All species are transitional

Hi robin,I agree that it seems unlikely that there are whole bunch of genes that control the amount of speckling on an animal's coat. But that isn't really the way that the speckling is being specified. The amount of speckling will likely be determined by some threshold mechanism that operates during development; Perhaps there is a hormone or something that causes pigmentation to develop when it reaches a local threshold, and this hormone is in a feedback loop with an inhibitor such that if one part of the skin is high in concentration of the hormone, it's neighbours will be low. Slight changes to the threshold level of the hormone's activity and slight changes to the decay rate, the feedback mechanism, etc. might well produce a huge variety of different specklinesses, from spots to stripes, dots, cheetah spots, etc. etc.This has been anlysed by simulation using cellular automata, see here and hereThese models are a bit more elegant and a bit more parsimonious than the straightforward gene->phenotype kind of thing you're thinking of. You know, there is not one gene per speckle or anything like that Mick

Mutations are required if the change in pigmentation is to be heritable. But those mutations don't have to change the genetic basis of the pigmentation system. The mutations can just be the bog-standard SNPs that exist in any population, we're not talking about major mutations or the origin of new enzymes or anything like that.Let me try to put it a different way.You have a system that lays pigment out on the surface of an animal. This system probably involves a large number of components, many different hormones, hormone receptors, enzymes, and these are all linked together in a number of feedback mechanisms that ensures the pigment is laid out in some specified manner consistently, across generations. Many of these components are not specific to pigmentation, they may be things like growth hormone that are involved in many different processes at different points in the life cycle of the animal. The system as a whole also depends on things like temperature and dietary intake of the animal (so for example if you don't eat carrots you might not be able to make orange spots, they might come out brown instead).What I want to suggest is that once you have a generic pigmentation system operating in an animal, you can get a huge variety of pigmentation patterns without adding any new parts to the pigmentation system, without evolving new components or developing new enzymes or anything like that; the mutations we're talking about are just the standard polymorphisms that exist in a population, single nucleotide changes etc.Now consider how a human being would design such a system, for laying out speckled patterns on a surface. I've used a real example of a human-designed system "photoshop", I just downloaded the demo. Photoshop contains an algorithm that "lays out pigment" on the computer screen in the shape of pseudo-random vertical stripes (It's found in the Filters>Render>Fibers menu). The algorithm takes two input parameters ("variance" and "strength") and generates the pattern for you. Here's some examples of the patterns you can get from this single algorithm by varying the parameters (variance increases from left to right, strength from top to bottom). It's clear that this single algorithm is capable of producing anything from clouds (top left) to blotches (top right), fine even stripes (bottom left) to speckles (bottom right). It's just one algorithm.How does this correspond to the biological system for pigment layout. I would say that the algorithm corresponds to the genetic basis of the trait - the collection of enzymes, hormones, pigments etc. and their interactions that underly pigment layout on the surface of the animal.The input parameters correspond to any pertinent variable involved in the pigment layout system, ranging from environmental conditions (i.e. diet quality) to properties of the system itself (i.e. solubility of an enzyme, decay rate of a hormone, reaction rate of an enzyme and its substrate, the time required to create one pigment molecule, etc).Once you have the generic "algorithm" in place, then you can get all sorts of patterns, (clouds, speckles, stripes) just by varying the parameters. Very slight, subtle changes in enzyme solubility or hormone half life can tweak the pattern in quite an extreme manner without necessitating any change to the genetic basis of the trait. The genetic basis of the trait (i.e. the algorithm) can be static, but can still generate a variety of patterns.Well, this is all hypothetical. Are there any examples of this in nature? Here are some photos of conus shells. The kind of mechanism I've proposed above seems quite a parsimonious explanation of this variability. It seems unlikely to me that major mutations (changing the genetic basis of the trait) were involved in each an every speciation event. It seems more likely that the pattern is tweaked by natural selection acting on small subtle mutations that exist naturally in populations.

Certainly. Remember that the algorithm is just the genetic basis of the coloration pattern - comprising enzymes, hormones, helper proteins, blah blah blah, and their interactions.I think it's interesting that in cats there is a "colour density" gene (and this is also present in rabbits, known as the "colour dilution" gene, and you can do a google search for it) which corresponds exactly to "saturation" or "strength" in image editing software. The existence of a "colour density" gene does imply, in my opinion, that there is a processing system encompassed in the genetic basis of the trait. Because "dilution" is not a concept that is embodied in genetic material.So there is this complex system, which has emergent properties like "saturation" that are based in the physiological properties of proteins (their solubility or whatever).But what I would emphasize is that massive changes to pigmentation (or any other trait) don't require massive changes to the genetic basis of that trait. An albino, for example, does not use a different genetic system to determine its colour than does a normal animal. And an albino can give birth to normal-coloured animals, showing that the genetic basis is retained.In cats and rabbits you also get coloration determined by temperature, which just shows that the pigmentation system has a level of complexity greater than that embodied in the genetic material. (Of course that is not to say the system as a whole is not the result of natural selection. The complexity and lability of the system is enabled by the genetic basis of the trait, that's all I'm saying).Here are some links on the genetics of rabbit coloration, cat coloration, dog coloration.mick