5.4: What are the two ways that variation can occur in bacteria? I. Evolution a. At the age of 22 Charles Darwin set out on the HMS Beagle for a scientific experiment in 1831 that lasted for 5 years where he came up with the theory of evolution by natural selection. b. Evolution: The process of cumulative change in the heritable characteristics of a population. c. Over time, if enough changes occur in a population, a new species can arise. If this happens, then the new species will not be able to interbreed with the species that they diverged from. II. Evidence for Evolution 1. Fossil Record a. Paleontologists: people who collect and classify fossils to help show Earth’s evolutionary past. They have found that: Life which existed more than 500 million years ago was very different than life today. Although Earth has had extensive oceans for most of its existence, fish fossils have only been found in rocks 500 million years old or younger (less than 15% of the history of life). Although most of the predators today are mammals such as bears, orca whales, big cats, wolves, etc., none existed at the time of the dinosaurs or before. Apart from organisms such as certain types of sharks, cockroaches or ferns, many living organisms today have no identical form in the fossil record. 2. Artificial Selection a. Artificial selection: the art and science of breeding domesticated animals provides a good record of recent changes in heritable characteristics. This is evidence that evolution is happening today due to small changes over time, although it is not the driving force of evolution in natural ecosystems. 3. Homologous Anatomical Structures a. Homologous Anatomical Structures: structures that are similar in function but are found in seemingly dissimilar species that suggest that the different species share a common ancestor. Ex: Pentadactyl limbs (5 fingers) that are found in bat wings and whale fins. III. Mechanism for Evolution 1. Too Many Offspring
a. Struggle for Survival: Many organisms produce more offspring than can survive. There is high demand for water, space, nutrients, and sunlight, but there is a limited supply. The consequence is high competition for resources in order to stay alive. b. Competition for resources such as food can lead to adaptive behaviors by organisms. Ex: chicks will push other eggs out of the nest so that they can eat, a tree will have a growth spurt so as to get more sunlight than its neighbors, and deforestation by humans. 2. Variations within Populations a. Organisms that reproduce sexually can have many DNA variations that make them unique. 3. Variation and Success a. Variation is closely related to how successful an organism is. Ex: colors for camouflage, beak shape, flower color, etc.IV. Causes of Variety a. Besides choice of mate, there are two main reasons why organism show variation: Mutations in DNA Sexual reproduction greatly promotes variation within a species 1. Mutations a. Mutations can cause diseases, but can also sometimes produce a characteristic which is advantageous. 2. Sexual Reproduction a. In sexual reproduction there are two way in which genes are mixed: Meiosis Fertilization b. When an egg cell is made during meiosis, it receives 50% of the mother’s genetic material and 50% of the father’s. When the cell splits, chromosomes are distributed randomly so each egg cell has a different combination of chromosomes.V. Natural Selection a. Evolution is not solely based on chance. Exactly which animals will survive and which ones will not is determined by their surroundings and the compatibility if their characteristics with those surroundings. The steps of evolution by natural selection are: Overproduction of offspring and, in those offspring, natural variation due to genetic differences (e.g. body size, morphology pigmentation, visual acuity, resistance to disease.) In the offspring: - Useful variations allow an individual to have a better chance of survival (e.g. hiding from predators, fleeing danger, and finding food.) - Harmful variations make it difficult to survive (e.g. inappropriate color fur for camouflage, heavy bones for birds, having such a big body size that there isn’t enough food to survive.)
Individuals with genetic characteristic that are poorly adapted for their environment tend to be less successful at accessing resources and have less of a chance of surviving to maturity. Individuals with genetic characteristics that are well adapted for their environment tend to be more successful at accessing resources and have a better chance to survive to maturity. Over many generations, the accumulation of changes in the heritable characteristics of a population results in evolution – the gene pool has changed. b. Darwin adopted the idea of ‘survival of the fittest.’VI. Examples of Natural Selection 1. Antibiotic Resistance in Bacteria a. Antibiotics: medications such as penicillin which kill or inhibit the growth of bacteria. b. Overuse of antibiotics can lead to the production of resistant strains of bacteria. Antibiotic resistance in bacteria develops in several steps: A person gets sick from a bacterial infection such as tuberculosis. Her doctor gives her an antibiotic to kill the bacteria. She gets better because the bacteria are largely destroyed. By a modification of its genetic makeup, however, one bacterium is resistant to the antibiotic. That bacterium is not killed by the antibiotic and it later multiplies in the patient’s body to make her sick again. She goes back to the doctor and uses the same antibiotic. This time, no results – she is still sick and asks her doctor what is wrong. The doctor prescribes a different antibiotic which (hopefully) works. But if the bacterium continues to change its genetic makeup, it could become resistant to all the antibiotics available. c. Bacteria reproduce asexually, so there is not often a genetic change. There are two ways that can change the genetic makeup of bacteria: Mutations Plasmid transfer – One bacterium donates genetic information to another in ring of nucleotides called a plasmid. Both the donating and receiving cells open their cell walls to pass the genetic material from one donor to the receiver. 2. Pesticide Resistance in Rats a. The following example illustrates how a population can adapt to its environment and how humans can be responsible for creating super-resistant creatures. Consider the following: Once applied in the fields, pesticides kill all the rats… or so the farmer thinks.
Due to natural variations, a few rats are slightly different and aren’t affected by the poison. The resistant rats survive and reproduce, making a new population in which some or all of the members possess the genetic resistance. Seeing rats again, the farmer puts out more poison; this time fewer rats die, To kill the resistant rats, a new pesticide must be used.5.5 Name the seven levels of classification from smallest to largest. Taxonomy is the practice and science of classification. There are seven levels of classification:(Domain), Kingdom, Phylum, Class, Order, Family, Genus and Species. This system allows to grouporganisms while also assigning them unique species names. Species is a group of organism which caninterbreed and produce fertile offspring. In order to assign a name to an organism we use the binomialnomenclature (genus & species). In addition, by using a dichotomous key (a series of binary questions),we can identify an organism.