3CHAPTER 23 THE EVOLUTION OF POPULATIONS Populations are the smallest unit capable of evolving not individual members 23 1 GENETIC VARIATION MAKES EVOLUTION POSSIBLE Variation in heritable traits is a prerequisite for evolution GENETIC VARIATION Individuals of a species vary in specific characteristics Variation could be observed genotypically or phenotypically Genetic variation at the whole gene level gene variability can be quantified as the average percentage of loci that are heterozygous Genetic Variation can also be measured at the nucleotide level nucleotide variability However little of this variation results in phenotypic variation because many variations occur in introns non coding sequences that lie between exons the regions retained in mRNA after RNA processing WITHOUT VARIAITION EVOLUTION CANNNOT OCCUR SOURCES OF GENETIC VARIATION Occurs when mutation gene duplication or other processes produce new alleles and new genes FORMATION OF NEW ALLELES Can arise by mutation a change in nucleotide sequence of an organism s DNA Only mutations that occur in cell lines that produce gametes can be passed on to offspring Plants and fungi have a higher chance of passing on mutations because many cell lines can make gametes however in animals most mutations occur in somatic cells ALTERING GENE NUMBER OR POSITION A key potential source of variation is the duplication of genes due to errors in meiosis such as unequal crossing over slippage during DNA replication or the activities of transposable elements Gene duplications that do not have severe effects can persist for several generations and be allowed to accumulate and ultimately expand the genome with new genes RAPID REPRODUCTION Mutation rates tend to be low in plants and animals and they are often even lower in prokaryotes However prokaryotes tend to have a short generational time period which can quickly generate genetic variation in their population Thus the reason it is difficult to combat HIV which has a generational timespan of 2 days and needs a cocktail of drugs to be combatted SEXUAL REPRODUCTION In organisms that reproduce sexually most of the genetic variation in a population results from the unique combination of alleles that each individual receives from its parents Sexual Reproduction shuffles the mutations that occurred at the nucleotide level and deals them to produce individual genotypes The three mechanisms that contribute to this shuffling are Crossing Over Independent Assortment of Chromosomes Fertilization During meiosis homologous chromosomes one inherited from each parent trade some of their alleles by crossing over Then these homologous chromosomes and the alleles they carry are distributed at random which ensures that sexual reproduction provides genetic variation 23 2 THE HARDY WEINBERG EQUATION CAN BE USED TO TEST WHETHER A POPULATION IS EVOLVING GENE POOLS AND ALLELE FREQUENCIES A POPULATION is a group of individuals of the same species that live in the same area that interbreed and produce fertile offspring A population s genetic make up can be characterized by describing the gene pool that consists of all copies of every type of allele at every locus in all members of the populations If there is one known allele for that locus then that allele is said to be fixed if there are two then that gene may be homozygous or heterozygous Each allele has a frequency in the population When studying a locus with two alleles the conventions to use are p to represent the frequency of one allele and q of the other allele THE HARDY WEINBERG EQUATION One way to asses whether natural selection or other factors are causing evolution at a particular locus is to determine what the genetic makeup of a population would be if it were not evolving at that locus HARDY WEINBERG EQUILLBRIUM If a population is not evolving allele and genotype frequencies will remain constant from generation to generation provided only Mendelian segregation and recombination are at work This is when a population is at HARDY WEINBERG EQUILLBRIUM To ensure that Hardy Weinberg Equilibrium is achieved there must be random mating H W Equilibrium could be best observed using this example Flowers need to alleles at for a particular locus to determine flower color C X C X A population of flowers has an allele frequency of 80 0 8 for red allele p and 20 0 2 for the white allele q The population has 1000 alleles Assuming random mating each egg and sperm have an 80 chance of creating a red flower and using the rule of multiplication 0 8 0 8 0 64 which is p 2 q 2 would be found multiplying 0 2 0 2 which would equal 0 04 Because there are 2 possibilities for having heterozygotes you would multiply 2 pq or in this case 2 0 2 0 8 which would equal 0 32 The equations used for a population in H W equilibrium are where the top equation is used to find the genotype frequencies and the bottom equation is used to find the allele frequencies CONDITIONS FOR HARDY WEINBERG EQUILLBRIUM Failure to meet these conditions results in evolutionary changes 23 3 NATURAL SELECTION GENETIC DRIFT AND GENE FLOW CAN ALTER ALLELE FREQUENCIES IN A POPULATION NATURAL SELECTION The concept of natural selection is based on differential success in survival and reproduction Individuals in a population exhibit variations in their heritable traits and those with better traits that are better suited to their environment tend to produce more offspring that those with bad traits The fruit fly has developed an allele that is resistant to DDT In the 1930 s the allele frequency was 0 in 1960 the allele frequency rose to 37 Because DDT is a strong selective force individuals who did not have resistance died and did not pass on their genes Resistant members ended up passing on their genes This is an example of ADAPTIVE EVOLUTION evolution that results in a better match between organisms and their environment GENETIC DRIFT Allele frequencies can fluctuate unpredictably from one generation to the next especially in small populations For example if you flip a coin 1000 times and land heads 700 times and tails 300 times you might become more suspicious However if you flipped it 10 times 7 heads and 3 tails may not be too unreasonable THE FOUNDER EFFECT This occurs when few members of a larger population become isolated and the small group establishes a new population The smaller population may have a significantly different gene pool This accounts for the relatively high frequency of certain inherited