Evolution is the change in the inherited traits of a population of organisms through successive generations. After a population splits into smaller groups, these groups evolve independently and may eventually diversify into new species. Ultimately, life is descended from a common ancestry through a long series of these speciation events, stretching back in a tree of life that has grown over the 3,500 million years of life on Earth. This is visible in anatomical, genetic and other likenesses between groups of organisms, geographical distribution of related species, the fossil record and the recorded genetic changes in living organisms over many generations. To distinguish from other uses of the word evolution, it is sometimes termed biological evolution, genetic evolution or organic evolution.Evolution is the product of two opposing forces: processes that constantly introduce variation in traits, and processes that make particular variants become more common or rare. A trait is a particular characteristic, such as eye color, height, or a behavior, that is expressed when an organism's genes interact with its environment. Genes vary within populations, so organisms show heritable differences (variation) in their traits. The main cause of variation is mutation, which changes the sequence of a gene. Altered genes, or alleles, are then inherited by offspring. There can sometimes also be transfer of genes between species.Two main processes cause variants to become more common or rare in a population. One is natural selection, which causes traits that aid survival and reproduction to become more common, and traits that hinder survival and reproduction to become more rare.Natural selection occurs because only a few individuals in each generation will survive, since resources are limited and organisms produce many more offspring than their environment can support. Over many generations, mutations produce successive, small, random changes in traits, which are then filtered by natural selection and the beneficial changes retained. This adjusts traits so they become suited to an organism's environment: these adjustments are called adaptations.Not every trait, however, is an adaptation. Another cause of evolution is genetic drift, which produces entirely random changes in how common traits are in a population. Genetic drift comes from the role that chance plays in whether a trait will be passed on to the next generation. Mechanisms:
The two main mechanisms that produce evolution are natural selection and genetic drift. Natural selection is the process which favors genes that aid survival and reproduction. Genetic drift is the random change in the frequency of alleles, caused by the random sampling of a generation's genes during reproduction. The relative importance of natural selection and genetic drift in a population varies depending on the strength of the selection and the effective population size, which is the number of individuals capable of breeding.Natural selection usually predominates in large populations, whereas genetic drift dominates in small populations. The dominance of genetic drift in small populations can even lead to the fixation of slightly deleterious mutations.As a result, changing population size can dramatically influence the course of evolution. Population bottlenecks, where the population shrinks temporarily and therefore loses genetic variation, result in a more. Evolution of life:
For more details on this topic, see time of evolution.Evolutionary tree showing the divergence of modern species from their common ancestor in the center.The three domains are colored, with bacteria blue, archaea green, and eukaryotes red.Despite the uncertainty on how life began, it is generally accepted that prokaryotes inhabited the Earth from approximately 3–4 billion years ago.No obvious changes in morphology or cellular organization occurred in these organisms over the next few billion years.The eukaryotes were the next major change in cell structure. These came from ancient bacteria being engulfed by the ancestors of eukaryotic cells, in a cooperative association called endosymbiosis.The engulfed bacteria and the host cell then underwent co-evolution, with the bacteria evolving into either mitochondria or hydrogenosomes.An independent second engulfment cyanobacterial-like organisms led to the formation of chloroplasts in algae and plants.It is unknown when the first eukaryotic cells appeared though they first emerged between 1.6 – 2.7 billion years ago.The history of life was that of the unicellular eukaryotes, prokaryotes, and archaea until about 610 million years ago when multicellular organisms began to appear in the oceans in the Ediacaran period.The evolution of multicellularity occurred in multiple independent events, in organisms as diverse as sponges, brown algae, cyanobacteria, slime moulds and myxobacteria.Soon after the emergence of these first multicellular organisms, a remarkable amount of biological diversity appeared over approximately 10 million years, in an event called the Cambrian explosion. Here, the majority of types of modern animals appeared in the fossil record, as well as unique lineages that subsequently became extinct.Various triggers for the Cambrian explosion have been proposed, including the accumulation of oxygen in the atmosphere from photosynthesis.About 500 million years ago, plants and fungi colonized the land, and were soon followed by arthropods and other animals.Insects were particularly successful and even today make up the majority of animal species.Amphibians first appeared around 300 million years ago, followed by early amniotes, then mammals around 200 million years ago and birds around 100 million years ago (both from "reptile"-like lineages). However, despite the evolution of these large animals, smaller organisms similar to the types that evolved early in this process continue to be highly successful and dominate the Earth, with the majority of both biomass and species being prokaryotes. THE ORIGIN OF LIFE: The origin of life is a necessary precursor for biological evolution, but understanding that evolution occurred once organisms appeared and investigating how this happens does not depend on understanding exactly how life began. The current scientific consious is that the complex biochemistry that makes up life came from simpler chemical reactions, but it is unclear how this occurred.Not much is certain about the earliest developments in life, the structure of the first living things, or the identity and nature of any ancestral gene pool.Consequently, there is no scientific consensus on how life began, but proposals include self-replicating molecules such as RNA,and the assembly of simple cells.
