For most of the 20th century, Ernst Mayr was the ornithologist. For over three years in the late 1920s, Mayr studied a blistering number of bird species in New Guinea; His careful and brilliant observations helped lay the foundations for the branch of biology, known as Systematics, the study of the diversification of life on earth. He was one of the architects of the Great Synthesis - which from the 1920s to the 1950s combined Darwin's biology and Mendel's laws. Mayr's contributions to this achievement were to emphasize the role of sexual selection (SS) in evolution and to insist that the principle object of natural selection (NS) was the individual organism, not the gene, the cell, the trait, the organ, the group or the species; This phrase -->"select for the phenotype"<-- [1] is inscribed into the Synthesis, and it remains the orthodox interpretation of what the principle object of NS is. [2]
Mayr (1999): "Darwin was a holist: for him the object, or target, of selection was primarily the individual as a whole. The geneticists, almost from 1900 on, in a rather reductionist spirit preferred to consider the gene the target of evolution. In the past 25 years, however, they have largely returned to the Darwinian view that the individual is the principal target."
Mayr (1997): "Since the genotype, interacting with the environment, is the cause of the phenotype, selection is automatically also for any component of the genotype contributing to the favored phenotype. Thus, selection is directly for the phenotype and indirectly for the genotype or parts of it."
Mayr (2000): "The basic theory [of evolution] has not really changed in the last 30-50 years, and I have a very strong feeling that it isn't going to change much in the next 30-50 years. We are fine-tuning the theory, for example, gaining a deeper understanding of the genetics of evolutionary change. If you look through the most prestigious scientific journals in evolutionary biology today, just about every paper is devoted to some aspect of DNA."
Mayr was sharp enough to have predicted that once science had a good understanding of genetics, sexual selection (in a number of species) would be deemed "the dominant driver of evolution". In the last two decades or so - due to an explosion in the knowledge of genetics - the theory of sexual selection has picked up a momentum and a salience which would have made Darwin's head spin. Recently, the sheer volume of papers written about sexual selection has come to dwarf other topics concerning evolution.
Mayr (1997): "Considering how many new kinds of selection for reproductive success are discovered year after year, I am beginning to wonder whether it is not even more important than survival selection, at least in certain higher organisms."
(Mayr coined the phrases "selection for reproductive success" and "selection for survival success", because he thought that they captured the nature of these evolutionary processes more accurately than Darwin's "sexual selection" and "natural selection" did.)
As historical background for SS being suspected to play a pivotal role in speciation, Darwin - writing in the Origin and the Descent - appears an amateur, enthused and winging it with his quaint descriptions of the mating interactions on the part of non-human organisms. Wildly, Darwin speculated that the presence of extreme "male ornaments" and intense female "preferences" for these ornaments (in higher taxa of organisms) "speed up" the spinning off of novel species. In the 1870s, Darwin did not know this to have been a feature of speciation, yet he more than dimly grasped its significance. For his "guess-work", Darwin was ridiculed. Mayr, Fisher and Dobzhansky were among his few defenders. It took over 140 years for Darwin's hypothesis to be sustained by solid, observational data. With the possible exception of Einstein, no scientist but Darwin can lay claim to such prescience and validation.
There are moments in the history of science (such as this), where one's breath is drawn from one's body; One becomes aware that great scientists have a window into human mind-independent aspects of the universe.
