To start the series let’s consider something quite simple. The species I will talk about is an imaginary one, I call it Spec. It lives in an isolated island, Isle, and Specs is the only animal present on it. Isle is a paradisiac island for Specs; virtually infinite surface and unlimited food, constant whether, and the Specs are almost perfectly adapted.
Specs’ life is divided in three main parts: food, sleep and sex.
They all sleep strictly the same time. And that’s invariable through the story I’m presenting here.
So, they eat and have sex. The first priority is food; a hungry Spec is unable to have sex, it can’t quite concentrate. Food is unlimited but it’s concentration isn’t optimal and Specs have to go around to gather what they need.
At time T0 all Specs are genetically identical, a perfect clone, as those you only may find in imaginary stories, and so they present exactly the same phenotype. From time T0 and further, mutations appears, but, to keep the story simple, they are possible in a single gene, velo. Velo is the gene which determines the maximal velocity of Specs.
It’s a quite important gene, because the faster a Spec is, the faster it can go around and find food and calm his hunger, thus increasing the time devoted to sex. So, mutations in velo gene, increasing velocity, are favored as they allow for longer sex periods and increase this way the probability to be inherited by the next generation.
Such mutations tends to accumulate and Specs become faster and faster over generations, up to the point Ts where the time spend for eating becomes insignificant compared to the time spend for sex. From now on, further mutation in velo gene don’t provide evolutionary advantage and thus they wouldn’t be favored. The velocity reached is Vmax.
From time T0 to time Ts there was a progressive and continuous increase of Specs’s velocity without any external factor acting; natural selection occurs, but driven by the probability of inheritance of the faster alleles of velo.
We have here a situation where random mutations on velo will be reflected over the generations as an increase of velocity of the individuals. One may expect that the process may be progressive at both the genetic and the phenotypic levels.
If you observe the increase of velocity of the Specs and don’t know anything about their genetics, you may have to postulate that there is some « Unknown Factor » making them faster and faster. Then you have two solutions: (a) considering some « Mysterious Internal Blueprint », due to « Supernatural Causation » or anything else one could imagine, or (b) inventing genetics and understand how Specs evolved from Spec SlowMotion (those Specs present at T0) to Spec AsFastAsPossible (those Specs living at time Ts.
Now, we know that even in the absence of directly involved, external, natural selection factors, Specs evolved following a strictly darwinian model, with completely random mutations at the genetic level, leading to a directional evolution of a phenotypical trait, velocity.
The selection factor in this case is the time available for sex. It exists without any external intervention, natural or supernatural, and will progressively vanish as the velocity of Specs increase to reach a plateau. In fact, the selection pressure will decrease proportionally to the increase of Specs’ ability to move fast.
And this is the first observation: phenotypical traits may evolve in a directional way, while mutations are perfectly random. The species is progressively adapting.
And there is a second observation: selection factors may be neither internal or external and their intensity may be directly dependent from the degree of evolution of a phenotypic trait. And selection factors may be invisible once the species is adapted. Here the selection factor is closely related to food’s density, which is an external factor. The schema proposed make the assumption that during Specs history food’s density don’t change.
Fig 1 :
Blue line corresponds to the mean velocity of the Specs; red line to the selection pressure. Top graph : increase of velocity and consequent dicrease of the selection pressure; no changes of foods density. Lower graph : Food’s density change during the period observed, reducing the selection pressure, which in a first time appears to be « negative« , then reach zero. The mean velocity of the Specs population drops in the absence of selective pressure (see below).
What if food’s density increase? Say to the point that feeding time is short enough to not hinder sex time. The selection pressure will drop, and the mutations accumulated will no more represent an evolutionary advantage. They will tend to disappear, simply, replaced by any other random mutations. The mean velocity of the Specs population will decrease and eventually reach the basal level again
No speciation observable here, just a succession of races of Specs.