The two main mechanisms that produce evolution are natural selection and genetic drift. Natural selection is the process which favors genes that aid survival and reproduction. Genetic drift is the random change in the frequency of alleles, caused by the random sampling of a generation's genes during reproduction. The relative importance of natural selection and genetic drift in a population varies depending on the strength of the selection and the effective population size, which is the number of individuals capable of breeding.Natural selection usually predominates in large populations, whereas genetic drift dominates in small populations. The dominance of genetic drift in small populations can even lead to the fixation of slightly deleterious mutations.As a result, changing population size can dramatically influence the course of evolution. Population bottlenecks, where the population shrinks temporarily and therefore loses genetic variation, result in a more. Evolution of life:
For more details on this topic, see time of evolution.Evolutionary tree showing the divergence of modern species from their common ancestor in the center.The three domains are colored, with bacteria blue, archaea green, and eukaryotes red.Despite the uncertainty on how life began, it is generally accepted that prokaryotes inhabited the Earth from approximately 3–4 billion years ago.No obvious changes in morphology or cellular organization occurred in these organisms over the next few billion years.The eukaryotes were the next major change in cell structure. These came from ancient bacteria being engulfed by the ancestors of eukaryotic cells, in a cooperative association called endosymbiosis.The engulfed bacteria and the host cell then underwent co-evolution, with the bacteria evolving into either mitochondria or hydrogenosomes.An independent second engulfment cyanobacterial-like organisms led to the formation of chloroplasts in algae and plants.It is unknown when the first eukaryotic cells appeared though they first emerged between 1.6 – 2.7 billion years ago.The history of life was that of the unicellular eukaryotes, prokaryotes, and archaea until about 610 million years ago when multicellular organisms began to appear in the oceans in the Ediacaran period.The evolution of multicellularity occurred in multiple independent events, in organisms as diverse as sponges, brown algae, cyanobacteria, slime moulds and myxobacteria.Soon after the emergence of these first multicellular organisms, a remarkable amount of biological diversity appeared over approximately 10 million years, in an event called the Cambrian explosion. Here, the majority of types of modern animals appeared in the fossil record, as well as unique lineages that subsequently became extinct.Various triggers for the Cambrian explosion have been proposed, including the accumulation of oxygen in the atmosphere from photosynthesis.About 500 million years ago, plants and fungi colonized the land, and were soon followed by arthropods and other animals.Insects were particularly successful and even today make up the majority of animal species.Amphibians first appeared around 300 million years ago, followed by early amniotes, then mammals around 200 million years ago and birds around 100 million years ago (both from "reptile"-like lineages). However, despite the evolution of these large animals, smaller organisms similar to the types that evolved early in this process continue to be highly successful and dominate the Earth, with the majority of both biomass and species being prokaryotes. THE ORIGIN OF LIFE: The origin of life is a necessary precursor for biological evolution, but understanding that evolution occurred once organisms appeared and investigating how this happens does not depend on understanding exactly how life began. The current scientific consious is that the complex biochemistry that makes up life came from simpler chemical reactions, but it is unclear how this occurred.Not much is certain about the earliest developments in life, the structure of the first living things, or the identity and nature of any ancestral gene pool.Consequently, there is no scientific consensus on how life began, but proposals include self-replicating molecules such as RNA,and the assembly of simple cells.
