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Evolution continued. Phylogenetic trees. When we recognise that organisms share homologous features, we group them together to indicate that they are related. The pattern of how organisms are related through evolutionary descent from a common ancestor is termed phylogeny .
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Phylogenetic trees • When we recognise that organisms share homologous features, we group them together to indicate that they are related. • The pattern of how organisms are related through evolutionary descent from a common ancestor is termed phylogeny.
Based on these shared homologous features, we can build a picture of the relationships of organisms in the form of a branching diagram. • Such diagrams are called phylogenetic trees.
Consider three vertebrates: fish, bat and seal • Theoretically, there are three possible ways that these organisms are related, and thus three possible phylogenetictrees.
Tree 1 suggests that the seal and fishare most closely related relative to the bat, based on their similar body shape, which suits their aquatic lifestyle. • The second tree indicates that the seal and bat are the two most closely related. They have similar bones in their forelimbs, have hair, suckle their young, are endotherms, and share many other features. • The third alternative tree has little to support it because the bat and fishhave few features in common (other than the fact that they are vertebrates as is the seal).
How do we choose between these three possibilities? • We base it on evidence of shared homologous features. • Tree 2 is the one in which we can have most confidencebecause seals and bats share homologous features (e.g. hair)—evidence of them being part of the group, mammals.
Phylogentic trees revisited • Anatomically characterizing an organism involves two main approaches: studying the morphology of animals and analyzing the fossil record. • Molecularly characterizing an organism uses various sequencing techniques to identify similarities in genetic information between organisms as expressed in nucleic acids or proteins.
Phylogenetic trees are constructed to record the hypothesized classifications of organisms. • If a group of organisms is hypothesized to share a common ancestor, the group is referred to as monophyletic. • If members of a group did not all evolve from a common ancestor, the group is referred to as polyphyletic.
UNDERSTANDING RELATEDNESS USING ANATOMICAL CHARACTERIZATION • Morphology • The morphology of an organism is simply a description of its physical characteristics. If the organism under study is extinct or impossible to resolve with modern microscopy techniques, observing morphology is unfeasible. • The fossil record • The fossil record contains fossilized remains and imprints whose age is estimated by the age of the surrounded rock. The oldest known fossils are believed to be approximately 3.5 billion years old and represent the existence of bacterium-like life. An example of a dating technique used to determine the ages of rocks and fossils on a scale of absolute time is radiometric dating.
How to create a phylogenetic tree Carefully look at the list of cars and trucks below: • car with three wheels and one seat • car with four wheels, four doors , front and back seat • car with four wheels, two doors, front seat only • car with four wheels, two doors, front and back seat, no top • truck with four wheels, two doors, one seat and a short bed • truck with four wheels, two doors, one seat and a long bed • truck with six wheels, two doors and only one seat • truck with six wheels, four doors, front and back seat In this activity we are going to consider these vehicles to be organisms that are all related in an evolutionary way.
As you study this list of vehicles think about the characteristics that can be used to show relationships. The number of wheels and the number of doors are two that come immediately to mind, but there are others as well.
Using a pencil, draw a branched phylogenetic tree starting at the bottom of a sheet of paper. • Start with what you think is the most primitive vehicle. • Remember that scientific knowledge grows by trial and error. All theories or interpretations are open to revision. • There will be a few questions that follow.
Vehicles: • car with three wheels and one seat • car with four wheels, four doors , front and back seat • car with four wheels, two doors, front seat only • car with four wheels, two doors, front and back seat, no top • truck with four wheels, two doors, one seat and a short bed • truck with four wheels, two doors, one seat and a long bed • truck with six wheels, two doors and only one seat • truck with six wheels, four doors, front and back seat
Questions • Explain several of the branching points on your tree. • Which vehicle seems to be the most primitive? Justify your answer. • Which vehicles seem to be the most advanced? Justify your answer.
