Microevolution: Unique Gene Pools It is likely that you taught first-year biology students about the “peppered moth”. Prior to starting this unit, assign students a task. Have them go to this website http://www.techapps.net/interactives/pepperMoths.swf and work their way through the components of the animations. The site is very well done and it will serve as nice introduction as well as a good review of basic evolutionary concepts and how man and the environment impact natural selection.

Charles Darwin Charles Darwin (1809-1882) is credited with proposing that the mechanism for the process of evolution is natural selection. Darwin spent five years on a voyage that took him around the world with the majority of his time spent in South America and its neighboring islands. Darwin published his theory with compelling evidence for evolution in his 1859 book On the Origin of Species, overcoming scientific rejection of earlier concepts of transmutation of species. http://en.wikipedia.org/wiki/Charles_Darwin contains FAR more information about Charles Darwin.

Charles Darwin He established that all species of life have descended over time from common ancestors, and proposed the scientific theory that this branching pattern of evolution resulted from a process that he called natural selection, in which the struggle for existence has a similar effect to the artificial selection involved in selective breeding. http://en.wikipedia.org/wiki/Charles_Darwin

Charles Darwin By the 1870s the scientific community and much of the general public had accepted evolution as a fact. However, many favored competing explanations and it was not until the emergence of the modern evolutionary synthesis from the 1930s to the 1950s that a broad consensus developed in which natural selection was the basic mechanism of evolution. In modified form, Darwin's scientific discovery is the unifying theory of the life sciences, explaining the diversity of life. http://en.wikipedia.org/wiki/Charles_Darwin

Darwin’s Observations Populations change over time as evidenced by the fossil record. There are always more offspring produced than the preceding generation. Populations, if left unchecked, grow at a geometric rate rather than an arithmetic rate. Darwin used an example involving elephants to illustrate the points above. He estimated that if elephants underwent unrestricted reproduction, that in 740-750 years there would be 19 million elephants produced from just one original pair. Darwin's example considers the "slowest breeder of all the animals", elephants. Also point out his math was a bit flawed since he neglected death rates AND did not assume a 50-50 split of male vs. female offspring!

Darwin’s Observations Students will most likely ask you, “What’s the difference between geometric rate and arithmetic rate?” The simple answer is arithmetic rates are linear, i.e. 2, 4, 6, 8, 10 etc. while geometric rates are not linear but rely on a different mathematical function such as an exponential function. These observations are what helped Charles Darwin formulate natural selection as the mechanism for evolution. Natural selection is the “theory of evolution” . Evolution does occur. Species do change over time as the fossil record demonstrates. http://www.idlex.freeserve.co.uk/idle/evolution/sex/elephant.html provides a nice explanation of the “math”

Darwin’s Elephant Problem “There is no exception to the rule that every organic being naturally increase at so high a rate that if not destroyed, the earth would soon be covered by the progeny of a single pair .... The Elephant is reckoned to be the slowest breeder of all known animals, and I have taken some pains to estimate its probable minimum rate of natural increase: it will be under the mark to assume that it breeds when thirty years old, and goes on breeding till ninety years old, bringing forth three pairs of young in this interval; if this be so, at the end of the fifth century there would be alive fifteen million elephants, descended from the first pair.” (Darwin, 1859 p.64) This site has a “calculator” that lets students set parameters relating to Darwin’s Elephant Problem so they can understand just how wrong Darwin was. http://www.athro.com/evo/elframe.html

Darwin’s Observations There is variation within a given species and the majority of this variation is inherited. This litter of kittens vary with respect to coat pattern and color. Any variation may, to some degree, affect the ability of an organism to reproduce and contribute genes to the gene pool, thus affecting evolutionary success. Species change over time. These changes are related to traits that are inherited or arise from an alteration of the genetic code. Some inherited traits are beneficial and contribute to survival. Whether a trait is beneficial or not is a function of the environment in which it lives. Emphasize that there is a difference between heredity and mutation.

