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Slow decomposition must begin so that the rock forms around it. The organism must fall into a body of water. The body must be buried in sediments. The body must absorb minerals and change into stone.
Exposed fossils are often subject to weathering, whereas those beneath the surface are protected. Organisms with hard parts, such as teeth, shells, and skeletons, are more likely to be preserved. Organisms with broad geographic distributions and large populations are more likely to be preserved than rare endemic species. Organisms living in certain habitats were more likely to be buried in sediments and preserved than organisms in other habitats.
mostly bacteria and simple one-celled plants and animals primarily sponges, algae, and seaweeds jellyfish and sponges, along with some tracks of various other organisms arthropods, mollusks, chordates, and a variety of soft-bodied organisms
Several duplications of the homeotic genes that control major body plans apparently arose about 540 MYA, creating new body plans and appendage configurations. Reproductive isolation was nearly nonexistent; many "species" interbred to form hybrids that went on to become new species. Many mutations accumulated due to high exposure to UV radiation, creating many new kinds of organisms. New genes appeared in many different organisms through a mechanism that has not been identified; these are associated with new body plans.
Hox loci are similar in structure and DNA sequence and are grouped in clusters, hence are assumed to have arisen through duplication; when a new Hox gene appears within a lineage, most of the descendent taxa have a homologous Hox locus. Phyla that branch off early, such as sponges and cnidarians, have simple body plans with relatively few Hox loci. In vertebrates, the Hox clusters themselves appear to be duplicated several times. Within phyla, there is no correspondence between the number of Hox genes and complexity.
Both genes are expressed early in development in both taxa, but later in development there are changes in both the location and timing of expression that are correlated with development of the mouse limbs. Both genes are expressed early in development in fish and mice; the lack of differences suggests that these genes are not important in the differing development of limbs and fins. Both genes were expressed early in development in fish, but neither was expressed early in development in mice. Instead, both were expressed later and in different patterns in the developing tetrapod limb. One gene, hoxd-11, was only expressed in zebra fish, whereas Shh was only expressed in mice, suggesting that limbs are not homologous with fins.
A fishless lake is colonized by a single fish species, which over a few thousand years gives rise to several species, each with a series of unique feeding adaptations. A small group of tree-dwelling lizards of a single species migrates to an uninhabited, treeless island and adapts to use the open grassland habitat. A colonizing species of fruit fly takes up residence on a new continent and displaces two closely related native species. A population of a cricket species takes up residence in a cave. Over many generations, the cave crickets eventually lose their eyes, like many other cave-dwelling animals.
It is a clock that is powered by biologically active molecules such as ATP. Because mutations occur at an predictable rate, you can count up the mutations within a sequence in two related organisms and estimate the time of their divergence. It is the inner clock that controls an animal’s diurnal rhythms. The accumulation of mutations that are favored by natural selection yields a highly accurate method of dating the amount of time it takes organisms to adapt to a new habitat.
that modern humans originated about 6000 years ago that modern humans originated less than 250,000 years ago that modern humans appeared about the same time as Neanderthals Molecular clocks cannot date the origin of Homo sapiens.
Morphological innovations disrupt natural selection, forcing species to find new adaptations. Morphological innovations can open up new adaptive options that may allow a species to colonize an underutilized resource. Morphological innovations often allow a species to replace competitors. Morphological innovations prevent adaptive radiations most of the time.
Each island was initially colonized by a different species that occupied its preferred habitat. Subsequent colonizing species were successful only if they could use an unoccupied habitat type. The phylogeny for each island was the same throughout the region because no evolution occurred, only colonization. Each island was colonized by a different species initially, and that species underwent a radiation, giving rise to new species that occupied different habitat types. There was no similarity, however, in the adaptations from one island to another. The phylogeny for each island was completely unique and totally unlike that of any other island. The first colonists on every island were always the same species, a twig-dwelling specialist. However, in every case that species gave rise to the same set of other species that specialized on different habitats. The phylogeny for each island had the same root and the same pattern of speciation. Different islands were initially colonized by different species that differed in habitat preference, but in both cases subsequent speciation produced a range of ecological specialists occupying similar habitat niches. The phylogeny for each island was unique, but ended up with species having similar sets of adaptations for each habitat.
