Processes of Evolution

Transcription

1 Processes of Evolution

2 Darwin s four postulates: 1. Individuals within species are variable in traits 2. Some of these variations (traits) are passed on to offspring (that is, these traits are heritable) 3. In every generation, more offspring are produced than can survive due to limits of the environment 4. Individuals with better variations (traits) have greater survival and reproduction. They are naturally selected.

3 Natural Selection Natural selection for various traits among individuals of a population affects which individuals survive and reproduce in each generation Process results in adaptation to the environment (increases fitness)

4 Adaptation Some heritable aspect of form, function, or behavior that improves the odds for surviving and reproducing Environment specific Outcome of natural selection

5 Populations Evolve Biological evolution changes populations, not individuals Traits in a population vary among individuals Evolution: change in the frequency of traits

6 The Gene Pool All the genes in a population Genetic resource that is shared (in theory) by all members of population

7 Variation in Phenotype Each gene in gene pool may have two or more alleles Individuals inherit different allele combinations leading to variation in phenotype Offspring inherit genes, not phenotypes

8 Variation in Populations

9 What Determines Alleles in a New Individual? Mutation Crossing over at meiosis I Independent assortment Fertilization Change in chromosome number or structure

10 Genetic Equilibrium Allele frequencies at a locus are not changing Population is not evolving

11 Five Conditions of Genetic Equilibrium No mutation Random mating Gene doesn t affect survival or reproduction Large population No immigration/emigration

12 Microevolutionary Processes Drive a population away from genetic equilibrium Small-scale changes in allele frequencies brought about by Natural selection Gene flow Genetic drift

13 Gene Mutations Infrequent but inevitable Each gene has own mutation rate Lethal mutations Neutral mutations Advantageous mutations

14 Results of Natural Selection Three possible outcomes: A shift in the range of values for a given trait in some direction Stabilization of an existing range of values Disruption of an existing range of values

15 Types of Natural Selection Directional Stabilizing Disruptive Natural selection also drives maintenance of phenotypic (and genetic) diversity Advantageous to have several forms (morphs) Sexual Selection Balancing Selection

16 Directional Selection Number of individuals Allele frequencies shift in consistent direction over time Number of individuals Range of values at time 2 Number of individuals Range of values at time 1 Range of values at time 3

17 Directional Selection Pinpointing the Target of Selection Populations of rock pocket mice have fur that matches the rocks on which they live Black basalt: dark fur Tawny granite: light fur

18 Pinpointing the Target of Selection DNA comparisons show that the two populations differ in Mclr gene sequence

19 Directional Selection Drought in Galapagos caused selection of larger beaks among ground finches (Grant and Grant 2003)

20 Directional Selection Pesticide Resistance Pesticides kill susceptible insects Resistant insects survive and reproduce If resistance has heritable basis, it becomes more common with each generation

21 Antibiotic Resistance Antibiotics first came into use in the 1940s Overuse has led to increase in resistant forms Most susceptible cells died out, while resistant forms multiplied

22 Stabilizing Selection Intermediate forms are favored and extremes are eliminated Number of individuals Number of individuals Range of values at time 1 Range of values at time 2 Number of individuals Range of values at time 3

23 Human Birth Weight percent of population percent mortality birth weight (pounds) human newborns rate of death

24 Range of values at time 3 Disruptive Selection Happens when forms at both ends of the range of variation are favored Intermediate forms are selected against Number of individuals Number of individuals Number of individuals Range of values at time 1 Range of values at time 2

25 Disruptive selection Individuals at both phenotypic extremes are favored. Example: African seedcrackers (birds) have two food sources hard seeds that require large beaks to crack, and smaller, softer seeds that smaller beaks are more suited to.

