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A homologous trait is any characteristic of organisms that is derived from a common ancestor. This is contrasted to analogous traits: similarities between organisms that were not in the last common ancestor of the taxa being considered but rather evolved separately. As defined by Owen (1843), a homology is a "structural correspondence", whereas an analogy is a "non-correspondent similarity".
Whether or not a trait is homologous depends on both the taxonomic and anatomical level at which the trait is examined. For example, the bird and bat wing are homologous as forearms in tetrapods. However, they are not homologous as wings, because the organ served as a forearm (not a wing) in the last common ancestor of tetrapods. By definition, any homologous trait defines a clade—a monophyletictaxon in which all the members have the trait (or have lost it secondarily); and all non-members lack it.
A homologous trait may be homoplasious – that is, it has evolved independently, but from the same ancestral structure – plesiomorphic – that is, present in a common ancestor but secondarily lost in some of its descendants – or (syn)apomorphic – present in an ancestor and all of its descendants.
A homologous trait is often called a homolog (also spelled homologue). In genetics, the term "homolog" is used both to refer to a homologous protein, and to the gene (DNA sequence) encoding it.
Homology of structures
Shared ancestry can be evolutionary or developmental. Evolutionary ancestry means that structures evolved from some structure in a common ancestor; for example, the wings of bats and the arms of primates are homologous in this sense. Developmental ancestry means that structures arose from the same tissue in embryonal development; the ovaries of female humans and the testicles of male humans are homologous in this sense.
Homology is different from analogy, which describes the relation between characters that are apparently similar yet phylogenetically independent. The wings of a maple seed and the wings of an albatross are analogous but not homologous (they both allow the organism to travel on the wind, but they didn't both develop from the same structure). Analogy is commonly also referred to as homoplasy, which is further distinguished into parallelism, reversal, and convergence.
From the point of view of evolutionary developmental biology (evo-devo) where evolution is seen as the evolution of the development of organisms, Rolf Sattler emphasized that homology can also be partial. New structures can evolve through the combination of developmental pathways or parts of them. As a result hybrid or mosaic structures can evolve that exhibit partial homologies. For example, certain compound leaves of flowering plants are partially homologous both to leaves and shoots because they combine some traits of leaves and shoots.
Homology of sequences in genetics
Homology among proteins and DNA is often concluded on the basis of sequence similarity, especially in bioinformatics. For example, in general, if two or more genes have highly similar DNA sequences, it is likely that they are homologous. But sequence similarity may also arise without common ancestry: short sequences may be similar by chance, and sequences may be similar because both were selected to bind to a particular protein, such as a transcription factor. Such sequences are similar but not homologous. Sequence regions that are homologous are also called conserved. This is not to be confused with conservation in amino acid sequences in which the amino acid at a specific position has been substituted with a different one with functionally equivalent physicochemical properties.
The phrase "percent homology" is sometimes used but is incorrect. "Percent identity" or "percent similarity" should be used to quantify the similarity between the biomolecule sequences. For two naturally occurring sequences, percent identity is a factual measurement, whereas homology is a hypothesis supported by evidence. One can, however, refer to partial homology where a fraction of the sequences compared (are presumed to) share descent, while the rest does not. For example, partial homology may result from a gene fusion event.
Many algorithms exist to cluster protein sequences into sequence families, which are sets of mutually homologous sequences. (See sequence clustering and sequence alignment.) Some specialized biological databases collect homologous sequences in animal genomes: HOVERGEN, HOMOLENS, HOGENOM.
Homologous sequences are of two types: orthologous and paralogous.
Homologous sequences are orthologous if they were separated by a speciation event: when a species diverges into two separate species, the divergent copies of a single gene in the resulting species are said to be orthologous. Orthologs, or orthologous genes, are genes in different species that are similar to each other because they originated from a common ancestor. The term "ortholog" was coined in 1970 by Walter Fitch.
The strongest evidence that two similar genes are orthologous is the result of a phylogenetic analysis of the gene lineage. Genes that are found within one clade are orthologs, descended from a common ancestor. Orthologs often, but not always, have the same function.
