Ernst Mayr and Related Topics

Natural selection

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Differential survival and reproduction of individuals due to differences in phenotype.

Aristotle considered whether different forms could have appeared, only the useful ones surviving.

Part of Thomas Malthus's table of population growth in England 1780–1810, from his Essay on the Principle of Population, 6th edition, 1826

Charles Darwin noted that pigeon fanciers had created many kinds of pigeon, such as Tumblers (1, 12), Fantails (13), and Pouters (14) by selective breeding.

Evolutionary developmental biology relates the evolution of form to the precise pattern of gene activity, here gap genes in the fruit fly, during embryonic development.

During the industrial revolution, pollution killed many lichens, leaving tree trunks dark. A dark (melanic) morph of the peppered moth largely replaced the formerly usual light morph (both shown here). Since the moths are subject to predation by birds hunting by sight, the colour change offers better camouflage against the changed background, suggesting natural selection at work.

1: directional selection: a single extreme phenotype favoured. 2, stabilizing selection: intermediate favoured over extremes. 3: disruptive selection: extremes favoured over intermediate. X-axis: phenotypic trait Y-axis: number of organisms Group A: original population Group B: after selection

Different types of selection act at each life cycle stage of a sexually reproducing organism.

The peacock's elaborate plumage is mentioned by Darwin as an example of sexual selection, and is a classic example of Fisherian runaway, driven to its conspicuous size and coloration through mate choice by females over many generations.

Selection in action: resistance to antibiotics grows though the survival of individuals less affected by the antibiotic. Their offspring inherit the resistance.

Ernst Mayr recognised the key importance of reproductive isolation for speciation in his Systematics and the Origin of Species (1942).

Evolution

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Change in the heritable characteristics of biological populations over successive generations.

In 1842, Charles Darwin penned his first sketch of On the Origin of Species.

DNA structure. Bases are in the centre, surrounded by phosphate–sugar chains in a double helix.

This diagram illustrates the twofold cost of sex. If each individual were to contribute to the same number of offspring (two), (a) the sexual population remains the same size each generation, where the (b) Asexual reproduction population doubles in size each generation.

Mutation followed by natural selection results in a population with darker colouration.

Simulation of genetic drift of 20 unlinked alleles in populations of 10 (top) and 100 (bottom). Drift to fixation is more rapid in the smaller population.

Homologous bones in the limbs of tetrapods. The bones of these animals have the same basic structure, but have been adapted for specific uses.

A baleen whale skeleton. Letters a and b label flipper bones, which were adapted from front leg bones, while c indicates vestigial leg bones, both suggesting an adaptation from land to sea.

Common garter snake (Thamnophis sirtalis sirtalis) has evolved resistance to the defensive substance tetrodotoxin in its amphibian prey.

Geographical isolation of finches on the Galápagos Islands produced over a dozen new species.

Tyrannosaurus rex. Non-avian dinosaurs died out in the Cretaceous–Paleogene extinction event at the end of the Cretaceous period.

The hominoids are descendants of a common ancestor.

As evolution became widely accepted in the 1870s, caricatures of Charles Darwin with an ape or monkey body symbolised evolution.

Defined by evolutionary biologist Ernst Mayr in 1942, the BSC states that "species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups."

Several major ideas about evolution came together in the population genetics of the early 20th century to form the modern synthesis, including genetic variation, natural selection, and particulate (Mendelian) inheritance. This ended the eclipse of Darwinism and supplanted a variety of non-Darwinian theories of evolution.

Modern synthesis (20th century)

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The early 20th-century synthesis reconciling Charles Darwin's theory of evolution and Gregor Mendel's ideas on heredity in a joint mathematical framework.

Darwin's pangenesis theory. Every part of the body emits tiny gemmules which migrate to the gonads and contribute to the next generation via the fertilised egg. Changes to the body during an organism's life would be inherited, as in Lamarckism.

Blending inheritance, implied by pangenesis, causes the averaging out of every characteristic, which as the engineer Fleeming Jenkin pointed out, would make evolution by natural selection impossible.

August Weismann's germ plasm theory. The hereditary material, the germplasm, is confined to the gonads and the gametes. Somatic cells (of the body) develop afresh in each generation from the germplasm.

Sewall Wright introduced the idea of a fitness landscape with local optima.

Drosophila pseudoobscura, the fruit fly which served as Theodosius Dobzhansky's model organism

E. B. Ford studied polymorphism in the scarlet tiger moth for many years.

Julian Huxley presented a serious but popularising version of the theory in his 1942 book Evolution: The Modern Synthesis.

Ernst Mayr argued that geographic isolation was needed to provide sufficient reproductive isolation for new species to form.

George Gaylord Simpson argued against the naive view that evolution such as of the horse took place in a "straight-line". He noted that any chosen line is one path in a complex branching tree, natural selection having no imposed direction.

Speciation via polyploidy: a diploid cell may fail to separate during meiosis, producing diploid gametes which self-fertilize to produce a fertile tetraploid zygote that cannot interbreed with its parent species.

Ant societies have evolved elaborate caste structures, widely different in size and function.

Evolutionary developmental biology has formed a synthesis of evolutionary and developmental biology, discovering deep homology between the embryogenesis of such different animals as insects and vertebrates.