Relatively recent evidence "strongly supports" Darwin's contention that sexual selection accelerates species-diversification. Fast-breeding populations of fishes in Africa have been observed to speciate within about 100 years, and processes leading to their speciation are closely linked to SS. From research in 2003:
"When Darwin proposed his theory of sexual selection, he was concerned mainly with explaining the widespread occurrence of exaggerated sexual ornaments and courtship displays, as these traits could not easily be explained by natural selection. He also noted that taxonomic groups with more pronounced sexual ornaments tended to have more species. This suggests that sexual selection elevates the rate at which populations diversify and give rise to new species. A new study of female mate preferences in five populations of an East African cichlid species strongly supports the connection between sexual selection and speciation." [3]
The more exaggerated the ornaments displayed by males of a higher taxon -> the more intense the female preferences are for the ornaments -> the more the genetic profile of the taxon shifts due to greater differential reproductive outcomes leading to reproductive isolation -> the more rapidly species arise from that taxon [relative to closely linked taxa where males show less exaggerated ornaments]
As predicted by Mayr in some species, sexual selection is the dominant driver of evolution. Darwin and Mayr were correct in connecting SS to speciation, but Mayr may not have fully appreciated (or accepted) how speciation occurs triggered by it; Geographic separation (as in allopatric speciation), where a daughter population breaks off from a parent population is not involved. Speciation in Tropheus (Cichlids) was observed to take place within a population's habitat, i.e. sympatrically. On many occasions, Mayr rejected sympatric speciation. [4]
Mayr devised a theory, called peripatric speciation, postulating that geographic separation, a decline in genetic variation and small population size are essential components for novel species to emerge. Under it, the "founder effect" (genetic drift in a daughter population which occurs after splitting from the parent population) is a crucial feature, leading to the appearance of new species. Mayr predicted that the genetic profile of the daughter would vary radically from its parent, and local selection pressures would favor homozygous individuals of the daughter population. His theory has been subject to numerous experimental tests, but results have not unambiguously confirmed it.
Mayr (1954): "One of the obvious effects of the sudden reduction of population size in the founder population will be a strong increase in the frequency of homozygotes. As a consequence, homozygotes will be much more exposed to selection and those genes will be favored which are specially viable in the homozygous condition." [5]
Mayr contributed to the development of the Biological Species Concept (BSC). Originally devised by Dobzhansky in 1937 to describe when speciation occurs, the BSC is widely accepted as the operational criterion for classifying organisms as members of the same species - on the basis of their actually or potentially interbreeding. Mayr amended Dobzhansky's definition throughout the 20th century. He insisted that Dobzhansky's use of it was too restrictive, because it referred only to a latter stage of an evolutionary process - speciation, rather than being the description of a species as a (discrete) population. [6]
Dobzhansky (1937): "...that stage of the evolutionary process at which the once actually or potentially interbreeding array of forms becomes segregated in two or more separate arrays which are physiologically incapable of inter-breeding."
In 1942, Mayr crafted the definition that is (for good or ill) generally accepted as the BSC. He defined species as: "... groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups."
Mayr later altered this definition to include an ecological aspect. He became interested in "ecological niche specialization" to help explain the level of observed species-diversity in nature. Mayr: "... a reproductive community of populations (reproductively isolated from others) that occupies a specific niche in nature."
On an historical note, Darwin did not accept that the term "species" referred to a "fundamental category of biological organisation". He regarded the use of term "species" as a means to "arbitrarily" classify a set of organisms which closely resemble each other.
Darwin: "... the term species, as one arbitrarily given for the sake of convenience to a set of individuals closely resembling each other,... it does not essentially differ from the term variety, which is given to less distinct and more fluctuating forms."
Against Darwin and since the Synthesis (around the late 1930s), species have been elevated to concete entities of evolutionary significance. However, given all of problems and contradictions involved with isolating species by the BSC, I'll wager that ol' Darwin was right - yet again.
References and Notes: 1. A phenotype is all of the characteristics of an individual organism that is "visible" to the environment, which can be selected for or selected against.
2. "The idea that a few people have about the gene being the target of selection is completely impractical; a gene is never visible to natural selection, and in the genotype, it is always in the context with other genes, and the interaction with those other genes make a particular gene either more favorable or less favorable. In fact, Dobzhanksy, for instance, worked quite a bit on so-called lethal chromosomes which are highly successful in one combination, and lethal in another. Therefore people like Dawkins in England who still think the gene is the target of selection are evidently wrong." [Mayr: What evolution is]"The two belief systems [differing positions held by geneticists and naturalists] had only one inconsistency--the object of natural selection. For the geneticists the object of selection had been the gene since the 1920s, but for most naturalists it was the individual."
