What is a species?

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What is a species?

• A species is a group of organisms which can
  reproduce to produce fertile offspring.
• This is the biological test for assessing the
  relationship between two organisms.
• In recent times, technology has clarified the
  relationship between groups of organisms. This
  has been possible through DNA hybridisation.

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Heredity versus Environment

• Some characteristics of organisms are clearly
  controlled by their genetic background. For
  example, the presence or absence of a widows peak
  or tongue rolling.
• Some characteristics of organisms are clearly
  controlled by their environment. For example,
  amputation of limbs, hair which is dyed.

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Heredity versus Environment

• However scientists are beginning to understand
  that many characteristics are the result of an
  interplay between the genetic background of an
  organism and its environment.
• A recent proposal coming from the genome project
  is that it seems that there may be far fewer
  genes in the human genome than we originally
  thought and that our phenotypes may be far more
  strongly influenced by our environment.

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The Gene

• Structurally, a gene is a sequence of nucleotide
  bases found on a strand of DNA.
• A gene is a functional unit of DNA (sequence of
  nucleotide bases) which controls for the
  production of a particular protein or polypeptide
  chain.

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Chromosomes

• Genes are found in sequences along the length of
  a double helix of DNA. When DNA condenses in the
  nucleus, it forms strands known as chromosomes.
• Each chromosome is made up of two strands known
  as chromatids.
• These chromatids are held together at the centre
  by a centromere.

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Chromosomes

• There are forty six chromosomes (23 pairs) in the
  human genome.
• However different organisms have genomes made up
  of different numbers of pairs of chromosomes.
• Organisms generally have a series of autosomal
  chromosomes which control general somatic
  characteristics.
• Organisms generally also have a pair or more of
  sex chromosomes which control features related to
  the reproductive structures and functions. They
  may also carry somatic characteristics.

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Chromosomes

• Note Nitrogenous bases refers only to the
  compounds adenine, guanine, cytosine, thymine.
• Nucleotide base is a term used to describe a
  combination of a base and the associated
  sugar-phosphates which made up the backbone of
  the double helix.

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Homologous Chromosomes

• This is a term used to describe a pair of
  chromosomes which carry identical information at
  each of the positions along their length.
• The position of a gene along the length of a
  chromosome is known as the locus.

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The Human Genome
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Diploid

• Diploid is a term used to describe a cell which
  contains two genes for each characteristic.
• Diploid cells can also be described as being 2n.
• One of the genes for each characteristic is
  paternal and the other is maternal.
• All somatic cells are diploid and are produced
  through the process of mitosis.

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Haploid

• Haploid is a term used to describe cells which
  contain only one gene for each characteristic.
• Haploid cells can also be described as being n.
• A haploid cell will be either a male of female
  gamete and will have been produced in the gonads
  of an organism.
• Haploid cells are produced through the process of
  meiosis.
• Gametes are haploid so that when fusion of
  gametes occurs at fertilisation, the two haploid
  gametes will combine to produce a diploid
  daughter cell.

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Variation through Sexual Reproduction

• Variation within the gene pool of a species
  occurs as a result of
• Sexual Reproduction
• Crossing Over
• Random Alignment

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Sexual Reproduction

• Sexual Reproduction involves the fusion of
  gametes, each of which carries information on all
  of the characteristics required to form a new
  organism. The interplay between these genes adds
  further variation.
• Sexual Reproduction The random mating of
  various individuals will result in an exchange
  and interaction of genetic material which will
  result in a unique individual with its own
  genotype.

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Sexual Reproduction
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Crossing Over

• Crossing Over This describes the situation in
  which a section of one chromosome will be crossed
  over and swapped with the same section from a
  homologous chromosome.
• This generally occurs at the mitotic stage of
  division in the process of meiosis.

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Crossing Over
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Random Alignment

• This is a situation in which homologous pairs of
  chromosomes do not align with each other when
  they accumulate along the equator of a cell.
• This occurs during the mitotic stage of division
  in the process of meiosis.

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Asexual Reproduction

• Asexual reproduction refers to all forms of
  reproduction which does not involve the fusion of
  gametes.
• These include
• Runner in plants
• Bulbs, corms and rhizomes in plants
• Propagation in plants
• Budding in plants and animals
• Regeneration in animals
• Parthanogenesis

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Asexual versus Sexual Reproduction

• Asexual reproduction has the advantage of be
  relatively low in energy cost and allowing the
  formation of large numbers of offspring without
  finding a mate.
• Sexual reproduction has the advantage of
  introducing genetic variation into the species.
  However, it does tend to be at high energy cost.

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The Best of Both Worlds
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Meiosis

• Meiosis is the process of cell division which
  occurs in the gonads of sexually reproducing
  organisms.
• Its primary function is to produce haploid cells
  from diploid cells.
• These cells each have one gene for every
  characteristic, which when fused with another
  gamete produces a cell which is diploid and has
  two genes for every characteristic (2n).

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Meiosis
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Gametogenesis

• Gametogenesis is the process which occurs in the
  gonads, whereby the haploid cells produced
  through meiosis go through a process of
  specialisation to become gametes which will be
  involved in the process of sexual reproduction.
• In humans, the process which occurs in the testes
  to produce sperm is known as spermatogenesis.
• In humans, the process which occurs in the
  ovaries to produce ova is known as oogenesis.