Adaptations and Fitness An adaptation is a genetically controlled trait that is favored by natural selection and gives the organism a reproductive advantage ensuring the trait is passed on to its descendants. This trait may also allow the individual to survive longer thus increasing the reproductive rate of that individual. Ask students to compare these two hares and identify the differences in their traits. The students should come up with coat color, length of ears, length of limbs, body shape, etc.

Adaptations and Fitness The antelope hare lives in the desert, and the snowshoe hare lives in the mountains. Explain how the differences in their traits enhance their ability to survive in their respective environments. Evolutionary success or fitness refers to the contribution of genes to the gene pool and NOT how long an organism lives. It’s one thing to “identify” traits and yet another to explain their importance or implication! The long limbs of the antelope hare help dissipate body heat and keep the hare cool. The brown coat color helps it to blend in with its environment, thus be less obvious to potential predators. The snowshoe hare has smaller ears and shorter limbs with a rounder body. This helps keep the hare warmer. The white coat color helps it to also blend in with its environment.

The Effect of Environmental Change Earth’s environment is NOT STATIC, but rather ever changing. As a consequence, traits or adaptations that were favorable may become unfavorable. The peppered moth, Biston betularia is native to England and exists in two forms, one is dark and the other light with a “peppered” appearance. Birds are its main predator. Prior to the industrial revolution, only 2% of the moths were dark. The industrial revolution produced vast amounts of sulfur dioxide and soot from the burning of coal which altered the environment. Fifty years later 95% of the moths were dark. Propose an explanation! Environmental changes often cause a shift in selection pressures. Traits that were once beneficial to a population of organisms may become detrimental and vice versa. http://www.techapps.net/interactives/pepperMoths.swf

Industrial Melanism England has since regulated the burning of coal and as a result, the trees are returning to their original state (A). Consequently, the coloring among the population of moths in Britain has shifted back so that the peppered moths are once again favored. Explanation: The trees were previously light and covered in lichens, thus peppered moths had the advantage of camouflage over dark moths. (You may have to point out the peppered moth near the top of photo A.) The SO2 gas produced from the industrial revolution killed the lichens. Furthermore, the soot produced during the burning of coal collected on tree trunks changing their appearance and darkening them. As a result, the darker moth is now more camouflaged and less likely to be eaten by birds.

Evolution Defined Evolution is defined as a change in the inherited characteristics of biological populations over successive generations. Evolutionary processes give rise to diversity at every level of biological organization, from the molecular to the macroscopic. As a result diversity is prevalent among molecules such as DNA as well as individual organisms and species of organisms. Students need to be clear on the approaching definitions and distinctions AND have some illustrative examples they can discuss on the AP Exam.

Microevolution Microevolution is simply a change in gene frequency within a population. Evolution at this scale can be observed over short periods of time such as from one generation to the next. Example: The frequency of a gene for pesticide resistance in a population of crop pests increases. Such a change might come about because natural selection favored the gene the population received new immigrants carrying the gene (gene flow) nonresistant genes mutated into a resistant version of the gene of random genetic drift from one generation to the next Remind students that a gene is a sequence of DNA nucleotides that specify a particular polypeptide chain and that genes code for proteins. Have students generate an example for each of the 4 example “causes” of microevolution. Many correct answers are possible! natural selection favored the gene (sickle cell anemia & malaria) the population received new immigrants carrying the gene (light skinned population crosses with dark skinned population resulting in hybrids) some nonresistant genes mutated to the resistant version (natural immunities to disease among populations) because of random genetic drift from one generation to the next (founder effect)

Microevolution A gene is a sequence of DNA nucleotides that specify a particular polypeptide chain. Genes code for proteins. An allele is a particular form of a gene. For example: B represents the allele for black coat color and b for white coat color. Selection acts on phenotype because differential reproduction and survivorship depend on phenotype not genotype. Natural selection acts on individuals, but only populations evolve. Really emphasize those last two bullets! When students write about evolution it is VERY important that they can “say what they mean and mean what they say”. These last two bullets belong in the response to any evolution free-response question!

