Biology Reference
In-Depth Information
364,000 generations of the organism for the chambered eye to develop by natural
selection. If we now suppose that each generation takes one year, which is a reason-
able time for a small aquatic animal, the eye could evolve in less than half a million
years. This compares with the minimal estimate of thirty million years between the
appearance of the first simple animals in the fossil record and the Cambrian explo-
sion some 540 million years ago. This calculation implies that about 500 million
years ago, the early vertebrates possessed eyes basically similar to our own.
Could we observe any of these small incremental changes in the laboratory
today? On the model described above, it takes about two hundred mutations
to change a flat surface into a slightly curved surface. If the generation time
of the organism is one year, it would need an experiment lasting about 36,000
years to observe this change. Even if the generation time of the organism was
one day, it would still take about one hundred years. This calculation illustrates
the basic problem with trying to convey the nature of evolutionary change to
the person-in-the-street - the rate of change is so slow as to be beyond human
comprehension.
An unsolved problem is how the first rhodopsin protein appeared and what
function it served. There are proteins called rhodopsins in Bacteria and Archaea,
involved in using light energy to move ions across the cell membrane, but this name
is misleading. These prokaryotic proteins are called rhodopsins because they have a
similar conformation to the eukaryotic rhodopsins, but there is no discernable simi-
larity in amino acid sequence between these proteins and the eukaryotic rhodopsins.
On the other hand, there are many Bacteria and Archaea that have not yet been stud-
ied, so it is possible that the origin of the eukaryotic rhodopsins will be found among
them.
Genetic Control of Eye Formation
In the 1970s so many different types of eye were known that it was proposed that the
eye has evolved independently at least forty times in different lineages of animals.
At that time, the available information was largely derived from studies of structures
visible in the light and electron microscopes. This picture was challenged by the
discovery in the 1990s that a single developmental regulatory gene, called P
ax6, is
required for eye development in species as different as insects, mice and humans.
There is now much more information available about the genetic control of eye
formation in a range of invertebrate and vertebrate species, leading to the proposal
that some common genetic factors underlie the evolution of different types of eye.
This conclusion reinforces the general conclusion that I have emphasised before in
this topic - all organisms are related to one another no matter how different they
appear to us. The
Pax6
gene links us to our early animal ancestors.
The first gene controlling eye formation was discovered in the fruit fly
Drosophila
in 1915. This gene was called
eyeless
because the result of a mutation in
this gene is to prevent the formation of eyes. It is common practice to name a gene in
