Foraminifera may perfectly illustrate evolution at work. As single celled organisms with a short life-cycle foraminifera and the composition of a specific foraminiferal fauna adopt rapidly to even small environmental changes. The great abundance of foraminifera in marine space and time in the long fossil record may be used to perfectly illustrate evolution.

The Theory
The theory of evolution wants to explain how we - meaning all living organisms - came to be in all our complexity and variation. Biological evolution is simply seen as a change in the frequency of genes through time. Charles Darwin (1809-1882) published 1859 his seminal work On the Origin of Species. Genes were not yet known but Darwin referred to the directly linked heritable characters, which are passed from parents to offspring.
Mutations
The passing process of genes is remarkably accurate but not 100% perfect, mistakes happen which are called mutations. These mutations cause differences between the single organisms, which result sometimes in differences in their fitness to reproduce.
Natural selection
In a group of organisms the environmental conditions will favour some individuals while others will not reproduce as well. If heritable characters are the cause a shift in their frequency within a population will occur. This process is called natural selection.
Time
Gene frequencies can remain constant for long periods of time and changes happen gradually. The huge variety and complexity of todays live on Earth needed a long time period to evolve. The fossil record shows indeed a very long time period, illustrated by findings of seashell fossils on high mountain tops. The earliest fossils - single celled organisms - are dated back to more than 3.500.000.000 years.
Evolvement of complex liveforms
The rising complexitiy of live needed long periods of only small changes in environmentals conditions to evolve. Abrupt changes caused in special the extinction of complex species, best illustrated by the extinction of the dinosaurs. Simple forms of live such as the foraminifera Reophax living in the sediment were almost not affected at all.

Open Sea offers little nutrition, which makes survival very difficult though competition is low and predators are rare. Planktonic foraminifers such as Globigerinoides use symbiont algae, who deliver energy through photosynthesis. In return Globigerinoides gives shelter for the little algae in its chambers, which need to be transparent, so that the sunlight may pass. Its globose construction allows sinking and up and downward drift in the watercolumn, probably needed to follow food, avoid predators and UV-radiation.

An elevated life position is advantageous as many competitors and predators live directly on the sea floor. Some foraminiferal tests show imprints of the substrate they were attached to, such as stems or leafs of algae and plants. Such a life-style depends on the possibility to attach to something, as found in seaweed meadows of shallow waters. A very small percentage of oceans-space offers such a habitat, which is vulnerable too.

Planispirally enrolled, round tests as build by Lenticulinas are advantagous to move through the upper and moderately condensed parts of the sediment. Recent specimens of planispiral genus Elphidium from around the world are observed to live on and in the sediment:

when severe changes of environmental conditions occur, is shown by the foraminiferal genus Reophax. It builds a simple row of chambers from pieces of sediment cemented together. A recent Reophax looks similar to a 30.000.000 years old Reophax from the Oligocene. There may have been no advantage to change its construction plan. The genus is in the fossil record since 460.000.000 years. After the Cretaceous/Tertiary extinction event when the dinosaurs went extinct, Reophax took advantage of the extinction of higher developed foraminifera. It peaked in worldwide abundance taking over the free living space.