diversity of organisms

• how can population isolation lead to speciation

  • This isolation means that members of the separate populations cannot breed together and gene flow cannot take place between them
  • If the environmental conditions affecting each population are different, then natural selection could act differently on each population and eventually lead to speciation
• genetic drift 


  random changes in allele frequencies in a population due to chance
• how can genetic drift lead to speciation

  • Once speciation has taken place, the two species can no longer breed to produce fertile offspring; they are reproductively isolated and are said to be separate species
• natural selection 


  the process by which organisms better adapted to their environment survive, reproduce and pass on their advantageous alleles to their offspring, while those less well adapted fail to do so
• Speciation can occur when gene flow does not occur between two populations of the same species
• The process of speciation occurs over very long time periods, and the differences between isolated populations accumulate gradually
• Distinguishing between populations and species

  • The process of speciation occurs over very long time periods, and the differences between isolated populations accumulate gradually
  • In most cases, it is likely that the ability of two populations to interbreed successfully declines gradually rather than a sudden cut-off point occurring, meaning that it is difficult to pinpoint the stage at which two separate populations have become two new species
  • The decision as to when to assign separate species status to two populations can therefore seem arbitrary (and subjective)
• diploid cell

  • a cell that contains two complete sets of chromosomes
  • Adult body cells are usually diploid cells
• haploid cell

  • contain one complete set of chromosomes (n), meaning that they have half the number of chromosomes compared to normal body cell
• • these haploid cells are called gametes and they are involved in sexual reproduction
  • In animals, they are the female egg cell and the male sperm cell
  • There are some unusual species which have haploid cells for other parts of their life cycle
• • During fertilisation the nuclei of haploid gametes fuse together to form the nucleus of a diploid zygote
  • Both gametes must contain the same number of chromosomes in order for the zygote to be viable.
  • For a diploid zygote this means that the gametes must be haploid
  • Every body cell that arises from the zygote will contain the same number of chromosomes
  • Exceptions to this include red blood cells, which have no nucleus and so contain no chromosomes
• Chromosome number is halved in gametes, and restored to diploid after fertilisation
• chromosome number

  • The number of chromosomes possessed by different species varies and is dependent upon changes that have occurred during that species' evolution
  • Each individual in a species always has the same number of chromosomes
  • There are a few rare instances where a chromosome mutation has occurred, giving a different chromosome number
  • Differences in chromosome number is one reason why organisms from different species are unable to breed together successfully
• stating the chromosome number give:

  • The number of chromosomes found in a diploid cell
  • The number of pairs of chromosomes found in a diploid cell
  • This will be the same as the haploid chromosome number
  • The number of chromosomes found in a haploid cell
• The diploid number must always be an even number

  This is because the diploid number (2n) must always be divisible by two to produce a whole haploid number (n)
• humans have 46 chromosomes and chimpanzees have 48
• chromosomes 


  strands of DNA
• gene

  • a section of DNA that codes for one polypeptide
• • Chromosomes that have undergone DNA replication have the appearance of an 'X' shape, where the 'legs' of the X are made up of two strands of DNA attached at a region known as the centromere
  • Chromosomes with a centromere located roughly in the middle are known as metacentric chromosomes
  • Chromosomes with the centromere near the end are acrocentric
  • Chromosomes that have been stained with a dye have a banded appearance
• karyogram


  an image that shows all of the chromosomes in a cell, arranged by size, shape, and banding pattern, and placed with their homologous pairs
• polypeptide


  a polymer consisting of a large chain of amino acids bonded together by peptide bonds
• A karyogram shows the karyotype of an individual
• karyotype


  The appearance of a complete set of an individual's chromosomes, including their number, size, shape, and banding
• homologous pairs 


  a pair of chromosomes(maternal and paternal) that are the same size and shape, and have the same gene loci. they are not necessarily identical as they may have different alleles
• A karyogram contains an individual's chromosomes arranged in homologous pairs. It shows a karyotype; the appearance of a complete set of chromosomes arranged by size, shape, and banding pattern
• making a karyogram

  • Cells are stained and viewed under a light microscope
  • Photographs are taken of the contents of the nucleus during metaphase of cell division
  • The photographs of the chromosomes are cut up and arranged by size, shape, and banding pattern
  • This can be done with paper and scissors or on a computer
• why do humans have 46 chromosones while chimpanzees have 48

  • A pair of chromosomes disappeared from the genome
  • A pair of chromosomes fused with another pair to form a single pair
• why is it unlikely that A pair of chromosomes disappeared from the genome in humans

  • The loss of an entire pair of chromosomes would have had a significant effect on the characteristics of human ancestors, and may have put their survival at risk
• Evidence that supports the hypothesis that Chromosomes in pairs 12 and 13 in a common ancestor fused to form the chromosomes in human pair 2

  • Human chromosome 2 contains a region of non-coding DNA known as satellite DNA that corresponds to the location of the centromere in chimpanzee chromosome 13; this could be a remnant of a centromere
  • The banding of the long arms of acrocentric chimpanzee chromosomes 12 and 13 corresponds to the banding of metacentric human chromosome 2
  • Human chromosome 2 contains telomeric DNA in the middle of the chromosome
• Evidence that supports the hypothesis that Chromosomes in pairs 12 and 13 in a common ancestor fused to form the chromosomes in human pair 2

  • Chimpanzee chromosomes 12 and 13, when placed end-to-end, match the length of human chromosome 2
  • The location of the centromere of chimpanzee chromosome 12 matches that of human chromosome 2
• Evidence that does not supports the hypothesis that Chromosomes in pairs 12 and 13 in a common ancestor fused to form the chromosomes in human pair 2

  • The length of chimpanzee chromosomes 12 and 13 combined is not a perfect match for human chromosome 2; there is a slight overlap
  • The location of the centromere of chimpanzee chromosome 13 does not match that of human chromosome 2
• Chromosome fusion evidence diagram


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• For a hypothesis to be testable, it must have the following characteristics

  • There needs to be access to evidence that supports it or refutes it
  • The hypothesis needs to be a testable statement
  • A hypothesis should not contain vague statements that use terms like 'may' or 'could
  • A hypothesis should not make predictions
  • A hypothesis should not draw causal conclusions
• how can the hypothesis be tested

  • the fusion of ancestral chromosomes 12 and 13 can be tested by examining chromosome evidence from humans and modern chimps
• genome

  • All of the genetic information in an organism
  • This refers to the DNA present within every cell of an organism
  • This includes genes that code for proteins as well as non-coding DNA sequences
  • Mitochondrial DNA and chloroplast DNA are included in the genome of eukaryotic cells
  • In a prokaryote cell, plasmid DNA is included in the genome
• Advances in technology have allowed scientists to determine the entire base sequence of the genes within an organism's genome
• DNA sequencing

  Determining the base sequence of DNA
• Genome-wide comparisons can now be made between individuals and between species
• • Comparisons show that there is a high level of genome similarity within, and even between, species
  • Humans share around 99.9 % of their DNA with other humans
  • Humans share around 99 % of their DNA with chimpanzees
• Organisms in the same species share most of their genome but variations such as single-nucleotide polymorphisms give some diversity.