In the first few decades of the 20th century

In the first few decades of the 20th century, most field naturalists continued to believe that Lamarckian and orthogenetic mechanisms of evolution provided the best explanation for the complexity they observed in the living world. But as the field of genetics continued to develop, those views became less tenable.^[94] Theodosius Dobzhansky, Sony VAIO PCG-5R1L battery a postdoctoral worker in T. H. Morgan's lab, had been influenced by the work on genetic diversity by Russian geneticists such as Sergei Chetverikov. He helped to bridge the divide between the foundations of microevolution developed by the population geneticists and the patterns of macroevolution observed by field biologists, with his 1937 book Genetics and the Origin of Species. Sony VAIO PCG-5R2L battery Dobzhansky examined the genetic diversity of wild populations and showed that, contrary to the assumptions of the population geneticists, these populations had large amounts of genetic diversity, with marked differences between sub-populations. The book also took the highly mathematical work of the population geneticists and put it into a more accessible form. Sony VAIO PCG-5S1L battery In Great Britain E.B. Ford, the pioneer of ecological genetics, continued throughout the 1930s and 1940s to demonstrate the power of selection due to ecological factors including the ability to maintain genetic diversity through genetic polymorphisms such as human blood types. Ford's work would contribute to a shift in emphasis during the course of the modern synthesis towards natural selection over genetic drift.^[ Sony VAIO PCG-5S2L battery Evolutionary biologist Ernst Mayr was influenced by the work of the German biologist Bernhard Rensch showing the influence of local environmental factors on the geographic distribution of sub-species and closely related species. Mayr followed up on Dobzhansky's work with the 1942 book Systematics and the Origin of Species, which emphasized the importance of allopatric speciation in the formation of new species. Sony VAIO PCG-5S3L battery This form of speciation occurs when the geographical isolation of a sub-population is followed by the development of mechanisms for reproductive isolation. Mayr also formulated the biological species concept that defined a species as a group of interbreeding or potentially interbreeding populations that were reproductively isolated from all other populations.^[92][93][97] Sony VAIO PCG-5T2L battery In the 1944 book Tempo and Mode in Evolution, George Gaylord Simpson showed that the fossil record was consistent with the irregular non-directional pattern predicted by the developing evolutionary synthesis, and that the linear trends that earlier paleontologists had claimed supported orthogenesis and neo-Lamarckism did not hold up to closer examination. Sony VAIO PCG-5T3L battery In 1950, G. Ledyard Stebbins published Variation and Evolution in Plants, which helped to integrate botany into the synthesis. The emerging cross-disciplinary consensus on the workings of evolution would be known as the modern evolutionary synthesis. It received its name from the 1942 book Evolution: The Modern Synthesis by Julian Huxley.^[92][93] Sony VAIO PCG-5T4L battery The evolutionary synthesis provided a conceptual core—in particular, natural selection and Mendelian population genetics—that tied together many, but not all, biological disciplines. It helped establish the legitimacy of evolutionary biology, a primarily historical science, in a scientific climate that favored experimental methods over historical ones.^[98] Sony VAIO PCG-61111L battery The synthesis also resulted in a considerable narrowing of the range of mainstream evolutionary thought (what Stephen Jay Gould called the "hardening of the synthesis"): by the 1950s, natural selection acting on genetic variation was virtually the only acceptable mechanism of evolutionary change (panselectionism), and macroevolution was simply considered the result of extensive microevolution. Sony VAIO PCG-61112L battery The middle decades of the 20th century saw the rise of molecular biology, and with it an understanding of the chemical nature of genes as sequences of DNA and of their relationship – through the genetic code – to protein sequences. At the same time, increasingly powerful techniques for analyzing proteins, such as protein electrophoresis and sequencing, brought biochemical phenomena into realm of the synthetic theory of evolution. Sony VAIO PCG-61411L battery In the early 1960s, biochemists Linus Pauling andEmile Zuckerkandl proposed the molecular clock hypothesis: that sequence differences between homologous proteins could be used to calculate the time since two species diverged. By 1969, Motoo Kimura and others provided a theoretical basis for the molecular clock, arguing that—at the molecular level at least—Sony VAIO PCG-71111L battery most genetic mutations are neither harmful nor helpful and that mutation and genetic drift (rather than natural selection) cause a large portion of genetic change: the neutral theory of molecular evolution.