UNIT 9 Evolution Part 1. Microevolution: Unique Gene Pools and Changing Allele Frequencies

Transcription

1 UNIT 9 Evolution Part 1 Microevolution: Unique Gene Pools and Changing Allele Frequencies

2 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. 2

3 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. 3

4 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! 4

5 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. 5

6 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. 6

7 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 7

8 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. 8

9 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. 9

10 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 10

11 Sources of Genetic Variation How does variation in a population or gene pool arise? 1. Mutations, gene duplication and chromosome fusion provide the raw material for evolution. 2. 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. 11

12 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! 12

13 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. 13

14 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. 14

15 Cytochrome c Comparison Molecular homology of cytochrome c (see three-letter code of amino acids) Human Gly Asp Val Glu Lys Gly Lys Lys Ile Phe Ile Met Lys Cys Ser Gln Cys His Thr Val Glu Lys Pig Val Gln - - Ala Chicken - - Ile Val Gln Dogfish Val - Val Gln - - Ala Asn Drosophila <<< Leu Val Gln Arg Ala Ala Wheat <<< - Asn Pro Asp Ala - Ala Lys Thr - - Ala Asp Ala Yeast <<< - Ser Ala Lys - - Ala Thr Leu - Lys Thr Arg - Glu Leu 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). 15

16 Hemoglobin Comparison 16

17 Human Impact on Gene Pools 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. 17

18 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. 18

19 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. 19

20 Antibiotics and Artificial Selection The increase in antibioticresistant 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) 20

21 MRSA or Methicillin-resistant 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. Staphylococcus aureus MDR bacteria can attack internal organs upon gaining entry into the body. 21

22 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 2 23 different kinds of gametes. Fertilization means that the uniqueness of an individual is Or the probability that two siblings will be genetically identical (excluding identical twins) is 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. 22

23 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. 23

24 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. 24

25 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. 25

26 Bottleneck Example Additionally, less than 50% of the eggs laid actually hatched in Populations outside IL do not experience the egg hatching problem. Bottleneck populations generally experience a severe reduction in genetic diversity within the population. 26

27 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 effects are very hard to study! 27

28 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. 28

29 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. 29

30 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. 30

31 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. 31

32 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. 32

33 Impact of Nonrandom Mating Nonrandom mating also changes allele frequency. Nonrandom mating implies a choice of mates which is more prevalent in animals. Random mating is less common. Time and space also factors into non-random mating. For example, pollen from Ohio is more likely to crosspollinate a nearby tree in Ohio, rather than Oregon. 33

34 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. 34

35 Sexual Selection 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. 35

36 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. 36

37 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.

38 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

39 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.

40 Frequency Frequency Frequency Possible Effect of Continual Directional Selection If continued, the variance may decrease. before after before after before after Phenotype (trait) Phenotype (trait) Phenotype (trait) 40

41 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. 41

42 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. 42

43 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. 43

44 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. 44

45 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? 45

46 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. 46

47 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. 47

48 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. 48

49 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. 49

50 Heterozygote Advantage 50

51 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. 51