26 Disruptive selection Selection favors birds with very large or very small bill Birds with intermediate-sized bill are less effective feeders lower bill 12 mm wide lower bill 15 mm wide Fig ,

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28 Sexual Selection Selection favors certain secondary sexual characteristics Through nonrandom mating, alleles for preferred traits increase Leads to increased sexual dimorphism

29 Sexual Selection in Birds

30 (a) Intersexual selection: Sexual dimorphism in a finch species (b) Intrasexual selection: Competing for mates Figure 13.29

31 Balanced Polymorphism Polymorphism: having many forms Occurs when two or more alleles are maintained at frequencies greater than 1 percent

32 Polymorphism

33 Sickle-Cell Trait: Heterozygote Advantage Allele Hb S causes sickle-cell anemia when heterozygous Heterozygotes are more resistant to malaria than homozygotes Malaria case Sickle-cell trait less than 1 in 1,600 1 in 400 1,600 1 in in in more than 1 in 64

34 Colorized SEM Areas with high incidence of malaria Frequencies of the sickle-cell allele 0 2.5% % % % % >12.5% Figure 13.30

35 Genetic Drift Random change in allele frequencies brought about by chance Effect is most pronounced in small populations Sampling error: fewer times an event occurs, greater the variance in outcome

36 Genetic drift occurs when chance events determine which alleles are passed to the next generation. It is significant only for small populations.

37 Genetic drift has four effects on small populations: 1. It acts by chance alone, thus causing allele frequencies to fluctuate at random. Some may disappear, others may reach 100% frequency (fixation).

38 Genetic Drift: Small Populations Frequency of b+ allele

39 Genetic Drift: Large Populations Frequency of b+ allele

40 Mechanisms of Evolution Genetic drift has four effects on small populations: 1. It acts by chance alone, thus causing allele frequencies to fluctuate at random. Some may disappear, others may reach 100% frequency (fixation).

41 Mechanisms of Evolution 2. Because some alleles are lost, genetic variation of the population is reduced. 3. Frequency of harmful alleles can increase if the alleles have only mildly deleterious effects. 4. Differences between populations can increase. Chance events may lead to allele fixation in one population and loss from another population.

42 Mechanisms of Evolution 2 and 3 can have dire consequences. Loss of genetic variation reduces the ability of the population to respond to changing environmental conditions. Increase of harmful alleles can reduce survival and reproduction. These effects are important for species that are near extinction.

43 Mechanisms of Evolution Prairie chicken populations in Illinois have been reduced by loss of habitat to farmland. In 1993, the population was <50. DNA from this population compared with museum specimens from the 1930s showed a decrease in genetic variation. 50% of eggs failed to hatch, suggesting fixation of harmful alleles.

44 Harmful Effects of Genetic Drift

45 Bottleneck A severe reduction in population size Causes pronounced drift Example Elephant seal population hunted down to just 20 individuals Population rebounded to 30,000 Electrophoresis revealed there is now no allele variation at 24 genes: all are homozygous

46 Founder Effect Effect of drift when a small number of individuals starts a new population By chance, allele frequencies of founders may not be same as those in original population Effect is pronounced on isolated islands

47 Founder Effect Albatross carries seed to island phenotypes of mainland population phenotype of island population Fig , p.191

48 Inbreeding Nonrandom mating between related individuals Leads to increased homozygosity Can lower fitness when deleterious recessive alleles are expressed

49 Gene Flow Physical flow of alleles into a population Tends to keep the gene pools of populations similar Counters the differences that arise from mutation, natural selection, and genetic drift

50 Gene Flow Blue jay carries acorn between oak populations

51 Gene flow: Alleles move between populations via movement of individuals or gametes. Gene flow has two effects: 1. Populations become more similar. 2. New alleles can be introduced into a population.

52 Gene Flow In the 1960s, new alleles that provide resistance to insecticides arose by mutation in mosquitoes in Africa or Asia. Mosquitos with the new alleles were blown by winds or transported by humans to new locations. The allele frequency increases rapidly in populations exposed to insecticides.

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54 Trophy Hunting and Inadvertent Evolution Humans have caused evolutionary changes in many organisms. Red foxes with silver-tipped fur declined because of preferential hunting. Antibiotics are a strong source of directional selection, leading to evolution of antibiotic resistance in bacteria.

