Evolutionary Theory Historical Context & Darwin's Revolution

1. Lecture 2: Evolution and Natural Selection Historical Context The Darwinian revolution The occurrence of evolution Natural selection as the mechanism The Logic of Natural Selection Population genetics—populations evolve not individuals. Microevolution: generation to generation changes in populations Genetic drift: the effects of chance in small populations Natural selection: differential survival and reproduction Mutation and recombination: the origin of new genotypes

2. CHAPTER 22 DESCENT WITH MODIFICATION: A DARWINIAN VIEW OF LIFE Section A: Historical Context for Evolutionary Theory 1. Western culture resisted evolutionary views of life 2. Theories of geological gradualism helped clear the path for evolutionary biologists 3. Lamarck placed fossils in an evolutionary context

3. On November 24, 1859, Charles Darwin published this famous book. The book drew a cohesive picture of life by connecting what had once seemed a bewildering array of unrelated facts. Darwin made two points in The Origin of Species: Today’s organisms descended from ancestral species. Natural selection provided a mechanism for evolutionary change in populations. On the Origin of species by means of Natural Selection

4. The Origin of Species challenged a worldview that had been accepted for centuries. The key classical Greek philosophers who influenced Western culture, Plato and Aristotle, opposed any concept of evolution. They all believed the old testament view that the creator had made each species perfectly adapted to its environment. Even Carlos Linnaeus the swedish botanist who invented our present system of classification and naming of organisms, did not believe in evolution. 1. Western culture resisted evolutionary views of life

5. Darwin’s views were influenced by fossils, the relics or impressions of organisms from the past, mineralized in sedimentary rocks. Sedimentary rocks form when mud and sand settle to the bottom of seas, lakes, and marshes. New layers of sediment cover older ones, creating layers of rock called strata. Fossils within layers show that a succession of organisms have populated Earth throughout time. Fig. 22.4 Fig. 22.2

6. James Hutton, a Scottish geologist, proposed that the diversity of land forms (e.g., canyons) could be explained by the gradual effects of mechanisms that we observe every day. Later, Charles Lyell, proposed a theory of uniformitarianism, that geological processes had not changed throughout Earth’s history. These theories implied that the earth was very old 2. Theories of geological gradualism helped clear the path for evolutionary biologists

7. In 1809, Jean Baptiste Lamarck published a theory of evolution based on his observations of fossil invertebrates in the Natural History Museum of Paris. Lamarck thought that he saw what appeared to be several lines of descent in the collected fossils and current species. Each was a chronological series of older to younger fossils leading to a modern species. 3. Lamarck placed fossils in an evolutionary context

8. Central to Lamarck’s mechanism of evolution were the concepts of use and disuse of parts and of inheritance of acquired characteristics. body parts used extensively to cope with the environment became larger and stronger, while those not used deteriorated. modifications acquired during the life of an organism could be passed to offspring. A classic example—the long neck of the giraffe in which individuals could acquire longer necks by reaching for leaves on higher branches and would pass this characteristic to their offspring.

9. Lamarck’s theory was a visionary attempt to explain both the fossil record and the current diversity of life through its recognition of the great age of Earth and adaptation of organisms to the environment. However, there is no evidence that acquired characteristics can be inherited. Acquired traits (e.g., bigger biceps) do not change the genes transmitted by gametes to offspring.

10. CHAPTER 22 DESCENT WITH MODIFICATION: A DARWINIAN VIEW OF LIFE Section B1: The Darwinian Revolution 1. Field research helped Darwin frame his view of life 2. The Origin of Species developed two main points: the occurrence of evolution and natural selection as its mechanism

11. The main mission of the five-year voyage of the Beagle was to chart poorly known stretches of the South American coastline. 1. Field research helped Darwin frame his view of life Darwin’s explorations ranged from the Brazilian jungles, the grasslands of the Argentine pampas, the desolation of Tiera del Fuego, and the heights of the Andes. Fig. 22.5

12. Darwin noted that the plants and animals of South America were very distinct from those of Europe. Organisms from temperate regions of South America were more similar to those from the tropics of South America than to those from temperate regions of Europe. Further, South American fossils more closely resembled modern species from that continent than those from Europe.

13. The origin of the fauna of the Galapagos, 900 km west of the South American coast, especially puzzled Darwin. On further study after his voyage, Darwin noted that while most of the animal species on the Galapagos lived nowhere else, they resembled species living on the South American mainland. It seemed that the islands had been colonized by plants and animals from the mainland that had then diversified on the different islands.

14. While on the Beagle, Darwin read Lyell’s Principles of Geology. Lyell’s ideas and his observations on the voyage lead Darwin to doubt the church’s position that the Earth was static and only a few thousand years old. Instead, he was coming to the conclusion that the Earth was very old and constantly changing.

