Adaptation is an essential concept for evolution. Each species appears to have adapted well to its environment. How did this happen?
Lamarck thought that adaptation was the consequence of the continuous efforts of individuals. The next generation inherits the unique traits obtained through continuous efforts with frequent usage. The sons of blacksmiths tended to have larger and stronger arms than the sons of weavers. He thought this observation could be convertible to species in nature. Each species is the creation of God and then adapted to its environment through frequent usage of specific organs and structures.
Darwin thought that adaptation was the consequence of the struggle for the survival of individuals. In each generation, only individuals that fit better into their environment survive and reproduce. Others should die because they have lost in their battle for survival. The essential requirement for this idea is the existence of inheritable phenotypic traits and a limited niche that selects survivors to reproduce the next generation. In this concept, through generations, each species will become better in its environment.
In the early 20th century, Darwin’s idea was merged with the rediscovered work of Mendel. Inheritable traits were interpreted by genes. The selection was interpreted by allelic frequency within a population. The alleles encoding advantageous phenotypic traits are enriched in a population because of successful survival and reproduction. This process facilitates adaptation to its environment. This is the foundation of neo-Darwinian theory.
This sounds correct. But is it really happening?
I know many studies of fieldwork and lab experiments support this idea. I had naively thought that adaptation was a correct concept and that adaptation was the driving force of evolution. Astonishingly, I'm afraid I have to disagree with that now. Adaptation only happens on specific occasions within species or subspecies. Usually, it demands human involvement. It is not the general driving force of evolution. Evolution, particularly in the evolution of multicellular organisms, is driven by another mechanism - chromosomal rearrangement. Reproductive isolation is the primary force of the fixation of phenotypic traits. Physical reproductive isolation has been the main consideration since Mayr proposed that. However, in diploid organisms, the meiotic block appears more critical for the fixation of phenotypic traits, speciation, and the creation of new forms. This is the idea that Richard Goldschmidt proposed 80 years ago in his book “The Material Basis of Evolution.” His theory has been dismissed and abandoned.
In neo-Darwinian theory, phenotypic traits are the target of selection. What controls the phenotype of an organism? Initially, people thought genetics. If the selected phenotype is genetically regulated, the allelic frequency in the next generation will increase in a population. Thus, the species will adapt better to the environment through generations. This is the foundation of neo-Darwinian theory.
However, the phenotypic characteristics are also regulated by epigenetic and genomic differences. Goldschmidt already depicted that. He noticed that very similar phenotypic characteristics can be observed in various species through different mechanisms. If the selected phenotypic characteristic is epigenetically regulated, the allelic frequency in the next generation will not change in a population. If the selected phenotypic characteristic is caused by genomic alteration, there are three possible outcomes. First, the genomic allele acts like a genetic allele. So, the allele enrichment could happen in a population. Second, infertility by meiosis block. Two homologous chromosomes cannot be aligned. Even if the selected phenotypic trait is advantageous for survival, those individuals cannot reproduce—the dead end. The third possibility is subfertility.
Interestingly, males and females have distinct sensitivities to meiosis block. Meiosis in females is less strict than in males. If a heterozygous individual carrying one original and one altered chromosome shows subfertility, 50% of its offspring are heterozygous again. They would show subfertility again, but they are not alone this time. If heterozygous sibling mating occurs, homozygous could be born with a 25% chance. Interestingly, it is reasonable to expect the fertility of the homozygous with the altered chromosome to be fully recovered. A new species emerges!
Mathematicians and population geneticists were the ones who created neo-Darwinian synthesis. They did not consider the unique processes of the diploid organism in-depth - meiosis and mating. In addition to meiosis, the diploid organism has a distinct feature compared to the haploid organism—the concept of dominant and recessive alleles. Not all phenotypic trait is directly inherited in the offspring, unlike haploids. If the allele is dominant, the offspring can reproduce the parental phenotype. However, if the allele is recessive, the chance that the offspring have the same parental phenotype will be slim.
