4.3.5 Classification and Phylogeny

Question 1

What is phylogeny?

Answer 1

In the past scientists classified organisms on the basis of shared visible features
Today scientists aim to classify organisms on the basis of phylogeny
Phylogeny can be defined as:
The evolutionary history of organisms

Classifying organisms according to their phylogeny means that species that share a more recent common ancestor are classified together, while species with a more distant common ancestor are classified in separate groups
Phylogenetic classification often means that historical organism groups need to be changed
E.g. grouping organisms on the basis of shared characteristics may result in birds and bats being classified together, but we know that these two organisms are not close evolutionary relatives
Advances in DNA, RNA and protein sequencing have allowed scientists to classify organisms according to their phylogeny more accurately than using visible characteristics
Molecular analysis allows scientists to build phylogenetic tree diagrams that show the relationships between organisms

Question 2

How can we use molecular evidence in classification?

Answer 2

Three types of sequence data are used to investigate evolutionary relationships
DNA
mRNA
Amino acids (of a protein)
Sequencing technology can determine the order of DNA bases, mRNA bases and amino acids within an organism’s genome
This technology is especially useful for comparison with an extinct species (using ancient DNA) or when distinguishing between species that are very physically similar
Scientists will choose specific proteins or sections of the genome for comparison between organisms
Looking at multiple proteins or multiple regions of the genome will allow for a more accurate estimate of evolutionary relatedness
Note the protein used needs to be present in a wide range of organisms and show sufficient variation between species
Cytochrome c is often used as it is an integral protein to respiration (in the electron transport chain) which is used by all eukaryotic organisms
For all types of sequence data it can be said that the more similar the sequences, the more closely related the species are
Two groups of organisms with very similar sequences will have separated into separate species more recently than two groups with less similarity in their sequences
Species that have been separated for longer have had a greater amount of time to accumulate mutations and changes to their DNA,mRNA and amino acid sequences
Sequence analysis and comparison can be used to create phylogenetic trees that show the evolutionary relationships between species

Question 3

How can we use DNA analysis and comparison?

Answer 3

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 share 98% of their genome (with slight differences to the differences seen in chimpanzees)