Evolution
In 1953, James Watson and Francis Crick described the double helix structure of DNA. This molecule encodes genetic information in a sequence of four bases: adenine, thymine, cytosine, and guanine. Before a cell divides, this long biopolymer is copied so that each resulting cell inherits the same sequence. Portions of the DNA molecule that specify a single functional unit are called genes. Different genes have different sequences of bases. Within cells, each long strand of DNA is called a chromosome. The specific location of a DNA sequence within a chromosome is known as a locus. If the DNA sequence at a locus varies between individuals, the different forms of this sequence are called alleles. DNA sequences can change through mutations, producing new alleles. When mutations occur, they may alter the product of a gene or prevent it from functioning. About half of the mutations in coding regions of protein-coding genes are deleterious. The other half are neutral. A small percentage confer a fitness benefit.
Natural selection acts on traits that enhance survival and reproduction. It embodies three principles: variation exists within populations, different traits confer different rates of survival, and these traits can be passed from generation to generation. More offspring are produced than can possibly survive, creating competition for resources. Organisms with advantageous traits are more likely to pass them on. For example, if an allele increases fitness, it has a higher probability of becoming common within the population. Conversely, less beneficial alleles become rarer. Genetic drift causes random fluctuations in allele frequencies. This process halts when an allele becomes fixed by disappearing or replacing others entirely. Even without selective forces, genetic drift can cause two separate populations to diverge into distinct groups. In 1967, researchers identified a GC-biased E. coli mutator strain. This discovery established the plausibility of mutational explanations for molecular patterns. Sexual reproduction increases genetic variation by shuffling genes through independent assortment and homologous recombination. John Maynard Smith described the two-fold cost of sex as a major evolutionary puzzle. Despite this cost, sexual reproduction remains the most common means among eukaryotes and multicellular organisms.
Microbial mat fossils have been found in 3.48 billion-year-old sandstone in Western Australia. Other early physical evidence includes graphite in 3.7 billion-year-old metasedimentary rocks discovered in Western Greenland. Scientists reported identifying 355 genes from the last universal common ancestor (LUCA) in July 2016. Prokaryotes inhabited Earth from approximately 3 to 4 billion years ago. No obvious changes in morphology occurred over the next few billion years. Eukaryotic cells emerged between 1.6 and 2.7 billion years ago. Bacteria were engulfed by eukaryotic cells in a cooperative association called endosymbiosis. The engulfed bacteria evolved into mitochondria or hydrogenosomes. Another engulfment of cyanobacterial-like organisms led to chloroplasts in algae and plants. Multicellular organisms began to appear around 1.7 billion years ago. About 538.8 million years ago, a remarkable amount of biological diversity appeared over 10 million years during the Cambrian explosion. Amphibians first appeared around 364 million years ago. Mammals emerged around 129 million years ago. Modern humans appeared around 250,000 years ago.
The Cretaceous, Paleogene extinction event caused non-avian dinosaurs to die out at the end of the Cretaceous period. The earlier Permian, Triassic extinction event was even more severe, driving approximately 96% of all marine species to extinction. Nearly all animal and plant species that have lived on Earth are now extinct. Present-day extinction rates are 100 to 1000 times greater than the background rate. Up to 30% of current species may be extinct by the mid 21st century. Speciation is the process where a species diverges into two or more descendant species. Allopatric speciation occurs when populations become geographically isolated. Peripatric speciation happens when small populations become isolated in new environments. Parapatric speciation results from evolution reducing gene flow between populations without physical separation. Sympatric speciation allows divergence without geographic isolation. In January 2016, scientists reported that a minor genetic change in GK-PID allowed organisms to transition from single cells to multicellular forms. About 800 million years ago, this shift occurred. The grass Anthoxanthum odoratum undergoes parapatric speciation in response to localized metal pollution.
Jean-Baptiste Lamarck published his transmutation theory in 1809. This scheme envisaged spontaneous generation producing simple life forms that developed complexity. Georges Cuvier insisted species were unrelated and fixed. Charles Darwin developed his theory of natural selection from 1838 onwards. Alfred Russel Wallace sent him a version of virtually the same theory in 1858. Their separate papers were presented together at an 1858 meeting of the Linnean Society of London. At the end of 1859, Darwin's publication On the Origin of Species explained natural selection in detail. Thomas Henry Huxley applied Darwin's ideas to humans using palaeontology and comparative anatomy. Othniel C. Marsh delivered a very influential speech before the annual meeting of the American Association for the Advancement of Science in 1877. He traced the evolution of vertebrates from fish all the way through humans. In 1865, Gregor Mendel reported that traits were inherited in a predictable manner. August Weismann made the important distinction between germ cells and somatic cells. The modern evolutionary synthesis combined evolution with Mendelian inheritance and population genetics in the early 20th century.
The Mexican tetra is an albino cavefish that lost its eyesight during evolution. Breeding different populations produced offspring with functional eyes. This helped identify genes required for vision and pigmentation. Viruses, bacteria, fungi, and cancers evolve to be resistant to host immune defences and pharmaceutical drugs. Many human diseases are not static phenomena but capable of evolution. In agriculture, pesticide and herbicide resistance pose similar problems. It is possible we are facing the end of the effective life of most available antibiotics. Simulations of evolution using evolutionary algorithms started in the 1960s. Ingo Rechenberg used evolution strategies to solve complex engineering problems. John Henry Holland popularized genetic algorithms. Evolutionary algorithms now solve multi-dimensional problems more efficiently than software designed by humans. They optimize system design and automatically evolve computer programs. Antibiotic resistance spreads when one bacterium acquires resistance genes and transfers them rapidly to other species. Horizontal gene transfer occurs from bacteria to eukaryotes like the yeast Saccharomyces cerevisiae and the adzuki bean weevil Callosobruchus chinensis.
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Common questions
When did James Watson and Francis Crick describe the double helix structure of DNA?
James Watson and Francis Crick described the double helix structure of DNA in 1953. This molecule encodes genetic information in a sequence of four bases: adenine, thymine, cytosine, and guanine.
What year were scientists reported identifying 355 genes from the last universal common ancestor LUCA?
Scientists reported identifying 355 genes from the last universal common ancestor LUCA in July 2016. Prokaryotes inhabited Earth from approximately 3 to 4 billion years ago before eukaryotic cells emerged between 1.6 and 2.7 billion years ago.
How many percent of all marine species went extinct during the Permian Triassic extinction event?
The earlier Permian Triassic extinction event drove approximately 96% of all marine species to extinction. Present-day extinction rates are 100 to 1000 times greater than the background rate with up to 30% of current species potentially extinct by the mid 21st century.
When did Charles Darwin develop his theory of natural selection and publish On the Origin of Species?
Charles Darwin developed his theory of natural selection from 1838 onwards and published On the Origin of Species at the end of 1859. Their separate papers were presented together at an 1858 meeting of the Linnean Society of London alongside Alfred Russel Wallace.
What year did the Mexican tetra cavefish lose its eyesight during evolution?
The Mexican tetra is an albino cavefish that lost its eyesight during evolution but no specific year for this loss is provided in the text. Breeding different populations produced offspring with functional eyes which helped identify genes required for vision and pigmentation.