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Subject: Music

This US group was formed in Los Angeles, California, in 1978 by Steve Allen (guitar, vocals) and Ron Flynt (bass, vocals), two expatriate musicians from Tulsa, Oklahoma. Drummer Mike Gallo ...

Experimental Evolution

Experimental Evolution   Reference library

Encyclopedia of Evolution

Reference type:
Subject Reference
Current Version:
2005
Subject:
Science and technology, Life Sciences, Social sciences, Anthropology
Length:
3,999 words
Illustration(s):
1

...The first article provides a summary overview contrasting experimental and comparative/historical approaches for studying evolution; the second article provides a case study of an evolutionary experiment in which populations of E. coli have been propagated and monitored for 20,000 generations. For related discussions, see Adaptation ; Artificial Selection ; Bacteria and Archaea ; Comparative Method ; Fitness ; Genetic Drift ; Natural Selection ; and Senescence . An Overview Experimental evolution is a scientific method in which populations of...

Convergent and Parallel Evolution

Convergent and Parallel Evolution   Reference library

Encyclopedia of Evolution

Reference type:
Subject Reference
Current Version:
2005
Subject:
Science and technology, Life Sciences, Social sciences, Anthropology
Length:
1,982 words
Illustration(s):
3

...to mere similarity owing to chance. At the molecular level, this distinction becomes very important because of the limited number of possible character states for nucleotides (one for each of the four nucleotides making up strands of DNA) and for amino acids (20 possible character states representing the 20 amino acids). The existence of convergent evolution in morphological, physiological, behavioral, and molecular changes indicates that there are significant constraints on the evolutionary process, and that these constraints are distributed across the...

Mammals

Mammals   Reference library

Encyclopedia of Evolution

Reference type:
Subject Reference
Current Version:
2005
Subject:
Science and technology, Life Sciences, Social sciences, Anthropology
Length:
5,871 words
Illustration(s):
1

...level seem to bear little resemblance to one another, yet certain features of the skeleton and dentition tie them into a single taxon. The armadillos, with their dorsal surfaces covered with a leathery skin embedded with dermal bone, present a distinctive appearance. There are 20 living species of armadillos, one of which reaches as far north as Oklahoma in the United States. Some species of armadillo are omnivorous, but others are specialized for feeding on ants and termites. The tree sloths are specialized for browsing on leaves in tropical forests. The...

Dinosaurs

Dinosaurs   Reference library

Encyclopedia of Evolution

Reference type:
Subject Reference
Current Version:
2005
Subject:
Science and technology, Life Sciences, Social sciences, Anthropology
Length:
3,672 words
Illustration(s):
2

...whose name means “birdlike foot,” is distinguished by elaborate dental modifications for grinding vegetation. Early members were small, about 1 to 2 meters (3–6 feet) in length, but some of the forms during the Cretaceous period (145–65 million years ago) reached nearly 20 meters (66 feet) length and as adults weighed several tons. Despite their suggestive name, ornithopod dinosaurs are not the ancestors of birds, and they became progressively less birdlike as time went on. The sister lineage of Ornithopoda is Marginocephalia, the “margin-headed”...

Malaria

Malaria   Reference library

Encyclopedia of Evolution

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Subject Reference
Current Version:
2005
Subject:
Science and technology, Life Sciences, Social sciences, Anthropology
Length:
2,563 words
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2

... et al., 1998 , 2000 ) as well as for mitochondrial genomes ( Conway et al., 2000 ). Based on these observations, the current distribution of P. falciparum throughout the world's tropical regions is believed to have derived from a small ancestral population within the past 20,000 to 60,000 years. Evidence exists to show that natural selection acts strongly on both the falciparum and its hosts. The human response is evident in the various hemoglobinopathies (e.g., sickle-cell trait) that have arisen to combat the Plasmodium . The P. falciparum ...

Stratigraphy

Stratigraphy   Reference library

Encyclopedia of Evolution

Reference type:
Subject Reference
Current Version:
2005
Subject:
Science and technology, Life Sciences, Social sciences, Anthropology
Length:
1,510 words
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2

...developed in the early 1800s, no one could tell the age and duration of the different geological periods. Some geologists thought that the age of the earth was nearly infinite ( Hutton noted that there was “no vestige of a beginning”), but other scientists gave the earth only 20 million years or less for its entire history. The discovery of radioactivity in 1896 , however, provided the first method of obtaining a numerical age for geological events. When a radioactive atom, such as uranium or rubidium, spontaneously decays by nuclear reactions, it gives...

Fitness

Fitness   Reference library

Encyclopedia of Evolution

Reference type:
Subject Reference
Current Version:
2005
Subject:
Science and technology, Life Sciences, Social sciences, Anthropology
Length:
2,777 words
Illustration(s):
1

...are relevant to their final frequencies among the survivors. Second, the magnitude of the change in the frequencies of the two forms within a generation is determined by the magnitude of the difference in these relative values; for example, if v 1 / v 2 were only 1.1 instead of 2.0, the frequency of tolerants among the survivors would be 52 percent instead of 67 percent. To determine the new frequencies of the two forms among the next generation, which is necessary to describe the course of evolutionary change, we would also need to know their mode of...

