Bio 301E Handout 2 - Evolution by Natural Selection Sept 2, 4

Evolution is a change in the heritable characteristics of a population over time. In population genetics evolution is measured as a change in gene frequency in a population over time. There are several causes of evolutionary change including mutation, effects of random events, non-random mating, & natural selection.

Charles Darwin and Alfred Wallace independently came up with a mechanism for evolutionary change, evolution by natural selection. Their essential points (with modern ideas from population genetics added in underlined phrases) were: (a) Individual members of a population differ from each other in many ways (and at least some of this variation has a genetic basis so can be inherited from generation to generation) (b) In each generation, some individuals in a population survive and reproduce successfully but others do not. Individuals with traits that allow greater reproductive success in their given habitat (greater evolutionary fitness) succeed more and pass more copies of their genes to the next generation. (c) As a result of differential reproductive success in individuals with different heritable traits, there is change in the population over time. (The frequency of specific alleles in a population can increase or decrease from generation to generation.)

1. People originally thought that species were fixed and life forms did not change over time. What were some observations that caused scientists to begin thinking that the earth was very old and life on earth had evolved over time? Most of the evidence for an ancient Earth is contained in the rocks that form the Earths crust. The rock layers themselves-like pages in a long and complicated history-record the surface-shaping events of th past, and buried within them are traces of life-the plants and animals that evolved from organic structures that existed perhaps 3 billion years ago. Thus, the results of studies of rock layers (stratigraphy), and of fossils (paleontology), coupled with the ages of certain rocks as measured by atomic clocks (geochronology), attest to a very old Earth.

2. The rock pocket mouse is a living example of Darwins process of natural selection. The rock pocket mouse is found in two color variants, or morphs: light and dark. In different environments, their visibility to predators such as owls varies. The dark morph is more vulnerable on light sandy desert, and the light morph on dark lava rock. Apply the steps of evolution by natural selection to explain how a trait may change in prevalence in a population over time. There are two forms of pocket mice: light-colored mice that live on sandy soil, and dark-colored mice that live on black lava rock. The dark mice came about through the process of evolution. Naturally occurring mutations to coat-color genes produced mice with dark fur. On black rocks, dark mice had an advantage over light mice: they were better-hidden from predators. They survived and reproduced, passing their dark fur genes to their offspring, which will survive today. Once a favorable variation occurs, it can quickly become the major form in a population. Each year, mice produce more offspring than will reach adulthood. Thanks to natural selection, the offspring with favorable characteristics are more likely to survive and reproduce.

3. Tell how a new mutation could be either deleterious, neutral, or beneficial under given circumstances. Give an example of each. Can you think of an example in which a trait might be harmful under some conditions and beneficial under different conditions? Mutations are permanent changes in the DNA. Most mutations are neutral; they either make no change in the expression of any gene, or the changes made do not affect the function of any gene product. A large percentage of DNA is not part of any gene, and has no known function. Changes in these portions of the DNA do not alter any gene and are thus silent. Not all portions of a protein are equally important for the function of that protein. Even a mutation which changes an amino acid in the final gene product may not make a difference in the function of the protein, or may alter the precise nature of the function of the gene product without alter of the usefulness of that product. These would be neutral mutations. Of those mutations which do make a difference, most have a negative effect. If an amino acid is altered in the active site region of a protein, it may decrease or destroy the ability of the protein to perform its function. This kind of mutation often leads to a classic recessive allele- an allele which does not reproduce a functional gene product. A small percentage of mutations may

actually improve the function of the gene product or may convey a new or expanded function to that product. These mutations provide the grist for the evolutionary mill. Mutations can be harmful in some situations and beneficial in others. For example, if a mutation in a mice called for a light-colored fur this could be beneficial or harmful depending on the type of environment in which the mouse lives. If the mouse lives on black volcanic rock than a mutation that would create a light-colored fur would be harmful because it would make the mouse stand out to predators. However, if the mouse were to live in a sandy area then this mutation would be beneficial because it would allow the mouse to blend in with the sand, which would prevent in from being eaten by some of the predators.

4. Distinguish these three modes of selection: stabilizing, directional, and disruptive.

5. Give three examples of evolution by natural selection happening today.

1. For example, some beetles are green and some are brown. Since the environment cant support unlimited population growth, not all individuals get to reproduce to their full potential. In this example, green beetles tend to get eaten by birds and survive to reproduce less often than brown beetles do The surviving brown beetles have brown baby beetles because this trait has a genetic basis. The more advantageous trait, brown coloration, which allows the beetle to have more offspring becomes more common in the population. If this process continues, eventually, all individuals in the population will be brown. 2. In an ecosystem, some girraffes have longs necks and others have short ones. If something caused low lying shrubs to die out, the giraffes with short necks would not get enough food. After a few generations, all the giraffes would have long necks.

3. A species of rats live in a certain type of tree with the branches evenly spaced. Smaller rats could not reach from branch to branch and larger rats would break the branches and fall. Soon, all rates were just the right size for the tree branches. 4. Deer mice that migrated to the sand hills of Nebraska changed from dark brown to light brown to better hide from predators in the sand. 5. Galapagos finches all have different types of beaks. During drought, the finches with the larger beaks survived better than those with smaller beaks. During rainy times, more small weeds were produced and the finches with smaller beaks fared better.

6. The process of speciation bridges microevolution (evolution within a population) and macroevolution (evolution of higher groups above the species level) and occurs when populations become genetically isolated from one another. New species form from preexisting species, and species are lost through extinction. Black pocket mice have NOT evolved to be a different species. What would it take in order for that to happen? One species does not turn into another or several other species not in an instant, anyway. The evolutionary process of speciation is how one population of a species changes over time to the point where that population is distinct and can no longer interbreed with the parent population. IN order for one population to diverge enough from another to become a new species, there needs to be something to keep the populations form mixing. Often a physical boundary divides the species into two (or more) populations and keeps them from interbreeding. if separated for long enough and presented with sufficiently varied environmental conditions, each populations takes its own distinct evolutionary path. Sometimes the division between the populations is never breached, and reproductive isolation remains intact purely for geographical reasons. It is possible, though, if the populations have been separate for long enough, that even if brought back together and given the opportunity to interbreed they wont, or they wont be successful if they try.

7. For each of these statements indicate whether it is true or false and, if false, correct the statement.

See Howard Hughes Medical Center Institute for questions and answers. (a) T/F____ Mutations are caused by selective pressure in the environment. False; mutations are caused by genes and your DNA. (b) T/F____The same mutation could be advantageous in some environments but deleterious in others. True; sometimes you can blend in and not be seen by predators but in other places, you can stand out and be seen easily. (c) T/F____ Individual organisms can evolve during a single lifespan. False; animals evolve through reproduction and cannot evolve during a single lifespan. Populations, not individual organisms, evolve. Evolutionary change is based on changes of heritable genetic makeup of populations over time. (d) T/F____The appearance of dark-colored volcanic rock caused the mutation for black fur to appear in the rock pocket mouse population. False; selective pressure does not cause mutations but rather sometimes whether a mutation is advantageous or deleterious in a particular environment. Students may also mention that the dark-colored volcanic rock played a role in making dark-colored fur (and the corresponding alleles or mutations) favored, but this is not necessary for a complete answer. (e) T/F____ The fittest organisms in a population are those that are strongest, healthiest, fastest, and/or largest False; the fittest organisms in a population are not necessarily those that are the strongest, healthiest, fattest, and/or largest. For example, sometimes the smallest organism might be best able to hide in small spaces between rocks, etc.

Population Genetics worksheet and Hardy-Weinberg problems on Friday Sept 4.