From killer whales to men. How culture affects the birth or evolution of a species

Among those who have a scholastic knowledge of evolutionary mechanisms, the idea that the engine of this process is the competition for resources, with the consequent selective advantage for those individuals who are able to adapt to different and less competitive ecological niches due to random mutations of their genotype.

Alternatively, school textbooks also show possible speciation due togeographical isolation of small founding populations, which due to genetic drift can move far away from their progenitors. These two simple mechanisms constitute the average knowledge that people have of the reasons for speciation, and therefore there are many who think that the only way in which it occurs is ultimately through competition and adaptation to new niches, geographical or more often ecological , which determine an abrupt or even gradual reproductive isolation and therefore the birth of a new species as genetic, behavioral and morphological differences accumulate. Giò Darwin, in reality, clearly illustrated a alternative mechanism, that of sexual selection, as a possible engine of speciationdue to the reproductive isolation caused by the selection of accessory characters demonstrating the prowess of the sex carrying those (often males) by the sex making the reproductive choice (often females), provided that the tastes of this latter sex diverge in starting population.

In reality, we now know much more about the possible mechanisms, both compared to Darwin’s times and compared to what many scholastic studies recall: the ways through which speciation can take place are many more, and they can be independent both from adaptation to a specific ecological niche, either from geographical isolation, from sexual selection, or from reproduction itself; Furthermore, speciation can be initiated by barriers between populations that do not correspond to differences in the DNA sequence of the constituent individuals. In plants, for example, simple DNA copy defects, with the production of individuals having accessory copies of the genetic makeup (polyploids), determines a forced reproductive isolation of these from their progenitors; however, since plants also reproduce vegetatively, even a single polyploid individual can give rise to a new population, interfertile and reproductively isolated, which can therefore easily differentiate into a new species. 15 percent of flowering plant species and 31 percent of ferns originated this way.

The change of genome architecture, in the absence of geographical isolation, can lead to speciation even in organisms that produce numerous offspring, as in African lake cichlid fish; the offspring of individuals with these accidental changes will be interfertile, and if the traits obtained are advantageous, a new species will arise. More rarely, as readers of this page know, an organism can acquire entire pieces of genome due to viral infections that integrate exogenous DNA; if this process takes place in the gametes, and if compatible insertions are generated in the opposite sexes, there is speciation for the development of new traits, as happened for the different placenta in different species of rodents and in primates.

Without a preliminary geographical or ecological isolation, Speciation can also occur when a particular genetic change affects the transmission of information between the sexes, altering the reception or emission of signals used for coupling. A change in the potassium channels of certain fishes that communicate by electrical signals in African lakes causes the emergence of different “electrical dialects”, and thus reproductive isolation and mutant speciation, in the same lake and without prior ecological adaptations. Even epigenetic modifications, in the form of differential and heritable DNA methylation, can lead to speciation, even in vertebrates, without geographical separation and without significant differences in the genome, inducing divergences in behavior and morphology sufficient for reproductive isolation; in this case, as in textbooks, diversification is accompanied by different ecological adaptations, but, unlike the model that is usually taught, the traits that generate reproductive isolation are not fixed in the DNA sequence.

Ultimately, as can be seen, the mechanisms, even competing ones, which can isolate a population within a species, to give rise to another, are very varied as regards the factor capable of determining this isolation; but there is one, which has not yet been mentioned, which I would like to bring here to the attention of the reader. Culturally transmitted traditions can have an impact on the choice of breeding partner; in this way, random differences in these traditions can create a reproductive isolation which secondarily, due to genetic drift, leads to the emergence of different species. I’m saying, essentially, that reproductive isolation can originate from incompatibility of learned and culturally transmitted behaviors in a population, generating subpopulations that will preferentially mate among individuals with the same cultural tradition, starting a process of speciation in the absence of any barrier original and without differences in ecological adaptation.

The first example of such a phenomenon occurred in Darwin’s finches: in these birds, the males learn the reproductive song from their father, transmitting the tradition to their sons, while the females mate only with individuals who sing the song of their own species, but with small variations compared to that of their father, to avoid mating with one’s direct relatives. Small learning errors on the part of the males generate those variations which are sufficient for the females to choose suitable mates, but always of their own species; but since, over time, these small errors can accumulate, in the end groups are produced with songs so different, as to be reproductively isolated. Thus singing, an evolving, culturally inherited trait, is an important factor in species recognition and mate choice. It constrains the mating of females, even when there is no genetic penalty for interbreeding, and thus can play a crucial role in species formation by promoting genetic isolation

The different clans of orca, a cosmopolitan animal, have cultural traditions in terms of dialect (different clans share very few vocalizations), specific hunting techniques (e.g. beaching for mammals in South America, making waves to drop seals in the water in the Arctic, bubble traps in Norway, stingray hunting in New Zealand, endurance-hunting to exhaust tuna around Gibraltar) and type of prey hunted (such as fish or mammals). Now, although in extreme cases an orca that has learned certain traditions can in some cases adapt, changing them – for example by consuming fish instead of mammals in captivity, after having seen a member of his clan die of hunger – it has been shown that he returns to learned behavior as soon as possible. Culturally diverse orca clans tend to be reproductively isolated, not only by language, but also by other adaptations. Although it is not possible to exclude that the cultural divergence was preceded by an ecological divergence, in orcas the different cultural traditions have led to a reproductive isolation even among groups living in the same areas; this isolation is of a sufficiently high level to suspect incipient speciation.

And in man? Although it is traditionally thought that adaptive cultural traditions, by decreasing the burden of environmental selection, slow down the evolution of our species, in reality, examples are well known in which cultural adaptations have caused rapid genomic evolution. For example, the spread of pastoral culture has led to the selection of lactose tolerance, just as the traditional consumption of plants with a high starch content in certain populations has led to the multiplication of genes that synthesize amylase. Our biological evolution, therefore, is also clearly influenced by cultural evolution; on the contrary, at least for the most technologically advanced modern populations, it is possible that the selection caused by culture is stronger than many other environmental factors (although not all, if we think, for example, of infectious agents). If and how, perhaps in the future, cultural segregation will reach such a level as to block the genetic flow between different human populations, then it is possible that our species will split into others; however, especially in an abundant species such as ours which among its adaptive traits has precisely that of the production of adaptive innovation through cultural exchange, to have reproductive isolation due to cultural traits we should imagine a future in which the defense of identity of a population passes through measures not unlike those that Nazism wanted to impose. Possible, sure, but hopefully still not too likely.