Lecture Outline - Chapter 27

27.1. Origin of Life (p. 516)

1. Origin of Earth:
   • a. Aggregated from dust particles and debris over 10 billion years ago.
   • b. Sun and planets form about 4.6 billion years ago.
   • c. Gravity and radioactivity produce heat, stratify earth into layers.
   • d. Iron and nickel molten core; silicate minerals for semiliquid mantle with volcanic lava upwelling through crust.
2. The Atmosphere Forms
   • a. Earth's size provides enough gravity to hold atmosphere.
   • b. Primitive atmosphere differed from current air; formed by volcanic outgassing.
   • c. Consisted mostly of:
     • i. water vapor (H₂O).
     • ii. nitrogen gas (N₂).
     • iii. small amounts of hydrogen gas (H₂).
     • iv. carbon dioxide (CO₂) and carbon monoxide (CO).
     • v. little, if any, free oxygen (O₂).
   • d. Lack of oxygen produced a reducing atmosphere; contrasts with today's oxidizing atmosphere.
   • e. As earth cooled, water vapor condensed to liquid water forming oceans. (Fig. 27.1)
   • f. If earth was closer or further, water would have evaporated or frozen.
3. Small Organic Molecules Evolve
   • a. Oparin in 1938 suggested organic molecules could be produced from primitive atmospheric gases in presence of outside energy sources:
     • i. heat from volcanoes.
     • ii. lightning.
     • iii. radioactivity from radioisotopes.
     • iv. light including ultraviolet radiation. (Fig. 27.1)
   • b. Stanley Miller in 1953 put Oparin's idea to test. (Fig. 27.1)
     • i. Simulated early atmosphere by providing:
       - methane (CH₄).
       - ammonia (NH₃).
       - hydrogen (H₂).
       - water (H₂O).

       ii. In closed system, heated mixture and circulated it past an electric spark.

       iii. After one week, experiment produced simple amino acids and organic acids.

       iv. More recent experiments provide similar results.
   • c. In primitive oxygen atmosphere free of bacteria, these molecules would accumulate over long time periods producing thick, warm organic soup.
4. Macromolecules Evolve and Interact (p. 517)
   • a. Three major hypotheses on origin of life:
     • i. RNA-first Hypothesis
       - only macromolecule RNA (ribonucleic acid) needed to progress to formation of first cell(s).
       - arose with discovery of ribozymes (RNA enzymes) that function in coding life processes and as proteins.
       - since some viruses use RNA genes, first genes could have been RNA.
       - first enzymes would be ribozymes also functioning as enzymes.
       - proponents call this an "RNA world."

       ii. Protein-first Hypothesis

       - Sidney Fox showed that amino acids polymerize abiotically when exposed to dry heat.

       - proteinoids are small polypeptides with catalytic properties and form.

       - microspheres form when proteinoids are returned to water; have properties of cells.

       - hypothesis assumes enzymes came before DNA genes; protein enzymes are needed for DNA replication.

       iii. Clay Hypothesis

       - Graham Cairns-Smith believes clay assists in polymerization of proteins and nucleic acids at the same time.

       - clay attracts small organic molecules; iron and zinc are inorganic catalysts for polypeptide formation.

       - RNA nucleotides and amino acids associate in such a way that polypeptides were ordered by and helped synthesize RNA; both polypeptides and RNA arise at same time.
5. A Protocell Evolves (p. 517)
   • a. Protocell occurs before first true cell.
   • b. Structure requires lipid-protein membrane and energy metabolism. (Fig. 27.2)
   • c. Lipids associated with microspheres form a lipid-protein membrane.
   • d. Coacervate droplets form when concentrated mixtures of macromolecules combine if appropriate conditions of temperature, ionic composition, and pH are present; supporting work of Oparin.
   • e. Coacervate droplets absorb substances from surrounding solution and form droplets called liposomes when phospholipid molecules are placed in a liquid environment.
   • f. Such a protocell would have contained only RNA functioning as both genetic material and enzymes.
6. Protocells Were Heterotrophic (p. 518)
   • a. Abundant organic molecules in the organic soup to serve as food; protocell likely was heterotroph.
   • b. Heterotrophs therefore believed to precede autotrophs (organisms that make their own food).
   • c. First protocells may have used preformed ATP; thereafter selection favored extracting energy from carbohydrates.
   • d. Since there was no free oxygen, we can assume that the protocell carried on a form of fermentation (glycolysis); evolution may have required millions of years.
7. A Self-Replication System Evolves
   • a. True cell has membrane-bounded structure, protein synthesis, and enzymes allowing DNA to replicate. (Fig. 27.3)
   • b. Mechanisms of genetics indicates information flows: DNA to RNA to protein; sequence probably developed in stages.
   • c. RNA-first hypothesis would have first true cells with RNA genes that directed and enzymatically carried out protein synthesis; reverse transcriptase uses RNA to form DNA which could have been origin of DNA genes.
   • d. Protein-first hypothesis requires protocell to develop sophisticated enzymes before synthesizing DNA and RNA; complexity of nucleic acid arising de novo is minimal and requires enzymes to synthesize nucleotides.
   • e. Cairns-Smith clay hypothesis has simultaneous evolution of polypeptides and RNA, an unlikely event but eliminating problem of which came first, protein or RNA?

