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- Shared Biochemistry
- Aminode: Identification of Evolutionary Constraints in the Human Proteome
- 12.2: Determining Evolutionary Relationships
Scientists collect information that allows them to make evolutionary connections between organisms. Similar to detective work, scientists must use evidence to uncover the facts. In the case of phylogeny, evolutionary investigations focus on two types of evidence: morphologic form and function and genetic. Organisms that share similar physical features and genetic sequences tend to be more closely related than those that do not.
Features that overlap both morphologically and genetically are referred to as homologous structures; the similarities stem from common evolutionary paths. For example, as shown in Figure Notice the structure is not simply a single bone, but rather a grouping of several bones arranged in a similar way in each organism even though the elements of the structure may have changed shape and size.
Some organisms may be very closely related, even though a minor genetic change caused a major morphological difference to make them look quite different. For example, chimpanzees and humans, the skulls of which are shown in Figure However, chimpanzees and humans show considerable anatomical differences, including the degree to which the jaw protrudes in the adult and the relative lengths of our arms and legs.
However, unrelated organisms may be distantly related yet appear very much alike, usually because common adaptations to similar environmental conditions evolved in both. An example is the streamlined body shapes, the shapes of fins and appendages, and the shape of the tails in fishes and whales, which are mammals.
These structures bear superficial similarity because they are adaptations to moving and maneuvering in the same environment—water. When a characteristic that is similar occurs by adaptive convergence convergent evolution , and not because of a close evolutionary relationship, it is called an analogous structure. In another example, insects use wings to fly like bats and birds. We call them both wings because they perform the same function and have a superficially similar form, but the embryonic origin of the two wings is completely different.
The difference in the development, or embryogenesis, of the wings in each case is a signal that insects and bats or birds do not share a common ancestor that had a wing. The wing structures, shown in Figure Similar traits can be either homologous or analogous. Homologous traits share an evolutionary path that led to the development of that trait, and analogous traits do not. Scientists must determine which type of similarity a feature exhibits to decipher the phylogeny of the organisms being studied.
Figure The bird and bat wings are homologous structures. However, the honey bee wing has a different structure it is made of a chitinous exoskeleton, not a boney endoskeleton and embryonic origin.
The bee and bird or bat wing types illustrate an analogy—similar structures that do not share an evolutionary history. This website has several examples to show how appearances can be misleading in understanding the phylogenetic relationships of organisms. With the advancement of DNA technology, the area of molecular systematics, which describes the use of information on the molecular level including DNA sequencing, has blossomed.
New analysis of molecular characters not only confirms many earlier classifications, but also uncovers previously made errors. Molecular characters can include differences in the amino-acid sequence of a protein, differences in the individual nucleotide sequence of a gene, or differences in the arrangements of genes. Phylogenies based on molecular characters assume that the more similar the sequences are in two organisms, the more closely related they are.
Different genes change evolutionarily at different rates and this affects the level at which they are useful at identifying relationships. Rapidly evolving sequences are useful for determining the relationships among closely related species. More slowly evolving sequences are useful for determining the relationships between distantly related species. To determine the relationships between very different species such as Eukarya and Archaea, the genes used must be very ancient, slowly evolving genes that are present in both groups, such as the genes for ribosomal RNA.
Comparing phylogenetic trees using different sequences and finding them similar helps to build confidence in the inferred relationships. Sometimes two segments of DNA in distantly related organisms randomly share a high percentage of bases in the same locations, causing these organisms to appear closely related when they are not.
For example, the fruit fly shares 60 percent of its DNA with humans. In addition to enhancing our understanding of the evolutionary history of species, our own included, phylogenetic analysis has numerous practical applications.
Two of those applications include understanding the evolution and transmission of disease and making decisions about conservation efforts. A study 3 of MRSA methicillin-resistant Staphylococcus aureus , an antibiotic resistant pathogenic bacterium, traced the origin and spread of the strain throughout the past 40 years.
The study uncovered the timing and patterns in which the resistant strain moved from its point of origin in Europe to centers of infection and evolution in South America, Asia, North America, and Australasia. The study suggested that introductions of the bacteria to new populations occurred very few times, perhaps only once, and then spread from that limited number of individuals.
This is in contrast to the possibility that many individuals had carried the bacteria from one place to another. This result suggests that public health officials should concentrate on quickly identifying the contacts of individuals infected with a new strain of bacteria to control its spread. A second area of usefulness for phylogenetic analysis is in conservation. Biologists have argued that it is important to protect species throughout a phylogenetic tree rather than just those from one branch of the tree.
Doing this will preserve more of the variation produced by evolution. For example, conservation efforts should focus on a single species without sister species rather than another species that has a cluster of close sister species that recently evolved. If the single evolutionarily distinct species goes extinct a disproportionate amount of variation from the tree will be lost compared to one species in the cluster of closely related species. A study published in 4 made recommendations for conservation of mammal species worldwide based on how evolutionarily distinct and at risk of extinction they are.
The study found that their recommendations differed from priorities based on simply the level of extinction threat to the species. The study recommended protecting some threatened and valued large mammals such as the orangutans, the giant and lesser pandas, and the African and Asian elephants. But they also found that some much lesser known species should be protected based on how evolutionary distinct they are.
These include a number of rodents, bats, shrews and hedgehogs. In addition there are some critically endangered species that did not rate as very important in evolutionary distinctiveness including species of deer mice and gerbils. While many criteria affect conservation decisions, preserving phylogenetic diversity provides an objective way to protect the full range of diversity generated by evolution.
How do scientists construct phylogenetic trees? Presently, the most accepted method for constructing phylogenetic trees is a method called cladistics. This method sorts organisms into clades, groups of organisms that are most closely related to each other and the ancestor from which they descended.
