Over time, these species diverge evolutionarily into new species that look very different from their ancestors that may exist on the mainland. Like anatomical structures, the structures of the molecules of life reflect descent with modification.
Evidence of a common ancestor for all of life is reflected in the universality of DNA as the genetic material and of the near universality of the genetic code and the machinery of DNA replication and expression. Fundamental divisions in life between the three domains are reflected in major structural differences in otherwise conservative structures such as the components of ribosomes and the structures of membranes.
In general, the relatedness of groups of organisms is reflected in the similarity of their DNA sequences—exactly the pattern that would be expected from descent and diversification from a common ancestor.
DNA sequences have also shed light on some of the mechanisms of evolution. For example, it is clear that the evolution of new functions for proteins commonly occurs after gene duplication events. These duplications are a kind of mutation in which an entire gene is added as an extra copy or many copies in the genome.
These duplications allow the free modification of one copy by mutation, selection, and drift, while the second copy continues to produce a functional protein.
This allows the original function for the protein to be kept, while evolutionary forces tweak the copy until it functions in a new way. The evidence for evolution is found at all levels of organization in living things and in the extinct species we know about through fossils.
Fossils provide evidence for the evolutionary change through now extinct forms that led to modern species. For example, there is a rich fossil record that shows the evolutionary transitions from horse ancestors to modern horses that document intermediate forms and a gradual adaptation o changing ecosystems.
The anatomy of species and the embryological development of that anatomy reveal common structures in divergent lineages that have been modified over time by evolution. The geographical distribution of living species reflects the origins of species in particular geographic locations and the history of continental movements. For example, certain compound leaves of flowering plants are partially homologous both to leaves and shoots because they combine some traits of leaves and some of shoots.
Homologous sequences are considered paralogous if they were separated by a gene duplication event; if a gene in an organism is duplicated to occupy two different positions in the same genome, then the two copies are paralogous.
A set of sequences that are paralogous are called paralogs of each other. Paralogs typically have the same or similar function, but sometimes do not. It is considered that due to lack of the original selective pressure upon one copy of the duplicated gene, this copy is free to mutate and acquire new functions.
Homology vs. This is because they are similar characteristically and even functionally, but evolved from different ancestral roots. Paralogous genes often belong to the same species, but not always. For example, the hemoglobin gene of humans and the myoglobin gene of chimpanzees are considered paralogs. This is a common problem in bioinformatics; when genomes of different species have been sequenced and homologous genes have been found, one can not immediately conclude that these genes have the same or similar function, as they could be paralogs whose function has diverged.
The opposite of homologous structures are analogous structures, which are physically similar structures between two taxa that evolved separately rather than being present in the last common ancestor.
Bat wings and bird wings evolved independently and are considered analogous structures. Genetically, a bat wing and a bird wing have very little in common; the last common ancestor of bats and birds did not have wings like either bats or birds. Wings evolved independently in each lineage after diverging from ancestors with forelimbs that were not used as wings terrestrial mammals and theropod dinosaurs, respectively. It is important to distinguish between different hierarchical levels of homology in order to make informative biological comparisons.
In the above example, the bird and bat wings are analogous as wings, but homologous as forelimbs because the organ served as a forearm not a wing in the last common ancestor of tetrapods. Analogy is different than homology. Although analogous characteristics are superficially similar, they are not homologous because they are phylogenetically independent.
Analogy is commonly also referred to as homoplasy. Convergent evolution occurs in different species that have evolved similar traits independently of each other. Sometimes, similar phenotypes evolve independently in distantly related species.
For example, flight has evolved in both bats and insects, and they both have wings, which are adaptations to flight. However, the wings of bats and insects have evolved from very different original structures. This phenomenon is called convergent evolution, where similar traits evolve independently in species that do not share a recent common ancestry.
Convergent evolution describes the independent evolution of similar features in species of different lineages. The two species came to the same function, flying, but did so separately from each other. Both sharks and dolphins have similar body forms, yet are only distantly related: sharks are fish and dolphins are mammals. Such similarities are a result of both populations being exposed to the same selective pressures.
Within both groups, changes that aid swimming have been favored. Thus, over time, they developed similar appearances morphology , even though they are not closely related. One of the most well-known examples of convergent evolution is the camera eye of cephalopods e.
Their last common ancestor had at most a very simple photoreceptive spot, but a range of processes led to the progressive refinement of this structure to the advanced camera eye.
Eye evolution : Vertebrates and octopi developed the camera eye independently. In the vertebrate version the nerve fibers pass in front of the retina, and there is a blind spot 4 where the nerves pass through the retina. This means that octopi do not have a blind spot. Convergent evolution is similar to, but distinguishable from, the phenomenon of parallel evolution. Parallel evolution occurs when two independent but similar species evolve in the same direction and thus independently acquire similar characteristics; for example, gliding frogs have evolved in parallel from multiple types of tree frog.
