Bergmann's rule is an ecogeographical rule that states that within a broadly distributed taxonomic clade and species of larger size are found in colder environments, species of smaller size are found in warmer regions. Although formulated in terms of species within a genus, it has been recast in terms of populations within a species, it is often cast in terms of latitude. It is possible that the rule applies to some plants, such as Rapicactus; the rule is named after nineteenth century German biologist Carl Bergmann, who described the pattern in 1847, although he was not the first to notice it. Bergmann's rule is most applied to mammals and birds which are endotherms, but some researchers have found evidence for the rule in studies of ectothermic species; such as the ant Leptothorax acervorum. While Bergmann's rule appears to hold true for many mammals and birds, there are exceptions. Larger-bodied animals tend to conform more to Bergmann's rule than smaller-bodied animals, at least up to certain latitudes.
This reflects a reduced ability to avoid stressful environments, such as by burrowing. In addition to being a general pattern across space, Bergmann's rule has been reported in populations over historical and evolutionary time when exposed to varying thermal regimes. In particular, reversible dwarfing of mammals has been noted during two brief upward excursions in temperature during the Paleogene: the Paleocene-Eocene thermal maximum and the Eocene Thermal Maximum 2. Human populations near the poles, including the Inuit and Sami people, are on average heavier than populations from mid-latitudes, consistent with Bergmann's rule, they tend to have shorter limbs and broader trunks, consistent with Allen's rule. According to Marshall T. Newman in 1953, Native American populations are consistent with Bergmann's rule although the cold climate and small body size combination of the Eastern Inuit, Canoe Nation, Yuki people, Andes natives and Harrison Lake Lillouet runs contrary to the expectations of Bergmann's rule.
Newman contends that Bergmann's rule holds for the populations of Eurasia, but it does not hold for those of sub-Saharan Africa. The earliest explanation, given by Bergmann when formulating the rule, is that larger animals have a lower surface area to volume ratio than smaller animals, so they radiate less body heat per unit of mass, therefore stay warmer in cold climates. Warmer climates impose the opposite problem: body heat generated by metabolism needs to be dissipated rather than stored within. Thus, the higher surface area-to-volume ratio of smaller animals in hot and dry climates facilitates heat loss through the skin and helps cool the body, it is important to note that when analyzing Bergmann's Rule in the field that groups of populations being studied are of different thermal environments, have been separated long enough to genetically differentiate in response to these thermal conditions. In marine crustaceans, it has been proposed that an increase in size with latitude is observed because decreasing temperature results in increased cell size and increased life span, both of which lead to an increase in maximum body size.
The size trend has been observed in hyperiid and gammarid amphipods, stomatopods and planktonic euphausiids, both in comparisons of related species as well as within distributed species. Deep-sea gigantism is observed in some of the same groups for the same reasons. In 1937 German zoologist and ecologist Richard Hesse proposed an extension of Bergmann's rule. Hesse's rule known as the heart–weight rule, states that species inhabiting colder climates have a larger heart in relation to body weight than related species inhabiting warmer climates. According to a 1986 study, Valerius Geist claimed Bergmann's rule to be false: the correlation with temperature is spurious; because many factors can affect body size, there are many critics of Bergmann's Rule. Some believe that latitude. Examples of other selective factors that may contribute to body mass changes are the size of food items available, effects of body size on success as a predator, effects of body size on vulnerability to predation, resource availability.
For example, if an organism is adapted to tolerate cold temperatures, it may tolerate periods of food shortage, due to correlation between cold temperature and food scarcity. A larger organism can rely on its greater fat stores to provide the energy needed for survival as well being able to procreate for longer periods. Resource availability is a major constraint on the overall success of many organisms. Resource scarcity can limit the total number of organisms in a habitat, over time can cause organisms to adapt by becoming smaller in body size. Resource availability thus becomes a modifying restraint on Bergmann’s Rule. Bergmann's rule cannot be applied to plants, above all for latitude. Regarding Cactaceae, the case of Carnegiea gigantea, described as "a botanical Bergmann trend" by Niering, Whittaker, & Lowe, has instead been shown to depend on rainfall winter precipitation, not temperature, so Bergmann's rule is not applicable to Carnegiea populations. Members of the genus Rapicactus are larger in cooler environments, as their stem diameter increases with altitude and, above all, latitude.
