Evolution Microevolution Changing Allele Frequencies Evolution is defined as a change in the inherited characteristics of biological populations over successive generations. Microevolution involves the change in allele frequencies that occur over time within a population. This change is due to four different processes: mutation, selection (natural and artificial), gene flow, and genetic drift. 2 Determining Allele Frequency Examine the frog population presented here. Their color is determined by a single gene, which has two alleles and phenotypically exhibits incomplete dominance. C G C G is green, C G C R is blue, and C R C R is red Calculate the allele frequency of the gene pool in the diagram. 3 Determining Allele Frequency These frogs are diploid, thus have two copies of their genes for color. Green (11) 22 0 Blue (2) 2 2 Red (3) 0 6 Total: 24 8 Frequency: p = 24 32 p = ¾ = 0.75 q = 8 32 q = ¼ = 0.25 If allelic frequencies change, then evolution is occurring. Let s suppose 4 green frogs enter the population (immigration). How do the frequencies change? 4 Immigration: Determining Allele Frequency Determining Allele Frequency Recall that currently: C G = 0.75 & C R = 0.25 Green (15) 30 0 Blue (2) 2 2 Red (3) 0 6 Total: 32 8 Frequency: p = 32 40 p = 8/10 = 0.80 q = 8 40 q = 2/10 = 0.20 How do the allelic frequencies change if 4 green frogs leave the population instead of enter the population? (emigration) 5 6 1
Emigration: Determining Allele Frequency Impact On Small vs. Large Population Recall that originally: C G = 0.75 & C R = 0.25 Green (7) 14 0 Blue (2) 2 2 Red (3) 0 6 Total: 16 8 Frequency: p = 16 24 p = 2/3 = 0.67 q = 8 24 q = 1/3 = 0.33 Before 4 frogs joined After 4 frogs joined Compare the effect on the small population to 4 frogs joining a much larger population. 7 8 Impact Large Population Impact Small Population Before 4 frogs joined larger population Green (22) 44 0 Blue (4) 4 4 Red (6) 0 12 Total: 48 16 Frequency: p = 48 64 p = 3/4 = 0.75 q = 16 64 q = 1/4 = 0.25 After 4 green frogs joined larger population Green (26) 52 0 Blue (4) 4 4 Red (6) 0 12 Total: 56 16 Frequency: p = 5 72 p = 56/72 = 0.78 q = 16 72 q = 16/72 = 0.22 Before 4 frogs joined Green (11) 22 0 Blue (2) 2 2 Red (3) 0 6 Total: 24 8 Frequency: p = 24 32 p = ¾ = 0.75 q = 8 32 q = ¼ = 0.25 After 4 green frogs joined Green (15) 30 0 Blue (2) 2 2 Red (3) 0 6 Total: 32 8 Frequency: p = 32 40 p = 8/10 = 0.80 q = 8 40 q = 2/10 = 0.20 In both cases the allele frequency for C G increases but it has a bigger impact on the smaller population. 9 10 Genetic Drift Small populations can experience changes in allele frequencies more dramatically than large populations. In very large populations the effect can be insignificant. Also in small populations genes can be lost more easily. When there is only one allele left for a particular gene in a gene pool, that gene is said to be fixed, thus there is no genetic diversity. Genetic Drift Genetic drift or allelic drift is the change in the frequency of a gene variant (allele) in a population due to random sampling in the absence of a selection pressure. Genetic drift is important when populations are dramatically reduced. Genes are lost and deleterious genes can also increase. When there are few copies of an allele, the effect of genetic drift is larger, and when there are many copies the effect is smaller. 11 12 2
Genetic Drift Genetic drift can be most profound in populations that are dramatically reduced (bottle neck populations) usually due to some environmental catastrophe. Also genetic drift occurs when a small population arrives at a new habitat such as an island. Bottleneck Example In 1900, the population of prairie chickens in Illinois was 100 million but by 1995, the population was reduced to around 50 in Jasper County due to over hunting and habitat destruction which caused the bottleneck to occur. A comparison of the DNA from the 1995 bird population indicated the birds had lost most of their genetic diversity. 13 14 Bottleneck Example Additionally, less than 50% of the eggs laid actually hatched in 1993. Populations outside IL do not experience the egg hatching problem. Bottleneck populations generally experience a severe reduction in genetic diversity within the population. 15 Bottleneck Example Genetic drift in smaller populations produces changes in allele frequency (evolution) whether it is due to a bottleneck or founder effect. A greater change of allele frequencies due to gene flow is evident in smaller populations. As populations rebound in number, their genetic diversity is still limited compared to the diversity that existed before the bottleneck event. Organism Year/Population Current Population Northern Elephant Seal 1890/30 Thousands Golden Hamster 1930/Single litter Millions American Bison 1890/750 360,000 Wisent European Bison 1900 s/12 3,000 16 Founder Effect The founder effect is the loss of genetic variation that occurs when a new population is established by a very small number of individuals from a larger population and is a special case of genetic drift. Founder Effect Biologist got their chance after a hurricane wiped out all the lizard species on certain islands in the Bahamas, scientists repopulated the small islands with two lizard pairs, one having long limbs and one having short limbs. Founder effects are very hard to study! 17 18 3