Mass extinctions, such as the K-T event, tend to take out large-bodied organisms, whereas smaller-bodied organisms tend to be removed by background extinction. Background extinctions occur only sporadically, whereas mass extinctions happen on a regular, periodic basis. Background extinctions are caused by rapid changes in the environment, whereas mass extinctions occur primarily in response to biological competition. During mass extinctions, adaptations for survival and competition make little difference in the likelihood of extinction, whereas the opposite is true of background extinctions.
Species are difficult to identify in the fossil record; for example, the shells of two species of mollusks that are preserved in the fossil record may look identical and so would be considered a single fossil "species." Families are a more basic unit of evolutionary change than species, and hence are a more appropriate subject for paleontological study. Species do not go extinct in the fossil record as often as families do, so one would not notice the pattern of mass extinctions if one were just counting species. Paleontologists just don’t have the time to count up species over that time period.
a worldwide change in climate caused by increased carbon dioxide in the atmosphere which led to melting of the ice caps and dramatic changes in sea level an asteroid impact that caused fires, acid rain, and a massive dust cloud that blocked sunlight for some time, causing rapid global cooling and low plant productivity an outbreak of a highly pathogenic virus against which reptiles have no defense, but mammals are apparently immune an outbreak of global volcanic activity that both directly, through eruption of magma, and indirectly, through ash clouds blocking the sun, caused massive mortality in plants and animals
Many marine invertebrates, such as clams and snails, were greatly reduced in diversity by the K-T event; new species took 4–8 million years to evolve and replace the extinct species. Many woody and flowering plant species were lost in the K-T event and were replaced by a radiation of fern species. Mammals survived and radiated after the K-T event with all major orders appearing within 5–10 million years, replacing the dinosaurs and marine reptiles. All large-bodied reptiles died out in the extinction and were replaced by smaller-bodied species of dinosaurs and crocodiles.
By chance, the K-T impact caused extinction of the dinosaurs but did not greatly affect the small, nocturnal, scavenging mammals. The release from competition allowed the mammals to diversify. Both mammals and dinosaurs survived the asteroid, but the mammals were much better adapted to the new environments created by the impact and therefore underwent adaptive radiation. Mammals were inherently better competitors than the dinosaurs; the K-T impact simply speeded up the replacement, which was already well under way. Mammalian diversity increased dramatically prior to the impact, but the population sizes of every species remained small until after the dinosaurs were extinct.
Earth is constantly bombarded with asteroids, like the one that hit the planet 65 million years ago; another such impact is likely to occur within the next 50 years. Widespread habitat destruction by humans, particularly in the tropical rain forests, seems to be accelerating extinction rates in many animal lineages. Human conservation efforts, coupled with the dramatic decline in human population levels since 1990, suggest that humans have less impact on natural ecosystems today than in the past and a mass extinction is unlikely. Mass extinctions only happen in response to major geological events; these cannot be predicted and so there is no way to assess the threat.
Two copies of a gene mean twice as much gene product; this means that rate limitations are relaxed and the organism can evolve a new body plan. Gene duplications arise in response to natural selection for diversification within a lineage. When a new species needs a new gene to adapt to a novel environment, it can produce a gene duplication to enable its adaptation. Gene duplications arise through hybridization of two species, giving rise to a new species; the combined genome carries two copies of a gene instead of one. When there is only one copy of a gene, natural selection may purge mutations, preventing it from evolving a new function. With a second copy, selection may be relaxed on that one copy and mutations conferring new properties may accumulate.
because they preserve a diverse array of microorganisms that we believe gave rise to multicellular life at the start of the Cambrian period because these fossils record the origin of animals from seaweeds and other photosynthetic organisms because these assemblages show precisely how animals began as single-celled organisms, then became two-celled, then four-celled, and so on to become the multicellular forms we know today because they capture a wide range of soft-bodied organisms that are rarely preserved in the fossil record at a time when the diversity of metazoans was greatly increasing
There was no life prior to the Cambrian explosion. Only unicellular organisms existed; no multicellular forms appear until 565 MYA in the fossil record. Bacteria, single-celled eukaryotes, and multicellular algae were all present one billion years ago. Several species of small, multicellular animals appear in the fossil record about one billion years ago, but none were terrestrial.