Orthologous sequences provide useful in
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Answers:1. water adheres to the skin of animals that sweat. The water, when it evaporates, lowers the temperature of the skin. This is known as evaporative cooling. It is why some animals sweat: to cool themselves. If water does not adhere to the skin, then it is pointless to sweat 2. Ice floats because it is lighter as a solid than a liquid. It is lighter because the ice crystals takes up more space than the water from which it forms. Since water makes up a large part of cells and organisms, this emergent property of water can be dangerous. Ice inside a cell will expand as it freezes, the ice can therefore make cells larger, possibly rupturing cell membranes and killing the animals. Therefore, most animals must do everything possible to prevent body temperatures from dropping below freezing. There are animals with anti-freeze compounds in their bloods to prevent freezing. There is one species of frog, the wood frog, which can survive freezing. This is so unusual that scientists and the public are marveling at its unique ability. 3. High specific gravity? That makes liquid water pretty heavy, so it can exist at ground level instead of floating on top of the atmosphere for example. When water vapor condenses, it is heavy enough so that it will fall. Rain creates fresh water, which many plants and animals rely on to live. 4. Most things dissolve in water, making many biochemical reactions easier to proceed within cells, by lowering the amount of energy needed to complete the reactions. This is important because high temperatures will cook the protein within us. By lowering the energy needed for reactions, we can stay cool and alive.
Answers:Any volume of water will increase by 9% when freezing. If you emergency water is likely to freeze then you can leave an expansion gap which might let your vessel survive the contents freezing, but you have to watch it. Freezing inside vessels usually starts from the edges and top which can negate the purpose of the expansion gap - the top forms a lid of ice and then as the water below freezes and expands, the expansion force can be directed at the vessels sides ( instead of into the expansion gap.) with resulting mess and expense. A cone shaped vessel will work, with the point of the cone at the base, the sloping sides directing the expansion upwards. We are encouraged to store emergency drinking water (to survive Earthquake damage) - we just use a lot of plastic drinks bottles and refill them every six months - thats 30 x 3 litre bottles. We keep them out of the sun, in the shed, not expecting freezing to be a problem ( we are close to the sea). I would think that polystyrene packing would be your answer. Leave an expansion gap,(just 90cc per litre) and place the container in a womb of insulating boards. Good luck - lets just both hope we never have to use our emergency water.
Answers:Yow. These are all complex questions, and *each one* would take a long description. But it's worth noting that *once life has started* ... all of them increase in complexity as a result of the process of evolution by natural selection. So your question really applies to the origins of the structures sufficient to sustain life ... i.e. the structures needed to support natural selection. The two KEY ones are #2 (Reproduction ... or more correctly, replication with inheritance) and #4 (Energy Utilization ... or more correctly, metabolism). I highly recommend the cover-story in the current (June '07) issue of Scientific American, which specifically addresses how, and in what order, replication and metabolism first arose in the earliest pre-life structures. All the others emerge as a consequence of these two (#2 and #4). Or more correctly, #2 and 4 lead to #1, 2, 5, and 6, which together are enough to support #7, which itself supports increases in 1-6. (I said this is complex.) As to how the others emerge as a consequence of #2 and #4: #1 Order is maintained (homeostasis) and increased through the expenditure of energy. E.g. the sodium-potassium pump in cellular membranes is just a structure that converts energy into maintaining homeostasis within the membrane, which is the same as maintaining the thermodynamic imbalance (order) between the inside and outside of the membrane. Order is increased through the process of growth and development (see #3). #3, Growth and Development is both energy expenditure and the execution of the structure of inheritance. E.g., specific sequences of nucleotides determine not only the order of amino acids of proteins, but how much of each protein to make, the order in which to make them, the assembly of these proteins into structures (membranes -> organelles -> cells and the cells -> tissues -> structures -> organs -> organism). All traceable to hierachies of instructions in the inheritance molecule (DNA or RNA) ... instructions whose job it is to turn other instructions on or off. #5, Response to Environment is precisely a conversion of energy. E.g., photopigments in the eye are simply proteins that change shape when they absorb photons of certain wavelengths, which begins a chain reaction of energy-driven events that eventually leads to muscles moving. #6, Homeostasis is just maintained order (see #1), so again, it is also maintained through expediture of energy. #7, Evolutionary Adaptation is explained by mutation + natural selection, which itself is explained by replication (but not completely error-free replication) + energy utilization occurring in environments with limited energy resources. I.e. competition for those energy resources means that those structures more efficient at getting those resources will, through inheritance, produce more structures just like them (but occasionally not exactly like them).
Answers:oki ~it exsists in 3 states ~boils at 100celcius ~freezes at 0 celcius ~state changes ~Adhesion : attracted to other water molecules ~Cohesion : can also be attracted to other materials ~surface tension these are jst general ones