Survival of the fittest. This is the famous phrase that people generally believe represents Charles Darwin’s idea of natural selection. However, this is not the phrase created by Darwin but by Herbert Spencer.
The essence of evolution is adaptation. Each organism, more precisely each species, appears adapting to its living environment very well. How did this happen? Why does a species look perfectly fit to its environment?
Adaptation is the keyword. A French naturalist, Jean-Baptiste Lamarck (1744-1829), saw the adaptation at the individual level. The characteristics more frequently used can develop more and become better. On the other hand, the ones not in use will less develop. Then, the characteristics developed by frequent usage in an individual will be inherited to the next generations. Adaptation is the consequence of ‘use and disuse’ and inheritance of acquired characteristics.
Darwin saw adaptation at the species level. He saw it as a population-based process. He recognized variation in a population, and some variation is inheritable. He knew various observations of breeders of domesticated animals and plants. He captured domestication as the selection of inheritable variation by humans. Finally, he visualized that life is the struggle for survival. The environment in which each species lives selects the survivors. Their survival is dictated by inheritable variation. The survivors should carry the traits better for survival. Darwin left three keywords: inheritance, variation and natural selection. Adaptation is the consequence of the enrichment of the inheritable variation in a population by natural selection.
In domestication, humans selected the traits they wanted. Something humans wish for is considered as good to be chosen. How did the environment select? Death or alive. Is there anything the environment wants to select? This is not the correct way of phrasing the question. We should use the word ‘permit’ instead of ‘select’. How did the environment permit?
This is much better. The environment is just permitting, not selecting. Then, does adaptation happen by permitting? The crucial point in this discussion is the threshold of ‘death or alive.’ If everyone in a population survives, this is not selection but permission. If only 10% survive, it sounds like a selection. If 90% survive, this sounds close to permission.
What controls the threshold of ‘death or alive’? Is there any predictive trait of ‘death or alive’? I can imagine lethal deficiencies such as malfunction of essential organs and white colour in a black environment, etc. This can predict death. However, I cannot imagine any trait guarantees their survival, except in anti-bacterial resistance and domestication. Both require human involvement.
To select the top 10%, whatever the top means, the selection should have uniform, consistent criteria and be applied to the whole population. Only in this condition is adaptation plausible. The orientation in fitness is recognized. Better and worse emerge. However, when this happens in nature, this is the time for the extinction of the species. No further adaptation. The dead end.
Suppose the bottom 10% is selected to death or not permitted to survive; how about the rest of 90%?
Can all 90% survive? This does not look real in nature. How about this? 10% are selected to death based on inherited traits. But only 10% survive from 90% of survivors by chance. As far as no lethal deficiency, the chance of survival is random and not linked with inherited traits. No orientation in fitness. All variations not causing lethal deficiency are permitted. Diversity increases.
This looks more real to me. The problem with this view is no adaptation. A species cannot adapt to its environment if we think this way. I agree. Adaptation happens only in unique, rare contexts but is not a general driver of evolution. Because of adaptation, maximization of individual fitness has been considered the agenda of live organisms. I'm afraid I have to disagree with all these views.
Adaptation and diversity are almost contradictory concepts to me. The current view is that because each environment is uniquely diversified, species adapted to each environment look diversified. Then, I have to ask a question. Did all species pass through the high threshold selection based on inherited traits? I doubt it.
Without adaptation, why does each species look perfectly fit to its environment? The inheritable changes always happen without any intention. The only changes not permitted are lethal deficiency. Up to here, everyone can agree. Without reproductive isolation or high threshold-directed selection, all genetic changes are diluted in a population. The enrichment of allelic frequency is often observed by genetic drift, not by selection, because consistent, directed, uniform selections are difficult.
Reproductive isolation/separation is the best way to fix unique inherited traits. Darwin noticed this as hybrid infertility. But that was too early to see the direct link to evolution. A German-American geneticist, Richard Goldschmidt (1878-1958), noticed that chromosomal rearrangement can become a reproductive barrier and a cause of inherited traits. If this happens, even if there is no phenotypic change, a new tiny group of organisms emerges that cannot reproduce with the original species.
The chance of survival of the new tiny group would be slim if they needed to compete against the original species. However, if associated phenotypic changes with the reproductive barrier happen to permit them to live in a new environment where no other species has taken, they could survive. I call this aptness. Aptness means permissiveness and preparedness.
The environment the new tiny group takes is not livable for the original species, like too hot or too cold, etc. This is not adaptation because there is no direction. It just happens to fit in an unoccupied space. Because of no competition, they can co-exist. Diversity increases.
When I look at the history of life, it looks different from the maximization of fitness and the survival of the fittest. Suppose we accept adaptation; a direction and concept of fitness need to emerge. Teleological thinking and agential thinking are inevitable. The goal of an organism is survival and reproduction. To achieve this goal, the maximization of fitness is the agenda because only survival of the fittest will be selected. As I discussed, adaptation is rare. It often demands human involvement.
In addition, many selectable phenotypic traits are epigenetically regulated. Sexual selection might only select epigenetic characteristics. Genetic adaptation in diploid organisms is very complicated if multiple genes are involved. Genetic drifts appear as a selection in Mendelian traits. Ultimately, the phenotypic variation is not solely created by genetics but epigenetics.
Reproductive isolation with aptness is the essence of speciation. Then, what is the driving force of evolution if adaptation is not? The minimization of lethal deficiency through increasing robustness. From haploids to diploids, from mono-cellular to multi-cellular organisms, they are not better. The new species, class or phylum is more robust and permits them to take on new open environments. Therefore, the old ones can still be around, and new ones co-exist.
Why did many species become extinct? Because of global changes in their environment. Uniform, consistent high-threshold selections. Nothing can be done by adaptation within a species. Only lucky aptness permits some species to survive. The continuity. This is the only reason why life exists on the earth. Survival and reproduction are not the goal but the requirement. All conditions permit survival and reproduction can stay and continue. The current live organisms are the ones who luckily found their niche when they were open. When the niche disappears, none can adapt but become extinct. Humans are the only species creating uniform, consistent environments, leading to unique adaptations and many species’ extinctions.