5.4 CLADISTICS
Nature of science: Use theories to explain natural phenomena—the theory of evolution by natural selection can explain the development of antibiotic resistance in bacteria. (2.1)
Understandings:
• Natural selection can only occur if there is variation among members of the same species.
• Mutation, meiosis and sexual reproduction cause variation between individuals in a species.
• Adaptations are characteristics that make an individual suited to its environment and way of life.
• Species tend to produce more offspring than the environment can support.
• Individuals that are better adapted tend to survive and produce more offspring while the less well adapted tend to die or produce fewer offspring.
• Individuals that reproduce pass on characteristics to their offspring.
• Natural selection increases the frequency of characteristics that make individuals better adapted and decreases the frequency of other characteristics leading to changes within the species.
Applications and skills:
• Application: Changes in beaks of finches on Daphne Major.
• Application: Evolution of antibiotic resistance in bacteria.
• Natural selection can only occur if there is variation among members of the same species.
• Mutation, meiosis and sexual reproduction cause variation between individuals in a species.
• Adaptations are characteristics that make an individual suited to its environment and way of life.
• Species tend to produce more offspring than the environment can support.
• Individuals that are better adapted tend to survive and produce more offspring while the less well adapted tend to die or produce fewer offspring.
• Individuals that reproduce pass on characteristics to their offspring.
• Natural selection increases the frequency of characteristics that make individuals better adapted and decreases the frequency of other characteristics leading to changes within the species.
Applications and skills:
• Application: Changes in beaks of finches on Daphne Major.
• Application: Evolution of antibiotic resistance in bacteria.
characteristics used for classification
Classifying organisms using molecular differences is called molecular systematics. As technology is improved and becomes more affordable, more and more specialists are using methods involving protein sequences and DNA.
clades
Cladistics is a system of classification that groups taxa together according to the characteristics that have evolved most recently. In this system, the concept of common descent is crucial to deciding into which groups to classify organisms. Cladistics is, therefore, an example of natural classification. To decide how close a common ancestor is, researchers look at how many primitive and derived traits the organisms share.
variations and phylogeny
Phylogeny is the study of the evolutionary past of a species. Species that are the most similar are most likely to be closely related, whereas those that show a higher degree of differences are considered less likely to be closely related. By comparing the similarities in the polypeptide sequences of certain proteins in different groups of animals, it is possible to trace their common ancestry. This has been done with the blood protein haemoglobin, with a mitochondrial protein called cytochrome c, and with chlorophyll, to name just three proteins.With advances in DNA sequencing, the study of nucleic acid sequences in an organism’s DNA, as well as its mitochondrial DNA, has been effective in establishing biochemical phylogeny. Changes in the DNA sequences of genes from one generation to the next are partly due to mutations, and the more differences there are between two species, the less closely related the species are.
Here is an imaginary example of a DNA sequence from four different organisms:
Here is an imaginary example of a DNA sequence from four different organisms:
- A A A A T T T T CCCCG G G G
- A A A A T T T A CCCCG G G G
- A A A A T T T A CCCG CG G G
- A A CA T CT A CCA G CCT G
the evolutionary clock
Differences in polypeptide sequences accumulate steadily and gradually over time, as mutations occur from generation to generation in a species. Consequently, the changes can be used as a kind of clock to estimate how far back in time two related species split from a common ancestor.
By comparing homologous molecules from two related species, it is possible to count the number of places along the molecules where there are differences. If the molecule is mitochondrial DNA, for example, we count the number of base pairs that do not match. Mitochondrial DNA is particularly interesting to study because, unlike DNA found in the cell’s nucleus, it is not shuffled and mixed during meiosis or fertilization: it is passed on directly from mother to child without modification. This is why we can be sure that any modifications in mitochondrial DNA are due solely to mutations.
By comparing homologous molecules from two related species, it is possible to count the number of places along the molecules where there are differences. If the molecule is mitochondrial DNA, for example, we count the number of base pairs that do not match. Mitochondrial DNA is particularly interesting to study because, unlike DNA found in the cell’s nucleus, it is not shuffled and mixed during meiosis or fertilization: it is passed on directly from mother to child without modification. This is why we can be sure that any modifications in mitochondrial DNA are due solely to mutations.
We can take this further. If we see that 83 nucleotide differences is approximately three times more than 26 differences, we can hypothesize that the split between species A and species C happened about three times further back in the past than the split between the species A and B. This is the idea of using quantitative biochemical data as an evolutionary clock to estimate the time of the speciation events (see Figure 5.16).
analogous and homologous traits
ANALOGY
In examining the traits of organisms in order to put them into their appropriate clades, thorough and systematic studies of their characteristics must be undertaken. Two types of characteristic that are considered are homologous characteristics and analogous characteristics.
As we saw earlier in this chapter, homologous characteristics are ones derived from the same part of a common ancestor. The five-fingered limbs found in such diverse animals as humans, whales, and bats are examples of homologous anatomical structures. The shape and number of the bones may vary, and the function may vary,
but the general format is the same, and the conclusion is that the organisms that possess these limbs had a common ancestor.
