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Sex and recombination
In asexual organisms, genes are inherited together, or linked, as they cannot mix with genes of other organisms during reproduction. In contrast, the offspring of sexual organisms... View MoreSex and recombination
In asexual organisms, genes are inherited together, or linked, as they cannot mix with genes of other organisms during reproduction. In contrast, the offspring of sexual organisms contain random mixtures of their parents' chromosomes that are produced through independent assortment. In a related process called homologous recombination, sexual organisms exchange DNA between two matching chromosomes. Recombination and reassortment do not alter allele frequencies, but instead change which alleles are associated with each other, producing offspring with new combinations of alleles. Sex usually increases genetic variation and may increase the rate of evolution.
The two-fold cost of sex was first described by John Maynard Smith. The first cost is that in sexually dimorphic species only one of the two sexes can bear young. This cost does not apply to hermaphroditic species, like most plants and many invertebrates. The second cost is that any individual who reproduces sexually can only pass on 50% of its genes to any individual offspring, with even less passed on as each new generation passes. Yet sexual reproduction is the more common means of reproduction among eukaryotes and multicellular organisms. The Red Queen hypothesis has been used to explain the significance of sexual reproduction as a means to enable continual evolution and adaptation in response to coevolution with other species in an ever-changing environment. Another hypothesis is that sexual reproduction is primarily an adaptation for promoting accurate recombinational repair of damage in germline DNA, and that increased diversity is a byproduct of this process that may sometimes be adaptively beneficial.
Mutation
Mutations are changes in the DNA sequence of a cell's genome and are the ultimate source of genetic variation in all organisms. When mutations occur, they may alter the product of a gene, or p... View MoreMutation
Mutations are changes in the DNA sequence of a cell's genome and are the ultimate source of genetic variation in all organisms. When mutations occur, they may alter the product of a gene, or prevent the gene from functioning, or have no effect. Based on studies in the fly Drosophila melanogaster, it has been suggested that if a mutation changes a protein produced by a gene, this will probably be harmful, with about 70% of these mutations having damaging effects, and the remainder being either neutral or weakly beneficial.
Mutations can involve large sections of a chromosome becoming duplicated (usually by genetic recombination), which can introduce extra copies of a gene into a genome. Extra copies of genes are a major source of the raw material needed for new genes to evolve. This is important because most new genes evolve within gene families from pre-existing genes that share common ancestors. For example, the human eye uses four genes to make structures that sense light: three for colour vision and one for night vision; all four are descended from a single ancestral gene.
New genes can be generated from an ancestral gene when a duplicate copy mutates and acquires a new function. This process is easier once a gene has been duplicated because it increases the redundancy of the system; one gene in the pair can acquire a new function while the other copy continues to perform its original function. Other types of mutations can even generate entirely new genes from previously noncoding DNA.
The generation of new genes can also involve small parts of several genes being duplicated, with these fragments then recombining to form new combinations with new functions. When new genes are assembled from shuffling pre-existing parts, domains act as modules with simple independent functions, which can be mixed together to produce new combinations with new and complex functions.[88] For example, polyketide synthases are large enzymes that make antibiotics; they contain up to one hundred independent domains that each catalyse one step in the overall process, like a step in an assembly line.
The geological time scale (GTS) divides and chronicles earth’s evolutionary history into various periods from the beginning to the present based on definite events that marked a major change in earth’... View MoreThe geological time scale (GTS) divides and chronicles earth’s evolutionary history into various periods from the beginning to the present based on definite events that marked a major change in earth’s physical, chemical and biological features.
Major changes in earth’s physical and biological history stretch over several millions of years and hence in GTS all the divisions are expressed in ‘million years (mya – million years ago).’
The primarily defined divisions of time are eons, the Hadean, the Archean, the Proterozoic and the Phanerozoic. The first three of these can be referred to collectively as the Precambrian supereon.
Each eon is subsequently divided into eras, which in turn are divided into periods, which are further divided into epochs.
SuperEon ==> Eon ==> Era ==> Period ==> Epoch
Hadean Eon
The Hadean eon (4,540 – 4,000 mya) represents the time before a reliable (fossil) record of life.
Temperatures are extremely hot, and much of the Earth was molten because of frequent collisions with other bodies, extreme volcanism and the abundance of short-lived radioactive elements.
A giant impact collision with a planet-sized body named Theia (approximately 4.5 billion years ago) is thought to have formed the Moon.
The moon was subjected to Late Heavy Bombardment (LHB – lunar cataclysm – 4 billion years ago).
During the LHB phase, a disproportionately large number of asteroids are theorised to have collided with the early terrestrial planets in the inner Solar System, including Mercury, Venus, Earth, and Mars.
Volcanic outgassing probably created the primordial atmosphere and then the ocean.
The early atmosphere contained almost no oxygen.
Over time, the Earth cooled, causing the formation of a solid crust, leaving behind hot volatiles which probably resulted in a heavy CO2 atmosphere with hydrogen and water vapour.
Liquid water oceans existed despite the surface temperature of 230° C because, at an atmospheric pressure of above 27 atmospheres, caused by the heavy CO2 atmosphere, water is still liquid.
Archean Eon
The beginning of life on Earth and evidence of cyanobacteria date to 3500 mya.
Life was limited to simple single-celled organisms lacking nuclei, called Prokaryota.
The atmosphere was without oxygen, and the atmospheric pressure was around 10 to 100 atmospheres.
