Theory of Evolution (CIE A Level Biology)

• A species can be defined as a group of organisms that are able to interbreed and produce fertile offspring
• Members of one species are reproductively isolated from members of another species
• In reality, it is quite hard to define ‘species’ and the determination of whether two organisms belong to the same species is dependent on investigation
• Individuals of the same species have similar behavioural, morphological (structural) and physiological (metabolic) features
• A common example used to illustrate this concept are mules; the infertile offspring produced when a male donkey and a female horse mate

The gene pool

• A gene pool is the collection of genes within an interbreeding population
  • A gene pool can be thought of as the sum of all the alleles at all of the loci within the genes of a population

Changes to the gene pool

• The allele frequencies within a gene pool can change over time due to processes such as:
  • Natural selection
  • Genetic drift
  • The founder effect
• When the gene pool within a population changes sufficiently over time, the characteristics of the species will also change
• The change can become so great that a new species evolves from an existing species
• For a population to have evolved into a separate species it must be genetically and reproductively isolated from the pre-existing species population
  • Reproductive isolation can occur due to mutations that lead to the incompatibility of gametes or sex organs, or differences in breeding behaviour.
  • When two populations are reproductively isolated, they can also be said to be genetically isolated from each other, meaning that they do not exchange genes with each other in the production of offspring
• Changes in the allele frequencies of isolated populations are not shared so they evolve independently of each other; this can lead to the formation of two groups that are no longer successfully able to interbreed and that are said to be separate species
  • The formation of new species in this way is known as speciation
• The evolution of a new species can take a very long time and many generations
• For organisms with a short generation time (such as bacteria), evolution of new species can be observed far more quickly

• DNA found in the nucleus, mitochondria and chloroplasts of cells can be sequenced and used to show evolutionary relationships between species
• The differences between the nucleotide sequences (DNA) 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

• DNA sequence analysis and comparison can also be used to create family trees that show the evolutionary relationships between species

Example of a family tree showing the relationship between primate species

DNA Analysis and Comparison

• DNA is extracted from the nuclei of cells taken from an organism
  • DNA can be extracted from blood or skin samples from living organisms or from fossils

• The extracted DNA is processed, analysed and the base sequence is obtained
• The base sequence is compared to that of other organisms to determine evolutionary relationships
  • The more similarities there are in the DNA base sequence, the more closely related (in that the less distant the species separation) members of different species are

• In 2005, the chimpanzee genome was sequenced, and when compared to the human genome it was discovered that humans and chimpanzees share almost 99% of their DNA sequences, making them our closest living relatives
  • In 2012, the sequencing of the bonobo genome also revealed that humans and bonobos also share 98% of their genome (with slight differences to the differences seen in chimpanzees)

The DNA base sequences of two closely related species being compared - Species X is the ancestor of Species Y

Mitochondrial DNA

• When analysing DNA from the mitochondria is important to remember that:
  • A zygote only contains the mitochondria of the egg and none from the sperm so only maternal mitochondrial DNA is present in a zygote
  • There is no crossing over that occurs in mtDNA so the base sequence can only change by mutation

• The lack of crossing over in mtDNA has allowed scientists to research the origins of species, genetic drift and migration events
• It has even been possible to estimate how long ago the first human lived and where
  • Mitochondrial Eve is thought to have lived in Africa ~200,000 years ago
  • The estimation of this date relies on the molecular clock theory which assumes there is a constant rate of mutation over time
  • The greater the number of differences there are between nucleotide sequences, the longer ago the common ancestor of both species existed
  • The molecular clock is calibrated by using fossils and carbon dating
  • A fossil of a known species is carbon-dated to estimate how long ago that organism lived
  • This mtDNA of this species is then used as a baseline for comparison with the mtDNA of other species

• Although for your exams you should say that only maternal mitochondrial DNA can be passed on or inherited by the zygote, recent research suggests that paternal mDNA may also be present in zygotes