Another mistake was that neo-Darwinian theory considered that the niche/environment was uniform and surrounded by a boundary. To make allele enrichment work in a population, phenotypic selection should occur uniformly and consistently. Local sporadic selections may enrich specific alleles locally or temporarily. However, without reproductive isolation, the selected alleles will be soon diluted into the population. Therefore, the physical isolation of populations is considered in neo-Darwinism. Geographical isolation often creates subspecies. Is it the consequence of natural selection? It is usually hard to conclude because this could also be due to genetic drift.
Fundamentally, what defines the size of the livable niche? Is the livability of the niche uniform inside of a sharp boundary or heterogenous with a gradual transition from livable to unlivable? The second one is more likely—no sharp boundary and not uniform. The size of the species niche is defined as the equilibration of the harshness of the outside niche and the permissiveness and readiness of each individual. Life challenges in the environment to a population are never uniform. They are local and temporal. Without reproductive isolation, the traits cannot be fixed but diluted. As a population, each species is highly robust. A robust system is resilience as a whole — keeping the original state without adaptation. Thus, it is consistent as a population. Uniform consistent selections are possible only on limited occasions, like at mass extinction or with human involvement, like domestication and antibiotic resistance in bacteria. The multiple extinctions in the life history are known by changes in oxygen tension and climate changes— No adaptation in the end because adaptation is difficult to happen in diploids.
Each species has a unique, distinct morphology. How much the phenotype is changeable? Some species have a wide range of sexual dimorphisms. But we can still recognize them because they are consistent. Why? Because they have consistent, robust developmental mechanisms to generate a consistent morphology.
Certain phenotypic traits are regulated by one or two genes, like coat colour. Easy to change. However, many morphological traits are the consequence of a developmental network consisting of highly complex biochemical interactions. A complex network is robust. In addition, diploidy provides a complete set of spares. In this sense, each diploid species is highly robust and more resistant to perturbation than haploid organisms. This simultaneously means that diploids are less adaptable than haploids.
If adaptation is the driving force, why did diploid organisms evolve? They are less adaptable. This is the crucial point. The environment does not select, and organisms do not adapt. Opposite! Organisms are apt, and then the environment permits. Organisms constantly change in random directions. Then, just by chance, if a change permits them to live in an environment no other organism occupies, they can survive as a new species. Kauffman described this as adjacent possible empty niches.
Reproductive isolation is essential to fix the unique traits. Otherwise, the traits will dilute into the original population. This process looks like adaptation retrospectively. But this is not an adaptation at all. Until a new species emerges, no one can predict what will emerge. All interpretations are retrospective. Through this process, life diversity increases, and niches for life expand.
What could be the driving force of evolution if adaptation is not? Minimization of lethal deficiency through increasing robustness. This permits the expansion of the niche. Importantly, no one knows the direction of the expansion until it happens. Retrospectively expanded. It is not aiming to expand. The consequence looks like ‘survival of the fittest’ retrospectively. But this process is not the competition at all. Maybe a loser in the competition is the one expanding the niche for their survival with their uniqueness. You can only propose the fitness matrix retrospectively. However, no one can provide a prospective fitness matrix because no one knows what will become the next niche.
Without reproductive isolation, this trait can spread in the population, and the species' niche expands. But if reproductive isolation is accompanied, this trait will be unique in a new group and quickly fixed. This could be a new species, class or phylum. Again, this is not planned. This is not the winner of the competition. Retrospectively, the new traits fit to live in the environment no other could have lived in - adjacent possible empty niches. Importantly, all reasoning explaining the emergence of a new population is retrospective.
What is the driving force of evolution? Increasing robustness to minimize lethal deficiency. Not maximizing individual fitness. No need to be a winner of competitions. Just survive for continuation by increasing robustness.
Find your niche that has yet to be taken, although you cannot know and predict what will be the niche for you. Everything is retrospective. Survive. Do your best what you like. This will take you to a place you have never imagined. That is your niche. Life is unpredictable frontiers.