Adaptation is an important concept in evolutionary biology. Each species appears adapting well to its environment. This is considered the consequence of natural selection. Individuals with better-adapted traits as variation in a population will have a higher chance to survive and reproduce. Therefore, the allelic frequency of the genes involved in controlling for the traits will be selected and enriched in a population. If this is true, the species keeps adapting better to its environment than its ancestors because Nature selects better.
I have substantial doubt about this notion. I wonder if selection happens. I cannot tell what better is in Nature. Once I doubt better, the direction for adaptation disappears. Probably, adaptation only occurs in some exceptional cases. On the other hand, in human societies, selection is possible and better exists. What causes the differences?
Then, what do you think about the difference between selecting the top 10% or the top 90%? In other words, the latter determines the bottom 10% for elimination.
To have ‘selection,’ we need to have a population with a measurable variation with criteria for selection. We cannot select anything from one. If all in the population are identical, it is nonsense to use the word ‘selection.’ If we can take all, it is also not ‘selection.’ Without any measurable variation or any criteria, ‘selection’ becomes random. In evolutionary biology, the consequence of random selection is called ‘genetic drift’ or ‘founder effect,’ the selection conducted without any criteria on measurable variation. Random sampling does not cause adaptation. For adaptation to occur, the selections need to be ‘uniform’ and ‘consistent.’
Natural selection is considered the process of ‘selection’ to adapt better to its environment. The driving mechanism is ‘death or alive.’ We know that a population of one species always has variations. Some of those variations are measurable and called phenotypic traits. Do any phenotypic traits of individuals contribute to the result of ‘death or alive’? Probably, many would answer this as yes. Then, the next question is if this is selecting the top 10% or the top 90% of the population.
How much do phenotypic traits of individuals contribute to their survival? Is the survival solely dependent on phenotypic traits? This is the crucial point if adaptation occurs or not. Adaptation is plausible if only the top 10% survive and are selected by the traits, and the selection is uniformly applied to the whole population for several generations. However, what do you think if the top 90% survive based on their traits, but 80% of those survivors randomly die, although both have a 10% survival rate? The traits associated with lethal deficiency would be removed, but adaptation is unlikely.
Uniform and consistent selections are rare in Nature but common in human societies and activities in which humans are involved. With a high threshold of ‘death or alive,’ we generated domesticated animals and plants and contributed to the emergence of antibiotic-resistant bacteria. In the history of life, there have been several times when ‘uniform’ and ‘consistent’ selections occurred, such as changes in oxygen tension and climate. I do not know if any species survived those challenges by adaptation. The question is if any variation within one species can provide sufficient and consistent differences for survival.
All multicellular organisms have chromosomes and go through the haploid-diploid life cycle. For animals, organisms we usually recognize are diploid and sperm and oocytes are called haploid gametes. For the successful reproduction of animals, meiosis and fertilization are essential. Meiosis is particularly interesting, the process of a diploid cell becoming two haploid daughters. During meiosis, two homologous chromosomes are paired and exchange their sequences before separating into two haploid gametes. If two homologous chromosomes are not similar enough due to inversion, translocation or others, meiosis fails. This does not mean the individual's death, but there is no chance for offspring. Notably, the success of meiosis is entirely independent of the phenotypic traits. This means that even if the phenotypic traits permit them to survive in the top 10% of the population, no progeny will be created—no enrichment in the population.
Another problem is that the phenotypic traits in parents are not copied in their offspring in diploid animals. Fertilization permits only 50% of inheritance from parents. The beneficial phenotypic traits that allow parents to survive do not always appear in their offspring. This is problematic if the challenge is uniform and consistent with a high threshold of ‘death or alive.’ In diploid organisms, robustness is higher than in haploid ones because detrimental mutations can be masked.
On the other hand, advantageous traits are easily diluted in a population. If a trait is regulated by a single gene, like the colour of organisms, the chance of inheritance would be higher in offspring with a high selection threshold. However, if a trait is regulated by multiple genes, like the height of humans, the reproducibility of the traits will become dependent on its partner. This property creates an interesting conflict for reproduction. The best way to keep the complex advantageous traits regulated by multiple genes is sibling mating. However, sibling mating reduces the masking ability of detrimental mutations. This is what humans performed during the domestication of animals and plants. The reduced reproductive ability of farm animals and the susceptibility to genetic diseases in specific breeds of dogs are good examples.