Founder Effect Before the hurricane, these islands supported populations of a Caribbean lizard, the brown anole, Anolis sagrei. After the hurricane, seven of the islands were thoroughly searched. No lizards were found. Founder Effect In May 2005, the researchers randomly selected one male and one female brown anole from lizards collected on a nearby larger island to found new anole populations on seven small islands. They then sat back and watched how those lizards evolved to get an up-close look at the Founder Effect. 19 20 Founder Effect During the next four years, the researchers repeatedly sampled lizards from the source island, from the seven experimental founder islands, and from 12 nearby islands that served as a control. The team found that all lizard populations adapted to their environment, yet retained characteristics from their founders. 21 A Human Founder Effect Example The Amish community was founded by a small number of colonist. The founding group possessed the gene for polydactyly (extra toes or fingers). The Amish population has increased in size but has remained genetically isolated as few outsiders become a part of the population. As a result polydactyly is much more frequent in the Amish community than it is in other communities. 22 Impact of Nonrandom Mating Two Types of Sexual Selection Nonrandom mating also changes allele frequency. Let s revisit our adorable frogs and suppose that 4 frogs migrate to a pond some distance from the main pond. It is likely that these 4 frogs will mate with one another, leaving the rest of the population in the main pond behind to also mate with one another. Nonrandom mating implies a choice of mates which is more prevalent in animals. Darwin wrote: The sexual struggle is of two kinds; in the one it is between individuals of the same sex, generally the males, in order to drive away or kill their rivals, the females remaining passive; whilst in the other, the struggle is likewise between the individuals of the same sex, in order to excite or charm those of the opposite sex, generally the females, which no longer remain passive, but select the more agreeable partners. 23 24 4
Sexual Selection Sexual selection of mates also affects allele frequency. The peacock provides a particularly well known example of intersexual selection, where ornate males compete to be chosen by females. The result is a stunning feathered display, which is large and unwieldy enough to pose a significant survival disadvantage. Female birds of many species choose the male. Males that are showier will better attract females. These males have a selective advantage even though they are more susceptible to predators. Sexual Selection 25 26 Sexual Selection Females that are drab, blend in to their surroundings and as a result, avoid predators which giving females a survival advantage. This illustrates that the importance of mating with the correct male outweighs the importance of being preyed upon. Sexual selection can lead to sexual dimorphism where there is a distinct difference between males and females. Hardy-Weinberg Equilibrium So, when is there no change in the allele frequency? When the population is said to be in Hardy-Weinberg Equilibrium, thus no evolution is occurring. FIVE Conditions of Hardy-Weinberg Equilibrium: 1. Population must be large so chance is not a factor. (No genetic drift). 2. Population must be isolated to prevent gene flow. (No immigration or emigration) 3. No mutations occur. 4. Mating is completely random with respect to time and space. 5. Every offspring has an equal chance of survival without regard to phenotypes. (No natural selection) 27 28 Hardy-Weinberg Equilibrium Condition #1 can be met. It is important to have large populations in order that the loss or addition of genes is not a factor. By contrast, small populations experience genetic drift. Additionally, if a small population moves to another area or becomes isolated, the gene pool will be different from the original gene pool. And the founder effect comes into play. Condition #2 can only be met if the population is isolated. If individuals immigrate or emigrate from the population, the allele frequencies change and evolution occurs. Condition #3 cannot ever be met since mutations always occur. Thus mutational equilibrium can never be met. Hardy-Weinberg Equilibrium Condition #4 can never be met. Mating is never random. Pollen from an apple tree in Ohio is more likely to pollinate a tree in Ohio than one in Washington state. Condition #5 can never be met. There will always be variation. Variation can help organisms survive longer and/or reproduce more effectively. Since 3 out of the 5 H-W conditions can never be met, evolution DOES occur and allele frequencies do indeed change. 29 30 5
