A-Level Biology OCR Notes

Classification

The Biological Species Concept- a species contains all organisms that are capable of breeding together to produce living, fertile offspring.
Classification is the process of sorting living things into groups.
Classification hierarchy comprises the taxa: domain, kingdom, phylum, class, order, family, genus and species.

Classifications are constantly updated as new methods are discovered to infer relationships e.g. physical characteristics to DNA sequencing, amino acid sequencing or cytochrome C comparisons

Animalia	Animals are eukaryotic, multicellular, heterotrophic and usually able to move around
Plantae	Plants are eukaryotic, autotrophic, multicellular, have a cellulose cell wall, and contain chlorophyll
Fungi	Fungi are eukaryotic, single-celled or multicellular, saprophytic, have a chitin cell wall and a multinucleate cytoplasm
Protoctista	Protoctists are eukaryotic, mostly free living, single-celled or multicellular, autotrophic or heterotrophic
Prokaryotae	Prokaryotes have smaller ribosomes, a naked loop of DNA, free living or parasitic, no nucleus or no other membrane bound organelles,

In a phylogenetic diagram, branch tips represent species at the end of their specific lineage, branching points represent common ancestors & The closer the branches, the closer the evolutionary relationship.

Natural Selection

Predation, disease and competition means that not all individuals within a population survive to get a chance to reproduce. This differential survival and reproduction is the process by which natural selection acts.
Genetic variation within a population creates intraspecific competition. The organisms with phenotypes that provides a selective advantage are more likely to survive and reproduce and thus pass on their favourable alleles to the next generation. This means that the proportional of individuals with the favourable allele will increase in the next generation (increase the allele frequency) within the population. This is evolution by natural selection.
Evolution is the change in allele frequencies in a population over time.
An adaptation is a trait that enhances survival in a habitat
- Anatomical adaptations are structural features e.g. roots with a large surface area
- Behavioural adaptations e.g. hibernation
- Physiological adaptations are changes to cellular processes e.g. fight or flight adrenaline response
Convergent evolution occurs when organisms that live in similar conditions are subject to the same selection pressures, resulting in similar adaptations
Evidence for the theory of natural selection comes from the fossil record and biological molecules, including mDNA
Evolution by natural selection has resulted in antibiotic resistance bacteria and pesticide resistance in insects

Variation

Interspecific variation is the differences between any two species
Intraspecific variation is the differences between members of the same species
Characteristics that show continuous variation are normally polygenic (determined by many gene loci that have additive effects on each other).
Characteristics that show discontinuous variation are usually monogenic (determined by a single gene loci).
Variation is due to genetic and environmental factors.
The main source of genetic variation is mutations, which can produce different alleles of genes.
Further sources of genetic variation include meiosis (independent assortment and crossing over) and the random fertilisation of gametes during sexual reproduction to create new allele combinations.
The environment can influence the way an organism’s genes are expressed. This can be because of biological factors such as predators or non-biological factors such as sunlight.