In diploid organisms, fixing complex phenotypic traits in a population regulated by multiple genes is complicated and easily diluted by mating. The best way to overcome this difficulty is to separate the individuals carrying the traits from the original population to constrain their reproductive activity. On the other hand, tremendous phenotypic variation is observed if this is accomplished. Look at various breeds of dogs. It isn't easy to believe that all belong to a single species. None of the other species show levels of stably inheritable variation in size, shape and temperament that dogs have.
Adaptation would happen if a single gene regulates the trait and is selectable with a consistently high threshold of ‘death or alive.’ However, adaptation is unlikely if multiple genes regulate the trait in diploid organisms. Robustness counteracts the phenotypic shift within a population. For diploid organisms, founder effects, which limit the number of founders, are the only way to stabilize the traits and observe the phenotypic shift.
Without adaptation, why does each species appear to be adapting well to its environment? I want to use the word ‘aptness.’ A species gains the traits suitable to fit the environment. Not selected by competition and not adapted. Just the intrinsic property of the species happens to suit for survival in the environment. A new species comes into the world because it gains the trait suitable for survival in an environment where any other species can survive. Random trials with a simple luck to fit into an unoccupied slot. No intention, no goal and no direction for better.
Richard Goldschmidt was highly insightful in this regard. He noticed the importance of meiosis for speciation. If chromosomal rearrangement occurs in one chromosome, it works as a reproductive barrier. Depending on the degree of chromosomal rearrangement, its consequence will be sterile, sub-fertile or have no impact on fertility. Sub-fertility is particularly interesting for speciation. First, males are more sensitive than females. Spermatogenesis is more susceptible than oogenesis against chromosomal rearrangement. Second, the sub-fertility is transient in heterozygous individuals carrying an old and new allele. Because the incompatibility between the two homologous chromosomes causes the fertility problem, once homozygous individuals with the new allele are generated, they will have no problem with reproduction.
It is not essential if chromosomal rearrangement is linked with any phenotypic trait. Notably, the new group of individuals that carry the homozygous of the rearranged chromosome is reproductively isolated within a few generations. They cannot share their genetic information efficiently anymore with the original population. Thus, they are a new species. I can think of three scenarios for the survival of this new species. First, they share the environmental resource with the original species. For this to work, the resources should be abundant. Second, the chromosomal rearrangement creates better survival traits in the original environment. Then, the new species can win the competition. Third, the chromosomal rearrangement creates the traits suitable to take the new open environment no other have taken. Aptness. These three scenarios are not mutually exclusive. However, the only third one can expand the niche, the livable environment for live organisms.
Goldschmidt depicted this process as “Hopeful Monsters”. A new species suddenly emerges from the original one. This was a highly unpopular idea then and even now.
Is there a goal? Is there any better? If you consider that a goal exists, you find paths and requirements to achieve the goal. Adaptation is possible because better and more efficiently can be measured to accomplish the goal. I do not think a goal exists in live organisms. Survival and reproduction are not goals but requirements for living organisms. As far as not creating lethal deficiency, all variation stays in the population. The genetic variation inevitably shows genetic drift, but the phenotypic traits are relatively stable because of robustness.
Something that appears to be a goal or a purpose in biology is an illusion. That is the stable equilibrated state maintained in a robust plastic network, an organism. All changes not disrupting the network are accommodated. If a change disrupts it, the organism dies and disappears. Teleological and agential thinking are only for the sake of human feeling for understanding. Newton eliminated teleological and agential thinking from physics. Darwin partially succeeded in eliminating them in biology. Teleological and agential thinking are still common in biology. They help understand biological phenomena, describing functions and roles, which are the words demanding the goal and purpose to be achieved. The actual requirements are the mechanisms creating equilibrium, not the final equilibrated state. Survive and reproduce. They are the only two requirements for the continuity of life.