Genetic Code

Genetic Code   Reference library

Encyclopedia of Evolution

Reference type:
Subject Reference
Current Version:
2005
Subject:
Science and technology, Life Sciences, Social sciences, Anthropology
Length:
3,000 words
Illustration(s):
2

...of Crick's closest colleagues). Because there are four possible bases in RNA (where DNA has adenine, guanine, cytosine, and thymine, RNA has adenine, guanine, cytosine, and uracil), there are 4 3 = 64 possible codons. However, because the genetic code specifies only 21 entities—20 amino acids and a “stop” signal—the genetic code is technically “degenerate” or redundant. That is, there is not a one-to-one correspondence between codons and amino acids. Instead, most amino acids are specified by more than one codon. Even before the general features of the code...

Vaccination

Vaccination   Reference library

Encyclopedia of Evolution

Reference type:
Subject Reference
Current Version:
2005
Subject:
Science and technology, Life Sciences, Social sciences, Anthropology
Length:
2,814 words
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1

...1. Summary of Epidemiological and Demographic Parameters Infection Region Date Mean Age at Infection, A (years) Reciprocal of Average Birth Rate, B (years) R 0 p c (%) Measles United Kingdom 1950s 5.0 70 15.0 93 Measles Senegal 1964 1.8 31 18.0 94 Smallpox West Africa 1960s 15.0 20 2.3 57 Smallpox India 1960s 12.5 30 3.4 70 Source: McLean, A. R. “Mathematical Modelling of the Immunization of Populations.” Reviews in Medical Virology 2 (1992): 141–152. The impact of perfect vaccines. The most straightforward situation is that of a vaccine that gives...

Density-Dependent Selection

Density-Dependent Selection   Reference library

Encyclopedia of Evolution

Reference type:
Subject Reference
Current Version:
2005
Subject:
Science and technology, Life Sciences, Social sciences, Anthropology
Length:
1,945 words
Illustration(s):
2

...In this case, the evolution of density-dependent traits did not affect the stability of population size. See also Demography ; Fitness ; Genetic Polymorphism ; Life History Theory: An Overview ; Population Genetics . Cody, M. “ A General Theory of Clutch Size. ” Evolution 20 (1966): 174–184. MacArthur, R. H. “ Some Generalized Theorems of Natural Selection. ” Proceedings of the National Academy of Sciences USA 48 (1962): 1893–1897. In this classic paper, MacArthur proposes that fitness be measured by the carrying capacity of the logistic equation at...

Heterozygote Advantage

Heterozygote Advantage   Reference library

Encyclopedia of Evolution

Reference type:
Subject Reference
Current Version:
2005
Subject:
Science and technology, Life Sciences, Social sciences, Anthropology
Length:
4,349 words
Illustration(s):
6

...( Cavalli-Sforza and Bodmer , 1971 ). Selection on the sickle-cell polymorphism can be presented in a model that uses the approximate genotypic fitnesses and allelic frequencies for an environment with falciparum malaria (Table 2). The fitnesses are estimated to be .85, 1.0 and .20 for the AA, AS, and SS genotypes, respectively. These fitnesses, when used in equations 3 and 4 above, predict equilibrium frequencies of 0.842 for A and 0.158 for S. These frequencies are approximately those seen in areas plagued by falciparum malaria. The genotypic frequencies...

Quantitative Genetics

Quantitative Genetics   Reference library

Encyclopedia of Evolution

Reference type:
Subject Reference
Current Version:
2005
Subject:
Science and technology, Life Sciences, Social sciences, Anthropology
Length:
4,115 words
Illustration(s):
4

...quantity i = S /√ V P is called the intensity of selection; it gives the number of standard deviation units by which the trait is altered by selection within a generation. Estimates of heritability are typically between 30 and 40 percent for morphological traits and are 10 to 20 percent for traits closely connected to fitness, such as survival or fecundity. Intensities of selection from studies of natural populations have a mode near 0 and a median around 0.1 standard deviations. Alternatively, one can standardize the response to selection by dividing by...

Diatoms

Diatoms   Reference library

Encyclopedia of Evolution

Reference type:
Subject Reference
Current Version:
2005
Subject:
Science and technology, Life Sciences, Social sciences, Anthropology
Length:
2,490 words
Illustration(s):
2

...individual diatom species is adding to our understanding of the true environmental optima of diatoms and also helps explain the adaptations of diatoms to new environments (see Vignette). Diatoms are among the most important photosynthetic organisms on earth. They produce at least 20–25 percent of the net free oxygen produced by plants. Even in highly productive salt-marsh systems, where grasses form the most abundant visible part of the food web, diatoms account for as much as half of the carbon flowing through the system. This is possible because of their...

