Evidence for Evolution

• The term ‘evolution’ has two meanings:

  • It can refer to the general theory of evolution. This is the way in which species have changed over time and continue to change
  • It can refer to the specific process by which evolution occurs - the theory of evolution by natural selection
• Although the general idea of evolution was acknowledged at the time, Charles 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 published his ideas in his book, On the Origin of Species (1859).
• Darwin made several key observations that helped him to develop the theory of evolution by natural selection, including:

  • There are always significantly more offspring than adults.  All organisms produce more offspring than can ever actually survive
  • Populations of organisms do fluctuate over time, but not significantly.  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
• Darwin's Theory of Evolution by Natural Selection:

  1. Individuals in a species show a wide range of variation
  2. Individuals with characteristics most suited to the environment have a higher chance of survival and more chances to reproduce
  3. These characteristics are passed to their offspring at a higher rate than those with characteristics less suited to survival
  4. Over many generations, these beneficial characteristics become more common in the population and the species changes (the species evolves)
  5. This idea of natural selection became known as ‘survival of the fittest’
• The theory of evolution by natural selection was only gradually accepted because:

  • There was much controversy surrounding these revolutionary new ideas
  • The theory challenged the idea that God made all the animals and plants that live on Earth
  • There was insufficient evidence at the time the theory was published to convince many scientists
  • The mechanism of inheritance and variation was not known until 50 years after the theory was published
• The theory of evolution by natural selection developed over time and from information gathered by many scientists. It is now widely accepted. Two key sources of evidence for this theory include:

  1. Fossil evidence
  2. Molecular evidence. Our understanding of genetics has clarified the mechanism by which natural selection can occur. It has been shown that characteristics are passed on to offspring in genes
• 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 organisms into a sequence through evolutionary time.
• Fossils also allow us to show similarities between extinct species, ancestral species and present-day species which indicates how closely related they are.
• 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.
• DNA found in the nucleus of cells can be sequenced and used to provide evidence of evolutionary relationships between species. DNA sequencing also allows us to observe how the genetic code of a species has changed as it has 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.