Investigating Diversity (AQA A-Level Biology): Revision Notes
Investigating Diversity
Understanding evolutionary relationships between organisms has evolved from simple observation to sophisticated molecular analysis. Modern techniques allow scientists to compare genetic diversity within and between species using various molecular approaches that provide more accurate evidence than traditional methods.
Comparison of observable characteristics
Traditional genetic diversity studies relied on observable characteristics that could be easily seen and measured. This approach was based on the principle that each visible trait is controlled by specific genes, with variations arising from different alleles and environmental factors.
However, this method has significant limitations. Many characteristics are polygenic, meaning they are controlled by multiple genes rather than single genes. This makes them difficult to distinguish clearly since they show continuous rather than discrete variation. Environmental factors can also modify how characteristics appear - for example, human height is genetically determined but can be influenced by nutrition during development.
The main problem with using observable characteristics is that differences may result from environmental conditions rather than genetic differences between individuals. Additionally, many traits show continuous variation, making it challenging to categorise organisms accurately for comparison purposes.
Comparison of DNA base sequences
Modern DNA sequencing technology has revolutionised the study of genetic diversity. Scientists can now determine the exact order of nucleotides in DNA using automated machines that produce computerised data for analysis.
In DNA sequencing, each of the four bases (adenine, thymine, guanine, cytosine) is tagged with a different coloured fluorescent dye. When DNA is sequenced, this creates a pattern of coloured bands that can be read by computer software to determine the precise nucleotide sequence.
This method provides direct access to genetic information since DNA determines the proteins of an organism, including enzymes that control features and characteristics. When organisms evolve, mutations cause changes in DNA sequences. Species that diverged more recently will have accumulated fewer mutations and therefore show greater similarity in their DNA sequences compared to more distantly related species.
The vast amount of information contained in DNA sequences - millions of base pairs in most organisms - provides detailed evidence about genetic diversity and evolutionary history. This makes DNA comparison far more reliable than observable characteristics for determining evolutionary relationships.
Comparison of mRNA base sequences
Messenger RNA (mRNA) sequences can also be compared between species to investigate genetic diversity. Since mRNA is transcribed directly from DNA using complementary base pairing, the base sequence of mRNA reflects the genetic information stored in DNA.
Comparing mRNA sequences provides another way to measure DNA diversity and therefore genetic diversity between organisms. This method is particularly useful because mRNA represents the genes that are actively being expressed, giving insight into functional genetic differences between species.
Comparison of amino acid sequences in proteins
The sequence of amino acids in proteins provides valuable information about genetic relationships since protein structure is determined by mRNA, which in turn is determined by DNA. The degree of similarity in amino acid sequences of the same protein between different species reflects how closely related those species are evolutionarily.
When comparing protein sequences, scientists count either the number of similarities or the number of differences between corresponding positions in the protein. Species with more similar amino acid sequences are more closely related than those with greater differences.
Worked Example: Comparing Protein Sequences
When comparing a specific protein across six different species, scientists can create a comparison table showing amino acid differences at each position. The species with the fewest differences are most closely related, while those with the most differences are most distantly related.
For instance, if Species A and B differ by only 2 amino acids, while Species A and C differ by 15 amino acids, then Species A is more closely related to Species B than to Species C.
Immunological comparisons of proteins
Immunological techniques provide another method for comparing proteins between species. This approach uses the immune system's ability to recognise foreign proteins through antibody-antigen reactions.
Worked Example: Immunological Comparison Process
The process works as follows:
- Serum albumin from species A is injected into species B
- Species B produces antibodies specific to the foreign albumin proteins
- Serum containing these antibodies is extracted from species B
- This serum is then mixed with serum from other species being compared
- Antibodies bind to similar antigens on proteins, forming a visible precipitate
The amount of precipitate formed indicates the degree of similarity between species. More precipitate formation suggests greater protein similarity and therefore closer evolutionary relationships. Less precipitate indicates more distant relationships between species.
This technique has been used to compare humans with other primates, showing that humans are most closely related to chimpanzees, followed by gorillas, then more distantly related to other primates and mammals.
Establishing evolutionary relationships
Multiple lines of evidence from different techniques help scientists establish accurate evolutionary relationships. Studies comparing primates using various methods demonstrate how different approaches can provide complementary evidence.
Haemoglobin studies comparing amino acid sequences at specific positions across different primate species show patterns of similarity that reflect evolutionary closeness. DNA base difference studies examining the first 200 bases of specific genes provide additional quantitative data about genetic relationships.
Immunological studies using antibody precipitation reactions offer a third line of evidence. When human serum proteins are tested against antibodies from other species, the amount of precipitation decreases as evolutionary distance increases - from chimpanzees (closest) to lemurs (most distant).
The use of multiple independent methods provides stronger evidence for evolutionary relationships than any single technique alone. When different approaches give consistent results, this increases confidence in the proposed evolutionary relationships between organisms.
Key Points to Remember:
- Observable characteristics have limitations due to polygenic inheritance and environmental influences, leading to their replacement by molecular methods
- DNA sequencing provides the most direct evidence of genetic relationships through precise nucleotide sequence comparison
- Protein comparisons (amino acid sequences and immunological tests) reflect underlying DNA differences and show evolutionary relationships
- Multiple methods used together provide stronger evidence than single techniques alone for establishing evolutionary relationships
- Closer evolutionary relationships are indicated by greater similarities in DNA, mRNA, and protein sequences between species