A new look at variation in Darwin's species of acorn barnacles

Darwin's studies on barnacles, begun to satisfy his curiosity about specimens from the Beagle , but later extended to the large collections of other naturalists, lasted for 8 years and produced a set of definitive monographs. Darwin was particularly troubled over the taxonomy of two groups of acorn barnacles, Balanus tintinnabulum and B. amphitrite , which he ultimately classified as clusters of wellmarked varieties. Recent studies, based on established taxonomic methods or on statistical treatment of morphometric data, suggest these are clusters of full species and that the 'intermediate' forms are just phenotypic variants. Darwin was also troubled by the small chthamalid intertidal barnacles, which he eventually grouped as varieties of a world-wide species, Chthamalus stellatur. Gel electrophoresis and morphometrics now show this to be a heterogenous assemblage of distinct species, some very different, others closer together, all with restricted geographical range, but all showing extreme variability in the characters normally used for classification.
Darwin's difficulties with the classification of the highly varying barnacles, and his anatomical studies on these peculiar animals, must have had considerable influence on the development of his theories about natural selection, more than is usually acknowledged by writers on evolution. These 'lost years' were in fact a period of intense zoological enquiry, and Darwin's change in attitude with regard to species and variation in nature underwent a big change, as can be seen from the letters and from the differences between the early drafts written before the barnacle work and the later Natural Selection and The Origin. Whether we regard the difficult groups of Balanus as clusters of species or clusters of varieties, they still draw attention to evolutionary processes as in Darwin's day.     

Extending Darwin's investigations on the barnacle life-history

Darwin's magnificent study of the stalked, sessile and fossil barnacles, perhaps his greatest work, was started not many years after the systematic position of cirripedes within the class Crustacea had been accepted. It was completed at a time when histology and microtomy were not developed and when living specimens could only be seen on occasional visits to the seaside. Yet 130 years later it remains the standard text. It was on a visit to Tenby that he observed that barnacles were sensitive to vibration and, in seeking an acoustic organ, he mistakenly seized upon the oviducal gland. This led to an ever increasing series of misinterpretations of the female generative mechanism. Associated with this error was the belief that the ovary and cement glands were homologous in both cyprid and adult. He later confessed to “having blundered terribly over the cement glands”, but it was probably his search for examples of organs changing their function during evolution that led to this blunder. Similarly, he convinced himself mistakenly that the ovigerous fræna, which hold fast the egg masses to the mantle lining in stalked barnacles, became modified to gills in sessile barnacles. Darwin's interest in barnacle reproduction led to his discovery of complemental males attached to hermaphrodite individuals. He proved that they were dwarf cirripedes and not parasites. His knowledge that male and hermaphrodite flowers could co-exist helped him to accept that these apparently unnecessary beings could be present also in barnacles. We now know that complemental males are not confined to stalked barnacles but are also present in certain balanids. Though Darwin did not include the dioecious parasitic group, the Rhizocephala, in his studies, he recognized the wide repertoire of sexual arrangements from hermaphroditism to dioecy which now provides challenging material to our further understanding of sex allocation, control and evolution. Since Darwin dealt mainly with dried or preserved specimens he did not describe any of the details of the liberation of nauplii or the searching behaviour of the cyprid. Current investigators have shown that the former involves a prostaglandin which activates the embryos. Searching cyprids have been shown to be able to recognize their own and other species. Charles would have seen in this phenomenon a relevance both to survival and to the evolution of epizoic barnacles. Despite Charles' belief that his barnacle work would be “for ever unapplied” it has in fact been the foundation on which all studies of commercially important barnacle fouling have been based.     

The genetics and evolution of female choice

Darwin based his theory of evolution on the central theme of 'the struggle for existence', 'the preservation of favoured races', 'the survival of the fittest'. In addition to this 'natural selection', he proposed a second type of selection, sexual selection. 'This depends, not on a struggle for existence, but on a struggle between males for possession of the females; the result is not death to the unsuccessful competitor, but few or no offspring. Males would compete for females, and females would choose between males. Since Darwin's work, many examples of natural selection have been observed in nature and in laboratory experiments, and knowledge of genetics has given natural selective theory a sound basis. The theory of sexual selection through male competition has also been widely accepted. However, the theory of sexual selection through female choice has had a much more chequered passage.     