Macroevolution Macroevolution is evolution on a scale of separated gene pools (not individuals). Think of it as an accumulation of changes which result in speciation (forming a new species). Macroevolutionary studies focus on change that occurs at or above the level of species, in contrast with microevolution, which refers to smaller evolutionary changes (typically described as changes in allele frequencies) within a species or population. The process of speciation may fall within the purview of either, depending on the forces thought to drive it. Macroevolution will be dealt with separately and the definition of macroevolution varies by textbook. -it can explain the evolution of more complicated things like the how the eye evolved -it can explain how speciation occurs -it can explain how populations on a broader scale evolve

More Evolution Terms Species-a group of interbreeding organisms that produce viable and fertile offspring in nature Gene pool-sum total of all the genes in a given species Allelic frequency-is the percent occurrence for a given allele

Sources of Genetic Variation How does variation in a population or gene pool arise? Mutations, gene duplication and chromosome fusion provide the raw material for evolution. Meiosis and sexual reproduction produce new recombinants of phenotypes upon which natural selection operates. The wisteria pictured on the right has a mutation causing it to produce white flowers instead of purple flowers. Emphasize the importance of meiosis and sexual reproduction as the driving force of evolution. Meiosis is responsible new phenotypic combinations upon which natural selection can act. Meiosis recombines alleles in new combinations  which results in unique gametes due to the way chromosomes line up on the metaphase plate and crossing over.  Meiosis coupled with fertilization produces offspring with different combinations of alleles.  The genetic complement that the zygote receives will be different from either parent and  is different from any sibling.   Identical twins are genetically identical to one another but the likelihood that two siblings (not identical twins) will be genetically identical is extremely remote 223 x 223.  Tell students that the genetic “shuffling” during meiosis is like getting a new hand when playing cards.

Types of Mutations MOST mutations are deleterious as well as recessive. Obviously, mutations occurring in somatic cells do not affect future generations. Only mutations occurring in gametes affect future generations. Mutations can occur at either the gene or chromosomal level. Mutations may cause a sheep to have a 5th leg. But this is not evolution!  Emphasize that mutations lead to genetic variation. These next slides explain each of these mutation types. Students should have prior knowledge of genes, DNA, and mutations from Pre-AP Biology.

Point Mutations: Synonymous vs. Nonsynonymous Point mutations occur when one nucleotide is substituted for another. The genetic code contains “synonyms” for the coding of amino acids. For example the DNA codons GGA, GGG, GGT, GGC all code for the amino acid proline. Therefore, as long as the codon has GG in positions 1 & 2, a mutation in position three has no consequence, proline will be coded for regardless. This sort of mutation is called a synonymous or silent mutation. Students should be aware of DNA, RNA, codons, amino acids and the mechanisms of protein synthesis. If it has been a year or two since their first biology course, you may have to refresh their memory. Also, it’s the synonyms (pun intended) that will confuse students—too bad we can’t decide on a single term to describe a single anomaly!

Point Mutations: Synonymous vs. Nonsynonymous Point mutations that do result in a different amino acid are called a nonsynonymous or missense mutations. Missense mutations can affect the protein in one of THREE ways: (Remember the new amino acid will have a different R group on the protein) It can result in a protein that does not function as well as the original protein. (This happens most often.) It can result in a protein that functions better than the original protein. It can result in a protein that functions like the original protein. This is usually because the R groups are similar. (both polar or both nonpolar, etc.) Emphasize that intermolecular force interactions between R groups establish the shape of a protein, thus establishing its function.

Gene Duplication Genes can be duplicated and occasionally the duplication moves a gene from one chromosome to another. Each gene will accumulate different mutations altering the protein that is subsequently synthesized. Myoglobin is a protein that binds with oxygen in the muscles. This gene has been duplicated and modified many times. It has given rise to the hemoglobin gene. Analogy: Evolution is more like editing a book rather than writing a book from scratch. Genes are duplicated and then moved; once duplicated genes are moved, they experience different mutations which can result in different proteins. As you might imagine, there can also be cutting and pasting or copying and pasting errors!

