Ch. 17 Part 2

Evolution Long-term change in the characteristics of a species Frequency of a particular allele within a population When a group of organisms change over time Changes in allele frequency within a population over many generations Occurs because natural selection gives some alleles a better chance at surviving than other Over many generations, populations gradually changed Get better adapted to environment

4 Main Principles of Natural Selection Variation exists within a population Organisms compete for limited resources Organisms produce more offspring than can actually survive Individuals with variations suitable for their habitat are the ones that SURVIVE and REPRODUCE

Three Major Types of Selection Stabilizing Selection Selection pressures are on the two extreme phenotypes Center range/mean phenotype (average) is favored Variation centered around mean value Environment is stable If organisms are well adapted to the environment, most common alleles in the population will lead to an advantage on the organisms and alleles wil continue to be passed on Directional Selection Aka EVOLUTIONARY SELECTION Occurs when new environmental factor or new allele appears within a population Results in change of characteristic in ONE particular direction ONE specific phenotype is favored Disruptive Selection Occurs when conditions favor both extreme phenotypes of a population Maintains different phenotypes with the population Polymorphism

1. Stabilizing Selection Favors the more common traits in a population as opposed to the more EXTREME or unusual traits Eliminates individuals that have EXTREME or unusual traits Maintains the existing population frequencies of common traits while selecting against all other trait variations Example: Infant birth weight

2. Directional selection Favors traits at one EXTREME of the range of possible traits Result: traits at the opposite end disappear If this continues for many generations, favored traits become more and more extreme Result: distinct changes in allele frequencies of the population Examples: Insecticide resistance Peppered moth

3. Disruptive Selection Environment favors more EXTREME or UNUSUAL traits over common traits Extreme or unusual traits at both ends of the spectrum are favored…common traits disappear over time Leads to BALANCED POLYMORPHISM Population divided into two phenotypes

Mutations CAUSES: Spontaneous Natural factors Ultraviolet radiation from the sun Exposure to chemicals Exposure to radiation Introduces new alleles into a gene pool that NEVER existed before VITAL b/c provides source for new variation What is a mutation: Change to an organisms genetic material (DNA) Change the NUCLEIC ACIDS that make up one or more genes Changes can produce new traits that can either HELP or HURT the survival of an organism BENEFICIAL Mutations help organism NEUTRAL Mutations have no effect on organism NEGATIVE Mutations hurt organisms chances for survival

New Environmental Factors Environmental changes can make certain phenotypes that were once NOT beneficial suddenly become beneficial Ex. Ice age Rabbit fur Agouti brown originally favored New environment makes white fur more beneficial Frequency of the allele for white fur increases at the expense of the allele for agouti brown fur Result after many generations? Many white furred rabbits Changes in environmental factors ONLY affect the likelihood of an allele surviving in a population Environmental factors do NOT affect the likelihood of an allele arising by mutations

Antibiotic Resistance Antibiotics chemicals produced by living organisms that inhibit or kill bacteria but do NOT harm human tissue Most produced by fungi Penicillin  1st antibiotic Produced by fungus Penicillium Treats wide range of bacteria Stops cell wall formation  prevents bacterial cell reproduction Kills all bacteria sensitive to penicillin Inhibitor of enzymes called glycopeptidases Glycopeptidases used to form cross-links between peptidoglycan molecules in cell walls of bacteria; makes cell wall rigid so cell do not burst when taking up water Hopefully the ENTIRE population What happens when entire population is NOT killed by antibiotic? Due to one or more individual bacteria carrying an allele that makes them resistant to penicillin NOT GOOD Bacterial DNA Single loop of DNA Only ONE copy of each gene Mutant alleles have IMMEDIATE effect on phenotype Provides bacteria with selective advantage (those with beneficial allele will survive and reproduce…those without will die) if there are ideal conditions Bacteria divide rapidly by BINARY FISSION

Antibiotic Resistance Arises when an existing gene with the bacterial genome changes (mutates) spontaneously to give rise to a nucleotide sequence that codes for a slightly different protein that is NOT affected by antibiotic DNA MUTATION!!!! Incorrect dosage or stopping a cycle of antibiotic treatment increase chances of antibiotic resistant bacteria

Penicillin Resistant Bacteria Produces enzymes that make penicillin ineffective against them B lactamase Group of Enzymes that breaks apart the penicillin molecule Penicillinase Enzyme that inactivates penicillin Staphylococcus Cause Staph Infection may cause disease due to direct infection or due to the production of toxins by the bacteria Methicillin-resistant Staphylococcus aureus MRSA type of Staphylococcus aureus that is resistant to the antibiotic methicillin and other drugs in this class (penicillin)

Human Use of Antibiotics Antibiotic use  change in environmental factors of bacteria Exert selection pressures on bacteria Enable more antibiotic bacteria t proliferate Constantly trying to find new antibiotics that bacteria are NOT resistant to More humans use antibiotics = greater selection pressures exerted on bacteria to evolve resistance to antibiotics BAD!

