BIOL 1020 – CHAPTER 23 LECTURE NOTES

Chapter 23: The Evolution of Population (Microevolution)

  1. Historical perspective – ideas on evolution and related topics up to Darwin

A.divine design and perfection model

  1. espoused by Aristotle, the ancient Greek philosopher (284-322 B.C.) whose ideas on biology dominated most thinking on the subject until the renaissance
  2. species were viewed on a scale from simple to complex
  3. all organisms were seen as moving toward perfection (which was associated with complexity)
  4. based on divine intervention and design (thus supernatural, outside the true realm of science)
  5. now discredited in biology, but still part of the social consciousness
  1. fossils
  2. fossils were known for centuries before Darwin
  3. fossils reveal organisms unlike any living today, and the idea that some fossils represent species that had become extinct was recognized even by Leonardo da Vinci (1452-1519)
  4. acquired traits
  5. mostly associated with Lamarck (1744-1829)
  6. still focused on a model of organisms driven toward complexity, but involved an explanation with natural causes
  7. postulated that changes or “acquired characteristics” during an organism’s life could be passed on to offspring
  8. famous example was Lamarck’s model for how giraffes developed long necks – he claimed that stretching of the neck in one generation would lead to offspring with longer necks
  9. understanding of genetic inheritance has led to rejection of acquired traits models
  10. birth of modern geology – Lyell, uniformitarianism, and the ancient Earth
  11. prior to the early 1800s, the world view of most was that the Earth is very young (around 6000 years old)
  12. in the early 1800s, geologists began to apply scientific reasoning to studies of geological processes, and quickly recognized that these processes require that the Earth be very old (billions of years) to occur naturally
  13. this “uniformitarian” model of geological processes was made famous by Lyell’s Principles of Geology, which influenced Charles Darwin
  14. the uniformitarian model is essentially the basis of geology today; confirming tests of this model include dating rocks using radioisotope ratios (more on that later)
  15. artificial selection
  16. it was well known that domesticated animals and plants had been breed over centuries by humans to produce different varieties, indicating that the characteristics of a species can be modified by selection
  17. some examples are different breeds of dogs and the “wild cabbage” lineage of cabbage, broccoli, cauliflower, Brussels sprouts, collards, kale, etc.
  18. population limits that would allow selection to act naturally were recognized
  19. Malthus (1766-1834) wrote the most influential works on this subject
  20. mathematically, populations will grow geometrically if unchecked
  21. food supplies rarely can be expected to grow faster than arithmetically, thus putting a limit on population growth

II.Darwin’s theory: evolution occurs by natural selection

  1. Darwin proposed that species evolve by natural selection; his theory of evolution was based on four general observations:
  2. overproduction – each species produces more offspring than will survive to maturity
  3. variation – individuals in a population vary, and some of the variation is heritable (this was expanded by others later, as genetics came to be understood)
  4. competition – there is competition among the individuals of a population for limited resources (struggle for existence)
  5. differential reproductive success – individuals that possess more favorable characteristics (in the pool of variation) are more likely to survive and reproduce; those with less favorable characteristics are less likely to survive and reproduce
  6. thus, natural selection will produce a population of individuals more suited to their environment through time
  7. when populations are separated (such as the geographic separation of islands from each other and a nearby continent), natural selection on two separate populations can produce two distinct populations with different characteristics – resulting in two separate species
  8. note that for this theory to explain the current variety of species on Earth, there is a need for a long amount of time for natural selection to produce the variety observed; thus, the idea of an ancient Earth hundreds of millions to billions of years old is crucial
  9. note also that this theory really has two major branches: microevolution, or changes of a population over time, and macroevolution, or the formation of species

III.The modern synthesis of evolutionary theory combines Darwin’s concept of natural selection with genetics