Something a little more realistic: • Hypothesize the appearance of the part of the morphological tree that shows the relationships between gorillas, chimpanzees, and humans. • On a sheet of notebook paper, they make a diagram of their hypotheses by drawing lines from Point A to each of the three organisms (G = gorilla, C = chimpanzee, H = human, A = common ancestor).
Modern research techniques allow biologists to compare the DNA that codes for certain proteins and to make predictions about the relatedness of the organisms from which they took the DNA. • You will use models of these techniques to test their hypotheses and determine which one is best supported by the data they develop.
Copy out the following strands of DNA: Label this strand "human DNA." This strand represents a small section of the gene that codes for human hemoglobin protein. Position 1 Position 20 A-G-G-C-A-T-A-A-A-C-C-A-A-C-C-G-A-T-T-A
Copy out the following strands of DNA: • Label this strand "chimpanzee DNA." This strand represents a small section of the gene that codes for chimpanzee hemoglobin protein. Position 1 Position 20 A-G-G-C-C-C-C-T-T-C-C-A-A-C-C-G-A-T-T-A
Copy out the following strands of DNA: • Label this strand "gorilla DNA." This strand represents a small section of the gene that codes for gorilla hemoglobin protein. Position 1 Position 20 A-G-G-C-C-C-C-T-T-C-C-A-A-C-C-A-G-G-C-C
Copy out the following strands of DNA: • Label this strand "common ancestor DNA." This DNA strand represents a small section of the gene that codes for the hemoglobin protein of a common ancestor of the gorilla, chimpanzee, and human. Position 1 Position 20 A-G-G-C-C-G-G-C-T-C-C-A-A-C-C-A-G-G-C-C
Compare the human DNA to the chimpanzee DNA by matching the strands base by base. • Count the number of bases that are not the same. • Record the data in a table. • Repeat these steps with the human DNA and the gorilla DNA.
Genetic linkage groups • Humans and cats are both mammals, although they belong to different groups. • Because they need many of the same gene products (enzymes) to function, cats and humans have many of the same genes.
Genetic linkage groups • One method of analysing how similar humans are to cats is to look at the linkage groups in both species. • A genetic linkage group is a group of genetic loci close together on the same chromosome.
Genetic linkage groups • Because they so close together, the genes rarely segregate. • The linkage groups of a number of genes are known for humans. • If groups of linked genes in humans are compared with the same genes in cats, they are also generally found to be linked.
Examples of the linkage relationship of genes in humans and the domestic cat.
Similarly, when genes of similar function in the vinegar flyDrosophila melanogaster and the Australian sheep blowflyLuciliacuprina are analysed, they are found to be linked in both species. • The simplest explanation for the similarity of linkage groups in humans and cats or among fliesis that these organisms have a common ancestry.
In summary • Homologous features between different organisms are evidence of evolution from a common ancestor. • Analogous features are features with the same function but have evolved independently in different groups of organisms. Comparative anatomy can reveal that they are structured differently. • Embryonic comparisons show that general features of large groups of organisms appear early in development. More specialised features, which distinguish the members of a group, appear later in development. • The construction of a phylogenetic tree based on the sharing of homologous characteristics by organisms is consistent with the theory of evolution.
In summary • A group of genes may be tightly linked and rarely segregate. They are inherited as a group. • The similarity of genetic linkage groups between species provides evidence that the species have a common ancestry.
Major players in modern evolution theory • Jean Baptiste Lamarck (1744–1829): • A French naturalist who was firstto publish a reasoned theory of evolution. In France, he is regarded as the ‘father of evolution’, but he died in poverty and was scorned because of his theory of the inheritance of acquired characteristics.
Major players in modern evolution theory • Charles Darwin (1809–1882) • An Englishman who started training in medicine. He sailed as a naturalist on the HMS Beagle, collecting information that led him to his theory of evolution by natural selection. Under the influenceof natural selection, those individuals in a variable population that are best suited to the environment have the greatest chance of surviving and reproducing.