15. After his return to Great Britain in 1836, Darwin began to perceive that the origin of new species and adaptation of species to the environment as closely related processes. For example, among the 13 types of finches that Darwin collected in the Galapagos, clear differences in the beak are adaptations to the foods available on their home islands. Fig. 22.6

16. Darwinism as a dual meaning. It refers to evolution as the explanation for life’s unity and diversity.—descent with modification It also refers to the Darwinian concept of natural selection as the cause of adaptive evolution—cumulative changes leading to diverse adaptations. 2. The Origin of Speciesdeveloped two main points: the occurrence of evolution and natural selection as its mechanism

17. Viewed from the perspective of descent with modification, the history of life is like a tree with multiple branches from a common trunk. Closely related species, the twigs of the tree, shared the same line of descent until their recent divergence from a common ancestor. • This evolutionary tree of the elephant family is based on evidence from fossils. Fig. 22.7

18. Observation #1: All species have such great potential fertility that their population size would increase exponentially if all individuals that are born reproduced successfully. Observation #2: Populations tend to remain stable in size,except for seasonal fluctuations. The logic of Darwin’s theory. Fig. 22.8 • Observation #3: Environmental resources are limited.

19. Inference #1: • Production of more individuals than the environment can support leads to a struggle for existence among the individuals of a population, with only a fraction of the offspring surviving each generation (Malthus 1798).

20. Observation #5: Much of this variation is heritable. • Observation #4: Individuals of a population vary extensively in their characteristics; no two individuals are exactly alike. Fig. 22.9

21. Inference #2: Survival in the struggle for existence is not random, but depends in part on the hereditary constitution of the individuals. • Those individuals whose inherited characteristics best fit them to their environment are likely to leave more offspring than less fit individuals. • Inference #3: This unequal ability of individuals to survive and reproduce will lead to a gradual change in a population, with favorable characteristics accumulating over the generations.

22. Darwin’s views on the role of environmental factors in the screening of heritable variation was heavily influenced by artificial selection. • Humans have modified a variety of domesticated plants and animals over many generations by selecting individuals with the desired traits as breeding stock. If artificial selection can in a few generations produce as much diversity as we see here among the cabbage family, then natural selection over millions of generations should produce enormous changes. Fig. 22.11

23. CHAPTER 23 THE EVOLUTION OF POPULATIONS Section A: Population Genetics Populations evolve, not individuals Evolution occurs as a result of changes in a population’s gene pool --defined by its allele frequencies

24. One obstacle to understanding evolution is the common misconception that organisms evolve, in a Darwinian sense, in their lifetimes. Natural selection does act on individuals by altering their chances of survival and their reproductive success. However, the evolutionary impact of natural selection is only apparent in tracking how a population of organisms changes over time. It is the population, not its individuals, that evolve. Populations evolve, not individuals

25. Evolution on the scale of populations, called microevolution, is defined as a change in the allele frequencies in a population. For example, the bent grass (Argrostis tenuis) in this photo is growing on the tailings of an abandoned mine, rich in toxicheavy metals. • Only plants with genes for metal tolerance survive. • Individual plants do not evolve to become more metal-tolerant during their lifetimes. Fig. 23.1

26. A population is a localized group of individuals that belong to the same species. One definition of a species (among others) is a group of populations whose individuals have the potential to interbreed and produce fertile offspring in a nature. Evolution occurs as a result of changes in a population’s gene pool --defined by its allele frequencies

27. Members of a population are far more likely to breed with members of the same population than with members of other populations. Individuals near the populations center are, on average, more closely related to one another than to members of other populations. Two populations of douglas fir separated by a river Fig. 23.2

28. CHAPTER 23 THE EVOLUTION OF POPULATIONS Section B: Causes of Microevolution 1. Microevolution is generation-to-generation change in a population’s allele frequencies 2. The two main causes of microevolution are genetic drift and natural selection

29. The total aggregate of genes in a population at any one time is called the population’s gene pool. It consists of all alleles at all gene loci in all individuals of a population. Each locus is represented twice in the genome of a diploid individual. Individuals can be homozygous or heterozygous for these homologous loci. If all members of a population are homozygous for the same allele, that allele is said to be fixed. Often, there are two or more alleles for a gene, each contributing a relative frequency in the gene pool.

30. For example, imagine a wildflower population with two flower colors. The allele for red flower color (R) is completely dominant to the allele for white flowers (r). Suppose that in an imaginary population of 500 plants, 20 have white flowers (homozygous recessive - rr). The other 480 plants have red flowers. Some are heterozygotes (Rr), others are homozygous (RR). Suppose that 320 are RR and 160 are Rr.

31. Because these plants are diploid, in our population of 500 plants there are 1,000 copies of the gene for flower color. The dominant allele (R) accounts for 800 copies (320 x 2 for RR + 160 x 1 for Rr). The frequency of the R allele in the gene pool of this population is 800/1000 = 0.8, or 80%. The r allele must have a frequency of 1 - 0.8 = 0.2, or 20%.