  1. although Mendel was a contemporary of Darwin, remember that his work was largely unrecognized until around 1900
  2. how traits are inherited was central to Darwin’s theory of evolution, and thus Darwin (and others) were keenly interested in finding working models of inheritance
  3. when genetic mechanisms came to be widely understood, they were quickly combined with Darwin’s model in the modern synthesis, also called Neo-Darwinism or the synthetic theory of evolution
  4. this model emphasizes the genetics of populations
  5. evolution is seen as working by natural selection on individuals to change the genetic makeup of populations over successive generations
  6. mutations play a key role in providing a source of genetic variation
  7. without genetic variation, evolution cannot occur
  8. mutations are necessary to produce genetic variation
  9. while many mutations have no impact and many others are harmful, it is critical to recognize that some mutations are advantageous
  1. Microevolution is a change in allele frequencies or genotype frequencies in a population over time
  2. population – a group of individuals capable or interbreeding and producing fertile offspring, and that are more or less isolated from other such groups
  3. gene pool – all alleles present in a population at a given time
  4. phenotype frequency – proportion of a population with a given phenotype
  5. genotype frequency – proportion of a population with a given genotype
  6. allele frequency – proportion of a specific allele in a population
  7. diploid individuals have two alleles for each gene
  8. if you know genotype frequencies, it is easy to calculate allele frequencies
  9. example:
  10. population (1000) = genotypes AA (490) + Aa (420) + aa (90)
  11. allele number (2000) = A (490x2 + 420) + a (420 + 90x2) = A (1400) + a (600)
  12. freq[A] = 1400/2000 = 0.70
  13. freq[a] = 600/2000 = 0.30
  14. note that the sum of all allele frequencies is 1.0

V.Genetic equilibrium in populations: the Hardy-Weinberg theorem

  1. the Hardy-Weinberg theorem describes the frequencies of genotypes in a population based on the frequency of occurrence of alleles in the population that is in a state of genetic equilibrium (that is, not evolving)
  2. the usual case for calculations: if allele “A” is dominant to “a”, and they are the only two alleles possible at the A-locus, then
  3. p = freq[A] = the frequency of occurrence of the A-allele in the population
  4. q = freq[a] = the frequency of occurrence of the a-allele in the population
  5. Then p + q = 1 (following the sum rule for probability)
  6. Allele associations follow the product rule for probability, so you multiply to predict the genotype frequencies:
  7. ( p + q ) x ( p + q )= p2 + 2 pq + q2
  8. p2 = frequency of homozygous dominant genotypes
  9. 2 pq = frequency of heterozygous genotypes
  10. q2 = frequency of homozygous recessive genotypes
  11. note that ( p + q ) x ( p + q )= 1 x 1 = 1, so p2 + 2 pq + q2 = 1
  12. Hardy-Weinberg equilibrium
  13. if the Hardy-Weinberg theorem can be used to accurately predict genotype frequencies from allele frequencies for a population then the population is in Hardy-Weinberg equilibrium or genetic equilibrium
  14. in such cases you can use data from one generation to predict the allele, genotype, and phenotype frequencies for the next generation
  15. such populations are not evolving, but are static instead
  16. the assumptions of this model are:
  17. large population size (due to statistical constraints, to minimize genetic drift)
  18. no migration – no exchange of alleles with other populations (no gene flow)
  19. no mutations of the alleles under study occur
  20. random mating of all genotypes
  21. no natural selection
  1. Microevolution thus can be described as deviation from Hardy-Weinberg equilibrium, where the allele or genotype frequencies in a population change over time
  2. consequences of small population size: genetic drift
  3. Consider taking a small sample of individuals from a larger population. If only two individuals were picked they cannot reflect the allele frequency in the larger population. Neither can 3, 4, or 5 individuals, but as the selected sample gets larger it becomes more likely that the sample reflects the allele frequency in the larger population.
  4. genetic drift is a change in gene frequencies of populations because of small population size
  5. genetic drift tends to decrease genetic variation within a population
  6. genetic drift tends to increase genetic variation between populations
  7. two general mechanisms lead to small population sizes
  8. genetic bottlenecks are created by dramatic reduction in population size – endangered species face a genetic bottleneck on a species-wide scale, and suffer lasting effects even if population size later recovers
  9. founder effect – when a new population is established, typically only a few individuals (founders) are involved in colonizing the new area; this is common for islands
  10. migration – when individuals leave or join a population
  11. migrating individuals carry their alleles with them (gene flow), usually resulting in changes in allele frequencies
  12. gene flow tends to decrease genetic variation between populations
  13. mutations increase variation in the gene pool of a species
  14. remember that mutations may be neutral, harmful, or beneficial
  15. even at the risk of harmful effects, mutations are necessary to increase variation in the population so that natural selection can produce organisms more suited to their environment
  16. nonrandom mating
  17. if individuals do not mate at random, then Hardy-Weinberg equilibrium is not achieved
  18. the most common cases of nonrandom mating involve mating between individuals of similar genotypes, either by choice or location
  19. such inbreeding does not change allele frequencies, but increases the frequency of homozygous genotypes
  20. inbreeding depression is seen in some cases, where inbred individuals have lower fitness that non-inbred individuals
  21. fitness – relative ability of a genotype to contribute to future generations
  22. fertility declines and high juvenile mortality associated with “unmasking” harmful recessive alleles can reduce fitness for inbred individuals
  23. hybrid vigor also leads to higher relative fitness for hybrids
  24. self-fertilization is the most extreme case of inbreeding
  25. assortive mating – mates are selected by phenotypes
  26. positive assortive mating – selection for the same phenotype; works like inbreeding for the genes governing that phenotype, and for loci closely linked to those genes
  27. negative assortive mating – selection for the opposite phenotype
  28. less common than positive assortive mating
  29. leads to a decrease in homozygous genotypes for the genes governing the selected phenotype, and for loci closely linked to those genes
  30. natural selection changes allele frequencies in a way that leads to adaptation to the environment
  31. fitness is the ability of an organism to compete successfully and pass its alleles on to the next generation
  32. populations undergoing natural selection are evolving, with alleles that contribute to better fitness increasing in frequency over successive generations
  33. natural selection only operates based on the current environment – as environmental conditions change, different alleles will be selected for
  34. sexual selection (mate choices based on inherited characteristics) is an aspect of natural selection
  35. there are 3 types of natural selection
  36. stabilizing selection – occurs in populations well adapted to their environments, selecting against phenotypic extremes
  37. this is probably the type of selection most commonly faced by populations
  38. example - human birth weight
  39. directional selection – permits species to adapt to environmental change by favoring selection of one extreme over the other; example – peppered moth
  40. disruptive selection – when more than one extreme phenotype is favored over intermediate phenotypes
  41. really a special case of direction selection, where there are trends in more than one direction
  42. can produce a genetic “split” in a population and thus serve as a mechanism for speciation
  43. example – pocket mice in New Mexico

