OCR AS Biology

Revision Notes

4.3.6 Evidence of Evolution

Evidence of Evolution

  • Regarding evolution, it is important to distinguish between two key ideas:
    • Firstly, the term ‘evolution’ can refer to the general theory of evolution. This refers to the way in which species have changed over time and continue to change
    • Secondly, the term ‘evolution’ may be used as a shorthand way of referring to the theory of evolution by natural selection (i.e. the specific process by which evolution occurs)
  • Although the general idea of evolution was acknowledged at the time, two biologists, named Chares Darwin and Alfred Russel Wallace, contributed greatly to developing the specific theory of evolution by natural selection
    • Wallace spent many years travelling in South America and South-East Asia and collecting specimens from these places
    • Darwin took part in a voyage around the world and collected specimens and information about many species
    • These experiences and the notes and samples they collected along the way led to both men publishing a joint scientific paper proposing their theory of evolution by natural selection in 1858

Darwin’s observations

  • Darwin made several key observations that helped him to develop the theory of evolution by natural selection, including:
    • All organisms produce more offspring than could ever actually survive (i.e. not all offspring survive)
    • Populations of organisms do fluctuate (change over time) but not significantly (i.e. their numbers usually stay fairly constant)
    • Populations of the same species of organisms show variation in characteristics between individuals (known as intraspecific variation)
    • Offspring inherit characteristics from their parents
  • The theory of evolution by natural selection is now widely accepted
  • Two key sources of evidence for this theory include:
    • Fossil evidence
    • Molecular evidence (our understanding of genetics has made clear the mechanism by which natural selection can occur and it has been shown that characteristics are passed on to offspring in genes)

Fossil evidence for the theory of evolution by natural selection

  • Fossils are preserved remains of organisms or other features left by organisms, such as footprints, burrows and faeces
    • We can tell from fossils that environments (and the organisms living in these environments) have changed significantly over millions of years
    • Fossils, as well as the rocks they are found in, can be dated, allowing us to accurately put fossil organisms into a sequence from oldest to youngest (i.e to see how the organisms changed through evolutionary time)
    • Fossils also allow us to show similarities between extinct species (i.e. how related they are) and even between now-extinct, ancestral species and present-day species
  • All this has provided evidence for the gradual change from simple life forms, such as Archaea and Bacteria, to complex Eukaryotic life forms and the evolutionary relationships between organisms

Molecular evidence for the theory of evolution by natural selection

  • DNA found in the nucleus of cells can be sequenced and used to provide evidence of evolutionary relationships between species and how the genetic code of species has changed as they have evolved
  • The differences between the nucleotide sequences in the analogous genes of different species can provide a lot of information:
    • The more similar the sequence the more closely related the species are
    • Two groups of organisms with very similar DNA will have separated into separate species more recently than two groups with less similarity in their DNA sequences
  • As a result, DNA sequence analysis and comparison can be used to create phylogenetic trees that show the evolutionary relationships between species
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