The Earth’s crust had cooled enough to allow the formation of continents.
The oldest rock formations exposed on the surface of the Earth are Archean.
Volcanic activity was considerably higher than today, with numerous lava eruptions.
The oceans were more acidic due to dissolved carbon dioxide than during the Proterozoic.
By the end of the Archaean, plate tectonics may have been similar to that of the modern Earth.
Liquid water was prevalent, and deep oceanic basins are known to have existed
The earliest stromatolites are found in 3.48 billion-year-old sandstone discovered in Western Australia.
The earliest identifiable fossils consist of stromatolites, which are microbial mats formed in shallow water by cyanobacteria.
Proterozoic Eon
It is the last eon of the Precambrian “supereon”.
It spans for the time of appearance of oxygen in Earth’s atmosphere to just before the proliferation of complex life (such as corals) on the Earth.
Bacteria begin producing oxygen, leading to the sudden rise of life forms.
Eukaryotes (have a nucleus), emerge, including some forms of soft-bodied multicellular organisms.
Earlier forms of fungi formed around this time.
The early and late phases of this eon may have undergone Snowball Earth periods (the planet suffered below-zero temperatures, extensive glaciation and as a result drop in sea levels).
Snowball Earth: The Snowball Earth hypothesis proposes that Earth’s surface became entirely or nearly entirely frozen at least once, sometime earlier than 650 Mya (million years ago).
It was a very tectonically active period in the Earth’s history.
It featured the first definitive supercontinent cycles and modern orogeny (mountain building).
It is believed that 43% of modern continental crust was formed in the Proterozoic, 39% formed in the Archean, and only 18% in the Phanerozoic.
In the late Proterozoic (most recent), the dominant supercontinent was Rodinia (~1000–750 Ma).
It was also during the Proterozoic that the first symbiotic relationships between mitochondria (found in nearly all eukaryotes) and chloroplasts (found in plants and some protists only) and their hosts evolved.
Phanerozoic Eon
The boundary between the Proterozoic and the Phanerozoic eons was set when the first fossils of animals such as trilobites appeared.
Life remained mostly small and microscopic until about 580 million years ago, when complex multicellular life arose, developed over time, and culminated in the Cambrian Explosion about 541 million years ago.
This sudden diversification of life forms produced most of the major life forms known today.
Plant life on land appeared in the early Phanerozoic eon.
Complex life, including vertebrates, begin to dominate the Earth’s ocean.
Pangaea forms and later dissolves into Laurasia and Gondwana.
Gradually, life expands to land and all familiar forms of plants, insects, animals and fungi begin appearing.
Birds, the descendants of dinosaurs, and more recently mammals emerge.
Modern animals—including humans—evolve at the most recent phases of this eon (2 million years ago).
The Phanerozoic eon is divided into three eras:
-the Palaeozoic, an era of arthropods, amphibians, fishes, and the first life on land;
-the Mesozoic, which spanned the rise, reign of reptiles, climactic extinction of the non-avian dinosaurs, the evolution of mammals and birds; and
-the Cenozoic, which saw the rise of mammals.
The Phanerozoic is divided into three eras: the Paleozoic, Mesozoic, and Cenozoic, which are further subdivided into 12 periods.
Sources of variation
Evolution can occur if there is genetic variation within a population. Variation comes from mutations in the genome, reshuffling of genes through sexual reproduction and migration ... View MoreSources of variation
Evolution can occur if there is genetic variation within a population. Variation comes from mutations in the genome, reshuffling of genes through sexual reproduction and migration between populations (gene flow). Despite the constant introduction of new variation through mutation and gene flow, most of the genome of a species is identical in all individuals of that species. However, even relatively small differences in genotype can lead to dramatic differences in phenotype: for example, chimpanzees and humans differ in only about 5% of their genomes.
An individual organism's phenotype results from both its genotype and the influence of the environment it has lived in. A substantial part of the phenotypic variation in a population is caused by genotypic variation. The modern evolutionary synthesis defines evolution as the change over time in this genetic variation. The frequency of one particular allele will become more or less prevalent relative to other forms of that gene. Variation disappears when a new allele reaches the point of fixation—when it either disappears from the population or replaces the ancestral allele entirely.
Before the discovery of Mendelian genetics, one common hypothesis was blending inheritance. But with blending inheritance, genetic variation would be rapidly lost, making evolution by natural selection implausible. The Hardy–Weinberg principle provides the solution to how variation is maintained in a population with Mendelian inheritance. The frequencies of alleles (variations in a gene) will remain constant in the absence of selection, mutation, migration and genetic drift.
Mesozoic and Cenozoic Eras
The Mesozoic and Cenozoic eras make up the youngest half of the Phanerozoic. The Triassic Period, the youngest period of the Mesozoic Era, was the time in which both mammals... View MoreMesozoic and Cenozoic Eras
The Mesozoic and Cenozoic eras make up the youngest half of the Phanerozoic. The Triassic Period, the youngest period of the Mesozoic Era, was the time in which both mammals and dinosaurs evolved. The Mesozoic ended with a major extinction at the close of the Cretaceous Period. All dinosaurs except birds disappeared in this extinction. Another mass extinction occurred near the end of the Triassic Period. The Cenozoic Era was a time in which communities became more modern in appearance. By the very end of the era (Neogene Period), Homo sapiens, our species, had evolved.
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