Rupert Riedl and the re-synthesis of evolutionary and developmental biology: body plans and evolvability

This paper reviews the scientific career of Rupert Riedl and his contributions to evolutionary biology. Rupert Riedl, a native of Vienna, Austria, began his career as a marine biologist who made important contributions to the systematics and anatomy of major invertebrate groups, as well as to marine ecology. When he assumed a professorship at the University of North Carolina in 1968, the predominant thinking in evolutionary biology focused on population genetics, to the virtual exclusion of most of the rest of biology. In this atmosphere Riedl developed his "systems theory" of evolution, which emphasizes the role of functional and developmental integration in limiting and enabling adaptive evolution by natural selection. The main objective of this theory is to account for the observed patterns of morphological evolution, such as the conservation of body plans. In contrast to other "alternative" theories of evolution, Riedl never denied the importance of natural selection as the driving force of evolution, but thought it necessary to contextualize natural selection with the organismal boundary conditions of adaptation. In Riedl's view development is the most important factor besides natural selection in shaping the pattern and processes of morphological evolution.     

Aesthetic evolution by mate choice: Darwin's really dangerous idea

Darwin proposed an explicitly aesthetic theory of sexual selection in which he described mate preferences as a 'taste for the beautiful', an 'aesthetic capacity', etc. These statements were not merely colourful Victorian mannerisms, but explicit expressions of Darwin's hypothesis that mate preferences can evolve for arbitrarily attractive traits that do not provide any additional benefits to mate choice. In his critique of Darwin, A. R. Wallace proposed an entirely modern mechanism of mate preference evolution through the correlation of display traits with male vigour or viability, but he called this mechanism natural selection. Wallace's honest advertisement proposal was stridently anti-Darwinian and anti-aesthetic. Most modern sexual selection research relies on essentially the same Neo-Wallacean theory renamed as sexual selection. I define the process of aesthetic evolution as the evolution of a communication signal through sensory/cognitive evaluation, which is most elaborated through coevolution of the signal and its evaluation. Sensory evaluation includes the possibility that display traits do not encode information that is being assessed, but are merely preferred. A genuinely Darwinian, aesthetic theory of sexual selection requires the incorporation of the Lande-Kirkpatrick null model into sexual selection research, but also encompasses the possibility of sensory bias, good genes and direct benefits mechanisms.     

Haldane’s view of natural selection

Among many things, J. B. S. Haldane is known for demonstrating how the principle of natural selection can be used to build a mathematical, and in particular quantitative, theory of evolution. However, to the end, he remained open to the idea of other evolutionary mechanisms. In his late writings, he repeatedly drew attention to situations in which natural selection did not operate, was hemmed in by constraints, or worked in a surprising manner. In this respect Haldane stands out among the architects of the Modern Synthesis.     

Sources for Imanishi Kinji’s views of sociality and evolutionary outcomes

Prior to the contribution of genetics or the modern evolutionary synthesis (MES) to natural selection theory, social ecologists searched for factors in addition to natural selection that could influence species change. The idea that sociality, not just biology, was important in determining evolutionary outcomes was prevalent in research in social ecology in the 1920s and 1930s. The influence of ‘tradition’ (or the transmission of learned behaviours between generations) and the view that animals are active in selecting their own environments, rather than passive organisms acted upon by chance, were given as much attention as natural selection theory in European ecology, while animal aggregation and cooperation studies were pursued in America. Imanishi Kinji’s personal library and his scientific notes and papers reveal that he was well aware of this literature and had been profoundly influenced by these earlier viewpoints prior to writing his view of nature in his first book, Seibutsu no Sekai (The World of Living Things, 1941). Evidence is presented to show that he developed his theories based partly on early western debates in social ecology while finding inspiration and a way to express his views in the writings of philosopher Nishida Kitarō and, perhaps, General J C Smuts. One of Imanishi’s lasting contributions is in the demonstrated results of over 40 years of subsequent ecological and ethological research by Imanishi and those trained by him that maintained the broader viewpoints on evolution that had been dropped from the western corpus of research by the 1950s. The current attempt to again get beyond natural selection theory is reflected in debates surrounding genetic and cultural evolution of cooperation, the biology of ‘traditions’ and the idea of ‘culture’ in animal societies. Imanishi Kinji is the Japanese name order, with family name first. Other Japanese names in the text are also written with family name first. A modified version of this paper appeared in Japanese in Seibutsu Kagaku, Vol. 57 No. 3, April 2006, pp 142–149.     

