Bio Exam 4

4/14 Lecture: Descent with Modification



Biological Evolution: heritable change in a population across many generations

  • Small scale - change in a single gene 

    • Microevolution 

  • Large scale - change above the species levels

    • Macroevolution 



Why Study Evolution? Learning what evolution is has immense practical value that extends beyond understanding biology and extends to understanding the world 



Evolutionary Medicine: studying and treating human illness and disease through the study of evolution 

  • Concepts such as adaptation and mutation inform therapies and strategies to combat pathogens like the flu 

  • Models developed by evolutionary biologists have shed light on genetic variation that may account for increased risks of genetic diseases like Alzheimer’s or heart disease

  • Knowing the evolutionary relationships among species allows scientists to choose appropriate organisms for the study of diseases such as HIV

  • Using the principles of natural selection to identify new drugs for detecting and treating diseases such as cancer



  • Evolution helps us understand the history and diversity of life - we are all related 



Evolutionary Changes In Everday Life: 

  • Antibiotic-resistant bacteria 

  • Drug-resistant bedbugs 



The Evolution of Evolutionary Thought: 

  1. Species are permanent, life is unchanging 

    1. Plato:

      1. Every organism was an example of perfect essence/type created by God

      2. Types were unchanging 

    2. Aristotle:

      1. Scale Naturae

      2. Imperfect → perfect, no room for movement 

  2. Humans discovered that there is change over time 

    1. Fossil = trace of organisms that lived in the past 






How Does The Earth Change? Uniformitarianism vs. catastrophism  



What Causes Species To Change Over Time?

  • Jean-Baptiste de Lamarck 

    • Best explanation for relationship to fossils is that LIFE EVOLVES, changes over time

    • Mechanism of evolution = inheritance of acquired characteristics

  • Darwin

    • Observations of thousands of different species shaped his idea about evolution

    • Organisms in temperate regions of SA were more like organisms in tropical regions of SA, not like organisms in temperature regions of Europe 

    • Extant = modern-day currently living organisms

    • Endemic = species unique to a defined geographic region

    • Gradients of change in species within & among continents, islands 

      • Different, but related species of flinches on Galapagos 

        • Beak size adapted to specific diet 

    • Characteristics of organisms → well-suited to their diverse environments



Descent with Modification: species present today descended from ancestral species 

Natural Selection: the mechanism responsible for change in a species over time 

Variation Among Individuals Within a Species:

  • Heritable 

  • Organisms produce more offspring than the environment can support 

    • Competition exists among individuals

    • Many offspring fail to survive and reproduce

  • Individuals whose inherited traits give them a higher probability of survival and reproduction in an environment tend to have more offspring than other individuals AND an unequal ability of individuals to survive and reproduce will lead to the accumulation of favorable traits in the populations over generations

  • The increasing frequency of favored traits in a population → evolution 



Fitness: the ability of an organism to pass its genes to the next generation

  • Includes surviving, finding a mate, producing offspring

  • Depends on the environment in which an organism lives
































Pre-Lecture Video 15: Mutations 



Mutation: extremely common, single base typo in cell replication, dna changes occurring in cells that can be passed down to offspring 

  • Not always bad 

  • Protein gene variations can have consequences

  • Spontaneous



Point-Substitution: common mutation, one base pair substituted for another 



Large Scale Mutations: 

  • Can occur during meiosis

  • Whole chromosomes can be replaced

  • Can have no effect or a large effect



Somatic Mutation: not passed to the next gen., an alteration of DNA that occurs after conception 



Germline Mutations: a gene change that occurs in the cell used in reproduction (egg or sperm), passed on to offspring



Adaptation: positive mutation



Deleterious Mutation: a mutation that can have harmful consequences, such as putting a person at higher risk for cancer





















4/15 Lecture: Evidence for Evolutionary Change 



Artificial Selection: selective breeding 

  • Darwin <3 pigeons 

  • Used in agriculture

  • Dog breeds 

  • Fox experiment 

    • Foxes selected for tameness had floppier ears over successive generations

    • A genetic link between phenotypic traits and alleles for behavior 



Homology: similarly due to common ancestry 

  • Homologous structures 

    • The similarity in structures between different organisms due to common ancestry 

    • Ancient structures became modified in different ways 



Vestigial Structures: anatomical features with no apparent function, but resemble structures of presumed ancestors 



Embryonic Homologies: developmental resemblances hint at common ancestry 

  • Hox genes 

    • Nearly every animal that's been tested has Hox sequences in its DNA

      • Hox genes arose very early in animal ancestry 



Molecular Homologies: similar molecular structure of proteins/DNA 



Analogous Structures: similar structures in organisms without shared ancestry, evolved independently to serve the same purpose 

