15 1 Microevolution 15 1 Microevolution cont A population is defined as all the members of a single species occupying a particular area and reproducing with one another Microevolution involves the evolutionary changes within a population While variation within a population is important to evolution it is not the only factor 15 1 Microevolution cont Evolution in a Genetic Context Evolution at the population level can be studied using population genetics In population genetics the various alleles at all the gene loci in all the individuals make up the gene pool of that population The gene pool of a population can be described in terms of gene frequencies Evolution in a Genetic Context cont Consider the following example for a population of 100 Drosophila fruit flies 36 of flies are homozygous dominant for long L wings 36 flies 16 are homozygous recessive for short l wings 16 flies 48 are heterozgous 48 flies Evolution in a Genetic Context cont So how many L and l alleles are in the population Number of L alleles Number of l alleles LL 2 L x 36 72 LL 0 l 0 Ll 1 L x 48 48 Ll 1 l x 48 48 ll 0 ll 32 0 L 120 L 2 l x 16 80 l 1 Evolution in a Genetic Context cont To determine the frequency of an allele in the population calculate the percentage of that allele from the total number of alleles in the population For the dominant allele L 120 200 0 6 Evolution in a Genetic Context cont These percentages represent the frequency of each allele in the gametes of this population If the mating in this population is assumed to be random then the genotypes in the subsequent generation can be determined using a Punnett square For the recessive allele l 80 200 0 4 Evolution in a Genetic Context cont In this Punnett square the cross indicates the possible alleles contributed by the population not an individual Evolution in a Genetic Context cont The important message here is that sexual reproduction alone does not change allele frequencies in a population This equilibrium illustrates the Hardy Weinberg principle sperm 0 6 L eggs 0 4 l 0 6 L 0 36 LL 0 24 Ll 0 4 l 0 16 ll 0 24 Ll Genotype frequencies 0 36 LL 0 48 Ll 0 16 ll Microevolution occurs when events other than sexual reproduction disrupt Hardy Weinberg equilibrium and cause the allele frequencies within a population to change Note that the frequencies of L and l remain the same in the subsequent generation Evolution in a Genetic Context cont Evolution in a Genetic Context cont Hardy Weinberg equilibrium is expressed as a simple binomial equation p2 2 pq q2 The letters p and q are used to represent the frequency of the two alleles in the population 2 Evolution in a Genetic Context cont Evolution in a Genetic Context cont Hardy Weinberg equilibrium is maintained in a population of sexual reproducing individuals if five conditions are met No net change in frequency due to mutations No gene flow migration of alleles in or out of the population Random mating must occur No genetic drift No natural selection Evolution in a Genetic Context cont These conditions are rarely if ever met in the real world Thus allele frequencies continually change and microevolution occurs Evolution in a Genetic Context cont In order for natural selection to act on allele frequencies the change must affect the phenotype associated with the gene A classic example of microevolution is industrial melanism The value of the Hardy Weinberg principle is that it describes the factors that cause evolution Evolution in a Genetic Context cont Evolution in a Genetic Context cont 3 Causes of Microevolution Deviations from the conditions of Hardy Weinberg equilibrium cause the allelic changes associated with microevolution Mutations Gene flow Nonrandom mating Genetic drift Natural selection Gene Flow Genetic Mutations Mutations are the raw material of evolutionary change Mutation introduces new variation into a population This variation is adaptive if it helps members of a population adjust to specific environmental conditions Gene Flow cont Gene flow or gene migration occurs when breeding members of a population leave a population or new members enter Gene migration can introduce new alleles to populations However continual gene flow between populations decreases differences in allele frequencies preventing speciation Nonrandom Mating Genetic Drift When males and females reproduce together strictly by chance it is called random mating Chance events that cause the allele frequency to change is called genetic drift Any behavioral activity that fosters the selection of specific mates is nonrandom mating Assortive mating occurs when organisms select mates with a similar phenotype Sexual selection favors traits that increase the likelihood of securing a mate The effect of genetic drift becomes increasingly important as the size of the population decreases 4 Genetic Drift cont Genetic Drift cont Another example of genetic drift is the bottleneck effect A bottleneck occurs when an event or a catastrophe drastically reduces the number of organisms in a population The variation in that population may also be reduced changing the allele frequencies within the population Genetic Drift cont Genetic Drift cont The founder effect is another example of genetic drift The founder effect occurs when combinations of alleles occur at a higher frequency in a population that has been isolated from a larger population Genetic Drift cont 15 2 Natural Selection Natural selection is the process that adapts populations to the environment Some aspects of the environment can involve biotic living components Competition for limiting resources Predation Parasitism 5 15 2 Natural Selection cont 15 2 Natural Selection cont Some aspects of the environment can involve abiotic nonliving components Weather and climate Temperature Moisture Types of Selection Directional Selection The variation within a population creates different phenotypes for a given trait When one extreme phenotype is favored by natural selection the distribution of the phenotype shifts in that direction The distribution of those phenotypes typically forms a normal distribution This type of selection is therefore called directional selection The effect of the three types of natural selection have different effects on this normal distribution Directional Selection cont Directional Selection cont 6 Stabilizing Selection Stabilizing Selection cont Stabilizing selection occurs when the intermediate or most common phenotype is favored This type of selection