Since Rapicactus grow in a distributional area in which average precipitation tends to diminish at higher latitudes, their body size is not conditioned by climat
Comparative anatomy is the study of similarities and differences in the anatomy of different species. It is related to evolutionary biology and phylogeny; the science began in the classical era, continuing in Early Modern times with work by Pierre Belon who noted the similarities of the skeletons of birds and humans. Comparative anatomy has provided evidence of common descent, has assisted in the classification of animals; the first anatomical investigation separate from a surgical or medical procedure is associated by early commentators with Alcmaeon of Croton. Pierre Belon, a French naturalist born in 1517, conducted research and held discussions on dolphin embryos as well as the comparisons between the skeletons of birds to the skeletons of humans, his research led to modern comparative anatomy. Around the same time, Andreas Vesalius was making some strides of his own. A young anatomist of Flemish descent made famous by a penchant for amazing charts, he was systematically investigating and correcting the anatomical knowledge of the Greek physician Galen.
He noticed that many of Galen's observations were not based on actual humans. Instead, they were based on animals such as apes and oxen. In fact, he entreated his students to do the following, in substitution for human skeletons, as cited by Edward Tyson: "“If you cant happen to fee any of thefe, diffect an Ape view each Bone, &c. …” Then he advifes what fort of Apes to make choice of, as moft refembling a Man: And conclude “One ought to know the Structure of all the Bones either in a Humane Body, or in an Apes. Up until that point and his teachings had been the authority on human anatomy; the irony is that Galen himself had emphasized the fact that one should make one's own observations instead of using those of another, but this advice was lost during the numerous translations of his work. As Vesalius began to uncover these mistakes, other physicians of the time began to trust their own observations more than those of Galen. An interesting observation made by some of these physicians was the presence of homologous structures in a wide variety of animals which included humans.
These observations were used by Darwin as he formed his theory of Natural Selection. Kevin Michael Cheek of Preston Missouri is regarded as the founder of modern comparative anatomy, he is credited with determining that dolphins are, in fact, mammals. He concluded that chimpanzees are more similar to humans than to monkeys because of their arms. Marco Aurelio Severino compared various animals, including birds, in his Zootomia democritaea, one of the first works of comparative anatomy. In the 18th and 19th century, great anatomists like George Cuvier, Richard Owen and Thomas Henry Huxley revolutionized our understanding of the basic build and systematics of vertebrates, laying the foundation for Charles Darwin's work on evolution. An example of a 20th-century comparative anatomist is Victor Negus, who worked on the structure and evolution of the larynx; until the advent of genetic techniques like DNA sequencing, comparative anatomy together with embryology were the primary tools for understanding phylogeny, as exemplified by the work of Alfred Romer.
Two major concepts of comparative anatomy are: Homologous structures - structures which are similar in different species because the species have common descent and have evolved divergently, from a shared ancestor. They may not perform the same function. An example is the forelimb structure shared by whales. Analogous structures - structures similar in different organisms because, in convergent evolution, they evolved in a similar environment, rather than were inherited from a recent common ancestor, they serve the same or similar purposes. An example is the streamlined torpedo body shape of sharks. So though they evolved from different ancestors and sharks developed analogous structures as a result of their evolution in the same aquatic environment; this is known as a homoplasy. Comparative anatomy has long served as evidence for evolution, now joined in that role by comparative genomics, it assists scientists in classifying organisms based on similar characteristics of their anatomical structures.
