EVIDENCE FOR EVOLUTION
=Evidence for Evolution= Evolution is the change in heritable traits of populations over successive generations. Over many generations new species can develop through a process called speciation. There is a wide range of evidence that support the idea that each of the species we see today evolved from a common ancestor. This evidence includes: *Fossil Evidence *Biogeography (species distribution) *Comparative anatomy *Comparative embryology *Genetic Evidence *Biochemical Evidence ==Fossil Evidence== Fossils are preserved remains or traces of animals, plants, and other organisms [image:http://i.imgur.com/TCtQXEi.png?1] Most fossils are found within layers of sedimentary rocks called strata. Deeper strata are usually older and therefore fossils from different time period can be compared. Analysis of fossils from different strata suggests that more complex, modern organisms evolved from simpler, more ancient organisms. The hominin (human) fossil record shows trends such as an increased tendency towards bipedalism (walking on two legs), smaller teeth / jaws and the development of a larger brain. Although people sometimes talk of a "missing link", in actual fact, the fossil record is full of intermediate species that no longer inhabit the Earth. [image:http://i.imgur.com/2UdzKB0.png?1] '''Transitional Fossils''' Major changes in lifestyle and anatomy would be subject to intense selection and so transitional (intermediate) forms would not be present for long periods of time. However, although less common, ''transitional'' fossils have been documented. For instance the acquisition of feathered wings by reptiles that would later evolve into birds (e.g. ''Archaeopteryx lithographica'' pictured left). ==Biogeography== [image:http://i.imgur.com/PJ7PG60.png?1] Biogeography is the study of species distributions. It examines how species have been distributed across different places at different times. The distribution of species shows a very clear pattern. More similar species tend to be found closer to one another geographically. The distribution of many animals and plants across different continents can be explained by continental drift (the movement tectonic plates). The continents were once all joined together in one giant super-continent. About 200-180 million years ago the southern half called Gondwanaland broke away. This would later split into what we now know as Antarctica, Africa, Australia, South America and India. These continents have some related species of plants and animals supporting the idea that a common ancestor once inhabited Gondwanaland. As regions separated, oceans became barriers to gene flow (inter-breeding) and different climates have caused each population to evolve into distinct species. However, they still share many features of their now extinct ancestors. ==Comparative Anatomy== Comparing the body structures (anatomy) of different species also supports the notion of a common ancestor. Closely related species have more anatomical (structural) similarities. Even less closely related species show evidence of underlying anatomical similarities, with common structural features that have been modified for a different function / purpose. [image:http://i.imgur.com/qGcQtih.png?1] Anatomical features that are derived from a common ancestor but have been adapted to a different purpose are called '''homologous structures'''. For instance the pentadactyl (5 digit) limb found in most vertebrates (animals with a spine) has the same general bone structure / pattern. However, the size and shape of each bone has been modified to serve a slightly different function. These "homologies" indicate that all of these species diverged from a common ancestor (see adaptive radiation) and that the basic limb plan has been adapted to meet the needs of different niches. [image:http://i.imgur.com/ovxSnDa.png?1] '''Vestigial organs''' Some animals possess inherited features that they no longer need. For instance whales still have the remains of a hip bone. It is significantly reduced (smaller), but serves no known function. This is evidence that whales have evolved from a once four-legged ancestor. The hind legs and hips which were no longer required have steadily become smaller and may one day be eliminated entirely. For now, whales are stuck with this "evolutionary baggage". '''''Analogous Structures''''' are features that have a very similar function but completely different anatomy. They normally occur when distantly related species occupy a similar environment. ==Comparative Embryology== [image:http://i.imgur.com/Jfc9AiB.png?2] All species start out as single celled organisms. Many species develop into much larger, more complex organisms after conception. If we compare the embryos of animals as they develop, we often find they are much more similar than their fully developed counterparts. Many of the anatomical differences between species only arise during our embryonic development. Different species often start with the same basic tissues or structures but they develop differently and are re-purposed into different structures as the organism develops. The more closely two species are related the later in development these differences usually emerge. This too supports the idea that we are descendants with modified structures that were inherited form a common ancestor. If you were to compare the embryos of these animals at what point do you think you could pick which one is human? ==Genetic Evidence== [image:http://i.imgur.com/jGfVsPP.png?1] The fact that the genetic code is universal to all living things suggests that we once had a common ancestor. Comparing the DNA sequence of two organisms can give us an idea of how closely related they are. For instance, your DNA sequence will be more similar to a direct relative than a stranger. Your DNA is more similar to other members of the same species than it is to other species. The more closely two DNA sequences match, the more recently they would have shared a common ancestor. By analysing the DNA from different species Scientists can start to generate family trees called '''''phylogenetic trees'''''. Scientists have devised a number of different ways to compare the DNA of different organisms such as: [https://www.pathwayz.org/Tree/Filter/SubTree/BIOTECHNOLOGY#!o999 DNA HYBRIDISATION], [https://www.pathwayz.org/Tree/Filter/SubTree/BIOTECHNOLOGY#!o1009 DNA PROFILING] and [https://www.pathwayz.org/Tree/Filter/SubTree/BIOTECHNOLOGY#!o1000 DNA SEQUENCING] ==Biochemical Evidence== Certain parts of our DNA sequence called genes each code for a unique sequence of amino acids called a polypeptide chain. These polypeptides fold into proteins that ultimately regulate our cellular functions thereby determining our characteristics. Evolution relies on mutations that alter the DNA sequence producing a new protein with an altered function. If the new function coveys some adaptive advantage it will be selected for (see [https://www.pathwayz.org/Tree/Filter/SubTree/PATHWAYZ/tag/41#!o289 natural selection]) However, not all mutations actually alter the amino acid sequence or structure of a protein. Therefore not every difference in the DNA sequence of two species represents an evolutionary change. Comparing the amino acid sequence or protein structures of two organisms gives a more accurate idea of their evolutionary relatedness.
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