Another example of a homologous characteristic is the presence of eyes. Such structures are seen in both vertebrates and invertebrates. Simple eyes found in molluscs such as the Nautilus function as pinhole cameras without a system of lenses, whereas highly evolved eyes like those of birds of prey use crystalline lenses, adjustable irises, and muscles to help focus on objects at different distances. Yet both types of eye have evolved from a common ancestor, because they all use one form or another of pigment cells and specialized nerve cells called photoreceptors that are light sensitive.Homology is observed in DNA sequences as well. Certain combinations of base pairs coding for similar proteins can be found in diverse organisms. As with homologous anatomical features, these sequences are evidence of a common ancestry.
HOMOLOGY
Homology is observed in DNA sequences as well. Certain combinations of base pairs coding for similar proteins can be found in diverse organisms. As with homologous anatomical features, these sequences are evidence of a common ancestry. The cytochrome c sequence studied in Section 3.1 is one example.
In contrast, analogous characteristics are those that may have the same function but they do not necessarily have the same structure and they are not derived from a common ancestor. Wings used for flying are an example: eagles, mosquitoes, bats, and extinct reptiles such as the pterosaurs all use (or used) wings to fly. Although
these organisms are all classified in the animal kingdom, they are certainly not placed in the same clade simply because of their ability to fly with wings. There are many other characteristics that must be considered.
Another example of an analogous characteristic is fins in aquatic organisms. Both sharks and dolphins have pectoral fins that serve a very similar function: helping them to swim well. But sharks are fish whereas dolphins are aquatic mammals, and the two are classified differently in both the Linnaean system and in cladistics.
In examining the traits of organisms in order to put them into their appropriate clades, thorough and systematic studies of their characteristics must be undertaken. Two types of characteristic that are considered are homologous characteristics and analogous characteristics.
As we saw earlier in this chapter, homologous characteristics are ones derived from the same part of a common ancestor. The five-fingered limbs found in such diverse animals as humans, whales, and bats are examples of homologous anatomical structures. The shape and number of the bones may vary, and the function may vary,
but the general format is the same, and the conclusion is that the organisms that possess these limbs had a common ancestor.
Another example of a homologous characteristic is the presence of eyes. Such structures are seen in both vertebrates and invertebrates. Simple eyes found in molluscs such as the Nautilus function as pinhole cameras without a system of lenses, whereas highly evolved eyes like those of birds of prey use crystalline lenses, adjustable irises, and muscles to help focus on objects at different distances. Yet both types of eye have evolved from a common ancestor, because they all use one form or another of pigment cells and specialized nerve cells called photoreceptors that are light sensitive.Homology is observed in DNA sequences as well. Certain combinations of base pairs coding for similar proteins can be found in diverse organisms. As with homologous anatomical features, these sequences are evidence of a common ancestry.
HOMOLOGY
Homology is observed in DNA sequences as well. Certain combinations of base pairs coding for similar proteins can be found in diverse organisms. As with homologous anatomical features, these sequences are evidence of a common ancestry. The cytochrome c sequence studied in Section 3.1 is one example.
In contrast, analogous characteristics are those that may have the same function but they do not necessarily have the same structure and they are not derived from a common ancestor. Wings used for flying are an example: eagles, mosquitoes, bats, and extinct reptiles such as the pterosaurs all use (or used) wings to fly. Although
these organisms are all classified in the animal kingdom, they are certainly not placed in the same clade simply because of their ability to fly with wings. There are many other characteristics that must be considered.
Another example of an analogous characteristic is fins in aquatic organisms. Both sharks and dolphins have pectoral fins that serve a very similar function: helping them to swim well. But sharks are fish whereas dolphins are aquatic mammals, and the two are classified differently in both the Linnaean system and in cladistics.
reclassification
From time to time, new evidence about a taxon requires a new classification. Either the taxon can be moved up or down the hierarchy (family to subfamily, for example), or from one family to another.
Plants commonly known as figworts used to be classified in the family Scrophulariaceae, and many of them have been used in herbal medicine. The name Scrophulariaceae, sometimes affectionately referred to by botanists as ‘scrophs’, comes from the time when plants were frequently named for the diseases they could be used to treat. The medical term ‘scrofula’ refers to an infection of the lymph nodes in the neck. Preparations made with figwort were given to patients who suffered from this infection, which was associated with tuberculosis.
Plants commonly known as figworts used to be classified in the family Scrophulariaceae, and many of them have been used in herbal medicine. The name Scrophulariaceae, sometimes affectionately referred to by botanists as ‘scrophs’, comes from the time when plants were frequently named for the diseases they could be used to treat. The medical term ‘scrofula’ refers to an infection of the lymph nodes in the neck. Preparations made with figwort were given to patients who suffered from this infection, which was associated with tuberculosis.
Theory of knowledge:
•Natural Selection is a theory. How much evidence is required to support a theory and what sort of counter evidence is required to refute it?
•Natural Selection is a theory. How much evidence is required to support a theory and what sort of counter evidence is required to refute it?