^[101] Studies of protein differences within species also brought molecular data to bear on population genetics by providing estimates of the level of heterozygosity in natural populations.^[102] Sony VAIO PCG-7141L battery From the early 1960s, molecular biology was increasingly seen as a threat to the traditional core of evolutionary biology. Established evolutionary biologists—particularly Ernst Mayr, Theodosius Dobzhansky and G. G. Simpson, three of the architects of the modern synthesis—were extremely skeptical of molecular approaches, especially when it came to the connection (or lack thereof) to natural selection. Sony VAIO PCG-7142L battery The molecular-clock hypothesis and the neutral theory were particularly controversial, spawning the neutralist-selectionist debate over the relative importance of mutation, drift and selection, which continued into the 1980s without a clear resolution. In the mid-1960s, George C. Williams strongly critiqued explanations of adaptations worded in terms of "survival of the species" (group selection arguments). Sony VAIO PCG-81113L battery Such explanations were largely replaced by a gene-centered view of evolution, epitomized by the kin selection arguments of W. D. Hamilton, George R. Price and John Maynard Smith.^[105] This viewpoint would be summarized and popularized in the influential 1976 book The Selfish Gene by Richard Dawkins.^[106] Models of the period seemed to show that group selection was severely limited in its strength; though newer models do admit the possibility of significant multi-level selection.^[107] Sony VAIO PCG-81114L battery In 1973, Leigh Van Valen proposed the term "Red Queen", which he took from Through the Looking-Glass by Lewis Carroll, to describe a scenario where a species involved in one or more evolutionary arms races would have to constantly change just to keep pace with the species with which it was co-evolving. Hamilton, Williams and others suggested that this idea might explain the evolution of sexual reproduction: Sony VAIO PCG-81115L battery the increased genetic diversity caused by sexual reproduction would help maintain resistance against rapidly evolving parasites, thus making sexual reproduction common, despite the tremendous cost from the gene-centric point of view of a system where only half of an organism's genome is passed on during reproduction.^[108][109] However, contrary to the expectations of the red queen hypothesis, Hanley et al. found that the prevalence, Sony VAIO PCG-81214L battery abundance and mean intensity of mites was significantly higher in sexual geckos than in asexuals sharing the same habitat.^[110] Furthermore, Parker, after reviewing numerous genetic studies on plant disease resistance, failed to find a single example consistent with the concept that pathogens are the primary selective agent responsible for sexual reproduction in their host.^[111Sony VAIO PCG-81312L battery ^]At an even more fundamental level, Heng^[112] and Gorelick and Heng^[113] reviewed evidence that sex, rather than enhancing diversity, acts as a constraint on genetic diversity. They considered that sex acts as a coarse filter, weeding out major genetic changes, such as chromosomal rearrangements, but permitting minor variation, such as changes at the nucleotide or gene level (that are often neutral) to pass through the sexual sieve. Sony VAIO PCG-8131L battery The adaptive function of sex, today, remains a major unresolved issue in biology. The competing models to explain the adaptive function of sex were reviewed by Birdsell and Wills.^[114] A principal alternative view to the red queen hypothesis is that sex arose, and is maintained, as a process for repairing DNA damage, and that genetic variation is produced as a byproduct.^[115][116] (Also see Evolution of sexual reproduction.) Sony VAIO PCG-8141L battery The gene-centric view has also led to an increased interest in Darwin's old idea of sexual selection,^[117] and more recently in topics such as sexual conflict andintragenomic conflict. W. D. Hamilton's work on kin selection contributed to the emergence of the discipline of sociobiology. The existence of altruistic behaviors has been a difficult problem for evolutionary theorists from the beginning.