Life And Time One of the first ways in which we learn to classify objects is into two groups: In casual encounters with the material universe, we rarely feel any difficulty here, since we usually deal with things that are clearly alive, such as a dog or a rattlesnake; or with things that are clearly nonalive, such as a brick or a typewriter.
Favourable variations are ones that increase chances for survival and procreation. Those advantageous variations are preserved and multiplied from generation to generation at the expense of less-advantageous ones.
This is the process known as natural selection. The outcome of the Vestigial drosophila crossed with white eyes is an organism that is well adapted to its environmentand evolution often occurs as a consequence.
Natural selection, then, can be defined as the differential reproduction of alternative hereditary variants, determined by the fact that some variants increase the likelihood that the organisms having them will survive and reproduce more successfully than will organisms carrying alternative variants.
Selection may occur as a result of differences in survival, in fertility, in rate of development, in mating success, or in any other aspect of the life cycle. All of these differences can be incorporated under the term differential reproduction because all result in natural selection to the extent that they affect the number of progeny an organism leaves.
Darwin maintained that competition for limited resources results in the survival of the most-effective competitors.
Nevertheless, natural selection may occur not only as a result of competition but also as a result of some aspect of the physical environmentsuch as inclement weather. Moreover, natural selection would occur even if all the members of a population died at the same age, simply because some of them would have produced more offspring than others.
Natural selection is quantified by a measure called Darwinian fitness or relative fitness. Fitness in this sense is the relative probability that a hereditary characteristic will be reproduced; that is, the degree of fitness is a measure of the reproductive efficiency of the characteristic.
Biological evolution is the process of change and diversification of living things over time, and it affects all aspects of their lives— morphology form and structurephysiologybehaviour, and ecology.
Underlying these changes are changes in the hereditary materials.
Evolution can be seen as a two-step process. First, hereditary variation takes place; second, selection is made of those genetic variants that will be passed on most effectively to the following generations. Hereditary variation also entails two mechanisms—the spontaneous mutation of one variant into another and the sexual process that recombines those variants see recombination to form a multitude of variations.
The variants that arise by mutation or recombination are not transmitted equally from one generation to another. Some may appear more frequently because they are favourable to the organism; the frequency of others may be determined by accidents of chance, called genetic drift.
The gene pool The gene pool is the sum total of all the genes and combinations of genes that occur in a population of organisms of the same species. It can be described by citing the frequencies of the alternative genetic constitutions. Consider, for example, a particular gene which geneticists call a locussuch as the one determining the MN blood group s in humans.
One form of the gene codes for the M blood group, while the other form codes for the N blood group; different forms of the same gene are called allele s.
The MN gene pool of a particular population is specified by giving the frequencies of the alleles M and N. Thus, in the United States the M allele occurs in people of European descent with a frequency of 0. In other populations these frequencies are different; for instance, the frequency of the M allele is 0.
The necessity of hereditary variation for evolutionary change to occur can be understood in terms of the gene pool. Assume, for instance, a population in which there is no variation at the gene locus that codes for the MN blood groups; only the M allele exists in all individuals.
Evolution of the MN blood groups cannot take place in such a population, since the allelic frequencies have no opportunity to change from generation to generation. On the other hand, in populations in which both alleles M and N are present, evolutionary change is possible. Genetic variation and rate of evolution The more genetic variation that exists in a population, the greater the opportunity for evolution to occur.
As the number of gene loci that are variable increases and as the number of alleles at each locus becomes greater, the likelihood grows that some alleles will change in frequency at the expense of their alternates. The British geneticist R. Fisher mathematically demonstrated a direct correlation between the amount of genetic variation in a population and the rate of evolutionary change by natural selection.Devil In The Dark () The Horta was an example of Silicon life.; Now we are really sailing off into terra incognito.
"Here be dragons" and all that. But if you have starships, you almost have to have aliens (Isaac Asimov's Foundation trilogy being the most notable exception).The "science" is called Astrobiology, the famous "science in search of a subject".
Evidence of common descent of living organisms has been discovered by scientists researching in a variety of disciplines over many decades, demonstrating that all life on Earth comes from a single timberdesignmag.com forms an important part of the evidence on which evolutionary theory rests, demonstrates that evolution does occur, and illustrates the processes that created Earth's biodiversity.
Evolution - The science of evolution: The central argument of Darwin’s theory of evolution starts with the existence of hereditary variation. Experience with animal and plant breeding had demonstrated to Darwin that variations can be developed that are “useful to man.” So, he reasoned, variations must occur in nature that are favourable .
A look at a large number of observed speciation events. Not only does this article examine in detail a number of speciation events, but it also presents a brief history of the topic of speciation.
A Definition of Life. The ubiquity of life In earlier chapters we considered the astronomical environment which extraterrestrial lifeforms must cope with. Other galaxies, stars, and countless planets appear amenable, if not perfectly hospitable, to life.
The study of the origin of life, called "abiogenesis" by many researchers in the field, is highly relevant to xenology and xenologists. By determining the conditions that existed on the primitive Earth, and by duplicating them in the laboratory, scientists can attempt to recreate events that must have occurred on this planet billions of years ago.