55 Hunting Resulted in the Decline of Silver Foxes

56 Connections in Nature: The Human Impact on Evolution Many human actions can alter the course of evolution. Pollutants and introduction of invasive species change aspects of the environment and alter selection pressures. Habitat fragmentation leaves isolated patches, which can affect evolutionary processes.

57 Evolutionary Effects of Habitat Fragmentation on a Hypothetical Species

58 Trophy Hunting and Inadvertent Evolution Bighorn sheep populations have been reduced by 90% by hunting, habitat loss, and introduction of domestic cattle. Hunting is now restricted in North America; permits to take a large trophy ram cost over $100,000.

59 Fighting over the Right to Mate

60 Trophy Hunting and Inadvertent Evolution Trophy hunting removes the largest and strongest males the ones that would sire many healthy offspring. In one population, 10% of males were removed by hunting each year. The average size of males and their horns decreased over 30 years of study.

61 Trophy Hunting Decreases Ram Body and Horn Size

62 Trophy Hunting and Inadvertent Evolution This is also being observed in other species: By targeting older, larger fish, commercial cod fishing has selected for genes that result in maturation at earlier ages and smaller size. Fish that mature earlier can reproduce before they are caught, but small fish produce fewer eggs.

63 Trophy Hunting and Inadvertent Evolution African elephants are poached for ivory; the proportion of the population that have tusks is decreasing. The unintended effects of human harvesting on these animals illustrate how populations can change, or evolve, over time.

64 The Human Impact on Evolution Human actions can alter the mechanisms of evolution: natural selection, genetic drift, gene flow. We know with certainty that our actions cause major environmental changes; we can infer that they are also causing evolutionary changes.

65 Table 12-1, p.192

66 Evolutionary Patterns, Rates, and Trends

67 Microevolutionary Processes Small-scale changes in allele frequencies brought about by Natural selection Gene flow Genetic drift

68 Crab Life Cycle Larval and juvenile stages molt repeatedly and grow in size egg

69 Macroevolution Major patterns and trends among lineages Rates of change in geologic time

70 Comparative Morphology Comparing body forms and structures of major lineages Guiding principle: When it comes to introducing change in morphology, evolution tends to follow the path of least resistance

71 Morphological Divergence early reptile pterosaur Change from body form of a common ancestor chicken bat Produces homologous structures porpoise penguin human

72 Morphological Convergence Individuals of different lineages evolve in similar ways under similar environmental pressures Produces analogous structures that serve similar functions

73 Morphological Convergence

74 Comparative Development Each animal or plant proceeds through a series of changes in form Similarities in these stages may be clues to evolutionary relationships Mutations that disrupt a key stage of development are selected against

75 Altering Developmental Programs Some mutations shift a step in a way that natural selection favors Small changes at key steps may bring about major differences

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77 Trochophore larva

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79 Molecular Evidence Biochemical traits shared by species show how closely they are related Can compare DNA, RNA, or proteins

80 Comparing Proteins Compare amino acid sequence of proteins produced by the same gene Human cytochrome c (a protein) Identical amino acids in chimpanzee protein Chicken protein differs by 18 amino acids Yeast protein differs by 56

81 Sequence Conservation Cytochrome c functions in electron transport Some sequences are identical in wheat, yeast, and primates

82 Sequence Conservation yeast wheat primate

83 Nucleic Acid Comparison Use single-stranded DNA or RNA Hybrid molecules are created, then heated The more heat required to break hybrid, the more closely related the species

84 Molecular Clock Assumption: Ticks (neutral mutations) occur at a constant rate Count the number of differences to estimate time of divergence

85 Biological Species Concept Species are groups of interbreeding natural populations that are reproductively isolated from other such groups. Ernst Mayr

86 Variable Morphology grown in water grown on land

87 Genetic Divergence Gradual accumulation of differences in the gene pools of populations Natural selection, genetic drift, and mutation can contribute to divergence Gene flow counters divergence