Major players in modern evolution theory • Alfred Russel Wallace (1823–1913) • An Englishman who travelled and collected specimens in the Amazon and the region of Indo-Malaya. While there, he independently came up with the same idea of evolution by natural selection as Darwin (which spurred Darwin on to publish his work On the Origin of Species).
At the time Lamarck, Darwin and Wallace were writing, people believed that species were fixedand did not change. • Lamarck was the firstto challenge this idea and publish a modern theory of evolution. • His theory stated that characteristics were inherited by subsequent generations and that characteristics within populations change over time. • However, he proposed that characteristics were acquired by organisms as the need arose—the neck of the giraffe got longer as the giraffe stretched its neck to reach higher and higher leaves on trees!
Darwin on to publish his work On the Origin of Species). • Darwin also believed in the inheritance of acquired characteristics because in his time no one knew about genes and chromosomes. • Mendel’s genetic theory provided the explanation that segregation of alleles allows variation to be passed from generation to generation. • While Mendel’s theory overcame the puzzle of Darwin and Wallace, it took almost three-quarters of a century before the contributions of Darwin, Wallace and Mendel were moulded together as the theory of evolution that is currently accepted.
Evolution • All the genes and their allelic forms in a population constitute a gene pool. • Evolution is the genetic change in the gene pool of a population over time. • During the course of evolution, organisms respond to environmental changes, some surviving and leaving offspring, others not.
Evolution • Thus, the particular allele carried by the most successful individuals in a population will increase in frequency over time. (A frequency is a percentage expressed as a decimal, e.g., 100% = 0.1). • Shifting allele frequencies in a population are what drive evolutionary change.
Why evolution? • Understanding how evolution occurs—the underlying mechanisms— gives us tools, for example, to make the most effective use of reduced levels of pesticides. • The evolution of insecticide resistance in heliothis moths is just one example.
The modern theory of evolution • Can be summarised in the following seven points: • Reproduction: Reproduction of organisms in a population produces descendant populations. • Excess of potential offspring: Parents have the potential to produce many more offspring than actually survive. • Variation: Members of a population vary. Variation that is genetically based (heritable) is passed on to offspring.
Selection: Environmental resources, such as food and nest sites, are limited, so there is competition between individuals. Individuals that can compete successfully will leave a greater proportion of offspring than less successful individuals. In this way their characteristics are selected. The limiting factor acts as a selection pressure. • Adaptation over time: Environments change over time. Heritable characteristics that suit a particular environment will be selected. Populations diverge over time and become adapted to new conditions. • Chance effects: In small populations, shifts in the frequency of certain characteristics can also occur by chance. • Divergence and speciation: When populations are geographically isolated and thus cannot interbreed, divergence over time may result in them becoming different species.
Genetic variation as a basis of evolution • Nearly all populations show variation between individuals for particular traits. • The human population, for example, shows considerable variation in hair, skin and eye colour (look around you at your classmates).
When members of a population show variation in a trait, such as floweror feather colour, the population is described as being polymorphic (poly meaning many, morph meaning form).
How does polymorphism (variation) arise in populations? • Mutations • Sexual Reproduction
Mutations • Mutations produce genotypic and consequently phenotypic differences between individuals. • The probability that a mutation occurs in any given generation is low. Therefore, while of critical importance in producing new genotypes, mutation alone does not account for significantchanges in the genetic make-up of a population. birds and the burning of their habitat too often.
Sexual Reproduction • Sexual reproduction, on the other hand, generates significantamounts of variation in every generation through genetic recombination, acting on the variation that already exists in a population. • Recombination occurs either between chromosomes (independent assortment) or within chromosomes (crossing over) during meiosis. • More new genotypic combinations are produced by recombination than are possible by mutation alone.
Asexual reproduction • In contrast to sexually reproducing organisms, some organisms reproduce asexually and form clones, with no genetic variation in a population. • Organisms that reproduce asexually are often found in environments that do not change very much over time.
