenine (A) purine
o Guanine (G) purine
o Cytosine (C) pyrimidine
o Thymine (T) pyrimidine
Linkages are purine to pyrimidine
o A–T
o G–C
(diagrams on slides – double helix, DNA replication)

Human genome project
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The DNA sequence in the entire human nucleus consists of about 2
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Hundreds of researchers completed the task of determining the order in 2003
Subsequently the challenge has been to identify where all the genes are located, estimated
that there are 30000 genes in the human genome
Applications include:
o Identify inherited characteristics
o Customise drugs to match genetic makeup

How can the genetic makeup of an individual influence its physical appearance (phenotype)?
By controlling protein synthesis
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Proteins are complex organic substances that play a vital role in organisms
They make up over 50% of the dry weight of organisms
Structural (connective) tissues in the body are composed of protein and enzymes (which
control reactions in the body) are also proteins

Proteins are composed of building blocks or sub units called amino acids

How does genetic material get passed on?
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By cell division
DNA is found mainly in the chromosomes
In somatic (non-reproductive) cells – in each cell humans have 23 identical (homologous)
pairs of chromosomes, 46 per nucleus
In the gametes – one of each pair of chromosome

Somatic human cells contain 46 chromosomes, so that when any new cell is added for growth or
repair the 46 chromosomes have to be passed on; this from of cell division is called mitosis and
genetic stability is required
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During
gamete (sperm or ova) formation this is accomplished by a form of cell division termed meiosis
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The fusion of the gametes during fertilisation leads to the formation of a zygote (with 46
chromosomes)
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Can be brought about by mistakes when DNA is copied or environmental factors
o Radiation
o Ultra-violet light
o Certain chemicals (e
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phenols)
Sickle cell anaemia – the alteration of one base on the DNA that codes for part of the human
haemoglobin molecule causes a single amino acid change which results in change in the
structure of haemoglobin which impairs function (when the molecules release oxygen they
become stacked together)

Sexual reproduction/meiosis
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Routine gene reshuffling
o Segregation of alleles – offspring receive one homologous chromosome from each
parent
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That means that new combinations occur in the gametes
o Crossing over – refers to a process which results in the exchange of genetic material
between the homologous chromosomes during meiosis as a result new
combinations of gene sequences are formed
Chromosome mutations
o This is when chromosomes get altered, usually by some mistake during meiosis
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o Aneuploidy occurs as a consequence of both of a pair of homologous chromosomes
going into the gamete during meiosis
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Not surprisingly this can have a profound effect on an organism’s
development
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Down’s syndrome sufferers
have an extra 21st chromosome

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Polyploidy is when all the pairs of chromosomes all go off into one cell during
meiosis
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If the diploid gamete then fuses with a haploid
gamete a triploid individual will result
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o Polyploidy is rare in animals and of little significance because the organism is usually
infertile
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In plants polyploidy is often associated with advantageous characteristics, for example
resistance to disease, higher rates of growth and hardiness and polyploids have therefore
been selected for in plant domestication
Many of our crop plants such as wheat are polyploids
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This is about five times the amount of DNA in the
human genome

Natural selection
Definition – the differential survival and reproduction of individuals carrying alternative inherited
characteristics/traits (different genotypes)
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The reproductive potential of populations is great (rabbits should cover the earth) but…
Populations tend to remain constant in size because…
Populations suffer a high mortality
Populations exhibit variation which leads to…
The differential survival and reproduction of individuals
Individuals characteristics are inherited by offspring
The composition of the population changes by the selective elimination of unfit individuals

Case study: industrial melanism
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Melanism = dark form
Peppered moth: Biston betularia
The typical and dark from belong to the same species and can still interbreed
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The first dark forms were reported in the 1850’s from
Manchester
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It would seem that the dark forms were better camouflaged against predators in industrial
areas where lichens are absent and the bark of trees became covered in soot
Since the mid 1950’s and the Clean Air Act, the habitat of the peppered moth in cities has
changed again and the frequency of the dark form has declined
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This is thought to be
important in small populations
Natural selection and genetic drift result in phyletic evolution (gradualism, microevolution)

Phyletic evolution
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The gradual transformation of one species into another through time (within a single
evolutionary lineage)
...
Strictly speaking it is
this process (which require the development of a barrier to prevent interbreeding) which is
referred to as speciation
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Genetic differences will then
build up through time as a result of natural selection (if environments are different)
or genetic drift
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g
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Known as speciation by
founders
o Within only a few likely to be present in the new population genetic drift is likely to
be very important
Archipelagos (groups of islands) like the Galapagos and Hawaiian islands are particularly rich
when compared to single isolated islands even if the latter is larger

Differentiation on island groups
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Initial invasion of islands
Isolation of islands populations due to water barriers
Differentiation of populations on separate islands
Reinvasion of populations on to the other islands in the group

Barriers can form
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Alternatively barriers can form dividing up the species into a number of isolated populations
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This is because rather
than affecting a single species the processes involved separates whole biotas
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Successful colonisation of a new host must be a rare
event and is likely to lead both to isolation from the original population and strong
selection pressure

Extinction
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Definition: extinction is to a species what death is to an individual: the irreversible and it
seems inevitable end of existence
One estimate is that over the history of life on earth 98% of species that have lived have
become extinct
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Either:
o Constructive – the old species becomes extinct as a result of competition from a rival
new species
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o Non-constructive – the old species goes extinct as a result of an unpredictable
challenge, and a new species later evolves to fill the ecological gap left
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Factors increasing the susceptibility of an organism to extinction:
o High up the food chain
o Diet or habitat specialist
o Restricted geographical range
o Low birth rates
o Plants are generally less vulnerable as they have the ability to regenerate from roots
and seeds can remain dormant for long periods
Mass extinctions: a period with an excessively high rate of extinction, amongst many
different organisms, on a global scale, during a relatively short-time interval
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10km
o Impacted at 20km second
o Left crater 180km diameter (Yucatan, Mexico)
o Released 10,000 times the energy of all present nuclear stocks
o Produced an iridium rich layer
How can such events cause extinctions on a global scale
o Dust and debris into the upper atmosphere drop in temperature and termination of
photosynthesis – domino effect on food chain
o Fireball in atmosphere fixes nitrogen – acid rain
o Ignite vegetation producing massive fires
o Produced a tidal wave (tsunami) which would have encircled the globe in hours
Intense volcanic activity: at K/T boundary on the Deccan plateau
Some gradualistic explanations
o Climate change: particularly through the impact on sea-level (rising and falling) and
the salinity and oxygen content of water
o Continental drift: the coming together of continents and the resultant competition
between organisms



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