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Closing remarks What is Universal Common Descent? All existing species originated gradually by biological, reproductive processes on a geological timescale. Modern organisms are the genetic descendants of one ancient, original species broadly defined as a communal population of organisms exchanging genetic material.
Genetical "gradualness", a much misunderstood term, is a mode of biological change that is dependent on population phenomena; it is not a statement about the rate or tempo of evolution.
Truly genetically gradual events are changes within the range of biological variation expected between two consecutive generations. Morphological change may appear fast, geologically speaking, yet still be genetically gradual Darwinpp. Though gradualness is not a mechanism of evolutionary change, it imposes severe constraints on possible macroevolutionary events.
Likewise, the requirement of gradualness necessarily restricts the possible mechanisms of common descent and adaptation, briefly discussed below.
Common Descent Can Be Tested Independently of Mechanistic Theories In this essay, universal common descent alone is specifically considered and weighed against the scientific evidence.
In general, separate "microevolutionary" theories are left unaddressed. Microevolutionary theories are gradualistic explanatory mechanisms that biologists use to account for the origin and evolution of macroevolutionary adaptations and variation.
These mechanisms include such concepts as natural selection, genetic driftsexual selection, neutral evolution, and theories of speciation. The fundamentals of genetics, developmental biology, molecular biology, biochemistry, and geology are assumed to be fundamentally correct—especially those that do not directly purport to explain adaptation.
However, whether microevolutionary theories are sufficient to account for macroevolutionary adaptations is a question that is left open.
Therefore, the evidence for common descent discussed here is independent of specific gradualistic explanatory mechanisms. None of the dozens of predictions directly address how macroevolution has occurred, how fins were able to develop into limbs, how the leopard got its spots, or how the vertebrate eye evolved.
None of the evidence recounted here assumes that natural selection is valid. None of the evidence assumes that natural selection is sufficient for generating adaptations or the differences between species and other taxa.
Because of this evidentiary independence, the validity of the macroevolutionary conclusion does not depend on whether natural selection, or the inheritance of acquired characaters, or a force vitale, or something else is the true mechanism of adaptive evolutionary change.
The scientific case for common descent stands, regardless. Furthermore, because it is not part of evolutionary theory, abiogenesis also is not considered in this discussion of macroevolution: In evolutionary theory it is taken as axiomatic that an original self-replicating life form existed in the distant past, regardless of its origin.
All scientific theories have their respective, specific explanatory domains; no scientific theory proposes to explain everything.
Quantum mechanics does not explain the ultimate origin of particles and energy, even though nothing in that theory could work without particles and energy. Neither Newton's theory of universal gravitation nor the general theory of relativity attempt to explain the origin of matter or gravity, even though both theories would be meaningless without the a priori existence of gravity and matter.
Similarly, universal common descent is restricted to the biological patterns found in the Earth's biota; it does not attempt to explain the ultimate origin of life.
Scientific theories are validated by empirical testing against physical observations. Theories are not judged simply by their logical compatibility with the available data. Independent empirical testability is the hallmark of science—in science, an explanation must not only be compatible with the observed data, it must also be testable.The Online Writing Lab (OWL) at Purdue University houses writing resources and instructional material, and we provide these as a free service of the Writing Lab at Purdue.
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Introduction. volution, the overarching concept that unifies the biological sciences, in fact embraces a plurality of theories and hypotheses. In evolutionary debates one is apt to hear evolution roughly parceled between the terms "microevolution" and "macroevolution". In Duchesne’s Upper School, your daughter expands her worldview, leads teams and groups, and grows in sophistication. The girls discover and refine their own unique voices as key contributors, authors, inventors, international travelers, and tireless volunteers. Comparative essay writing: creating a good introductory paragraph. Writing a comparative essay can be challenging since you are making significant comparisons using two related subjects. The essay itself is somewhat demanding since you have to detail each subject, then mention what is similar and different about each.
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