Fluctuating Asymmetry

Fluctuating Asymmetry   Reference library

Encyclopedia of Evolution

Reference type:
Subject Reference
Current Version:
2005
Subject:
Science and technology, Life Sciences, Social sciences, Anthropology
Length:
2,548 words
Illustration(s):
1

...“ Fluctuating Asymmetry: Measurement, Analysis, Patterns. ” Annual Review of Ecology and Systematics 17 (1986): 391–431. Contains the standard techniques used for the analysis of FA. Perrett, D. I. , et al. “ Symmetry and Human Facial Attraction. ” Evolution and Human Behaviour 20 (1999): 295–307. A recent study of how human facial symmetry influences attraction. Roy, B. A. , and M. L. Stanton . “ Asymmetry of Wild Mustard, Sinapsis arvensis (Brassicaceae) in Response to Severe Physiological Stress. ” Journal of Evolutionary Biology 12 (1999): 440–449....

Phylogenetic Inference

Phylogenetic Inference   Reference library

Encyclopedia of Evolution

Reference type:
Subject Reference
Current Version:
2005
Subject:
Science and technology, Life Sciences, Social sciences, Anthropology
Length:
10,174 words
Illustration(s):
4

...of discrete data—that is, data for which there exist only a finite number of different states that the character can take (contrast this with, for example, a trait like body size that can adopt an effectively infinite number of different states). Proteins can adopt one of only 20 different amino acids at each site, and gene sequences can have one of only four different bases (A, C, G, or T) at each site in the sequence. The attraction of this discrete sequence data was that it was possible to write down simple and mathematically tractable models of how a...

Language

Language   Reference library

Encyclopedia of Evolution

Reference type:
Subject Reference
Current Version:
2005
Subject:
Science and technology, Life Sciences, Social sciences, Anthropology
Length:
6,265 words
Illustration(s):
1

...novel vocalizations, or approximations to the words of any human language. The human brain clearly is specialized to regulate speech production. As we talk, the sequence of phonemes (meaningful sounds, roughly approximated by the letters of the alphabet) transmits words at rates of 20 to 30 phonemes per second. In contrast, the human auditory system fuses discrete nonspeech sounds into a buzz at rates exceeding 15 per second. The rapid transmission rate of speech is achieved by encoding—that is, melding phonemes into syllables transmitted at a slower rate that...

Sex, Evolution of

Sex, Evolution of   Reference library

Encyclopedia of Evolution

Reference type:
Subject Reference
Current Version:
2005
Subject:
Science and technology, Life Sciences, Social sciences, Anthropology
Length:
6,244 words
Illustration(s):
5

...populations, or stochastic and require finite (and usually small) populations or certain population structures. In the rest of this section, we briefly summarize the most popular hypotheses for sex; a thorough review is provided by Kondrashov ( 1993 ). Although more then 20 hypotheses have been suggested and new ones are published with regularity, we will not (and could not hope to) cover all of these. In addition, many require extremely restrictive assumptions and so are not likely to be generally important, and it is not unusual for new models to be...

Disease

Disease   Reference library

Encyclopedia of Evolution

Reference type:
Subject Reference
Current Version:
2005
Subject:
Science and technology, Life Sciences, Social sciences, Anthropology
Length:
9,315 words
Illustration(s):
1

...146 amino acid protein. The result is a protein that tends to aggregate under conditions of hypoxia (low levels of oxygen in the blood), and results in a characteristic sickle shape to the red blood cells. In malarial regions of Africa, the incidence of the β S allele approaches 20 percent, much higher than expected under a mutation-selection balance unless the selection actually favors β S . There is now quite direct evidence that heterozygotes for β S have reduced mortality by malaria, so that despite the deleterious consequences of β S homozygosity, the...

Genetic Load

Genetic Load   Reference library

Encyclopedia of Evolution

Reference type:
Subject Reference
Current Version:
2005
Subject:
Science and technology, Life Sciences, Social sciences, Anthropology
Length:
3,825 words
Illustration(s):
1

...can define w opt as the survival rate of the best genotype: it is 50 percent. We calculate mean fitness for the absolute survival rates ( w̄ = 40 percent) with the absolute numbers which again comes out as 0.2 in the example. The fitness of an average member of the population is 20 percent lower than it could have been. Genetic load can arise for a number of reasons; they are all cases in which the members of a population are less well adapted than they could be. [ See Adaptation for a related discussion of adaptive imperfection .] The distinctive feature...

Hybrid Zones

Hybrid Zones   Reference library

Encyclopedia of Evolution

Reference type:
Subject Reference
Current Version:
2005
Subject:
Science and technology, Life Sciences, Social sciences, Anthropology
Length:
3,039 words
Illustration(s):
2

...The zone is fairly narrow but varies in width from place to place. The cline width, in which a character changes gradually from pure parallelus to pure erythropus , varies among characters (e.g., that for an X-chromosome C-band is <2km, while that for song syllable length is >20km). Most characters have broadly coincident clines, but a few are displaced from the center (Figure 2). These details vary among transects in different places across the Pyrenees, which reflects the complex interaction of many genes hybridizing over many generations in different...

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