Bacteria can pass off genes to other bacteria increase proliferation of resistant bacteria

Pollution from Sulfur Dioxide Released into the Air = no more lichens Industrial Melanism Pollution from Sulfur Dioxide Released into the Air = no more lichens Example of changing environmental factor Peppered Moth Biston betularia in UK and Ireland Night-flying During the day found on branches of tress to camouflage from insect eating birds (sight hunters) Before 1849 Pale wings with dark markings In 1849 Black (melanic) moth found Population of melanic moth increased in certain areas Difference between Speckled and Melanic Moths One gene Normal speckled coloring recessive allele c Black coloring  dominant allele C Frequency of dominant C allele increased in areas near industrial cities Frequency of recessive c allele increased in non-industrial areas Selection pressure causing change in allele frequency in industrial areas  predation by birds Unpolluted (non-industrial) areas  green, brown, and grey lichens covered tree branches = good cover for speckled moths Polluted (industrial) areas  sulfur dioxide affects lichens  lichens do NOT grow on trees = dark bark presides = good cover for melanic moths

1970s decrease in pollutants = change in melanic allele frequencies Mutated C alleles were always present in moth population Those with dominant C allele never survived because they were always spotted by the predatory birds and eaten (and never passed off their genes) Dominant C allele only increased its frequency when having dark pigment gave moths advantage in polluted areas Mutations to the C allele NOT caused by pollution Changes in environmental factors ONLY affect the likelihood of an allele surviving in a population Environmental factors do NOT affect the likelihood of an allele arising by mutations

Malaria Review Caused by protoctist parasite Plasmodium Insect vector  mosquito (salivary glands) Mosquito bites  Plasmodium enters blood Parasite enters RBCs  multiplies in RBCs  illness and death

Sickle Cell Anemia and Malaria HBS (sickled hemoglobin) and HBA (normal hemoglobin) alleles Possible genotypes: HBSHBS (homozygous dominant, sickled RBCs) Great selective disadvantage  less likely to survive and reproduce HBSHBA (heterozygous dominant, some sickled RBCs, carriers) Less likely to suffer from serious attack of malaria Contain about 1/3rd the number of Plasmodium parasites is their blood compared to homozygous normal HBAHBA HBAHBA (homozygous dominant  normal) More likely to suffered from serious attack of malaria Parts of the world with increased cases of MALARIA have higher frequency of HBS (sickled hemoglobin) allele

Two Strong Selection Pressures Acting on 2 Alleles HBSHBS Selection against these genotypes b/c they become seriously anemic HBAHBA Selection against these genotypes b/c they are more likely to die from malaria HBAHBs Strong selection advantage Do not suffer from sickle cell anemia Less likely to suffer from malaria Both alleles remain in population where MALARIA is an important environmental factor

More noticeable when a small # of individuals are separated from the rest of the population Form small sample of original population Not likely to have same allele frequencies as large population Further genetic drift in small population  alters allele frequencies even more Evolution of small population may be very different from evolution of larger population Process called FOUDNERS EFFECT (occurs in small, isolated populations) Genetic Drift

Genetic Drift Founder Effect Bottleneck Effect When allele frequency of group of migrating individuals, by CHANCE, are different from their original population Bottleneck Effect Occurs when a population undergoes a dramatic decease in size Regardless of cause of bottleneck (natural disaster, predation, disease), small population is now very vulnerable to genetic drift Certain alleles have greater effect on population than other alleles The genes of the next generation will be the genes of the “lucky” individuals, not necessarily the healthier or “better adapted” individuals. That, in a nutshell, is genetic drift.

Genetic Drift changes in the allele frequency within a population that occur by chance genes of the next generation will be the genes of the “lucky” individuals, not necessarily the healthier or “better adapted” individuals No guarantee that the new population will be better suited to its environment than the original population Effect of Genetic drift is very strong and dramatically influences evolution of small populations (fewer than 100) Before After The genes of the next generation will be the genes of the “lucky” individuals, not necessarily the healthier or “better adapted” individuals. That, in a nutshell, is genetic drift.

Allele frequencies measure genetic variation. how common allele is in population can be calculated for each allele in gene pool Calculate the allele frequency for G(Green frogs) in the population Calculate the allele frequency for g (brown frogs) in the population

How to calculate the ALLELE Frequency in a Gene Pool Allele X or Allele x for certain TRAIT Allele frequency X = # of allele X in population (gene pool) total # of alleles (X + x) in population (gene pool)

Hardy Weinberg Principle (HWP) Phenotypes controlled by 2 alleles only (A and a) Three possible genotypes: Homozygous DOMINANT (AA) Heterozygous DOMINANT (Aa) Homozygous RECESSIVE (aa) Two phenotypes: Dominant (can be AA or Aa) Recessive (HAS to be aa) HWP allows the proportions of each genotype in a large, randomly mating population to be calculated Total population = 100% or 1 Frequencies within population are rations or percentages of whole population (decimals or percentages) P = frequency of dominant allele in population Q = frequency of recessive allele in population P + Q = 1 (all the alleles in the population) P2 = frequency of homozygous dominant allele in population Q2 = frequency of homozygous recessive allele in frequency in population (easy to recognize) 2pq = frequency of heterozygous genotypes in population

Condition when the HWP does NOT apply to a population Purpose of HWP Condition when the HWP does NOT apply to a population When to use HWP: Determining ratios of different genotypes in population allows predication of their ratios in the next generation to be compared with observed results Use Chi-Squared test to analyze significance Significant differences mean: Evidence of directional selection occurring in population (only if migration and non-random mating can be discounted) Small populations Significant selective pressures against one particular genotype Migration of Individuals carrying one of the two alleles into OR out of population Non-random mating

Practice Problem

Practice Problem