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Spermatogenesis and Oogenesis
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Spermatogenesis and Oogenesis
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Sex Determination in Mammals

• We are aware that humans have 22 pairs of
  autosomal chromosomes and 1 pair of sex
  chromosomes.
• In females the two sex chromosomes are known as
  the XX chromosomes.
• Males have a genotype of XY for their sex
  chromosomes.

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Sex Determination in Mammals

• It is important to note, that despite theories to
  the contrary, there is a 50 chance of an
  offspring being male and 50 chance of female.

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Sex Determination in Mammals
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Mutations

• A mutation is a spontaneous change in the gene
  composition of an organism.
• There are a number of types of mutations which
  can be caused by a wide range of mutagens. These
  mutagens include
• Ionising radiation
• Viruses and microorganisms
• Environmental poisons
• Alcohol and diet

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Types of Mutation

• There are a number of different types of
  mutations which can affect the genome of an
  individual in the following ways
• Changes to a particular gene
• Deletion of a particular gene
• Changes to a section or entire chromosome
• Changes to the number of chromosomes.

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Point Mutation

• These are mutations which affect a particular
  series of nucleotide bases or a gene.
• Instances where there is a mismatch of the bases
  are known as Tautormerism.
• Missense Substitution refers to instances where a
  particular base pair is substituted for a
  different base pair. This will often lead to a
  change in the amino acid translated at this point.

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Point Mutation

• Nonsense Substitution refers to situations in
  which insertions, rather than substitutions, of
  base pairs occurs.
• This can have a much more significant effect
  since all base pairs from this point on are
  effectively out of position or sequence.
• This can potentially lead to a range of amino
  acids not being produced.

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Point Mutation

• Reading Frame Shift refers to instances where the
  beginning and end of a gene is incorrectly coded.
  This results in a message stopping prematurely
  or not stopping at the appropriate time
  effectively joining more than one genes worth of
  information together.

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Chromosome Mutation

• Translocation refers to situations in which there
  is a movement of genes between two different
  chromosomes.
• Inversion refers to situations in which segments
  of material are rotated and rejoined in the same
  locus range.
• Duplication refers to situations where one
  homologous chromosome loses a segment of material
  and this is added to its homologue, effectively
  providing a duplication of a segment of material.
• Deletion refers to the loss of a segment of a
  chromosome.

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Aneuploidy

• This is a situation caused by the non-disjunction
  of chromosomes during meiosis.
• This results in a gamete which contains more or
  less genetic material than a normal haploid cell.
• Many of the sex chromosome syndromes, such as
  Klinefelter Syndrome, Turners Syndrome and Jacob
  Syndrome, are all examples of aneuploidy

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Polyploidy

• This refers to situations in which an individual
  contains three or four times the haploid number
  of chromosomes.
• Polyploidy has been a significant contributor to
  the development of many of our current commercial
  crops of plants, such as wheat. These strains of
  plant tend to have exaggerated characteristics
  from the ancestor, such as large seed heads etc.

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Mutations and Change

• Mutations provide the valuable raw material for
  change within a species.
• This variation can give rise to a trait which is
  naturally selected as favourable in a particular
  set of environmental conditions.
• This in turn will lead to a species which is
  genetically stronger and better suited to the
  conditions in which it lives.

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Natural Selection

• Natural selection is the process whereby
  particular variations of a trait are more
  successful than other variations of the same
  trait.
• This means that the individuals exhibiting the
  more successful trait will be more likely to
  produce offspring, and these offspring are also
  more likely to be successful because they will
  also carry the successful trait.

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Natural Selection

• The key requirements for natural selection to
  occur are
• Variation within the genome of a species
• A selective pressure in the environment.
• Repeated and excessive reproduction

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Natural Selection

• If the selective pressure from the environment is
  too great and natural attrition is greater than
  the rate at which births occur, then a species
  may in fact die out before a trait can be
  naturally selected.

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Natural Selection

• Video Four How does evolution really work?
• http//www.pbs.org/wgbh/evolution/educators/teachs
  tuds/svideos.html

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Speciation

• Speciation is the process whereby natural
  selection occurs independently, under different
  environmental pressures, in two populations of
  the same species.
• If the two populations continue to adapt to their
  different environments and do not interbreed,
  they may in fact become different species. They
  will be unable to interbreed to produce fertile
  offspring.

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Speciation
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Allopatric Speciation

• Allopatric speciation occurs when there is an
  environmental or geographical barrier to
  reproduction between two populations of the same
  species.

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Sympatric Speciation

• Sympatric speciation occurs when a species
  develops an intrinsic isolating mechanism, which
  prevents interbreeding between two populations.
  An example of this is a reproductive strategy
  which does not permit mixing of the two gene
  pools.

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Evolution

• With each natural selection there is a slight
  change in the characteristics of a species.
• Over extremely long periods of time the
  accumulation of many natural selections results
  in significant changes in the characteristics of
  a species this is known as evolution.

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Evidence for Evolution

• There is a wide range of evidence which provides
  insight into the processes of evolution and the
  relationship between organisms.
• These include
• Geological distribution (stratification)
• Fossil analysis
• Comparative anatomy
• DNA hybridisation

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Evidence for Evolution

• Video Three How do we know evolution happens?
• http//www.pbs.org/wgbh/evolution/educators/teachs
  tuds/svideos.html

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Humans - a case study.

• Video Five Did Humans Evolve?
• http//www.pbs.org/wgbh/evolution/educators/teachs
  tuds/svideos.html

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What effect does evolution have today?

• Video Six Why does evolution matter now?
• http//www.pbs.org/wgbh/evolution/educators/teachs
  tuds/svideos.html