Sexual selection: Another Darwinian process

Why was sexual selection so important to Darwin? And why was it de-emphasized by almost all of Darwin's followers until the second half of the 20th century? These two questions shed light on the complexity of the scientific tradition named “Darwinism”. Darwin's interest in sexual selection was almost as old as his discovery of the principle of natural selection. From the beginning, sexual selection was just another “natural means of selection”, although different from standard “natural selection” in its mechanism. But it took Darwin 30 years to fully develop his theory, from the early notebooks to the 1871 book The Descent of Man, and Selection in Relation to Sex. Although there is a remarkable continuity in his basic ideas about sexual selection, he emphasized increasingly the idea that sexual selection could oppose the action of natural selection and be non adaptive. In time, he also gave more weight to mate choice (especially female choice), giving explicit arguments in favor of psychological notions such as “choice” and “aesthetic sense”. But he also argued that there was no strict demarcation line between natural and sexual selection, a major difficulty of the theory from the beginning. Female choice was the main reason why Alfred Russel Wallace, the co-discoverer of the principle of natural selection, engaged in a major controversy with Darwin about sexual selection. Wallace was suspicious about sexual selection in general, trying to minimize it by all sorts of arguments. And he denied entirely the existence of female choice, because he thought that it was both unnecessary and an anthropomorphic notion. This had something to do with his spiritualist convictions, but also with his conception of natural selection as a sufficient principle for the evolutionary explanation of all biological phenomena (except for the origin of mind). This is why Wallace proposed to redefine Darwinism in a way that excluded Darwin's principle of sexual selection. The main result of the Darwin–Wallace controversy was that most Darwinian biologists avoided the subject of sexual selection until at least the 1950 s, Ronald Fisher being a major exception. This controversy still deserves attention from modern evolutionary biologists, because the modern approach inherits from both Darwin and Wallace. The modern approach tends to present sexual selection as a special aspect of the theory of natural selection, although it also recognizes the big difficulties resulting from the inevitable interaction between these two natural processes of selection. And contra Wallace, it considers mate choice as a major process that deserves a proper evolutionary treatment. The paper's conclusion explains why sexual selection can be taken as a test case for a proper assessment of “Darwinism” as a scientific tradition. Darwin's and Wallace's attitudes towards sexual selection reveal two different interpretations of the principle of natural selection: Wallace's had an environmentalist conception of natural selection, whereas Darwin was primarily sensitive to the element of competition involved in the intimate mechanism of any natural process of selection. Sexual selection, which can lack adaptive significance, reveals this exemplarily.     

A Critique of R.D. Alexander's Views on Group Selection

Group selection is increasingly being viewed as an important force in human evolution. This paper examines the views of R.D. Alexander, one of the most influential thinkers about human behavior from an evolutionary perspective, on the subject of group selection. Alexander's general conception of evolution is based on the gene-centered approach of G.C. Williams, but he has also emphasized a potential role for group selection in the evolution of individual genomes and in human evolution. Alexander's views are internally inconsistent and underestimate the importance of group selection. Specific themes that Alexander has developed in his account of human evolution are important but are best understood within the framework of multilevel selection theory. From this perspective, Alexander's views on moral systems are not the radical departure from conventional views that he claims, but remain radical in another way more compatible with conventional views.     

