* A synapomorphy is a common property shown by two or more groups of organisms, that can be traced back to the most recent ancestor that both the groups evolved from. However, this character may not be shown by other closely-related groups, because some of them may have evolved further, or lost the character altogether.
* Synapomorphic traits are those which have appeared only in the last common ancestor for the first time, and not in more primitive organisms. This helps researchers find out which ancestral organism first evolved a particular character, that is now seen in different species or populations. It helps them establish evolutionary relationships between different groups of organisms, like birds, reptiles, and mammals, that show similar properties.
* Synapomorphies play an important role in the system of 'cladistics', which groups organisms into different categories, or 'clades', depending on their common ancestors. Only synapomorphic traits can be used to link various groups together, because if a property shared by different organisms is not ancient, it means that they share a recent, common ancestor who developed it.
* Humans and cats, despite being from different groups (primates and carnivores respectively), show similarity in their forearm bone structure. Though they evolved by different paths, these common traits were passed down to both groups by the last common ancestor that they shared. Similarly, the bones in bat wings have a similar arrangement to human arm bones, with the bones at the top being very similar to those of our hands.
* The category of 'land plants' includes different groups, like the Coleochaete (a type of algae), liverworts (flowerless plants), conifers, and angiosperms (flowering plants). Despite this, only liverworts, conifers, and angiosperms show a multicellular sporophyte (spore-producing stage), while the Coleochaete does not. This is a synapomorphy passed down to them by the last ancestor that they shared.
* The genus Homo includes all great apes, such as Homo erectus (upright man), Homo neanderthalensis (neanderthals), and Homo sapiens (modern man). Despite the differences between them, they all shared the similar property of having large brain-cases, indicating their higher intelligence. This is because, their most recent and common ancestor―Australopithecus―evolved this trait for the first time, and passed it down to them. Of these species, we are the only ones still surviving today.
* The superclass Tetrapoda includes all four-footed animals, such as reptiles (like lizards and crocodiles), amphibians (like salamanders and frogs), birds, and mammals. Despite their enormous diversity, all these animals show four limbs and an amniotic egg (embryo development inside an egg), which indicates that they evolved from a common ancestor. Interestingly, some synapomorphies are seen only in mammals, such as hair and mammary glands, and are absent in other tetrapods.
Since current evolutionary theory says that traits arise (are derived) in lineages through evolution, only synapomorphies can be used to establish relationships. Autapomorphies contain no information about relationships (because they don't group organisms together); symplesiomorphies should not be used to unite taxa. For example, if you are comparing a clubmoss, a fern and a flowering plant, the trait "free-sporing" (as opposed to "non-free-sporing") can not be used to group the clubmoss and the fern into one group, since this feature is ancestral to both, that is, inherited from the ancestor of all land plants. On the other hand, the trait "megaphyll" (as opposed to "microphyll") unites the fern and the flowering plant into a group, excluding the clubmoss.
Organisms that are united by one or more synapomorphies share a common ancestor which possessed these derived traits. They belong to a monophyletic group (Figure 2.4; A) in which all descendants of the common ancestor have to be included. This is also referred to as a "natural" or "evolutionary" group or as a lineage. In modern evolutionary biology, we work hard to recognize only monophyletic groups. If a group does not include all the descendants of a common ancestry, the group is termed paraphyletic (Figure 2.4; B), or a grade. An example of this is the Bryophyta, which includes liverworts, mosses and hornworts, but not the vascular plants. Another example may be "gymnosperms" when used to refer to non-angiosperm seed plants. If the group includes some or all of the descendants, but not the common ancestor, it is called polyphyletic (Figure 2.4; C). For example, a group of all epiphytic plants regardless of their ancestry would be extremely polyphyletic. A sister group (or sister taxon) is defined as the closest relative to a monophyletic group as determined by one or more synamorphies uniting the groups. (Figure 2.5 on next page).
* The parsimony principle is basic to all science and tells us to choose the simplest scientific explanation that fits the evidence. In terms of tree-building, that means that, all other things being equal, the best hypothesis is the one that requires the fewest evolutionary changes.