1. Evolution: all changes that have occurred in living things since beginning of life.
2. If history of earth is condensed to one 24-hour day:
   • a. Earth is formed at beginning at midnight.
   • b. Prokaryotes appear about 5:00 in morning.
   • c. Eukaryotes present by 4:00 p.m. in afternoon.
   • d. Invasion of land starts about 10:00 p.m. at night.
   • e. Humans appear 30 seconds before midnight at end of day.
3. Fossil Evidence
   • a. Fossils are remains, traces or other direct evidence of past life forms.
   • b. Most fossils form from burial of plants and animals in sediment; soft parts are more often consumed or decomposed but may leave imprints if buried rapidly.
   • c. Most fossils are embedded in sedimentary rock, weathered particles that provide strata from lower older layers to upper newer layers.
   • d. Paleontologists study the fossil record based on boundaries between strata, where one mix of fossils gives way to another.
   • e. Transitional links are intermediate between major groups.
     • i. Archeopteryx has features intermediate between primitive reptiles and birds.
     • ii. Eustheopteron is fish ancestral to amphibians.
     • iii. Seymouria is amphibian ancestral to reptiles.
     • iv. Therapsids are reptiles ancestral to mammals.
     • v. Fossil links combined with modern comparative anatomy allows us to deduce vertebrate descent:
       fish --> amphibian --> reptile --> bird and mammal.
4. Geological Time Scale
   • a. Geological history of earth is divided into eras, periods, and epochs. (Table 27.1)
   • b. Fossil record provides relative dating of rock layers; top layers of rock are younger than lower layers.
   • c. Absolute dating method uses radioactive isotopes.
     • i. Isotopes each have particular half-life or time it takes for half of isotope to decay and become nonradioactive.
     • ii. Carbon-14 (¹⁴C) used to date organic matter; half decays to ¹⁴N each 5,730 years; limited to about last 50,000 years.
     • iii. Half of potassium-40 (⁴⁰K) decays to argon-40 (⁴⁰Ar) each 1.3 million years; estimates age of younger rocks.
     • iv. Uranium-238 decays to lead-207; estimates age of older rocks.
5. Mass Extinctions
   • a. Extinction is death of all members of species in wild; mass extinctions are extinctions of many species in short time.
   • b. Five mass extinctions in fossil record define end of:
     • i. Ordovician
     • ii. Devonian
     • iii. Permian
     • iv. Triassic
     • v. Cretaceous
   • c. Following extinctions, remaining groups expand to fill habitats vacated by extinct species.
   • d. Extinction of dinosaurs at end of Cretaceous.
     • i. Proposed in 1977 that Cretaceous extinction was caused by asteroid impact.
     • ii. Cretaceous-Tertiary border has high level of iridium, rare in earth's crust but common in meteorites.
     • iii. Calculations of effects of nuclear bomb explosions ("nuclear winter") compare with worldwide climate cooling expected from large asteroid impact.
     • iv. Worldwide layer of soot also defines iridium layer.
     • v. Huge meteorite crater of correct age found in Caribbean Ocean and Yucatan peninsula; suspected site of impact of meteor that resulted in dinosaur extinction .
   • e. Marine animal fossil record indicates mass extinctions occur every 26 million years; corresponds to movement of solar system within Milky Way galaxy.
6. Biogeographical Evidence (p. 522)
   • a. Biogeography is study of distribution of plants and animals throughout the world.
   • b. Current distribution of organisms reflects evolutionary history; organisms evolve in one location and spread out into other regions; for example, no rabbits are found in South America--they originated elsewhere and did not reach South America.
   • c. Physical factors, including location of continents, limit population range.
   • d. Continental drift states that continents have slowly moved over time.
     • i. Explains close puzzle-piece fit of east coast of South America with west coast of Africa, and other continent edges.
     • ii. Explains distribution of seed ferns throughout southern continents.
     • iii. Explains distribution of early reptiles across many continents from time when land was conjoined.
     • iv. Explains divided distribution of mammals that evolved after continents parted.
7. Anatomical Evidence (p. 523)
   • a. Many organisms share a unity of plan; for example, vertebrate forelimbs contain same sets of bones used for different functions in bat wings, whale fins, etc.
   • b. Simplest explanation is having a common ancestor whose basic forelimb plan was modified in succeeding groups as each continued along its own evolutionary pathway.
   • c. Homologous structures are similar structures derived through descent from a common ancestor. (Fig. 27.7)
   • d. Analogous structures have similar functions but differ in anatomy and did not derive from the same ancestral structure; for instance, an insect wing and a bird wing.
   • e. Vestigial structures are reduced and functionless anatomical features that are fully developed and functional in other ancestral groups.
     • i. Flightless birds have vestigial wings.
     • ii. Snakes have remnants of a pelvic girdle.
     • iii. Humans have a tail bone but no tail.
     • iv. Vestigial structures are evidence of an organism's evolutionary history.
8. Related species share embryological development. (Fig. 27.8) (p. 523)
   • a. All vertebrates exhibit notochord during development.
   • b. All vertebrates, including humans, exhibit paired pharyngeal pouches.
     • i. In fishes and amphibians, these become functioning gills.
     • ii. In humans, they become the eustachian tubes, middle ear cavity, tonsils, and thyroid and parathyroid glands.
   • c. Simplest explanation is that fish notochord and pharyngeal pouches are primitive fish features and fish are ancestral to other vertebrates.
9. Biochemical Evidence
   • a. Almost all living organisms use the same basic biochemicals: DNA, ATP, many identical enzymes, DNA triplet code, 20 amino acids, introns, and hypervariable regions.
   • b. Similarity of biochemistry is explained by descent from common ancestor.
   • c. DNA base sequences differences in DNA between a number of organisms shows less difference the more closely related they are; for example, 2.5% difference between humans and chimpanzees but 42% difference between humans and lemurs. (Table 27.2)
   • d. Amino acid sequences of cytochrome c show similarity between human and monkey, distance from human to duck and greater distance to Candida yeast.
   • e. Data are understandable assuming humans and chimpanzees share a more recent common ancestor than do humans and lemurs, ducks, or yeast.
   • f. Biochemical evidence is generally consistent with anatomical similarity of organisms.