88 Genetic Divergence parent species time A time B time C time D daughter species

89 Reproductive Isolation Cornerstone of the biological species concept Speciation is the attainment of reproductive isolation Reproductive isolation arises as a by-product of genetic change

90 Reproductive Isolating Mechanisms Prevent pollination or mating Block fertilization or embryonic development Cause offspring to be weak or sterile

91 Prezygotic Isolation Mechanical isolation Temporal isolation Behavioral isolation Ecological isolation Gametic mortality

92 Mechanical Isolation Wasp and zebra orchid

93 Temporal Isolation Cicada

94 Albatrosses Behavioral Isolation

95 Postzygotic Mechanisms Early death Sterility Low survival rates

96 Models for Speciation Allopatric speciation Sympatric speciation Parapatric speciation

97 Allopatric Speciation Speciation in geographically isolated populations Some sort of barrier arises and prevents gene flow Effectiveness of barrier varies with species

98 Allopatric Speciation

99 Extensive Divergence Prevents Inbreeding Species separated by geographic barriers will diverge genetically If divergence is great enough it will prevent inbreeding even if the barrier later disappears

100 Archipelagos Island chains some distance from continents Galapagos Islands Hawaiian Islands Colonization of islands followed by genetic divergence sets the stage for speciation

101 Speciation on 1 A few individuals of a species on the mainland reach isolated island 1. Speciation follows genetic divergence in a new habitat an Archipelago Later in time, a few individuals of the new species colonize nearby island 2. In this new habitat, speciation follows genetic divergence. 1 2 Speciation may also follow colonization of islands 3 and 4. And it may follow invasion of island 1 by genetically different descendents of the ancestral species

102 Hawaiian Islands Volcanic origins, variety of habitats Adaptive radiations: Honeycreepers: in absence of other bird species, they radiated to fill numerous niches

103 Ancestral Type Housefinch (Carpodacus) Fig d13, p.209

104 Speciation in Hawaiian Honeycreepers Akepa (Loxops coccineus) Fig d1, p.209

105 Speciation without a Barrier Sympatric speciation Species forms within the home range of the parent species Parapatric speciation Neighboring populations become distinct species while maintaining contact along a common border

106 Sympatric Speciation in African Cichlids Studied fish species in two lakes Species in each lake are most likely descended from single ancestor No barriers within either lake

107 Sympatric Speciation in African Cichlids Feeding preferences localize species in different parts of lake

108 Speciation by Polyploidy Change in chromosome number (3n, 4n, etc.) Offspring with altered chromosome number cannot breed with parent population Common mechanism of speciation in flowering plants

109 Possible Evolution of Wheat Triticum monococcum (einkorn) Unknown species of wild wheat T. turgidum (wild emmer) T. aestivum (one of the common bread wheats) T. tauschii (a wild relative) Figure 18.9 Page AA X 14BB 14AB 28AABB X 14DD 42AABBDD cross-fertilization, followed by a spontaneous chromosome doubling

110 Parapatric Speciation Populations in contact along a common border giant velvet worm blind velvet worm

111 We re All Related All species are related by descent Share genetic connections that extend back in time to the prototypical cell

112 Patterns of Change in a Lineage Cladogenesis Branching pattern Lineage splits, isolated populations diverge Anagenesis No branching Changes occur within single lineage Gene flow throughout process

113 Evolutionary Trees species 1 species 2 species 3 ancestral stock Summarize information about relationships among groups

114 Gradual Model Species emerge through many small changes accumulating over time Fits well with evidence from certain lineages in fossil record

115 Punctuation Model Speciation model in which most changes in morphology are compressed into brief period near onset of divergence Supported by fossil evidence in some lineages

116 Adaptive Radiation Burst of divergence Single lineage gives rise to many new species New species fill vacant adaptive zone Adaptive zone is way of life Cenozoic radiation of mammals