* Hypothesis 1 requires six evolutionary changes and Hypothesis 2 requires seven evolutionary changes, with a bony skeleton evolving independently, twice. Although both fit the available data, the parsimony principle says that Hypothesis 1 is better — since it does not hypothesize unnecessarily complicated changes.
* This principle was implicit in the tree-building process we went through earlier with the vertebrate phylogeny. However, in most cases, the data are more complex than those used in our example and may point to several different phylogenetic hypotheses. In those cases, the parsimony principle can help us choose between them.
* One reliable method of building and evaluating trees, called parsimony, involves grouping taxa together in ways that minimize the number of evolutionary changes that had to have occurred in the characters. The idea here is that, all other things being equal, a simple hypothesis (e.g., just four evolutionary changes) is more likely to be true than a more complex hypothesis (e.g., 15 evolutionary changes). So, for example, based on the morphological data, the tree at left below requires only seven evolutionary changes and, based on the available evidence, is a better hypothesis than the tree at right, which requires nine evolutionary changes.
In evolutionary biology, convergent evolution is the process whereby organisms not closely related (not monophyletic), independently evolve similar traits as a result of having to adapt to similar environments or ecological niches.
It is the opposite of divergent evolution, where related species evolve different traits.
On a molecular level, this can happen due to random mutation unrelated to adaptive changes; see long branch attraction.
In cultural evolution, convergent evolution is the development of similar cultural adaptations to similar environmental conditions by different peoples with different ancestral cultures.
An example of convergent evolution is the similar nature of the flight/wings of insects, birds, pterosaurs, and bats.
All four serve the same function and are similar in structure, but each evolved independently.
Some aspects of the lens of eyes also evolved independently in various animals.
Convergent evolution is similar to, but distinguishable from, the phenomena of evolutionary relay and parallel evolution.
Evolutionary relay refers to independent species acquiring similar characteristics through their evolution in similar ecosystems, but not at the same time (e.g. dorsal fins of extinct ichthyosaurs and sharks).
Parallel evolution occurs when two independent species evolve together at the same time in the same ecospace and acquire similar characteristics (extinct browsing-horses and extinct paleotheres).
Structures that are the result of convergent evolution are called analogous structures or homoplasies; they should be contrasted with homologous structures, which have a common origin.
* Biogeography — the study of how species are scattered across the planet, and how they got that way.
* Wallace had already accepted evolution when he began his travels in 1848 through the Amazon and Southeast Asia. On his journeys, he sought to demonstrate that evolution did indeed take place, by showing how geography affected the ranges of species. He studied hundreds of thousands of animals and plants, carefully noting exactly where he had found them. The patterns he found were compelling evidence for evolution. He was struck, for example, by how rivers and mountain ranges marked the boundaries of many species' ranges. The conventional explanation that species had been created with adaptations to their particular climate made no sense since he could find similar climatic regions with very different animals in them.
* Wallace came to much the same conclusion that Darwin published in the Origin of Species: biogeography was simply a record of inheritance. As species colonized new habitats and their old ranges were divided by mountain ranges or other barriers, they took on the distributions they have today
* Wallace pushed the study of biogeography to grander scales than Darwin. As he traveled through Indonesia, for example, he was struck by the sharp distinction between the northwestern part of the archipelago and the southeastern, despite their similar climate and terrain. Sumatra and Java were ecologically more like the Asian mainland, while New Guinea was more like Australia. He traced a remarkably clear boundary that snaked among the islands, which later became known as "Wallace's Line." He later recognized six great biogeographical regions on Earth, and Wallace's Line divided the Oriental and the Australian regions.
* The biogeographic regions of the world that Wallace recognized roughly coincide with the continents themselves. But in the twentieth century, scientists have recognized that biogeography has been far more dynamic over the course of life's history. In 1915 the German geologist Alfred Wegener (left) was struck by the fact that identical fossil plants and animals had been discovered on opposite sides of the Atlantic. Since the ocean was too far for them to have traversed on their own, Wegener proposed that the continents had once been connected. Only in the 1960s, as scientists carefully mapped the ocean floor, were they able to demonstrate the mechanism that made continental drift possible — plate tectonics.