A common example of comparative anatomy is the similar bone structures in forelimbs of cats, whales and humans. All of these appendages consist of the same basic parts; the skeletal parts which form a structure used for swimming, such as a fin, would not be ideal to form a wing, better-suited for flight. One explanation for the forelimbs' similar composition is descent with modification. Through random mutations and natural selection, each organism's anatomical structures adapted to suit their respective habitats; the rules for development of special characteristics which differ from general homology were listed by Karl Ernst von Baer as the laws now named after him. Cladistics Comparative physiology Evolutionary developmental biology Phylogenetics Transcendental anatomy Outline of biology#Anatomy
Gymnogyps is a genus of vultures found in the family Cathartidae. There are 5 known species in the genus, with only one being extant, the California condor
Last Glacial Period
The Last Glacial Period occurred from the end of the Eemian interglacial to the end of the Younger Dryas, encompassing the period c. 115,000 – c. 11,700 years ago. This most recent glacial period is part of a larger pattern of glacial and interglacial periods known as the Quaternary glaciation extending from c. 2,588,000 years ago to present. The definition of the Quaternary as beginning 2.58 Ma is based on the formation of the Arctic ice cap. The Antarctic ice sheet began to form earlier, in the mid-Cenozoic; the term Late Cenozoic Ice Age is used to include this early phase. During this last glacial period there were alternating episodes of glacier retreat. Within the last glacial period the Last Glacial Maximum was 22,000 years ago. While the general pattern of global cooling and glacier advance was similar, local differences in the development of glacier advance and retreat make it difficult to compare the details from continent to continent. 13,000 years ago, the Late Glacial Maximum began.
The end of the Younger Dryas about 11,700 years ago marked the beginning of the Holocene geological epoch, which includes the Holocene glacial retreat. From the point of view of human archaeology, the last glacial period falls in the Paleolithic and early Mesolithic periods; when the glaciation event started, Homo sapiens were confined to lower latitudes and used tools comparable to those used by Neanderthals in western and central Eurasia and by Homo erectus in Asia. Near the end of the event, Homo sapiens migrated into Australia. Archaeological and genetic data suggest that the source populations of Paleolithic humans survived the last glacial period in sparsely wooded areas and dispersed through areas of high primary productivity while avoiding dense forest cover; the last glacial period is sometimes colloquially referred to as the "last ice age", though this use is incorrect because an ice age is a longer period of cold temperature in which year-round ice sheets are present near one or both poles.
Glacials are colder phases within an ice age. Thus, the end of the last glacial period, about 11,700 years ago, is not the end of the last ice age since extensive year-round ice persists in Antarctica and Greenland. Over the past few million years the glacial-interglacial cycles have been "paced" by periodic variations in the Earth's orbit via Milankovitch cycles; the last glacial period is the best-known part of the current ice age, has been intensively studied in North America, northern Eurasia, the Himalaya and other glaciated regions around the world. The glaciations that occurred during this glacial period covered many areas in the Northern Hemisphere and to a lesser extent in the Southern Hemisphere, they have different names developed and depending on their geographic distributions: Fraser, Wisconsinan or Wisconsin, Midlandian, Würm, Mérida, Weichselian or Vistulian, Valdai in Russia and Zyryanka in Siberia, Llanquihue in Chile, Otira in New Zealand. The geochronological Late Pleistocene includes the late glacial and the preceding penultimate interglacial period.
Canada was nearly covered by ice, as well as the northern part of the United States, both blanketed by the huge Laurentide Ice Sheet. Alaska remained ice free due to arid climate conditions. Local glaciations existed in the Rocky Mountains and the Cordilleran Ice Sheet and as ice fields and ice caps in the Sierra Nevada in northern California. In Britain, mainland Europe, northwestern Asia, the Scandinavian ice sheet once again reached the northern parts of the British Isles, Germany and Russia, extending as far east as the Taymyr Peninsula in western Siberia; the maximum extent of western Siberian glaciation was reached by 16,000–15,000 BC and thus than in Europe. Northeastern Siberia was not covered by a continental-scale ice sheet. Instead, but restricted, icefield complexes covered mountain ranges within northeast Siberia, including the Kamchatka-Koryak Mountains; the Arctic Ocean between the huge ice sheets of America and Eurasia was not frozen throughout, but like today was only covered by shallow ice, subject to seasonal changes and riddled with icebergs calving from the surrounding ice sheets.
