home

Bird is the word: Ernst Mayr

nekhbet

For most of the 20th century, Ernst Mayr was the ornithologist. In the late 1920s - over a period of three years, Mayr studied a blistering number of bird species in New Guinea. His careful, patient and brilliant observations helped lay the foundations for the branch of biology, known as Systematics. Systematics is the study of the diversification of life forms 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) is the individual organism - not the gene, the cell, the trait, the organ, the group or the species.

This phrase -->"select for the phenotype"<-- is inscribed into the Synthesis, and it remains the orthodox interpretation of what the principle object of NS is. [1]

Mayr
"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." -- 1999 [2]

"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." -- 1997

"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." -- 2000

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
"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." -- 1997

(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" does.)

As historical background for SS being suspected to play a crucial 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" (in higher taxa of organisms) speeds-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 required 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.

Relatively recent evidence "strongly supports" Darwin's hypothesis 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 speciation are closely linked to SS.

Ken Kraaijeveld and Andrew Pomiankowski:
"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 greater the females preferences for the ornaments -> the more the genetic profile of the taxon shifts due to greater differential reproductive outcomes leading to the reproductive isolation of that taxon -> the more rapidly species spin-off from that taxon [relative to closely linked taxa where males show less exaggerated ornaments]

As predicted by Mayr - in some species, SS is the dominant driver of evolution. Darwin and Mayr were correct in linking SS to speciation, but Mayr may not have fully appreciated (or accepted) how speciation occurs - as 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 - outright. [4]

my note
Homo Heidelbergensis is a well accounted for hominid species - 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?)

Mayr devised a theory: peripatric speciation. He postulated that geographic separation, a decline in genetic variation and small population size are essential components for novel species to emerge. Under peripatric speciation, 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 differ radically from its parent and that local selection pressures would favor homozygous individuals of the daughter population. His theory has been subject to numerous experimental tests, but results have not confirmed it.

Mayr
"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." -- (1954) [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
"...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." -- (1937)

Mayr crafted the definition of species that is (for good or ill) generally accepted as the BSC:

Mayr
"... groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups." -- (1942)

Mayr later altered this definition to include an ecological aspect. He became interested in "ecological niche specialization" to help explain the level of the 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 rejected that the term "species" referred to a "fundamental category of biological organisation". He regarded the application of the term "species" as a means to "arbitrarily" classify a set of organisms - which closely resemble each other.

Darwin
"I look at the term species, as one arbitrarily given for the sake of convenience to a set of individuals closely resembling each other, and that it does not essentially differ from the term variety, which is given to less distinct and more fluctuating forms. The term variety, again, in comparison with mere individual differences, is also applied arbitrarily, and for mere convenience sake." -- 1859

Against Darwin and since the Synthesis (around the late 1930s), species have been elevated to concete entities of evolutionary significance. However - given all of the problems and contradictions involved in isolating species with the BSC, I'll wager that Darwin was right - yet again.

Mayr rejected that species were objects of selection, favoring the term "species replacement" - while Darwin (though he vacillated) held out the possibility that species selection had occurred.

Mayr
"[] In the early post-Darwinian period when thinking about selection was rather confused, it was often said that such and such a character had evolved because it was "good for the species". This is quite misleading. The selected character had originated because it benefited certain individuals of a species and had gradually spread to all others. The species as an entity does not answer to selection."

"[] I hesitate to use the term species selection and prefer to call such events species turnover or species replacement because the actual selection takes place at the level of competing individuals of the two species." -- 1997

The criteria, given in textbooks, for species selection is as follows: (1) adaptive radiation (2) mass extinction (3) key innovation(s) and (4) body size. The classic case for species selection is that ~66Myr ago dinosaur species were wiped-out but mammal species survived and proliferated. The term, "key innovation(s)" (3), is problematic, but key innovation(s) are generally described as "aspects of organismal phenotype that promote diversification" - which further the "success" of the taxa originating the novel traits. (However, I suspect that - when scientists lay down criteria as above then dicker about them semantically, they are heading down the wrong path.)