Traits arising through convergent evolution are analogous structures, in contrast to homologous structures, which have a common origin, but not necessarily similar function. The British anatomist Richard Owen was the first scientist to recognize the fundamental difference between analogies and homologies.
Bat and pterosaur wings are an example of analogous structures, while the bat wing is homologous to human and other mammal forearms, sharing an ancestral state despite serving different functions. The opposite of convergent evolution is divergent evolution, whereby related species evolve different traits. On a molecular level, this can happen due to random mutation unrelated to adaptive changes. Some organisms possess structures with no apparent function which appear to be residual parts from a past ancestor.
For example, some snakes have pelvic bones despite having no legs because they descended from reptiles that did have legs. Another example of a structure with no function is the human vermiform appendix. These unused structures without function are called vestigial structures.
Other examples of vestigial structures are wings which may have other functions on flightless birds like the ostrich, leaves on some cacti, traces of pelvic bones in whales, and the sightless eyes of cave animals. Vestigial appendix : In humans the vermiform appendix is a vestigial structure; it has lost much of its ancestral function.
There are also several reflexes and behaviors that are considered to be vestigial. The arrector pili muscle, which is a band of smooth muscle that connects the hair follicle to connective tissue, contracts and creates the goose bumps on skin. Vestigial structures are often homologous to structures that function normally in other species. Therefore, vestigial structures can be considered evidence for evolution, the process by which beneficial heritable traits arise in populations over an extended period of time.
The existence of vestigial traits can be attributed to changes in the environment and behavior patterns of the organism in question. In some cases the structure becomes detrimental to the organism. Whale Skeleton : The pelvic bones in whales are also a good example of vestigial evolution whales evolved from four-legged land mammals and secondarily lost their hind legs.
Letter c in the picture indicates the undeveloped hind legs of a baleen whale. If there are no selection pressures actively lowering the fitness of the individual, the trait will persist in future generations unless the trait is eliminated through genetic drift or other random events.
Although in many cases the vestigial structure is of no direct harm, all structures require extra energy in terms of development, maintenance, and weight and are also a risk in terms of disease e.
The vestigial versions of a structure can be compared to the original version of the structure in other species in order to determine the homology of the structure.
Homologous structures indicate common ancestry with those organisms that have a functional version of the structure. Vestigial traits can still be considered adaptations because an adaptation is often defined as a trait that has been favored by natural selection. Adaptations, therefore, need not be adaptive, as long as they were at some point. The biological distribution of species is based on the movement of tectonic plates over a period of time. Biogeography is the study of the geographic distribution of living things and the abiotic factors that affect their distribution.
Abiotic factors, such as temperature and rainfall, vary based on latitude and elevation, primarily. As these abiotic factors change, the composition of plant and animal communities also changes. Ecologists who study biogeography examine patterns of species distribution. No species exists everywhere; for example, the Venus flytrap is endemic to a small area in North and South Carolina. An endemic species is one which is naturally found only in a specific geographic area that is usually restricted in size.
Other species are generalists: species which live in a wide variety of geographic areas; the raccoon, for example, is native to most of North and Central America. Since species distribution patterns are based on biotic and abiotic factors and their influences during the very long periods of time required for species evolution, early studies of biogeography were closely linked to the emergence of evolutionary thinking in the eighteenth century.
Some of the most distinctive assemblages of plants and animals occur in regions that have been physically separated for millions of years by geographic barriers. Biologists estimate that Australia, for example, has between , and , species of plants and animals. For permineralization to occur, the organism must be covered by sediment soon after death, or soon after the initial decay process.
The degree to which the remains are decayed when covered determines the later details of the fossil. Fossils usually consist of the portion of the organisms that was partially mineralized during life, such as the bones and teeth of vertebrates or the chitinous or calcareous exoskeletons of invertebrates.
However, other fossils contain traces of skin, feathers or even soft tissues. Fossils may also consist of the marks left behind by the organism while it was alive, such as footprints or feces. These types of fossils are called trace fossils, or ichnofossils, as opposed to body fossils. Past life may also leave some markers that cannot be seen but can be detected in the form of biochemical signals; these are known as chemofossils or biomarkers.
The totality of fossils, both discovered and undiscovered, and their placement in fossiliferous fossil-containing rock formations and sedimentary layers strata is known as the fossil record. Opponents of evolution point to gaps in the fossil record as proof that the theory is invalid. They say the fossil record fails to show what are called "transitional forms," generally the in-between stages as one type of creature evolved into another.
The fossil record certainly has gaps, mostly because the conditions required to create fossils have been rare ever since life began on Earth.
A very small percentage of animals that have lived and died ever became fossils. Thus, many pieces of the puzzle are missing; some will never be found.