^[118] Sony VAIO PCG-8152L battery Significant progress was made in 1964 when Hamilton formulated the inequality in kin selection known asHamilton's rule, which showed how eusociality in insects (the existence of sterile worker classes) and many other examples of altruistic behavior could have evolved through kin selection. Other theories followed, some derived from game theory, such as reciprocal altruism.^[119] Sony VAIO PCG-8161L battery In 1975, E.O. Wilson published the influential and highly controversial book Sociobiology: The New Synthesis which claimed evolutionary theory could help explain many aspects of animal, including human, behavior. Critics of sociobiology, including Stephen Jay Gould and Richard Lewontin, claimed that sociobiology greatly overstated the degree to which complex human behaviors could be determined by genetic factors. Sony VAIO PCG-9131L battery They also claimed that the theories of sociobiologists often reflected their own ideological biases. Despite these criticisms, work has continued in sociobiology and the related discipline of evolutionary psychology, including work on other aspects of the altruism problem. One of the most prominent debates arising during the 1970s was over the theory of punctuated equilibrium. Sony VAIO PCG-3E1M battery Niles Eldredge andStephen Jay Gould proposed that there was a pattern of fossil species that remained largely unchanged for long periods (what they termed stasis), interspersed with relatively brief periods of rapid change during speciation.^[122][123] (Also seeSpeciation.) Improvements in sequencing methods resulted in a large increase of sequenced genomes, Sony VAIO PCG-9Z1L battery allowing the testing and refining of evolutionary theories using this huge amount of genome data.^[124] Comparisons between these genomes provide insights into the molecular mechanisms of speciation and adaptation.^[125][126] These genomic analyses have produced fundamental changes in the understanding of the evolutionary history of life, such as the proposal of the three-domain system by Carl Woese.^[127] SONY VAIO PCG-81111V battery Advances in computational hardware and software allow the testing and extrapolation of increasingly advanced evolutionary modelsand the development of the field of systems biology.^[128] One of the results has been an exchange of ideas between theories of biological evolution and the field of computer science known as evolutionary computation, SONY VAIO PCG-81211V battery which attempts to mimic biological evolution for the purpose of developing new computer algorithms. Discoveries in biotechnology now allow the modification of entire genomes, advancing evolutionary studies to the level where future experiments may involve the creation of entirely synthetic organisms. Microbiology was largely ignored by early evolutionary theory. SONY VAIO PCG-51111V battery This was due to the paucity of morphological traits and the lack of a species concept in microbiology, particularly amongst prokaryotes.^[130] Now, evolutionary researchers are taking advantage of their improved understanding of microbial physiology and ecology, produced by the comparative ease of microbial genomics, to explore the taxonomy and evolution of these organisms.^[131] SONY VAIO PCG-51111M battery These studies are revealing unanticipated levels of diversity amongst microbes.^[132][133] One important development in the study of microbial evolution came with the discovery in Japan in 1959 of horizontal gene transfer.^[134] This transfer of genetic material between different species of bacteria came to the attention of scientists because it played a major role in the spread of antibiotic resistance.^[135SONY VAIO PCG-51112M battery ^] More recently, as knowledge of genomes has continued to expand, it has been suggested that lateral transfer of genetic material has played an important role in the evolution of all organisms.^[136] These high levels of horizontal gene transfer have led to suggestions that the family tree of today's organisms, the so-called "tree of life", is more similar to an interconnected web or net.^[137][138] SONY VAIO PCG-51211M battery Indeed, the endosymbiotic theory for the origin of organelles sees a form of horizontal gene transfer as a critical step in the evolution of eukaryotes such as fungi, plants, and animals.