1. Evolution is a great unifying theory of biology.
2. The word "theory" is reserved for a concept supported by large numbers of observations; has not yet been found lacking.
3. How to Detect Evolution
   • a. Population defined as all members of a single species occupying an area at same time.
   • b. In sexually reproducing populations, gene pool represents all the various allele frequencies of various genes in all members.
4. Hardy-Weinberg Law
   • a. Allele frequencies in a sexually reproducing population come to an equilibrium that is maintained each generation, as long as certain conditions are met.
   • b. Conditions that must be met: random mating, no mutations, no gene flow, no genetic drift, and no selection.
   • c. Reproduction alone does not alter allele frequencies over time.
   • d. Binomial Expression (Fig. 27.9)
     • i. Used to calculate the genotype and allele frequencies of a population.
     • ii. p² + 2pq + q² = 1.00 where p and q represent alleles of one gene loci.
     • iii. p + q = 1
     • iv. Therefore: p² = the homozygous dominant individuals, q² = homozygous recessive individuals, and 2pq = heterozygous individuals where recessive is hidden in phenotype.
   • e. In real life, allele frequencies in gene pool of population do change from one generation to next; therefore evolution has occurred.
   • f. Hardy-Weinberg law tells us what factors cause evolution--those that violate conditions mentioned.
5. Microevolution (Fig. 27.10)
   • a. Accumulation of small changes in gene pool over a relatively short period of two or more generations.
   • b. In classic observations and experiments, dark colored moths went from being 10% of population to 80% when soot colored trees and switched success of predators.
6. Five Agents of Evolutionary Change (p. 527)
   • a. Mutations
     • i. Provides new alleles and therefore underlies all other mechanisms.
     • ii. High levels of molecular variation are rule in natural populations; Drosophila are polymorphic at over 30% of gene loci, individual flies are heterozygous at 12% percent of loci.
     • iii. Many mutations are not ordinarily detected in adapted organism.
     • iv. Seemingly harmful mutations can be source of variation for better adaptation to a new or changing environment.
     • v. Daphnia normally lives between 20 - 27^oC; mutation allows survival between 25 - 30^oC.
   • b. Genetic Drift
     • i. Involves changes in allele frequencies of a gene pool due to chance.
     • ii. In small populations, there is greater chance that rare genotypes might not participate in next generation. (Fig. 27.11)
     • iii. Founder Effect
       - When few individuals found a colony, only a fraction of total genetic diversity of original gene pool is represented.
       - For example, Amish of Lancaster, Pa. have recessive allele causing dwarfism and higher proportion of polydactylism (1-in-14 compared to 1-in-1,000). (Fig. 27.12)

       iv. Bottleneck Effect

       - Populations subjected to near extinction because of a natural disaster endure a bottleneck.

       - Chance determines which few genotypes participate in production of the next generation.