"Elliot Sober showed how one could resolve this conflict. He pointed out that one must discriminate between selection of an object and selection for an object. The answer to the question of what is being selected for specifies the particular properties for which a given object of selection is favored. However, a particular gene can favor an individual without being the object of selection because it gives properties to the individual that favor its selection. It is a selection for these properties." [Mayr: 80 Years of Watching the Evolutionary Scenery]
3. "The rapid rate of cichlid speciation is explained by Darwin's theory of sexual selection and the evolution of apparently "maladaptive" traits due to female choice. Female selection for bright colors and elaborate bowers provides the key to understanding the extraordinary number of cichlid species. Sexual selection is an accelerator of speciation. Slight variations in female preferences among males of different populations can lead to complete reproductive isolation. Females of one species of rock-dwelling cichlids prefer blue males, whereas females of sibling species prefer males with red fins. Females of a drab sand-dwelling species prefer males with the largest sand castle, while others prefer males with a complex array of multiple castles." [McKaye: Sexual selection in Tropheus]
4. Speciational Evolution or Punctuated Equilibria, Ernst Mayr (1992): "Owing to his adoption of sympatric speciation, however, Darwin never needed to consider the geographical component in his theorizing."
"Unfortunately, by the time Darwin published the Origin (1859), he had adopted sympatric speciation (Mayr, 1982a). When he said that a new species might originate as a local variety, he did not necessarily mean an isolated population."
Homo Heidelbergensis is a well accounted for species of hominid - with specimens ranging from Europe to Africa to Asia. Yet, allopatric speciation is out of the question, given that there is no evidence of an expansion from Africa (or elsewhere) around the time it arose. Sympatric speciation remains a controversial "process" (especially to M. Wolpoff), and had sympatric speciation been involved, hominid populations on three continents arrived at Heidelbergensis simultaneously. (Could a variant of Darwin's conjecture be convincingly advanced to help unravel this riddle?)
5. Change of genetic environment and evolution (1954), pp. 169-170 in Evolution as a Process: "We come thus to the important conclusion that the mere change of the genetic environment may change the selective value of a gene very considerably. Isolating a few individuals (the "founders") from a variable population which is situated in the midst of the stream of genes which flows ceaselessly through every widespread species will produce a sudden change of the genetic environment of most loci. This change, in fact, is the most drastic genetic change (except for polyploidy and hybridization) which may occur in a natural population, since it may affect all loci at once. Indeed, it may have the character of a veritable "genetic revolution." Furthermore, this "genetic revolution," released by the isolation of the founder population, may well have the character of a chain reaction. Changes in any locus will in turn affect the selective values at many other loci, until finally the system has reached a new state of equilibrium."
6. Mayr (2004) "Curiously, there is no chapter on speciation in Dobzhansky's book [Genetics and the Origin of Species]. His description of the isolating mechanisms was erroneous. Isolating mechanisms are genetic properties of individuals, yet he included geographic barriers among them. In addition, his putative species definition refers to the stage of a process, but is not the description of a (species) population. I was able to correct this in my 1942 book."
Further reading:
Ernst Mayr, Genetics and Speciation (2004), by William B. Provine
Sexual selection and speciation (2001), Tami M. Panhuis, Roger Butlin, Marlene Zuk and Tom Tregenza
HAPPY BIRTHDAY: 80 Years of Watching the Evolutionary Scenery (2004), by Ernst Mayr
The Objects of Selection (1997), by Ernst Mayr
Ernst Mayr and the modern concept of species (2005), by Kevin de Queiroz
Punctuated equilibria: an alternative to phyletic gradualism (1972), by Eldredge, N. & Gould, S.J.