A 21st century tree of life showing horizontal gene transfers among prokaryotes and the saltational endosymbiosis events that created the eukaryotes, neither fitting into the 20th century's modern synthesis

Inputs to the modern synthesis, with other topics (inverted colours) such as developmental biology that were not joined with evolutionary biology until the turn of the 21st century

The synthesis was defined differently by its founders, with Ernst Mayr in 1959, G. Ledyard Stebbins in 1966 and Theodosius Dobzhansky in 1974 offering differing numbers of basic postulates, though they all include natural selection, working on heritable variation supplied by mutation.

Species

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Basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity.

A region of the gene for the cytochrome c oxidase enzyme is used to distinguish species in the Barcode of Life Data Systems database.

The cladistic or phylogenetic species concept is that a species is the smallest lineage which is distinguished by a unique set of either genetic or morphological traits. No claim is made about reproductive isolation, making the concept useful also in palaeontology where only fossil evidence is available.

A chronospecies is defined in a single lineage (solid line) whose morphology changes with time. At some point, palaeontologists judge that enough change has occurred that two species (A and B), separated in time and anatomy, once existed.

A cougar, mountain lion, panther, or puma, among other common names: its scientific name is Puma concolor.

The type specimen (holotype) of Lacerta plica, described by Linnaeus in 1758

Ernst Mayr proposed the widely used Biological Species Concept of reproductive isolation in 1942.

Palaeontologists are limited to morphological evidence when deciding whether fossil life-forms like these Inoceramus bivalves formed a separate species.

Horizontal gene transfers between widely separated species complicate the phylogeny of bacteria.

John Ray believed that species breed true and do not change, even though variations exist.

Carl Linnaeus created the binomial system for naming species.

Blackberries belong to any of hundreds of microspecies of the Rubus fruticosus species aggregate.

The butterfly genus Heliconius contains many similar species.

The Hypsiboas calcaratus–fasciatus species complex contains at least six species of treefrog.

Seven "species" of Larus gulls interbreed in a ring around the Arctic.

Opposite ends of the ring: a herring gull (Larus argentatus) (front) and a lesser black-backed gull (Larus fuscus) in Norway

A greenish warbler, Phylloscopus trochiloides

Presumed evolution of five "species" of greenish warblers around the Himalayas

Ernst Mayr emphasised reproductive isolation, but this, like other species concepts, is hard or even impossible to test.

Description of rare animals (写生珍禽图), by Song dynasty painter Huang Quan (903–965)

Taxonomy (biology)

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Scientific study of naming, defining and classifying groups of biological organisms based on shared characteristics.

Title page of Systema Naturae, Leiden, 1735

Evolution of the vertebrates at class level, width of spindles indicating number of families. Spindle diagrams are typical for evolutionary taxonomy

The same relationship, expressed as a cladogram typical for cladistics

The basic scheme of modern classification. Many other levels can be used; domain, the highest level within life, is both new and disputed.

Type specimen for Nepenthes smilesii, a tropical pitcher plant

A comparison of phylogenetic and phenetic (character-based) concepts

Thus, Ernst Mayr in 1968 defined "beta taxonomy" as the classification of ranks higher than species.

In the Kaneshiro model, a sample of a larger population results in an isolated population with less males containing attractive traits. Over time, choosy females are selected against as the population increases. Sexual selection drives new traits to arise (green), reproductively isolating the new population from the old one (blue).

Peripatric speciation

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Mode of speciation in which a new species is formed from an isolated peripheral population.

The concept of peripatric speciation was first outlined by the evolutionary biologist Ernst Mayr in 1954.

Punctuated equilibrium

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Theory that proposes that once a species appears in the fossil record, the population will become stable, showing little evolutionary change for most of its geological history.

Alternative explanations for the punctuated pattern of evolution observed in the fossil record. Both macromutation and relatively rapid episodes of gradual evolution could give the appearance of instantaneous change, since 10,000 years seldom registers in the geological record.

Their paper built upon Ernst Mayr's model of geographic speciation, I. Michael Lerner's theories of developmental and genetic homeostasis, and their own empirical research.

The Central Valley in California prevents the two salamander populations from interacting with each other which is an example of habitat isolation. After many generations the two salamander gene pools will become mutated caused by natural selection. The mutation will change the DNA sequence of the two populations enough that the salamander populations can no longer successfully breed between each other making the populations of salamander become classified as different species.

Reproductive isolation

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The mechanisms of reproductive isolation are a collection of evolutionary mechanisms, behaviors and physiological processes critical for speciation.

The flowers of many species of Angiosperm have evolved to attract and reward a single or a few pollinator species (insects, birds, mammals). Their wide diversity of form, colour, fragrance and presence of nectar is, in many cases, the result of coevolution with the pollinator species. This dependency on its pollinator species also acts as a reproductive isolation barrier.

In coral reefs, gamete incompatibility prevents the formation of numerous inter-species hybrids.

Mules are hybrids with interspecific sterility.

Reproductive isolation can be caused by allopatric speciation. A population of Drosophila was divided into sub populations selected to adapt to different food types. After some generations the two sub populations were mixed again. Subsequent matings occurred between individuals belonging to the same adapted group.

Zoologist Ernst Mayr classified the mechanisms of reproductive isolation in two broad categories: pre-zygotic for those that act before fertilization (or before mating in the case of animals) and post-zygotic for those that act after it.

Systematics and the Origin of Species

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Systematics and the Origin of Species from the Viewpoint of a Zoologist is a book written by zoologist and evolutionary biologist Ernst Mayr, first published in 1942 by Columbia University Press.

Species concept