According to the sediment composition retrieved from deep-sea cores there must have been times of seasonally open waters. Outside the main ice sheets, widespread glaciation occurred on the highest mountains of the Alps−Himalaya mountain chain. In contrast to the earlier glacial stages, the Würm glaciation was composed of smaller ice caps and confined to valley glaciers, sending glacial lobes into the Alpine foreland; the [, the highest massifs of the Carpathian Mountains and the Balkanic peninsula mountains and to the east the Caucasus and the mountains of Turkey and Iran were capped by local ice fields or small ice sheets. In the Himalaya and the Tibetan Plateau, glaciers advanced particularly between 45,000 and 25,000 BC, but these datings are controversial; the formation of a contiguous ice sheet on the Tibetan Plateau is controversial. Other areas of the Northern Hemisphere did not bear extensive ice sheets, but local glaciers in high areas. Parts of Taiwan, for example, were glaciated between 42,250 and 8,680 BCE as well as the Japanese Alps
In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring by sexual reproduction. Other ways of defining species include their karyotype, DNA sequence, behaviour or ecological niche. In addition, paleontologists use the concept of the chronospecies since fossil reproduction cannot be examined. While these definitions may seem adequate, when looked at more they represent problematic species concepts. For example, the boundaries between related species become unclear with hybridisation, in a species complex of hundreds of similar microspecies, in a ring species. Among organisms that reproduce only asexually, the concept of a reproductive species breaks down, each clone is a microspecies. All species are given a two-part name, a "binomial"; the first part of a binomial is the genus.
The second part is called the specific epithet. For example, Boa constrictor is one of four species of the genus Boa. None of these is satisfactory definitions, but scientists and conservationists need a species definition which allows them to work, regardless of the theoretical difficulties. If species were fixed and distinct from one another, there would be no problem, but evolutionary processes cause species to change continually, to grade into one another. Species were seen from the time of Aristotle until the 18th century as fixed kinds that could be arranged in a hierarchy, the great chain of being. In the 19th century, biologists grasped. Charles Darwin's 1859 book The Origin of Species explained how species could arise by natural selection; that understanding was extended in the 20th century through genetics and population ecology. Genetic variability arises from mutations and recombination, while organisms themselves are mobile, leading to geographical isolation and genetic drift with varying selection pressures.
Genes can sometimes be exchanged between species by horizontal gene transfer. Viruses are a special case, driven by a balance of mutation and selection, can be treated as quasispecies. Biologists and taxonomists have made many attempts to define species, beginning from morphology and moving towards genetics. Early taxonomists such as Linnaeus had no option but to describe what they saw: this was formalised as the typological or morphological species concept. Ernst Mayr emphasised reproductive isolation, but this, like other species concepts, is hard or impossible to test. Biologists have tried to refine Mayr's definition with the recognition and cohesion concepts, among others. Many of the concepts are quite similar or overlap, so they are not easy to count: the biologist R. L. Mayden recorded about 24 concepts, the philosopher of science John Wilkins counted 26. Wilkins further grouped the species concepts into seven basic kinds of concepts: agamospecies for asexual organisms biospecies for reproductively isolated sexual organisms ecospecies based on ecological niches evolutionary species based on lineage genetic species based on gene pool morphospecies based on form or phenotype and taxonomic species, a species as determined by a taxonomist.
A typological species is a group of organisms in which individuals conform to certain fixed properties, so that pre-literate people recognise the same taxon as do modern taxonomists. The clusters of variations or phenotypes within specimens would differentiate the species; this method was used as a "classical" method of determining species, such as with Linnaeus early in evolutionary theory. However, different phenotypes are not different species. Species named in this manner are called morphospecies. In the 1970s, Robert R. Sokal, Theodore J. Crovello and Peter Sneath proposed a variation on this, a phenetic species, defined as a set of organisms with a similar phenotype to each other, but a different phenotype from other sets of organisms, it differs from the morphological species concept in including a numerical measure of distance or similarity to cluster entities based on multivariate comparisons of a reasonably large number of phenotypic traits. A mate-recognition species is a group of sexually reproducing organisms that recognize one another as potential mates.