Applying the H-W Model Applying the H-W Model Here we go with our frogs again! Let s suppose that in a population of 100 frogs, 36 were green (C G C G ), 48 were blue (C G C R ) and 16 were red (C R C R ) and there was total random mating. Green (36) 72 0 Blue (48) 48 48 Red (16) 0 32 Total: 120 80 Frequency: p = 120 200 p = 3/5 = 0.60 q = 80 200 q = 2/5 = 0.40 Thus, it can be assumed that 60% of all the gametes (eggs and sperm) should carry the C G allele and 40% of the gametes should carry the C R allele. 31 C G 0.60 C R 0.40 C G 0.60 C R 0.40 C G C G 0.36 C G C R 0.24 C G C R 0.24 C R C R 0.16 A population Punnett square is shown above. It indicates that the next generation should have the following offspring distribution: 36% green (C G C G ), 48% blue(c G C R ), 16% red (C R C R ). When the second generation gets ready to reproduce, the results will be the same as before. Green (36) 72 0 Blue (48) 48 48 Red (16) 0 32 Total: 120 80 Frequency: p = 120 200 p = 3/5 = 0.60 q = 80 200 q = 2/5 = 0.40 32 Applying the H-W Model Deriving the H-W Model So, the allele frequency remains at 0.40 C G and 0.60 C R thus no evolution is taking place. Let s suppose that there is an environmental change that makes red frogs more obvious to predators. How is the population affected and now the population consists of 36 green, 48 blue, and 6 red frogs? Green (36) 72 0 Blue (48) 48 48 Red (6) 0 12 Total: 120 60 Frequency: p = 120 180 p = 2/3 = 0.66 q = 60 180 q = 1/3 = 0.33 Now, allele frequencies are changing and there is an advantage to being green or blue but NOT red. Evolution is indeed occurring. 33 C G 0.60 C R 0.40 C G 0.60 C R 0.40 C G C G 0.36 C G C R 0.24 Examine this Punnett square again. If p represents the allele frequency of C G (dominant) and q represents the allele frequency of C R (recessive) then two equations for a population in Hardy-Weinberg equilibrium can be derived where the following genotypes are represented by: C G C G = p 2 C R C R = q 2 C G C R = 2pq Mathematically then p + q = 0.60 + 0.40 = 1 (1 st H-W equation) So, the Punnett square effectively crossed (p + q ) (p + q ) which gives p 2 + 2pq + q 2 = 1 (2 nd H-W equation) C G C R 0.24 C R C R 0.16 34 Natural Selection Natural Selection is the only mechanism that consistently causes adaptive evolution. Evolution by natural selection is a blend of chance and sorting. Chance in the context of mutations causing new genetic variations Sorting in the context of natural selection favoring some alleles over others This favoring process causes the outcome of natural selection to be anything but random! Natural Selection consistently increases the frequencies of alleles that provide reproductive advantage and thus leads to adaptive evolution. Relative Fitness There are animal species in which individuals, usually males, lock horns or otherwise compete through combat for mating privileges. Reproductive success is usually far more subtle! Relative fitness is defined as the contribution an individual makes to the gene pool of the next generation relative to the contributions of other individuals. 6
Frequency Frequency Frequency 1/4/2018 Three Modes of Natural Selection Natural selection can alter the frequency distribution of heritable traits in three ways depending on which phenotype is favored: Directional Selection Disruptive Selection Stabilizing Selection Directional Selection Directional selection occurs when conditions favor individuals exhibiting one extreme of a phenotypic range. Commonly occurs when the population s environment changes or when members of a population migrate to a new (and different) habitat. before Possible Effect of Continual Directional Selection If continued, the variance may decrease. after before after before after Disruptive or Diversifying Selection Disruptive selection occurs when conditions favor individuals at both extremes of a phenotypic range over individuals with intermediate phenotypes. The intermediates in the population have lower relative fitness. Phenotype (trait) Phenotype (trait) Phenotype (trait) 39 40 Disruptive or Diversifying Selection Disruptive selection occurs when conditions favor individuals at both extremes of a phenotypic range over individuals with intermediate phenotypes. The intermediates in the population have lower relative fitness. Stabilizing Selection Stabilizing selection removes extreme variants from the population and preserves intermediate types. This reduces variation and tends to maintain the status quo for a particular phenotypic character. 41 42 7
