Wellwisher said:
If the mutation rate is so low, how is it possible for humans, for example, to show so much superficial variety, such as facial and body features, or finger prints and unique eyes for computer ID scans?
How to generate gene driven morphological variety: Billions of mutations, recombined in many billions of possible combinations, are tested by natural selection, each generation, for their ability to survive and reproduce, in their environment. Many of the variants that you might find in a population will not be strongly selectively advantageous or disadvantageous, so there will not be a strong selection for or against them, and they will be found in future generations. This can result in a lot of variability.
Nature vs. Nurture: There have been
many analyses of how much variability in organisms is due to their inherited genetic instructions as opposed to other causes (usually considered nurture, meaning events specific to their life, affecting how they develop).
Realizing genetically identical organisms have a degree of variability among themselves, makes it possible to calculate a ration of effects on a final phenotype by comparing the variability of the trait in identical siblings with the variability among unrelated individuals with a "similar" genetic complement (like non-identicle twins or siblings).
There are many studies. Seems like they are usually around 50%-50% in the effect of Nature vs. Nurture on different traits, but there's a lot of variability.
There are cases were it will be all nature (a mutation in a gene product directly causing a phenotype, like sickle cell) and some that can be caused or strongly influenced by environmental influences (like loss of an appendage or modifying the color of fish by what you feed them).
The environmental variability can be attributed to many things:
For example,
Development: Not all processes in development (going from a fertilized egg to an adult) are genetically determined at a molecular level. There are many cases in development where systems of operation are set-up and let run (such as sets of cells at certain locations in a developing embryo, they produce, release bind, and respond to small amounts of communication molecules to signal among themselves). A cell here and there may die, get misplaced, not get a signal, or not respond normally and the system can adapt and still produce a functionally useful product (adult form, able to reproduce).
These kinds of adaptive processes are repeated, over and over during development, as structures generated by one developmental process (like gastrulation endoderm, mesoderm, and ectoderm), are used as a basis for the generation of another level of structural detail (like the generating the nervous system).
Small changes in earlier stages can cascade into larger changes in later stages, like gastrulation problems, can lead to later developing nervous system problems, like spina bifida. The later structures are contingent on earlier structures.
Their communication system let's the developing system (higher level than cells) adapt to its different situation. It should be noted that in the axon and synapse do more than just release neurotransmitters. There a lot of signals going in both directions among the players interacting in this situation. In times of disturbance (wounding) the signals will change, cells will respond by changing their physiology and which genes they are expressing and how that is controlled.
Cells are not just dumb little bags. They are sophisticated information processors that can sense things around them, respond physically and biochemically, and move change position and their OS.
These kinds of adaptive processes also allow a developmental process to control greater numbers of cells (in a larger space, in a larger organism) without having to individually programming each individual cell (cells in an animal can excede the number of base pairs in the genome).
From a selection point of view, there would be no selection against a system that operates this way, unless it did not work well and produced bad results (fewer reproducing offspring). Perhaps there would be a burden of the building and maintaining the signalling systems in the various different types of cells involved (but these singnalling systems are found in many other cellular systems). Selection in favor of such a mechanism could be attributed to its greater
robustness in the face of environmental challenges.
This category would include things like your wound scenario.
Wellwisher said:
Does the brain have a role in this mechanism, providing feedback for cellular differentiation control, from our unique POV.
Wellwisher said:
Do they help to regulate replication in other cells and therefore help control mutation rates? If you cut your hand, you slice through nerves and the local skin cells lose process control. They replicated faster until wired back up, with the scar leaving a trace of change, due to loss of process control.
Influence of the nervous system on injury responses: There are some cases where injured nerves have been shown to release substances that promote regeneration/repair responses. This usually affects things involved things like
regenerating a limb or promoting cell division to repair a wound.
At a cellular level, a
neuron can tell a regenerating muscle cell where to put its part (post-synaptic side) of the synapse.
Influence of somatic mutations on evolution:
In animals, any mutations of somatic cells (cells of the body (soma) that will not be genetically involved in reproducing) are not going to contribute to evolutionary change of the breeding population because mutations in cells in the body will not end up in the next generation. To get new mutations into the next generation, the cells with the mutations will have to be in the gonads (ovaries and testes), and only some of those cells will be the reproductive germ cells. The others will just be there to keep the germ cells happy (physiologically speaking).
However, mutations in somatic cells may cause cancer.
The germ cells are usually a protected set of cells (selected to be protected because they are the seat of genetic transmission (to the next generation)). Their lineage is usually separated from the rest of the cells in the body and under go relatively few cell divisions until the animal starts reproducing.
There are
primitive animals where the germ cell lineages are not separated from those of somatic cells (sponges, coelenterates). Some can also reproduce by budding, when somatic mutations could transmit to the next generation. Their reproductive cells are endodermally derived pluripotent stem cells. These kinds of animals can regenerate from having their bodies cut in half, so germ cell distribution would be adaptive.
Wellwisher said:
Do they help to regulate replication in other cells and therefore help control mutation rates?
Regulating replication and mutation rates are two separate things.
I suppose an overly rapid rate of division could result in cell division before DNA copying is done or before chromosomes are properly lined up and separated, but normally there are lots of molecular mechanisms in cell division (mitosis) to prevent these things.
Wellwisher said:
Brain cells or rather neurons never replicate after an early age.
Yes, but:
A few cases are now known (for >20 years) where
new brain cells have been produced in mammals. Its not the brain cells dividing however, its precursor cells dividing and making new baby neurons.