The historical context of natural selection: The case of Patrick Matthew

Conclusions It should be evident from the foregoing discussion that one man's natural selection is not necessarily the same as another man's. Why should this be so? How can two theories, which both Matthew and Darwin believed to be nearly identical, be so dissimilar? Apparently, neither Matthew nor Darwin understood the other's theory. Each man's viewpoint was colored by his own intellectual background and philosophical assumptions, and each read these into the other's ideas. The words sounded the same, so they assumed the concepts must als be the same.¹²³ As Ghiselin has pointed out, historians attempting to evaluate Darwin's predecessors have been similarly blinded by a preoccupation with words, without regard to their proper context.¹²⁴ In the case of Matthew, the practice of quoting only brief passages from the appendix to Naval Timber and Arboriculture, without relating them to the rest of his work, has suggested a greater resemblance to Darwin's theory than actually exists.It is clear, both from the use which Matthew made of his ideas and from the philosophical roots of his natural world view, that he could not have arrived at the concept of natural selection by the same thought process which Darwin employed. His discussion of natural selection is presented not as an argument, but as an axiom. No theory is proposed, no evidence marshaled to support it. Natural selection is stated as a fact, a Law of Nature, unquestioned, and presumably, unquestionable.Despite his clamor for recognition as the discoverer of natural selection, Matthew recognized and acknowledged this very fundamental difference between Darwin and himself. In a letter to the Gardener's Chronicle of May 12, 1860, he wrote:To me the conception of this law of Nature came intuitively as a self-evident fact, almost without an effort of concentrated thought. Mr. Darwin here seems to have more merit in the discovery than I have had—to me it did not appear a discovery. He seems to have worked it out by inductive reason, slowly and with due caution to have made his way synthetically from fact to fact onwards; while with me it was by a general glance at the scheme of Nature that I estimated this select production of species as an a priori recognisable fact—an axiom, requiring only to be pointed out to be admitted by unprejudiced minds of sufficient grasp.¹²⁵ In the same letter, Matthew maintained that his ideas had not been accepted because the age was not ripe for such ideas.¹²⁶ Nor, he said, was the present age. He considered the inability of most of Darwin's critics to grasp his theory to be incurable. Yet he did not argue that natural selection should be accepted because of the evidence, but rather, that it should be accepted on faith:Belief here requires a certain grasp of mind. No direct proof of phenomena embracing so long a period of time is within the compass of short-lived man. To attempt to satisfy a school of ultra skeptics, who have a wonderfully limited power of perception of means to ends... would be labour in vain.... They could not be brought to conceive the purpose of a handsaw though they saw its action, if the whole individual building it assisted to construct were not presented complete before their eyes... Like a child looking upon the motion of a wheel in an engine they would only perceive and admire... without noticing its agency in... affecting the purposed end.¹²⁷ Here, then, is the final irony. In a passage urging acceptance of Darwin's theory, a theory which was to banish design and purpose from the natural world, we find echoes of Paley and of Providence.Loren Eiseley has lamented the fact that Matthew did not bring his views into the open, because the amount of ground he was able to cover in a few paragraphs suggests that he might have been able to sustain a longer treatise.¹²⁸ Now that the intellectual and historical context of Matthew's ideas are known, this statement is no longer tenable. Matthew was not a scientist, and his books were not written as biological treatises. His discussions of natural selection were not attempts to cover ground in advancing a particular scientific theory, but were simply reflections of his own assumptions about the natural world.Furthermore, despite Matthew's acceptance of evolution and natural selection, his biological thought was basically conservative on points where Darwin's was radical. Where Matthew saw a series of stable worlds interrupted by violent upheavals, Darwin saw a continuous process of change in an ever-fluctuating world. Where Matthew conceived of species in terms of Aristotelian classes and essences, Darwin revolutionized our concept of species by treating them as populations. Where Matthew saw a world of design and beauty functioning according to natural laws laid down by benevolent Providence, Darwin abolished design and Providence from nature and ushered in a world which cycles ever onward according to laws of chance and probability.It is not even particularly useful to point to Matthew as evidence that evolution was in the air prior to 1859.¹²⁹ His ideas did not represent the first wave of a coming revolution, but were the product of his own personal philosophical outlook, as expressed in the context of the biological thought of the 1830's. Matthew is important in the history of ideas, not simply because he accepted the concept of evolution or thought of something resembling natural selection, but because he did so without overthrowing, in his own mind, any of the basic philosophical assumptions which had underlain biological science since Aristotle. In recognizing Matthew's failure to do so, we are in a position to appreciate more fully the significance of the Darwinian Revolution.     

Introduction to the symposium--barnacle biology: essential aspects and contemporary approaches

Barnacles have evolved a number of specialized features peculiar for crustaceans: they produce a calcified, external shell; they exhibit sexual strategies involving dioecy and androdioecy; and some have become internal parasites of other Crustacea. The thoroughly sessile habit of adults also belies the highly mobile and complex nature of their larval stages. Given these and other remarkable innovations in their natural history, it is perhaps not surprising that barnacles present a spectrum of opportunities for study. This symposium integrates research on barnacles in the areas of larval biology, biofouling, reproduction, biogeography, speciation, population genetics, ecological genomics, and phylogenetics. Pioneering comparisons are presented of metamorphosis among barnacles from three major lineages. Biofouling is investigated from the perspectives of biochemical and biomechanical mechanisms. Tradeoffs in reproductive specializations are scrutinized through theoretical modeling and empirical validation. Patterns of endemism and diversity are delineated in Australia and intricate species boundaries in the genus Chthamalus are elucidated for the Indo-Pacific. General methodological concerns with population expansion studies in crustaceans are highlighted using barnacle models. Data from the first, draft barnacle genome are employed to examine location-specific selection. Lastly, barnacle evolution is framed in a deep phylogenetic context and hypothetical origins of defined characters are outlined and tested.     