Neutral Mutations Naturally evolving proteins gradually accumulate mutations while continuing to fold into stable structures. This process of neutral evolution is an important mode of genetic change and forms the basis for the molecular clock. Cytochrome c is a small protein found on the mitochondrial membrane. Between mammals and reptiles there are 15 different amino acids or mutations. This is illustrating that there are some proteins that have been around for a “long time”. That being the case, over time, these proteins experience mutations that do not affect the function of a protein. For example both human and dog hemoglobin found in red blood cells carry oxygen, yet the two proteins vary only slightly in their amino acid sequencing. Cytochrome c is an example of this as well. The average rate of these mutations can be used as an evolutionary clock.

Neutral Mutations Mammals and reptiles diverged 265 million years ago. That means on average cytochrome c mutated every 17 million years. In comparing the evolution of other organisms and their cytochrome c one mutation every 17 million years holds true.

Changes in Cytochrome C Explain that a pseudogene is a gene that has been duplicated but certain mutations have rendered this gene nonfunctional so it is never transcribed or translated. It is part of the genome that is simply conserved. Above is a comparison ancestral cytochrome c and human cytochrome c. This gene has been highly conserved as it is a protein used in the electron transport chain of the mitochondria. Missense mutations occur more frequently in pseudogenes (genes that have been duplicated, then mutated and are no longer functional) than in functional genes.

Cytochrome c Comparison A dash indicates that the amino acid is the same one found at that position in the human molecule. All the vertebrate cytochromes (the first four) start with glycine (Gly). The Drosophila, wheat, and yeast cytochromes have several amino acids that precede the sequence shown here (indicated by <<<). In every case, the heme group of the cytochrome is attached to Cys-14 and Cys-17 (human numbering). Assign the Amino Acid Sequences and Evolutionary Relationships activity as homework!

Hemoglobin Comparison This is a comparison between the differences in the amino acid sequence of human hemoglobin and different species. The last three species do not have a distinction between a and b chains. There is an inverse relationship between the difference in the amino acid sequence and how closely related the organisms are to humans. The b chain of hemoglobin has 146 amino acids. Ask the students to speculate as to why soybeans might have a protein similar to hemoglobin. Leghemoglobin removes oxygen that would kill a bacteria living in the root of the soybean. This bacterium “fixes” nitrogen in the roots of the soybean or takes nitrogen from the air and changes it into a usable form that the plant can use.

Hemoglobin Comparison A nice visual representation of the biochemical differences.

Frameshift Mutation A frameshift mutation occurs as a result of either an insertion or deletion of a nucleotide. This changes the amino acid sequence of the protein from that point forward. Almost all frame shift mutations are deleterious. Recently, bacteria were found growing in a pool of nylon wastes. (Flavobacterium) These bacteria were actually digesting the nylon waste. Upon examining the genome of these bacteria, it was found there was a frameshift mutation in their DNA that caused the production of three different enzymes that could digest the nylon. Interesting example but emphasize that most frameshift mutations do not result in a functional protein.

Evolution of Hemoglobin Gene Take some time to explain this slide and the evolution of hemoglobin genes. It drives home the point that genes are duplicated and experience different mutations.

Chromosomal Rearrangement There have also been major changes in chromosome structure that result in changes within populations which can, in turn, result in the emergence of new species. These include: inversions deletions duplication translocations fusions Chromosomal rearrangement is another source of genetic variation.

Chromosomal Rearrangement Compare the karyotype of a human (H) and a chimpanzee (C). Notice the great apes have 24 pairs of chromosomes compared to 23 pairs of chromosomes in a human. Why the difference? Chromosome #2 in the human is the result of a fusion of two chimpanzee chromosomes. Chromosomal rearrangement is another source of genetic variation.

Human Impact on Gene Pools Ask students to give examples of artificial selection. They should come up with dog breeding, horse breeding, disease-resistant crops, etc. It is well documented that humans have had an impact on certain gene pools. For example, humans have selected for certain desirable traits within the mustard family and cultivated different agricultural products for human consumption.