VII.Genetic variation must exist for natural selection to occur

  1. the ultimate source of genetic variation is mutations
  2. once variation exists, it can be affected by independent assortment and genetic recombination during gamete formation
  3. consider the cross AaBb x AaBb – 9 different genotypes arise
  4. this involves only 2 alleles at 2 loci; if there were 6 alleles possible at just 5 loci, over 4 million genotypes are possible
  5. thus, given that there are thousands of genes in an organism, and that many alleles are possible at most of these loci, it becomes clear that in nature there is great genetic variability
  6. the demonstrated presence of two or more alleles at a given locus is genetic polymorphism
  7. biologists have produced tools for studying the genetic polymorphism of populations at the molecular level (RFLP, DNA sequencing, etc.)
  8. these tools can be used to demonstrate and study polymorphism in populations without necessarily knowing the specific genes involved
  9. genetic polymorphisms can be maintained by heterozygote advantage or hybrid vigor
  10. when either the homozygous dominant or recessive is more suited to an environment than the heterozygote, the homozygous genotype will be more likely to be fixed in the population
  11. but when heterozygous genotypes have advantage over either of the homozygous genotypes, variation tends to increase in the population
  12. example - sickle cell anemia and malaria resistance
  13. genetic polymorphismscan be maintained due to frequency-dependent selection
  14. there are cases where the frequency of a given genotype affects the degree to which it is or isn't selected in the population
  15. example - predator/prey relationships, where individuals with a rare phenotype may be ignored by a predator, but as they become more abundant the selective advantage decreases because the predator is more likely to notice them
  16. much of the genetic variation in a population will produce no selective advantage, i.e. it is neutral
  17. the role of neutral variation in evolution is debated today
  18. remember that what is neutral in one context may not be neutral in another context, so as environments change some previously neutral variation may be acted on by natural selection

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