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A few notes:
Stiffler, Hekstra and Ranganathan (2015) Evolvability as a Function of Purifying Selection in TEM-1 β-Lactamase. Cell 160:882 2015 https://www.sciencedirect.com/science/article/pii/S0092867415000781?via=ihub
Johnson et al. (2019) Higher fitness yeast genotypes are less robust to deleterious mutations. bioRxiv. https://www.biorxiv.org/content/10.1101/675314v1
As noted, while evolutionary theory certainly originated with Darwin, you are correct that his thoughts on evolution are quite old (they pre-date knowledge of genes and genetics, which are crucial to our modern understanding of evolution). If modern evolutionary theory has a name, it would be generally referred to as the "modern synthesis," referring to a theory that synthesizes Darwin's model of natural selection with our modern understanding of genetics and molecular biology. A brief description of the modern synthesis can be found here: http://www.talkorigins.org/faqs/modern-synthesis.htmlBut that doesn't address all the issues and I am confident that other people have augmented that basic theory with 'add-ons' and really I don't know what they are. Can biologists here tell us what those are?
It is important to remember that the fitness of an organism is a function of its environment. An organism that is the most fit in one environment is not necessarily the most fit in a different environment. Sometime, specialization to gain fitness advantages in one particular environment may cause fitness disadvantages in a different environment (e.g. the example of the cost of antibiotic resistance genes in an antibiotic free environment which was mentioned earlier). These trade offs between fitness and "robustness" or "evolvability" (i.e. ability to withstand environmental change) have been experimentally observed in laboratory models of evolution, for example in the following studies:1. In answer to the OP's point, I think one of the points that has to arrive is that, surely, the theory is actually 'survival of the generally fit' rather than the fittest. I mean, if mutation is random, as the OP proposal (which seems to make sense to me), this will give a range of characteristics, some of which may not be particularly beneficial at a given point in time. But as environments and predation changes those 'hidden' characteristics may manifest and all of a sudden within the generic mix of those individuals, those that 'had been' the fittest all of a sudden, possibly instantaneously, die out. For example, say a species living in an arid area has some of its individuals 'uselessly' born with webbed feet but it doesn't disadvantage them too much and it just happens to be a characteristic, some have more webbed feet than others. The fastest runners don't have webbed feet and so do a bit better and slowly the population are moving towards 'no webbed feet', but still it is not such a disadvantage that those individuals die out. .... and then their habitat floods ...! So I think the substance of this is not merely 'the fittest' and 'survival' but actually if a species can successfully maintain the widest set of random characteristics then it is more likely to survive. As such, this is not survival of the fittest individuals, this is survival of the species with the widest set of tolerances to environmental changes. I am sure this has been proposed by evolutionary biologists and already has a name, which would address the OPs question.
Stiffler, Hekstra and Ranganathan (2015) Evolvability as a Function of Purifying Selection in TEM-1 β-Lactamase. Cell 160:882 2015 https://www.sciencedirect.com/science/article/pii/S0092867415000781?via=ihub
Johnson et al. (2019) Higher fitness yeast genotypes are less robust to deleterious mutations. bioRxiv. https://www.biorxiv.org/content/10.1101/675314v1
The rate of evolution is definitely not linear and there are periods (especially those associated with widespread environmental changes) that are associated with much higher rates of speciation (e.g. the Cambrian explosion). This relates to the previous point about the role of environment in determining fitness. Changes to the environment will alter the relative fitness of the populations in an ecosystem, precipitating changes in the populations in that environment. The idea that evolutionary change can happen in intense bursts of change followed by relative periods of stability is known as punctuated equilibrium (https://en.wikipedia.org/wiki/Punctuated_equilibrium). The extent to which "evolution by jerks" (punctuated equilibrium) vs "evolution by creeps" (gradualism) describes most evolutionary change is a subject of debate in the field.3. Rate of change of species; A common ancestor of ours to other primates (as far as I know) is the Miocene Proconsul (https://www2.palomar.edu/anthro/earlyprimates/early_2.htm) which is said to have lived from 21 to 14 million years ago. But if it took 7 million years to become a new species then why is that an unrepresentative timescale for the 14 million years since then? I mean, surely there are more than two evolutionary 'species' steps from proconsul ape to us? Maybe not? I don't know? What do updated evolutionary theories say about this? I mean, taking that thought further, if we look at the first 'modern humans' 250,000 years ago, if one was here today they could integrate into society, we'd be able to have offspring with them and (presumably with training!) talk together. Yet that is 'only' 1/40th or so between proconsul ape and us, and it looks to me like there are going to be more than 40 times the number of differences between early homo sapiens and proconsul ape? Again are there any modern theories to explain what would initially appear to be, presumably, sudden accelerations in changes of species?
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