^[139][140] The endosymbiotic theory holds that organelles within the cells of eukorytes such as mitochondria and chloroplasts, had descended from independent bacteria that came to live symbiotically within other cells. SONY VAIO PCG-51212M battery It had been suggested in the late 19th century when similarities between mitochondria and bacteria were noted, but largely dismissed until it was revived and championed by Lynn Margulis in the 1960s and 70s; Margulis was able to make use of new evidence that such organelles had their own DNA that was inherited independently from that in the cell's nucleus. SONY PCG-7134M battery In the 1980s and 1990s the tenets of the modern evolutionary synthesis came under increasing scrutiny. There was a renewal of structuralist themes in evolutionary biology in the work of biologists such as Brian Goodwin and Stuart Kauffman, which incorporated ideas from cybernetics and systems theory, and emphasized the self-organizing processes of development as factors directing the course of evolution. SONY PCG-7131M battery The evolutionary biologist Stephen Jay Gould revived earlier ideas ofheterochrony, alterations in the relative rates of developmental processes over the course of evolution, to account for the generation of novel forms, and, with the evolutionary biologist Richard Lewontin, wrote an influential paper in 1979 suggesting that a change in one biological structure, or even a structural novelty, Sony Vaio VGN-CR13/W Battery could arise incidentally as an accidental result of selection on another structure, rather than through direct selection for that particular adaptation. They called such incidental structural changes "spandrels" after an architectural feature.^[142] Later, Gould and Vrba discussed the acquisition of new functions by novel structures arising in this fashion, calling them "exaptations".^[143] Sony Vaio VGN-CR13G Battery Molecular data regarding the mechanisms underlying development accumulated rapidly during the 1980s and '90s. It became clear that the diversity of animal morphology was not the result of different sets of proteins regulating the development of different animals, but from changes in the deployment of a small set of proteins that were common to all animals.^[144] Sony Vaio VGN-CR13G/B Battery These proteins became known as the "developmental toolkit".^[145] Such perspectives influenced the disciplines ofphylogenetics, paleontology and comparative developmental biology, and spawned the new discipline of evolutionary developmental biology also known as evo-devo. One of the tenets of the modern evolutionary synthesis was that macroevolution Sony Vaio VGN-CR13G/L Battery (the evolution of phylogenic clades at the species level and above) was solely the result of the mechanisms of microevolution (changes in gene frequency within populations) operating over an extended period of time. During the last decades of the 20th century some paleontologists raised questions about whether other factors, Sony VAIO PCG-5N1M battery such as punctuated equilibrium and group selection operating on the level of entire species and even higher level phylogenic clades, needed to be considered to explain patterns in evolution revealed by statistical analysis of the fossil record. Near the end of the 20th century some researchers in evolutionary developmental biology suggested that interactions between the environment and the developmental process might have been the source of some of the structural innovations seen in macroevolution, Sony VAIO PCG-5S1M battery but other evo-devo researchers maintained that genetic mechanisms visible at the population level are fully sufficient to explain all macroevolution.Epigenetics is the study of heritable changes in gene expression or cellular phenotype caused by mechanisms other than changes in the underlying DNA sequence. By the first decade of the 21st century it had become accepted that Epigenetic mechanisms were a necessary part of the evolutionary origin of cell differentiation.^[150] Sony Vaio VGN-CR13G/W Battery Although epigenetics in multicellular organisms is generally thought to be a mechanism involved in differentiation, with epigenetic patterns "reset" when organisms reproduce, there have been some observations of transgenerational epigenetic inheritance. This shows that in some cases non genetic changes to an organism can be inherited and it has been suggested that such inheritance can help with adaptation to local conditions and affect evolution.^[151][1Sony Vaio VGN-CR13G/P Battery ^52] Some have suggested that in certain cases a form of Lamarckian evolution may occur. Teilhard de Chardin's ideas have been seen by advocates of the Gaia theory proposed by James Lovelock, which holds that the living and nonliving parts of Earth can be viewed as a complex interacting system with similarities to a single organism,^[154] as being connected to Lovelock's ideas.^[155] Sony Vaio VGN-CR13G/R Battery The Gaia hypothesis has also been viewed by Lynn Margulis^[156] and others as an extension of endosymbiosis and exosymbiosis.^[157] This modified hypothesis postulates that all living things have a regulatory effect on the Earth's environment that promotes life overall.Sony VAIO PCG-31111M battery,Sony VAIO PCG-31112M battery,Sony VAIO PCG-31113M battery