32. Four factors can alter the allele frequencies in a population: genetic driftnatural selection, gene flow, mutation 2. The two main causes of microevolution are drift and natural selection • Natural selection is the only factor that generally adapts a population to its environment. • Selection always favors the disporportionate propagation of favorable traits. • The other three may effect populations in positive, negative, or neutral ways.

33. Genetic drift occurs when changes in gene frequencies from one generation to another occur because of chance events that occur when populations are small.

34. Genetic drift occurs when changes in gene frequencies from one generation to another occur because of chance events that occur when populations are small. For example, one would not be too surprised if a coin produced seven heads and three tails in ten tosses, but you would be surprised if you saw 700 heads and 300 tails in 1000 tosses - you expect 500 of each. The smaller the sample, the greater the chance of deviation from an idealized result. Genetic drift at small population sizes often occurs as a result of two situations: the bottleneck effect or the founder effect.

35. For example, in a small wildflower population with a stable size of only ten plants, genetic drift can completely eliminate some alleles. Fig. 23.4

36. The bottleneck effect occurs when the numbers of individuals in a larger population are drastically reduced by a disaster. By chance, some alleles may be overrepresented and others underrepresented among the survivors. Some alleles may be eliminated altogether. Genetic drift will continue to impact the gene pool until the population is large enough to minimize the impact of chance factors. Fig. 23.5

37. Bottlenecking is an important concept in conservation biology of endangered species. Populations that have suffered bottleneck incidents have lost at least some alleles from the gene pool. This reduces individual variation and adaptability. For example, the genetic variation in the three small surviving wild populations of cheetahs is very low when compared to other mammals. Their genetic variation is similar to highly inbred lab mice! Fig. 23.5x

38. The founder effect occurs when a new population is started by only a few individuals that do not represent the gene pool of the larger source population. At an extreme, a population could be started by single pregnant female or single seed with only a tiny fraction of the genetic variation of the source population. Genetic drift would continue from generation to generation until the population grew large enough for sampling errors to be minimal. Founder effects have been demonstrated in human populations that started from a small group of colonists.

39. Natural selection accumulates and maintains favorable genotypes in a population. In our wildflower example, if herbivorous insects are more likely to locate and eat white flowers than red flowers, then plants with red flowers (either RR or Rr) are more likely to leave offspring than those with white flowers (rr). If pollinators were more attracted by red flowers than white flowers, red flowers would also be more likely to leave more offspring. Either mechanism, differential survival or differential reproduction, will increase the frequency of the R allele in the population and decrease that of the r allele.

41. A mutation is a change in an organism’s DNA. A new mutation that is transmitted in gametes can immediately change the gene pool of a population by substituting the mutated allele for the older allele. For any single locus, mutation alone does not have much quantitative effect on a large population in a single generation. An individual mutant allele may have greater impacts later through increases in its relative frequencies as a result of natural selection or genetic drift.

42. While mutations at an individual locus is a rare event, the cumulative impact of mutations at all loci can be significant. Each individuals has thousands of genes, any one of which could experience a mutation. Populations are composed of thousands or millions of individuals that may have experienced mutations. Over the long term, mutation is a very important to evolution because it is the original source of genetic variation that serves as the raw material for natural selection.

43. Why do we consider evolution to be of pivotal importance in understanding organismal biology? • It allows us to make sense of both the unity and the diversity • that we see among living things • Natural selection allows us to understand the process of adaptation and to see how the process of adaptation to a changing environment contributes to diversification. • Evolution allows us to understand the history of life on earth • and to understand how gradually the changing earth has influenced living organisms and how organisms in turn have dramatically influenced the global environment. • Evolution has changed the way we look at ourselves.

44. Why do we consider evolution to be of pivotal importance in understanding organismal biology? • It allows us to make sense of both the unity and the diversity • among living things • Natural selection allows us to understand the process of adaptation and to see how the process of adaptation to a changing environment contributes to diversification • Evolution allows us to understand the history of life on earth • and to understand how gradually the changing earth has influenced living organisms and how organisms in turn have dramatically influenced the global environment. • We are also beginning to understand that subtle evolutionary adaptations of organisms to each other can happen within a few generations, and that this makes organisms function better within • complex ecosystems, and makes the systems more stable. • Finally, evolution has changed the way we look at ourselves.

45. Lecture 2: Evolution and Natural Selection Historical Context The Darwinian revolution The occurrence of evolution Natural selection as the mechanism The Logic of Natural Selection Population genetics Microevolution: generation to generation changes in populations Genetic drift: the effects of chance in small populations Natural selection: differential survival and reproduction Mutation and recombination: the origin of new genotypes

46. Terms you encountered in this lecture Fossil, sedimentary rocks, gradualism, uniformitarianism, natural selection, evolutionary tree, population, allele frequency, genetic drift, gene flow, mutation, founder effect, bottleneck, genotype, phenotype.