Evolution continued

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.

• 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 phylogenetic trees.

• Tree 1 suggests that the seal and fish are 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 fish have 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 confidence because 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 treeCarefully 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 20A-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 20A-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 20A-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 20A-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 comparisons

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 fly Drosophila melanogaster and the Australian sheep blowfly Lucilia cuprina 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 flies is 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.

Evolution—genetic change over time

Major players in modern evolution theory

• Jean Baptiste Lamarck (1744–1829):– A French naturalist who was first to 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 influence of 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 fixed and did not change.

• Lamarck was the first to 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 flower or 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 significant changes 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

significant amounts 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.

Selection

• It is because populations are variable that evolutionary mechanisms can bring about change over time.

• The main mechanism is by selection. • Selection occurs because some individuals,

with particular favourable features, have a greater chance than others of leaving fertile offspring.

Survival of the fittest

• You have probably heard the expression ‘survival of the fittest’.

• When selection occurs, the evolutionary success of an individual depends on how fit he or she is.

• The ‘fittest’ are those phenotypes that are best suited to the environment, relative to other phenotypes present.

• In evolutionary terms, biological fitness is measured as the relative proportion of offspring that an individual leaves to the next generation.

Survival of the fittest

• The inherited characteristic that allows the individual to survive and reproduce is called an adaptation.

• The adaptation of the resistant heliothis moths is their biochemical resistance to insecticide.

• Fit individuals are said to be adapted to an environment in which pyrethroids are used.

Survival of the fitest

• Fitest is always relative because the impact of natural selection depends on both the environment and the genotypes present in a population.

• The relative fitness of a phenotype in one environment may be different from the relative fitness of that phenotype in different environmental conditions.

Natural and artificial selection

• If the fitness of an individual is determined by the natural environment (e.g., succulent plants in coastal salty environments), evolutionary change occurs as a result of natural selection.

• If humans decide which animals or plants should leave offspring to the next generation, as is the case in animal and plant breeding programs, changes in the phenotypic and genotypic make- up of the population occur because of artificial selection.

• Based on a common ancestral population of horses, artificial selection by horse breeders has produced a number of breeds of very different phenotype. Examples of breeds are

• Humans have deliberately selected horses with particular characteristics for breeding, resulting in a number of different breeds arising.

• Based on a common ancestral population of horses, artificial selection by horse breeders has produced a number of breeds of very different phenotype.

Artificial selection in dogs

• ‘Man’s best friend’, the dog, shows an amazing range of variation, probably more than any other domestic animal.

• The domestic dog is descended from the wolf Canis lupus. Archaeological evidence indicates that dogs have been domesticated for at least 14 000 years.

• The many breeds of dogs are an indication of the considerable genetic variation inherent in Canis lupus.

Defects in dogs

• Boxers have been selected for their muscular body and flattened face. Their short nose and jaw led to respiratory and dental problems in many cases.

• The breed also suffers a higher than normal incidence of cancer.

• Cocker spaniels have sore eyes due to extra lashes on the inside of the eyelid, and commonly have severe bacterial ear infections resulting from lack of air circulation and high humidity caused by floppy and hairy ears.

Selection does not generate variation

• It is important to remember that selection acts on the variation already present in a population, largely due to genetic recombination.

• Variation initially comes from random mutations and is not acquired through some need.

Selection in action

• The ideas discussed so far—that evolutionary change results from selection acting on genetic variation in populations over time—can be illustrated by the following two case studies:– Case study 1: Drunken Drosophila – Case study 2: The Australian sheep blowfly

Case study 1: Drunken Drosophila

• As far as winemakers are concerned, the vinegar fly Drosophila melanogaster is inappropriately named as it is often found in winery cellars and in surrounding vineyards.

• Laboratory tests show that concentrations of ethanol (6% by volume) do not greatly affect D. melanogaster but kill other species such as the closely related D. simulans.

• In seepage from casks in the cellar, where D. melanogaster feeds and breeds, ethanol concentrations can be in excess of 6% by volume.

• Tolerance of ethanol is under genetic control. Drosophila melanogaster carries the genes for alcohol tolerance and D. simulans does not.

• Field experiments show that D. melanogaster is particularly attracted towards the cellars during the vintage period when grapes are crushed and fermentation of grape sugars to alcohol occurs.

• There is also evidence that tolerance of ethanol varies between different populations of D. melanogaster.

• Flies found in the cellar can tolerate higher concentrations of ethanol than those collected in the nearby vineyard.

• In contrast, populations of D. melanogaster collected from areas 100 kilometres from the vineyard have the lowest levels of tolerance to ethanol.

• This result shows that tolerance to alcohol can be selected for in D. melanogaster.

This result shows Alcohol tolerance, a genetically inherited trait, increases in populations of Drosophila melanogaster the closer the population is to an alcohol source in wine cellars and vineyards.

Case study 2: The Australian sheep blowfly

• The Australian sheep blowfly (Lucilia cuprina) is a significant pest of the wool and sheep meat industries.

• The blowfly is the primary cause of flystrike in Australia.

• During flystrike, larvae develop on sheep, causing open wounds and influencing sheep survival, fleece production and weight gain.

• The damage is done by the pest during the larval stage of its lifestyle.

Chemical control of the pest

• Various management programs have been used to control the blowfly, but the major form of control is through the use of chemical insecticides.

• These are applied to the sheep by dipping or jetting (spraying) to provide protection from flystrike.

• In the case of the use of dieldrin, control was excellent between 1955 and 1958.

• When it was first used, this insecticide gave approximately twelve weeks protection from flystrike.

• However, the evolution of insecticide resistance by the blowfly reduced the protection period and effective control of the pest ceased.

• Within two years the protection period had decreased to two weeks, too short a period for dieldrin to be useful in blowfly control.

• This required the introduction of a new insecticide to achieve satisfactory protection from the pest.

Genetic resistance to dieldrin

• Resistance to dieldrin is determined by a single gene, with two alleles RR and RS.

• Genotype RRRR specifies a highly resistant phenotype that survives concentrations of dieldrin that kill the other blowfly phenotypes.

• Genotype RSRS specifies the sensitive, susceptible phenotype (killed by the insecticide).

• It is reasonable to assume that the RR allele was at a very low frequency in the sheep blowfly population before the introduction of dieldrin for blowfly control.

Natural selection at work

• The effect of natural selection on the pattern of dieldrin resistance is easy to explain.

• Susceptible individuals are poisoned by the insecticide and either die or, at the very least, have their normal development severely disrupted.

• In either case, susceptible blowflies have little chance of becoming adults and of leaving fertile offspring in the next generation; their fitness is greatly reduced relative to resistant phenotypes.

• The fl exibility of a species to adapt to environmental change depends on a number of characteristics such as population size and generation time. The Australian sheep blowfl y has shown this fl exibility. Evolution occurs when natural selection acts on genetically based phenotypic variation present in natural populations.

Selection pressures

• Selection pressures include all the abiotic and biotic components of the external and internal environment of an organism that influence population change over time.

• Selection factors include not only pesticides but also climate, soil, fire, availability of food resources, predators, parasites and so on.