Mathematics,genetics and evolution

The importance of mathematics and statistics in genetics is well known. Perhaps less well known is the importance of these subjects in evolution. The main problem that Darwin saw in his theory of evolution by natural selection was solved by some simple mathematics. It is also not a coincidence that the re-writing of the Darwinian theory in Mendelian terms was carried largely by mathematical methods. In this article I discuss these historical matters and then consider more recent work showing how mathematical and statistical methods have been central to current genetical and evolutionary research.     

Darwin's evolutionary philosophy: The laws of change

The philosophical or metaphysical architecture of Darwin's theory of evolution by natural selection is analyzed and diflussed. It is argued that natural selection was for Darwin a paradigmatic case of a natural law of change — an exemplar of what Ghiselin (1969) has called selective retention laws. These selective retention laws lie at the basis of Darwin's revolutionary world view. In this essay special attention is paid to the consequences for Darwin's concept of species of his selective retention laws. Although Darwin himself explicity supported a variety of nominalism, implicit in the theory of natural selection is a solution to the dispute between nominalism and realism. It is argued that, although implicit, this view plays a very important role in Darwin's theory of natural selection as the means for the origin of species. It is in the context of these selective retention laws and their philosophical implications that Darwin's method is appraised in the light of recent criticisms, and the conclusion drawn that he successfully treated some philosophical problems by approaching them through natural history. Following this an outline of natural selection theory is presented in which all these philosophical issues are highlighted.     

Bateson's two Toronto addresses, 1921: 2. Evolutionary faith

William Bateson's plenary address to the American Association for the Advancement of Science meeting in Toronto in 1921 was titled "Evolutionary Faith and Modern Doubts." In it he expressed his deep-seated skepticism about the causes of evolution (and in particular, his dissatisfaction with Darwinian natural selection) while reaffirming his belief in the reality of evolution itself. The address led to controversy at both the scientific and popular levels. Scientific criticism centered on Bateson's rejection of natural selection; popular controversy, as evidenced by contemporary newspaper clippings, was very widespread, not least because religious fundamentalists misrepresented Bateson in their campaign against evolution. I draw attention to the forgotten case of F.E. Dean, a superintendent of schools at Fort Sumner, New Mexico, who in 1922 was forced to resign his post for merely challenging a resolution of the local school board banning the teaching of evolution in schools. Dean deserves to be remembered, along with John T. Scopes, as an early hero of the continuing fight for the right to teach evolution in U.S. schools.     

Was Darwin a creationist?

Throughout the Origin of Species, Darwin contrasts his theory of natural selection with the theory that God independently created each species. This makes it seem as though the Origin offers a scientific alternative to a theological worldview. A few months after the Origin appeared, however, the eminent anatomist Richard Owen published a review that pointed out the theological assumptions of Darwin's theory. Owen worked in the tradition of rational morphology, within which one might suggest that evolution occurs by processes that are continuous with those by which life arises from matter; in contrast, Darwin rested his account of life's origins on the notion that God created one or a few life forms upon which natural selection could act. Owen argued that Darwin's reliance on God to explain the origins of life makes his version of evolution no less supernatural than the special creationist that Darwin criticizes: although Darwin limits God to one or a few acts of creation, he still relies upon God to explain life's existence.     