       - For example, cheetahs have uniform enzymes, indicating this bottleneck caused certain alleles to be lost from a population.
7. Gene Flow
   • a. Is movement of alleles between populations by interbreeding, migration, dispersal of seeds, cross- pollination, etc.
   • b. Keeps gene pools similar and prevents close adaptation to local environment.
8. Nonrandom Mating
   • a. Occurs when individuals pair up not by chance but according to genotypes or phenotypes.
   • b. Includes inbreeding, mating between relatives.
   • c. Increases proportions of both homozygotes at all gene loci; decreases proportion of heterozygotes.
9. Natural Selection (p. 530)
   • a. Process by which populations adapt to their environments.
   • b. Charles Darwin, father of evolution, explained evolution by natural selection.
   • c. Fitness of individual is measured by how reproductively successful its offspring are in next generation.
   • d. Gene mutations are ultimate source of variation; recombination of alleles and crossing over, independent assortment of chromosomes, and fertilization provide variation.
   • e. Natural selection acts on phenotype; most traits acted on by natural selection are polygenic.
10. Types of Selection
    • a. Stabilizing Selection (Fig. 27.13)
      • i. Favors the intermediate phenotype.
      • ii. Selects against the extremes.
      • iii. For example, birth weight of human infants between average 3.1 kg and 3.5 kg have better chance of surviving than weights on either extreme.
    • b. Directional Selection (Fig. 27.14)
      • i. Occurs when extreme phenotype is favored; distribution curve shifts in that direction.
      • ii. Example: Evolution of the Horse
        - Hyracotherium was about the size of dog; adapted for forest life with teeth for browsing, short legs, etc.
        - gradual increase in body size and length of legs as population adapted to environment changing to grassland.
        - modern horse Equus evolved larger size and one-toed hooves for speed, grinding teeth, etc.

        iii. Industrial melanism of moths show shift from light to dark color due to selection by birds feeding on contrasting moths from sooty environment.

        iv. Pesticide effects on insects and antibiotics on bacteria are directional selection agents producing resistant strains.
    • c. Disruptive Selection (Fig. 27.15)
      • i. Two or more extreme phenotypes are favored over intermediate phenotype.
      • ii. British land snails in low vegetation areas are eaten by thrushes if they have dark shells, but opposite effect occurs in forest areas that have light bands on shells; result is mixed population.
11. Variations are Maintained
    • a. Sickle-cell disease is an example of a balanced polymorphism where a favored heterozygote keeps two homozygotes present in population.
    • b. Two alleles for sickle cell are severe problem; two alleles for normal blood make person susceptible to serious malaria.
    • c. Heterozygous individual for sickle-cell trait has adaptive advantage but will continue to produce two homozygotes that are not adaptive.

1. Species is group of interbreeding populations that share a gene pool and are reproductively isolated from other species.
2. Subpopulations of same species exchange genes; different species do not exchange genes.
3. Reproductive isolation of gene pools occurs by premating isolating mechanisms where mating is never attempted, or postmating isolating mechanisms where offspring do not develop. (Table 27.3)
4. Two Means to New Species (p. 533)
   • a. Speciation occurs when one species gives rise to two species through reproductive isolation. (Fig. 27.16)
   • b. Geographic isolation occurs when subpopulations are separated from each other by a new river, canal, etc.
   • c. If enough variation arises between the separate populations due to independent mutations, drift, selection, etc., they will be reproductively isolated when the populations are reunited; this is called allopatric speciation.
   • d. If a population separates into two reproductively isolated groups without geographic isolation, as with doubled chromosome numbers in some plants, this is called sympatric speciation.
5. Adaptive Radiation (p. 535)
   • a. Adaptive Radiation is proliferation of species by adaptation to new ways of life.
   • b. Classic example is Darwin's finches. (Fig. 27.17)
     • i. Seen by Darwin on his voyage to Galápagos Islands.
     • ii. Islands contained much larger variety of finches than Ecuador, South America, 600 miles to east.
     • iii. Islands also contained far greater variation, 13 different-sized finches with different bills adapted to particular food-gathering methods.
     • iv. Different conditions on islands selected for variation while providing some isolation.
   • The Pace of Speciation
     • a. "Living fossils" is term for organisms that have changed little from earlier times; are usually well- adapted to unchanging environment.
     • b. Examples of little-changed organisms includes coelacanth, horseshoe crab, crocodile.
     • c. Stasis is a time of limited change in a lineage.
     • d. Phyletic gradualism is older model of evolution by slower gradual change; gradualism makes dramatic transitional fossils unlikely.
     • e. Punctuated equilibrium is newer model of evolution, with stasis punctuated with rapid change or speciation.
     • f. Because speciation is rapid in this model, transitional links may be rare as fossils.