Where did I come from?


Estimating Biological Relationships - Why Evolution Matters

Genetic methods can be used to study how organisms are related one to another.

Genetic Changes Accumulate

Genetic changes accumulate slowly in a species' genome as time passes. When that species becomes split into two species, each of those daughter species will continue to accumulate change, but the changes will be different in each species. These daughter species will continue to share the changes which their ancestor acquired. So, species contain both unique genetic differences and shared differences, when compared with other related species.

 

sequenceevolution.png

 

Can you find where the daughter species A and B share genetic changes inherited from their ancestor?

Inferring Relationships from Genetic Differences

We are able to study the relationships among species by determining the genetic differences among them.
Imagine this splitting process happening several times in successive daughter species. Genetic differences will accumulate. However, when scientists obtain DNA sequences from many different organisms and compare them, they will find that some are more similar to one another than they are others. This genetic similarity is mostly because closely related species will share a recent ancestor from whom they inherited a common genetic makeup.

 

phylogenetics.png

 

In this diagram you can see how genetic changes are inherited through several ancestral species. We observe genetic sequences from the four species shown at the bottom. They are all descended from their common Ancestor 1, at the top.

One of Ancestor 1's daughter species is Ancestor 2, and that species differed by a single change from Ancestor 1. Ancestor 1's other daughter species is Species 1, and there have been two changes in that history.

Ancestor 2 split to form Ancestor 3 and Species 2, and subsequently Ancestor 3 gave rise to Species 3 and 4.

Each change is shown as if we knew exactly what happened. However, scientists only see Species 1 - 4. They have to use reasoning to work out what the changes must have been. By this process they estimate what genetic sequences occurred in the ancestors, and the history of species splits.

These splits represent the evolutionary relationships among the species. One way of estimating the history of splits is to count the differences among the observed species, and then to use statistical methods to draw the diagram.

Usually scientists don't compare whole genomes, but choose more manageable portions of them.