快快！！！進化論

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演化論也稱進化論，是用來解釋生物演化的理論，而演化是指生物在世代與世代之間具有變異的現象。從古希臘時期直到19世紀的這段時間，曾經出現一些零星的思想，認為一個物種可能是從其他物種演變而來，而不是從地球誕生以來就是今日的樣貌。

拉馬克的理論曾經在19世紀到20世紀初的西歐相當流行，並影響了後來包括達爾文在內的許多學者。而1850年代出現的達爾文主義，雖然能夠提出演化的機制與證據，但由於當時孟德爾遺傳學仍受到忽略，使演化理論遇到瓶頸。因此直到1930年代的現代綜合理論（新達爾文主義）建立之後，拉馬克主義才逐漸衰微。不過蘇聯由於政治人物的喜好，使得他們沿用拉馬克主義直到1960年代。

生物學上的驟變（saltation）是指生物相鄰的兩個世代之間，具有顯著差異。驟變說（saltationism）則認為生物的變異，是「非偶然」且「非漸進」的，甚至只需要一個步驟便能形成新物種。這種觀念與後來的新達爾文主義相違背，在目前是屬於非主流的演化思想。

史蒂芬·古爾德與艾崔奇提出的疾變平衡（punctuated equilibrium），經常受人誤解為一種驟變理論。雖然這個理論認為，物種形成的速度可能比原來所設想的更不平均，可能在某些時期相對更快，但是這是以地質時間而言，也就是仍然需要數十萬年（而不是動輒百萬年以上）。因此並不屬於驟變說。

中性理論全稱為分子演化的中性理論。是日本遺傳學家木村資生所提出。這個理論認為在分子遺傳學的層次上，基因的變化大多數是中性突變。由於中性突變對生物個體既沒有好處也沒有壞處，因此並不受自然選擇影響。現今的演化生物學家認為，自然選擇理論與中性理論是能夠並立且互補。

In biology, evolution is the process of change with time in the inherited characteristics, or traits, of a population of organisms. Heritable traits are encoded in the genetic material of an organism (usually DNA). Evolution results from changes in this genetic material (mutation) and the subsequent spread of these changes in the population, and explains the observed changes over time in the fossil record.

Natural selection, one of the processes that drives evolution, results from improved reproductive success by individuals best adapted to survive and reproduce in a given environment. These successful survivors and reproducers pass their beneficial, heritable traits to the next generation. If these traits increase the evolutionary fitness of an organism, they will be more likely to survive and reproduce than other organisms in the population. In doing so, they pass more copies of those (heritable) traits on to the next generation, causing advantageous traits to become more common in each generation; the corresponding decrease in fitness for deleterious traits results in their becoming rarer.In time, this results in adaptation: the gradual accumulation of new beneficial traits and the preservation of existing ones results in a population of organisms becoming better suited to its environment and ecological niche.

Though natural selection is decidedly non-random in its manner of action, other more capricious forces have a strong hand in the process of evolution. Genetic drift results in heritable traits becoming more or less common simply due to random chance. This aimless process may overwhelm the effects of natural selection in certain situations (especially in small populations).

Differences in environment, and the element of chance in what mutations happen to arise and which ones survive, can cause different populations (or parts of populations) to develop in divergent directions. With enough divergence, two populations can become sufficiently distinct that they may be considered separate species, in particular if the capacity for interbreeding between the two populations is lost. Evidence such as the wide distribution of the canonical genetic code indicates that all known cellular organisms are ultimately descended from a common ancestral population.

While the idea of evolution (as opposed to the fixity of species) is ancient, the modern concept of evolution by natural selection was first set out by Alfred Wallace and Charles Darwin in a joint paper to the Linnean Society, followed by the publication of Darwin's 1859 book, On the Origin of Species. In the 1930s, the modern evolutionary synthesis combined Darwin's natural selection with Gregor Mendel's genetics. As more and more evidence was collected and understanding of the processes of evolution improved, evolution became the central organising principle of biology.

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