Mendel's experiments: A reinterpretation

Conclusion My conclusion is that Mendel deliberately, though without any real falsification, tried to suggest to his audience and readers an unlikely and substantially wrong reconstruction of the first and most important phase of his research. In my book I offer many reasons for this strange and surprising behavior,⁵³ but the main argument rests on the fact of linkage. Mendelian genetics cannot account for linkage because it was based on the idea of applying probability theory to the problem of species evolution. Central to the theory is the law of probability according to which the chance occurrence of a combination of independent events is the product of their separate probabilities. This is the common basis of Mendel's first and second laws, but this is why Mendel's second law on independent assortment is enunciated in too general a way. From Morgan's work we now know that characters may not always be independent if their genes are located very close one to the other on the same chromosome. And this was also the basis of Mendel's personal drama: he surely observed the effects of linkage, but he had no theoretical tools with which to explain it. So he presented his results in a logical structure consistent with the central idea of his theory. Had he described the real course of his experiments he would have had to admit that his law worked for only a few of the hundreds of Pisum characters — and it would thus have been considered more of an exception than a rule. This is why he insisted on the necessity of testing the law on other plants, and this is why in his second letter to Carl Nägeli he admits that the publication of his data was untimely and dangerous.⁵⁴.We can argue that already in 1866 Mendel was less confident that his so-called second law had the same general validity as the first — and that later he lost his confidence altogether. Contemporary testimony indicates that in the end he became as skeptical as all his contemporaries as to the scientific relevance of his theory.⁵⁵ But he was wrong. His research is in no way the fruit of methodological mistakes or forgery, and it remains a landmark in the history of science. He was only the victim of a strange destiny in which the use of probability theory was responsible, at the same time, for the strength and for the weakness of his theory. We must still consider him the father and founder of genetics.     

Experimental elimination of parasites in nature leads to the evolution of increased resistance in hosts

A reduction in the strength of selection is expected to cause the evolution of reduced trait expression. Elimination of a parasite should thus cause the evolution of reduced resistance to that parasite. To test this prediction in nature, we studied the fourth- and eighth-generation descendants of guppies (Poecilia reticulata) introduced into four natural streams following experimental elimination of a common and deleterious parasite (Gyrodactylus spp.). After two generations of laboratory rearing to control for plasticity and maternal effects, we infected individual fish to assess their resistance to the parasite. Contrary to theoretical expectations, the introduced guppy populations had rapidly and repeatably evolved increased resistance to the now-absent parasite. This evolution was not owing to a resistance-tolerance trade-off, nor to differences in productivity among the sites. Instead, a leading candidate hypothesis is that the rapid life-history evolution typical in such introductions pleiotropically increases parasite resistance. Our study adds a new dimension to the growing evidence for contemporary evolution in the wild, and also points to the need for a re-consideration of simple expectations from host–parasite theory. In particular, our results highlight the need for increased consideration of multiple sources of selection and pleiotropy when studying evolution in natural contexts.     

Darwin's legacy: why biology is not physics, or why evolution has not become a common sense

Cosmology and evolution together have enabled us to look deep into the past and comprehend evolution-from the big bang to the cosmos, from molecules to humans. Here, I compare the nature of theories in biology and physics and ask why physical theories get accepted by the public without necessarily comprehending them but biological theories do not. Darwin's theory of natural selection, utterly simple in its premises but profound in its consequences, is not accepted widely. Organized religions, and creationists in particularly, have been the major critic of evolution, but not all opposition to evolution comes from organized religions. A great many people, between evolutionary biologists on one hand and creationists on the other, many academics included, who may not be logically opposed to evolution nevertheless do not accept it. This is because the process of and the evidence for evolution are invisible to a nonspecialist, or the theory may look too simple to explain complex traits to some, or because people compare evolution against God and find evolutionary explanations threatening to their beliefs. Considering how evolution affects our lives, including health and the environment to give just two examples, a basic course in evolution should become a required component of all our college and university educational systems.     

Olfactory Mechanisms Affording Protection from Attack to Juvenile Mice (Mus musculus L.)

In the course of their evolution domestic animals were selected by man not only for physical characteristics and physiological properties, but also for certain behaviour patterns. This permits us to trace the development of new behaviours during historically documented time. In three pigeon breeds — Swing Pouter, Birmingham Roller, Rhine Ringbeater — the development and evolution of breed-specific flight manoevers with courtship function under the influence of human selection was examined, and their course reconstructed. Behaviour patterns which in wild animals would have been eliminated through natural selection were able to develop under human protection and selection.     

Population density effects on longevity

Population density, or the number of adults in an environment relative to the limiting resources, may have important long and short term consequences for the longevity of organisms. In this paper we summarize the way in which crowding may have an immediate impact on longevity, either through the phenomenon known as dietary restriction or through alterations in the quality of the environment brought on by the presence of large numbers of individuals. We also consider the possible long term consequences of population density on longevity by the process of natural selection. There has been much theoretical speculation about the possible impact of population density on the evolution of longevity but little experimental evidence has been gathered to test these ideas. We discuss some of the theory and empirical evidence that exists and show that population density is an important factor in determining both the immediate chances of survival and the course of natural selection.