Introduction (AQA A-Level Biology): Revision Notes
Introduction
The living world displays remarkable biodiversity that extends beyond just the variety of different species. This diversity encompasses the range of different individuals within single species, along with the variety of tissues, organs and cells that comprise each organism. Understanding this diversity requires examining the underlying genetic basis that shapes all life forms.
Biodiversity operates at multiple levels - from the molecular diversity within individual organisms to the vast array of different species across ecosystems. This multi-layered complexity makes biology both fascinating and challenging to study.
The genetic basis of diversity
Genes represent specific regions on DNA molecules, with each gene located at a particular position called a locus. DNA serves as an extraordinary information storage system, containing vast amounts of data encoded in its nucleotide base sequences. The sequence of bases within each gene carries coded genetic information that determines the corresponding sequence of amino acids in the organism's proteins.
A remarkable feature of life is that the genetic code operates universally across all living organisms. This universal nature provides indirect evidence supporting the theory that all organisms have evolved from common ancestors. The consistency of the genetic code allows for the reliable transfer of genetic information between generations.
The universality of the genetic code is one of the strongest pieces of evidence for common evolutionary origin. The fact that the same genetic code operates in bacteria, plants, animals, and fungi suggests all life descended from a single ancestor.
DNA stability and information transfer
DNA maintains exceptional stability, which enables the faithful reproduction of parental characteristics in offspring. This stability ensures genetic information passes accurately from one generation to the next. However, this raises an important question: if DNA is so stable and reliable, how does genetic diversity arise?
This apparent paradox - the need for both stability and variation - is resolved through the specific mechanisms that introduce controlled amounts of genetic change while maintaining overall DNA integrity.
Sources of genetic variation
Several mechanisms introduce variety into genetic material:
Sexual reproduction creates variation by combining different sets of genes from two parents. During meiosis, chromosomes undergo random shuffling, leading to unique combinations of genetic material in gametes.
Mutations represent spontaneous random changes to DNA structure. Although relatively rare, mutations form the foundation of genetic change and evolutionary processes. These random alterations provide the raw material upon which natural selection can act.
Environmental factors also play a role by modifying the expression of an individual's genetic characteristics, though the genes themselves remain unchanged.
Evolution and natural selection
Within any population, individuals show variation in their characteristics. Some individuals will be better adapted to their current environmental conditions than others. These better-adapted individuals have increased survival and reproductive success, making them more likely to pass their alleles to subsequent generations.
This process of natural selection allows populations to evolve over time, potentially leading to the formation of new species. The diversity within populations provides the variation necessary for evolutionary change to occur.
Natural selection does not create new variations - it acts on existing genetic diversity within populations. Without pre-existing variation, evolution cannot occur, which is why genetic diversity is crucial for species survival.
Measuring diversity
Scientists can quantify diversity at different levels:
Within-species diversity can be assessed by comparing differences in nucleotide base sequences in DNA or amino acid sequences in proteins. These comparisons reveal the genetic variation present within populations.
Community-level biodiversity can be measured using diversity indices or species richness calculations. These mathematical approaches help scientists quantify and compare the variety of species within different ecological communities.
Diversity indices often incorporate both the number of different species (richness) and how evenly distributed those species are (evenness). A community with many species but dominated by just one has lower diversity than a community with the same number of species distributed more evenly.
Foundation knowledge requirements
Success in this unit builds upon several key concepts from earlier study:
- Sterile techniques for preparing culture media and transferring microorganisms
- Meiosis as the type of cell division producing gametes
- The role of sexual reproduction in generating variation through gamete fusion
- DNA structure as a double helix composed of deoxyribonucleic acid
- Genes as specific DNA sections coding for particular protein combinations
- The relationship between alleles and population characteristics
- Quantitative sampling methods using quadrats and transects
Key Points to Remember:
- DNA serves as a stable information storage system that uses a universal genetic code across all life forms
- Genetic variation arises through sexual reproduction, meiosis, and mutations, despite DNA's inherent stability
- Natural selection acts on population variation, allowing better-adapted individuals to pass on their alleles more successfully
- Diversity can be measured both within species (through DNA/protein comparison) and between communities (using diversity indices)
- Practical skills including aseptic techniques and mathematical analysis are essential for studying genetic diversity