For example, in Figure Consequently, all of these organisms also have amniotic eggs and make a single clade, also called a monophyletic group. Clades must include the ancestral species and all of the descendants from a branch point. Lizards, rabbits, and humans all descend from a common ancestor in which the amniotic egg evolved. Thus, lizards, rabbits, and humans all belong to the clade Amniota.
Vertebrata is a larger clade that also includes fish and lamprey. Which animals in this figure belong to a clade that includes animals with hair? Which evolved first: hair or the amniotic egg?
Clades can vary in size depending on which branch point is being referenced. The important factor is that all of the organisms in the clade or monophyletic group stem from a single point on the tree.
Cladistics rests on three assumptions. The first is that living things are related by descent from a common ancestor, which is a general assumption of evolution. The second is that speciation occurs by splits of one species into two, never more than two at a time, and essentially at one point in time. This is somewhat controversial, but is acceptable to most biologists as a simplification. The third assumption is that traits change enough over time to be considered to be in a different state.
It is also assumed that one can identify the actual direction of change for a state. In other words, we assume that an amniotic egg is a later character state than non-amniotic eggs. This is called the polarity of the character change. We know this by reference to a group outside the clade: for example, insects have non-amniotic eggs; therefore, this is the older or ancestral character state.
Cladistics compares ingroups and outgroups. An ingroup lizard, rabbit and human in our example is the group of taxa being analyzed. An outgroup lancelet, lamprey and fish in our example is a species or group of species that diverged before the lineage containing the group s of interest.
If a characteristic is found in all of the members of a group, it is a shared ancestral characterbecause there has been no change in the trait during the descent of each of the members of the clade. Although these traits appear interesting because they unify the clade, in cladistics they are considered not helpful when we are trying to determine the relationships of the members of the clade because every member is the same. In contrast, consider the amniotic egg characteristic of Figure Only some of the organisms have this trait, and to those that do, it is called a shared derived character because this trait changed at some point during descent.
This character does tell us about the relationships among the members of the clade; it tells us that lizards, rabbits, and humans group more closely together than any of these organisms do with fish, lampreys, and lancelets. The same trait could be either ancestral or derived depending on the diagram being used and the organisms being compared.
Scientists find these terms useful when distinguishing between clades during the building of phylogenetic trees, but it is important to remember that their meaning depends on context.
Constructing a phylogenetic tree, or cladogram, from the character data is a monumental task that is usually left up to a computer. The computer draws a tree such that all of the clades share the same list of derived characters. But there are other decisions to be made, for example, what if a species presence in a clade is supported by all of the shared derived characters for that clade except one?
One conclusion is that the trait evolved in the ancestor, but then changed back in that one species. Also a character state that appears in two clades must be assumed to have evolved independently in those clades. These inconsistencies are common in trees drawn from character data and complicate the decision-making process about which tree most closely represents the real relationships among the taxa.
To aid in the tremendous task of choosing the best tree, scientists often use a concept called maximum parsimony, which means that events occurred in the simplest, most obvious way. Computer programs search through all of the possible trees to find the small number of trees with the simplest evolutionary pathways. Starting with all of the homologous traits in a group of organisms, scientists can determine the order of evolutionary events of which those traits occurred that is the most obvious and simple.
Practice Parsimony: Go to this website to learn how maximum parsimony is used to create phylogenetic trees be sure to continue to the second page. To build phylogenetic trees, scientists must collect character information that allows them to make evolutionary connections between organisms.
Aminode: Identification of Evolutionary Constraints in the Human Proteome
Activity Summary Students will compare the sequence of amino acids in a gene shared between humans and six other organisms and infer evolutionary relationships among the species. This gene produces a type of protein called a transcription factor , which turns other genes "on" or "off. In animals, the FOXP2 gene is especially active during embryonic development in the brain, gut, heart, and lungs, but scientists are still unraveling which genes it regulates in each of these tissues. As explained in the NOVA scienceNOW segment, FOXP2 also plays a role in the processes involved in human speech and birdsong: people with an altered form of the gene have difficulty with many aspects of speech, and birds whose FOXP2 activity is disrupted have trouble learning songs. Despite these and other observations, scientists still don't know which other genes FOXP2 regulates or what its function is in the numerous other species that share this gene with birds and humans. That FOXP2 is so widespread raises additional questions, not only about its role in other organisms, but also how the gene differs from one organism to the next.
Molecular Evidence. Common Composition: The code used to translate nucleotide sequences into amino acid sequences is essentially the same in all organisms. Moreover, proteins in all organisms are invariably composed of the same set of 20 amino acids. This unity of composition and function is a powerful argument in favor of the common descent of the most diverse organisms. Click Image.
12.2: Determining Evolutionary Relationships
Scientists collect information that allows them to make evolutionary connections between organisms. Similar to detective work, scientists must use evidence to uncover the facts. In the case of phylogeny, evolutionary investigations focus on two types of evidence: morphologic form and function and genetic. Organisms that share similar physical features and genetic sequences tend to be more closely related than those that do not. Features that overlap both morphologically and genetically are referred to as homologous structures; the similarities stem from common evolutionary paths.
Identify the differences in the amino-acid sequences of the cytochrome c and hemoglobin molecules of several species. Infer the evolutionary relationships among several species by comparing amino-acid sequences of the same protein in. You are a zoologist who specializes in the classification of vertebrates according to their evolutionary relationships. In your.
Wilfred A. The American Biology Teacher 1 February ; 72 2 : — In a flexible multisession laboratory, students investigate concepts of phylogenetic analysis at both the molecular and the morphological level.
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