  • Evidence for convergent evolution

    • Developmental resemblances hint at common ancestry 

  • Echolocation, eyes, flight, opposable thumbs, carbon fixation in plants, plant carnivory 



Biogeography: focuses on the distribution of species 

  • Species on ocean islands tend to resemble species of the nearest mainland, even if the environment is different

  • Many species on islands are endemic 

    • Islands are engines of evolution 



Fossil Record: linkages between extinct and extant species



  1. Natural selection does not give organisms what they need, it only acts on existing variation 

  2. Not all genetic variation makes an organism more fit 

  3. Populations, not individuals, are the units of evolution

  4. Evolution can lead to more complex structures → but complexity does not = BETTER structures 

  5. Natural selection does not make perfect organisms 



Mechanisms of Microevolution: 



Genetic Drift: random events that cause allele frequencies to fluctuate unpredictably from one generation to the next 

  • Important in small populations 

  • Can greatly reduce or eliminate alleles 



Genetic Drift - Founder Effect: individuals isolated from main population 

  • In “founding/new” population

    • Loss of alleles 

  • Fixed Allele: an allele that is the only variant in a population

  • Example: Amish 



Genetic Drift - Bottleneck Effect: population goes through a period in which its size decreases 

  • Example: northern elephant seals 



Gene Flow: genetic exchange due to the migration of fertile individuals or gametes between populations 

  • Reduces differences BETWEEN populations

  • Increases genetic diversity WITHIN populations

  • Can decrease or increase the fitness of populations





















4/17 Lecture: Mechanisms of Microevolution 



Mutation: 

  • Heritable change in DNA

    • Random 

    • Rare- not big change in allele frequency in 1 generation

  • Mostly neutral 

  • Sometimes bad / but sometimes good 



Natural Selection: 

  • Accumulates and maintains favorabel genotypes in a population 

  • Must have genetic variation 



Modes of Natural Selection: 

  • Directional selection

    • Favors individuals at one extreme of a phenotypic distribution

    • Individuals of one extreme experience poor reproductive success 

  • Disruptive selection (diversifying selection)

    • Favors individuals with extreme phenotypes

  • Stabilizing selection

    • Favors individuals with intermediate phenotypes 

    • Average is good; extremes are not 

    • Reduction in variation 

  • Balancing selection 

    • Maintains genetic diversity in population 

    • Frequency-dependent-selection: The fitness of a phenotype depends on how common it is in the population

    • Natural selection does not always cause the elimination of “weak” or less fit alleles 

  • Sexual selection: 

    • Acts on traits that affect reproductive success

    • Survival doesnt always mean reproduction → animal must find a mate to pass on genes 



Polymophism: alternate phenotypes

  • Different eye color, left handedness vs. right handedness 

  • Heritable, but not Mendelian, there are many genes involved + very complex 

  • Potentially influences by development in utero 








Speciation: How Species Arise - bridge between microevolution and macroevolution 



Biological Species Concept: a group of individuals that have the potential to interbreed and produce fertile offspring 

  • Ex: dogs

  • Has major limitations 

    • What about extinct forms of life?

    • What about asexual reproduction? 

    • What about self-fertility? 

  • Ring Species:

    • Gene flow occurs between neighboring populations, but at the ends of the “ring” the populations dont interbreed 

    • Where to mark the point of speciation?



Reproductive Barriers:

  • Prezygotic reproductive barriers: no mating attempted 

    • Temporal isolation = not at the right time to mate 

    • Habitat isolation = not in the right place or time to mate 

    • Behavioral isolation = different mating calls 

    • Mechanical isolation = morphological features prevent successful mating 

    • Gametic isolation = gametes do not unite 

  • Postzygotic barriers: mating and fertilization occur; zygote forms 

    • Hybrid inviability = hybrid development impaired 

    • Hybrid sterility = hybrid cannot reproduce

    • Hybrid breakdown = 1st generation hybrids fertile, 2nd generation is sterile 

  • Can lead to speciation - limiting gene flow 



Geography In Speciation: 

  • Allopatric speciation: 

    • geographic separation → no gene flow 

    • Natural selection, genetic drift → populations diverge 

    • reproductive isolation

    • Regions with more geographic barriers → more species 

  • Sympatric Speciation:

    • Speciation without geographic isolation 

    • Reproductive barrier evolves first 

    • One common mechanism = polyploidy 

    • Polyploidy: gametes with extra chromosomes due to error in cell division 

      • Change in # of chromosome sets can create new species