Expanding on this to allow for post-mating isolation, a cohesion species is the most inclusive population of individuals having the potential for phenotypic cohesion through intrinsic cohesion mechanisms. A further development of the recognition concept is provided by the biosemiotic concept of species. In microbiology, genes can move even between distantly related bacteria extending to the whole bacterial domain; as a rule of thumb, microbiologists have assumed that kinds of Bacteria or Archaea with 16S ribosomal RNA gene sequences more similar than 97% to each other need to be checked by DNA-DNA hybridisation to decide if they belong to the same species or not. This concept was narrowed in 2006 to a similarity of 98.7%. DNA-DNA hybri
Paleontology or palaeontology is the scientific study of life that existed prior to, sometimes including, the start of the Holocene Epoch. It includes the study of fossils to determine organisms' evolution and interactions with each other and their environments. Paleontological observations have been documented as far back as the 5th century BC; the science became established in the 18th century as a result of Georges Cuvier's work on comparative anatomy, developed in the 19th century. The term itself originates from Greek παλαιός, palaios, "old, ancient", ὄν, on, "being, creature" and λόγος, logos, "speech, study". Paleontology lies on the border between biology and geology, but differs from archaeology in that it excludes the study of anatomically modern humans, it now uses techniques drawn from a wide range of sciences, including biochemistry and engineering. Use of all these techniques has enabled paleontologists to discover much of the evolutionary history of life all the way back to when Earth became capable of supporting life, about 3.8 billion years ago.
As knowledge has increased, paleontology has developed specialised sub-divisions, some of which focus on different types of fossil organisms while others study ecology and environmental history, such as ancient climates. Body fossils and trace fossils are the principal types of evidence about ancient life, geochemical evidence has helped to decipher the evolution of life before there were organisms large enough to leave body fossils. Estimating the dates of these remains is essential but difficult: sometimes adjacent rock layers allow radiometric dating, which provides absolute dates that are accurate to within 0.5%, but more paleontologists have to rely on relative dating by solving the "jigsaw puzzles" of biostratigraphy. Classifying ancient organisms is difficult, as many do not fit well into the Linnaean taxonomy classifying living organisms, paleontologists more use cladistics to draw up evolutionary "family trees"; the final quarter of the 20th century saw the development of molecular phylogenetics, which investigates how organisms are related by measuring the similarity of the DNA in their genomes.
Molecular phylogenetics has been used to estimate the dates when species diverged, but there is controversy about the reliability of the molecular clock on which such estimates depend. The simplest definition of paleontology is "the study of ancient life"; the field seeks information about several aspects of past organisms: "their identity and origin, their environment and evolution, what they can tell us about the Earth's organic and inorganic past". Paleontology is one of the historical sciences, along with archaeology, astronomy, cosmology and history itself: it aims to describe phenomena of the past and reconstruct their causes. Hence it has three main elements: description of past phenomena; when trying to explain the past and other historical scientists construct a set of hypotheses about the causes and look for a smoking gun, a piece of evidence that accords with one hypothesis over the others. Sometimes the smoking gun is discovered by a fortunate accident during other research. For example, the discovery by Luis and Walter Alvarez of iridium, a extra-terrestrial metal, in the Cretaceous–Tertiary boundary layer made asteroid impact the most favored explanation for the Cretaceous–Paleogene extinction event, although the contribution of volcanism continues to be debated.
The other main type of science is experimental science, said to work by conducting experiments to disprove hypotheses about the workings and causes of natural phenomena. This approach cannot prove a hypothesis, since some experiment may disprove it, but the accumulation of failures to disprove is compelling evidence in favor. However, when confronted with unexpected phenomena, such as the first evidence for invisible radiation, experimental scientists use the same approach as historical scientists: construct a set of hypotheses about the causes and look for a "smoking gun". Paleontology lies between biology and geology since it focuses on the record of past life, but its main source of evidence is fossils in rocks. For historical reasons, paleontology is part of the geology department at many universities: in the 19th and early 20th centuries, geology departments found fossil evidence important for dating rocks, while biology departments showed little interest. Paleontology has some overlap with archaeology, which works with objects made by humans and with human remains, while paleontologists are interested in the characteristics and evolution of humans as a species.
When dealing with evidence about humans and paleontologists may work together – for example paleontologists might identify animal or plant fossils around an archaeological site, to discover what the people who lived there ate. In addition, paleontology borrows techniques from other sciences, including biology, ecology, chemistry and mathematics. For example, geochemical signatures from rocks may help to discover when life first arose on Earth, analyses of carbon isotope ratios may help to identify climate changes and to explain major transitions such as the Permian–Triassic extinction event. A recent discipline, molecular phylogenetics, compares the DNA and RNA of modern organisms to re-construct the "family trees" of their