To investigate diversity the ecosystem has to be sampled. There are three main ways of doing this.

Random Sampling

• Used to reduce bias.
• Draw a map of the area with a numbered grid over it, or lay tape measures along the edges of the area to be sampled.
• Place quadrats at co-ordinates after using a random number generator to generate the coordinates.
• Make sure to use a large number of quadrats (at least 10). This ensures the estimate is representative.

Systematic Sampling

Using transects- these can be a tape measure or a rope laid across the habitat to be sampled.

• E.g., belt transects. A quadrat is placed at regular intervals along the tape measure.
• Used to detect changes in species across gradients (e.g., light, moisture, soil type).
• Again, make sure to use a large number of quadrats and carry out multiple repeats of the transect.

Mark Release Recapture

• Used to sample mobile organisms (animals).
• Requires the population to remain constant in size and marking to not impact survival. More about this in year 13 module 7.
  

 Simpson’s Index of Diversity (D)

• N = Total number of organisms of all species.

• n = Number of individuals of each species.

Why use this? It accounts for both: Species richness- the number of different species and species evenness- the number of individuals in each species.

For a detailed explanation of how to use this diversity index- see the study note on the index of diversity.

Modern methods are more accurate than simply looking at observable traits.

Comparing DNA base sequences

• Closely related species have more similar DNA base sequences. This is because they evolved from a more recent common ancestor and so there has been less time for mutations to accumulate.
• Higher % similarity in base sequence = closer relationship / more recent common ancestor.
• Lower % similarity in base sequence = less closely related / shared a less recent common ancestor.

Comparing mRNA sequences

• mRNA reflects the DNA coding regions (exons), as mRNA is produced using the DNA template strand during transcription.
• Used when DNA isn’t available (e.g., ancient samples or partial material).
• mRNA base sequences will be more similar between individuals than DNA as there are no introns in mRNA so variation in these regions would not be recognised.

Comparing amino acid sequences

• Based on protein structure, e.g., cytochrome C (an enzyme used in aerobic respiration).
• Cytochrome C is a useful protein to use for comparisons as it is used in respiration and therefore present in all eukaryotic cells.
• More similar amino acid sequences = more closely related organisms/ a more recent common ancestor.
• The more similar the amino acid sequence the more similar the DNA base sequence must be. Remember, the sequence of bases in DNA determines the sequence of amino acids in a protein.
• However, studying amino acid sequences would not reflect the true genetic variation in DNA as the genetic code is degenerate. Therefore, two species may have the same amino acid sequence in a protein but different DNA base sequences.

Old Methods Used:

• Observable traits (e.g., height, flower colour).
• Problem: These can be influenced by the environment, not just genetics.

E.g., two plants may look different because of sunlight or mineral availability, even if genetically identical.

• Analogous Structures: features of different species that are similar in function but not necessarily in structure.
• Problem: These structures do not derive from a common ancestor.

E.g., The wings of birds and insects have the same function but have vastly different structures.

Modern molecular techniques provide greater accuracy by comparing actual genetic material.