* Biogeographers now recognize that as continents collide, their species can mingle, and when the continents separate, they take their new species with them. Africa, South America, Australia, and New Zealand, for example, were all once joined into a supercontinent called Gondwanaland. The continents split off one by one, first Africa, then New Zealand, and then finally Australia and South America. The evolutionary tree of some groups of species — such as tiny insects known as midges — show the same pattern. South American and Australian midges, for example, are more closely related to one another than they are to New Zealand species, and the midges of all three land masses are more closely related to one another than they are to African species. In other words, an insect that may live only a few weeks can tell biogeographers about the wanderings of continents tens of millions of years ago.
* Adaptive radiation is the development of many species derived from a single ancestral population. The Hawaiian silversword `ohana (family) is probably the foremost example of adaptive radiation among plants in the world. Over the course of millions of years, the descendants of the pioneer plant evolved into 28 distinct species in three genera, occupying many different habitats.
*Scientists believe that the entire silversword family probably descended from a member of the sunflower family, similar to Muir's Tarweed, from California. The barbed fruit of this tarweed may have been carried to Hawai`i on the feathers of a bird. Since this tarweed came from an alpine shrubland, it most likely became established in Hawai`i in a similar kind of habitat.
* Since so few organisms successfully colonized Hawai`i, many diverse habitats were available. Over time, descendants of the tarweed slowly adapted to many of these habitats. Beneficial mutations enabled the plant forms to change and become quite different from the ancestor. The end result was extensive and spectacular adaptive radiation.
* Today, plants of the silversword family occupy every terrestrial habitat in Hawai`i--from wet forests to dry forests and from near sea level to alpine shrublands. Although these plants are still closely related (all species are able to hybridize, they often look extremely different from one another.
* The silversword family tree is divided into three genera: Argyroxiphium (5 species), Wilkesia (2 species), and Dubautia (21 species). Dubautia species are the most diverse. All members of the silversword family are endemic to Hawai`i; 82% are to a single island. Approximately one half of these single island endemic species are further limited in distribution, often growing only in one area or microclimate. It is a very special `ohana!
* The size and the form of a plant are adaptations to the habitat in which it grows. For example, large trees, shrubs and vines occur in wet areas; small shrubs, mats rosettes tend to occur in drier areas. Rainfall, sunlight, temperature, and elevation all affect the size and form of a plant. Plants growing in bogs are unable to use the stagnant, acidic water, so their roots are shallow. These plants display similar adaptations to plants growing in very dry areas
* Neoteny (/niːˈɒtɪni/ /niːˈɒtni/ or /niːˈɒtəni/, (also called juvenilization) is the delaying or slowing of the physiological (or somatic) development of an organism, typically an animal. Neoteny is found in modern humans. In progenesis (also called paedogenesis), sexual development is accelerated.
* Both neoteny and progenesis result in paedomorphism (or paedomorphosis), a type of heterochrony. Some authors define paedomorphism as the retention of larval traits, as seen in salamanders.
* Both neoteny and progenesis cause the retention in adults of traits previously seen only in the young. Such retention is important in evolutionary biology, domestication, and evolutionary developmental biology.
* Progenesis is the speeding up of the germ line. The result is paedomorphosis - reproduction happens in what was ancestrally a juvenile morphologic stage
* Neoteny is the slowing down of somatic development. The result is that reproduction happens in what was ancestrally a juvenile morphologic stage.
* Humans have also been argued to be neotenous. As adults, we are morphologically similar to the juvenile forms of great apes. This paedomorphosis, if it is real (and there is a serious argument that it is not), would be neotenous rather than progenetic because our age of breeding has not shifted earlier relative to other apes; our age of first breeding is actually later than other apes. Our somatic development therefore has not simply slowed down while reproductive development has stayed the same: what happened was that our somatic development slowed down even more than our reproductive development.
Christy C. Hayhoe, Doug Hayhoe, Jeff Major, Maurice DiGiuseppe Kenneth R. Miller, Levine Kenneth R. Miller, Levine