Sexual Selection A form of selection in which individuals with certain inherited characteristics are more likely than other individuals to obtain mates. Can result in sexual dimorphism which is a difference between the two sexes with regard to secondary sexual characteristics. Intrasexual vs. Intersexual Selection How does sexual selection operate? Intrasexual selection within the same sex, individuals of one sex compete directly for mates of the opposite sex. Males are famous for this! 43 44 Intrasexual vs. Intersexual Selection Intersexual selection (mate choice) individuals of one sex are choosy. Often these are females that select mates based on their showiness. Preserving Genetic Variation Some of the genetic variation is populations represents neutral variation, differences in DNA sequence that do not confer a selective advantage or disadvantage. There are several mechanisms that counter the tendency for directional and stabilizing selection to reduce variation: Diploidy Balancing Selection Hererzygote Advantage Frequency-Dependent Selection 45 46 Diploidy In diploid eukaryotes each organism has two copies of every gene and a considerable amount of genetic variation is hidden from selection in the form of recessive alleles. Often alleles are recessive and less favorable than their dominant counterparts. By contrast, haploid organisms express every gene that is in their genome. What you see is what you get. It reduces genetic variability. Diploidy Recessive alleles persist by propagation in heterozygous individuals. This latent variation is exposed to natural selection only when both parents carry the same recessive allele and two copies end up in the same zygote. As you might expect, this happens rarely if the allelic frequency of the recessive allele is very low. Why is heterozygote protection of potentially negative recessive alleles important to species survival? 47 48 8
Balancing Selection Balancing selection occurs when natural selection maintains two or more forms in a population. This type of selection includes heterozygote advantage and frequency-dependent selection. Heterozygote advantage involves an individual who is heterozygous at a particular gene locus thus has a greater fitness than a homozygous individual. Heterozygote Advantage A well-studied case is that of sickle cell anemia in humans, a hereditary disease that damages red blood cells. Sickle cell anemia is caused by the inheritance of a variant hemoglobin gene (HgbS) from both parents. In these individuals, hemoglobin in red blood cells is extremely sensitive to oxygen deprivation, and this causes shorter life expectancy. 49 50 Heterozygote Advantage A person who inherits the sickle cell gene from one parent, and a normal hemoglobin gene (HgbA) from the other, has a normal life expectancy. However, these heterozygote individuals, known as carriers of the sickle cell trait, may suffer problems from time to time. 51 Heterozygote Advantage The heterozygote is resistant to the malarial parasite which kills a large number of people each year in Africa. There exists a balancing selection between fierce selection against homozygous sickle-cell sufferers, and selection against the standard HgbA homozygotes by malaria. The heterozygote has a permanent advantage (a higher fitness) wherever malaria exists. 52 Heterozygote Advantage Frequency-Dependent Selection The fitness of a phenotype depends on how common it is in the population. In positive frequency-dependent selection the fitness of a phenotype increases as it becomes more common. In negative frequency-dependent selection the fitness of a phenotype increases as it becomes less common. For example in prey switching, rare morphs of prey are actually fitter due to predators concentrating on the more frequent morphs. 53 54 9
Balanced Polymorphism Balanced polymorphism occurs in a given population when two distinct types (or morphs) exists and the allele frequencies do not change. This may be due to Variation in the environment where one morph may be favored over another. One morph may be better adapted to a certain time of the year over the other. The lady bird beetle has 2 morphs. The red variety is more abundant in the spring and winter, whereas the black morph is more abundant in the summer and fall. Why Natural Selection Cannot Fashion Perfect Organisms 1. Selection can act only on existing variations. Natural selection favors only the fittest phenotypes among those in the population, which may not be the ideal traits. New advantageous alleles do not arise on demand. 2. Evolution is limited by historical constraints. Each species has a legacy of descent with modification from ancestral forms. Evolution does not scrap the ancestral anatomy. For example in birds and bats, an existing pair of limbs took on new functions for flight as these organisms evolved from nonflying ancestors. 55 56 Why Natural Selection Cannot Fashion Perfect Organisms 3. Adaptations are often compromises. The loud call that enables a frog to attract mates also attracts predators. 4. Chance, natural selection and the environment interact. Chance can affect the subsequent evolutionary history of populations. A storm can blow birds hundreds of kilometers over an ocean to an island, the wind does not necessarily transport those individuals that are best suited to the environment! Created by: Carol Leibl Director of Science Content National Math and Science Dallas, TX 57 10