Artificial Selection When humans manipulate a gene pool it is called artificial selection. There are often consequences involved in such manipulations. For example in agriculture, farmers try to increase crop production, which may lead to many farmers growing only one variety of a particular crop such as corn. This leads to a loss of genetic diversity. If a disease attacks that particular variety of corn, the farmers growing that variety lose their entire crop. Emphasize the point that artificial selection can lead to the loss of genetic diversity. This has led to the creation of the Svalbard Global Seed Vault which on an island off of Norway. It is a place where seeds are kept and maintained to preserve genetic diversity.

Antibiotics and Artificial Selection When antibiotics are applied to a population of microorganisms to treat an infection, some of the microorganisms may be naturally immune to the drug. Why? A random mutation occurred in the genetic code of the microorganism conferring its resistance. These resistant microorganisms continue to flourish and cause disease. The only remaining option a physician has is to treat the infection with a different antibiotic and hope that none of the surviving microorganisms possess a different random mutation that makes them resistant to the second antibiotic as well. This concept of “things” becoming resistant to some sort of human treatment as a result of artificial selection is important. Examples of this include antibiotic resistant bacteria, herbicide resistant weeds and insecticide resistant insects.

Antibiotics and Artificial Selection The increase in antibiotic-resistant bacteria has caused doctors to reduce the number of prescriptions written for antibiotics in general. About 70% of pathogenic bacteria are resistant to at least one antibiotic and are called “super bugs” or MDR bacteria. (multidrug resistant) The mechanism pictured is that of penicillin. (note the artist’s use of the “P”s). It is illustrating how the drug attacks a bacterial pathogen. There are many related topics of discussion you can explore with students if time permits such as: Are we overusing antibiotics? Is it a good idea to let your immune system struggle for a few days at the first sign of a cold? If fever is our immune system’s first response to infection, what purpose is it serving?

MRSA or Methicillin-resistant Staphylococcus aureus MDR bacteria do not respond to “first line of defense” antibiotics. These types of bacteria are most commonly found in hospitals. Skin boils or similar lesions that do not heal often result. MDR bacteria can attack internal organs upon gaining entry into the body. Depending upon where you live, MDR bacteria are also commonly found on sports equipment in locker rooms as well as nail salons.

Reducing or Eliminating Gene Pools Human activities often augment genetic drift and diminish gene flow for many species. This reduces genetic variation thereby disrupting adaptive processes both locally and globally within a species. This impact is illustrated within populations of collared lizards (Crotaphytus collaris) living in the Missouri Ozarks. Forest fire suppression has reduced habitat and disrupted gene flow in this lizard, thereby altering the balance toward drift and away from gene flow. This balance can be restored by managed landscape burns.  Just interesting…Collared lizards in the wild have been the subject of a number of studies of sexual selection. In captivity if two males are placed in the same cage they will fight to the death. Males have a blue-green body with a light brown head. Females have a light brown head and body.

Effect of Sexual Reproduction Sexual reproduction recombines genes in new ways. This results in unique offspring that differ from either parent or sibling. Humans make 223 different kinds of gametes. Fertilization means that the uniqueness of an individual is 223  223. Or the probability that two siblings will be genetically identical (excluding identical twins) is 446. Ask students “Why TWO to the 23rd?” First, they should know that humans have 46 chromosomes which can be arranged into 23 pairs with the “last” pair being XX if female and XY if male. Since gametes are haploid, they have only 23 chromosomes. We use 2 because there are 2 chromosomes in a pair, and you have 23 pairs. In order to figure out the probability you raise 2 to the 23rd power, which gives you 8.3 million different possibilities for only the egg or sperm. So, if you take 8.3 million times 8.3 million, you get 64 trillion different possibilities for fertilization, without even considering crossing over. That’s a lot of unique phenotypes upon which natural selection can act! Sexual reproduction is like shuffling a deck of cards and every time getting a new and unique hand dealt. It is the major driving force of evolution.

Created by: Carol Leibl Science Content Director National Math and Science