Chapter 15: Evolution—Evidence and Theory

15-1 The Fossil Record

15-2 Theories of Evolution

15-3 Evolution in Process

I. Nature of Fossils (physical EVIDENCE for evolution)• Organisms appeared, LASTED for periods of time, BUT disappeared.

• NOTE: MICROSCOPY Too much DETAIL for fossils to be simple rock formations (Robert Hooke, 1700s).

15-1 The Fossil Record

(1) Fossil (e.g., Types: sedimentary fossils, molds and casts)• TRACE of a dead organism, usually with HARD MINERALS replacing tissues of an organism (leaving ROCK-LIKE structures).

(2) Sediment (fossils form INSIDE sedimentary rock)• Dust, sand, OR mud deposited in LAYERS by wind or water;

NOTE: Traps the HARDEST body parts of an organism to become fossilized (shell, bones, teeth, woody stems).

(3) Mold• HOLLOW fossil formed by sediment ENCASING a dead organism— becomes filled with HARD minerals, forming a CAST of an organism.

(4) Cast• Rock-like MODEL of an organism resulting from a MOLD filled in with HARD minerals.

II. Distribution of Fossils (Nicholas Steno, 1669) • Earth’s GEOLOGICAL and BIOLOGICAL history are recognized by Steno’s Law of Superposition.

(1) Law of Superposition• SUCCESSIVE layers of sediment are deposited on TOP of one another by wind and water GEOLOGICAL STRATA.

(2) Stratum (pl. strata)• Geological layer showing its RELATIVE age with respect to OTHER strata above (YOUNGER) and below (OLDER).

(3) Relative Age (estimates VARY due to DEPOSITION and EROSION)• Age with respect to OTHER strata based upon its POSITIONING;

(also can infer the AGE of the FOSSILS contained).

(1) What may you be able to conclude if you find identical examples of a fossilized organism in TWO adjacent geological strata?

Critical Thinking

(4) Absolute Age (a form of quantitative data)• Determined by RADIOACTIVE DATING of materials isolated from within a stratum.

(A) Succession of Forms (Prokaryotes Modern Humans)• Fossil-bearing strata show species have CHANGED over time, possibly becoming EXTINCT (replaced) by newer forms.

(1) Extinct (opposite = extant)• NO LONGER present due to SELECTIVE PRESSURES and confirmed through fossilized remains.

(2) Mass Extinction (fossil evidence)• Period where LARGE numbers of species disappeared due to DRASTIC environmental pressures.

(i.e., Widespread volcanic activity, or asteroid impacts (ash & dust theory))

(B) Biogeography (Extinct VS. Extant)• NEW organisms ARISE in environments where SIMILAR forms already inhabit (shared selective pressures yield similar phenotypes).

I. Jean Baptiste de Lamarck’s Explanation (of HOW species change—1800s)• Claimed ACQUIRED traits COULD be passed on to offspring (i.e., Theory of Acquired Characteristics).

15-2 Theories of Evolution

(1) Acquired Traits (CANNOT be passed down)• Are NOT determined by genes BUT by the ENVIRONMENT.

(2) If a favorable trait INCREASES the lifespan of an organism WITHOUT affecting reproductive success, does it contribute to EVOLUTION?

Critical Thinking

II. The Beginning of Modern Evolutionary Thought (Darwin and Wallace)• GENETIC traits (NOT acquired traits) could be PASSED DOWN from parent to offspring via genes.

(1) Natural Selection (drives the process of EVOLUTION)• Individuals BEST adapted will reproduce MORE thus OVER time, MOST FIT are those with MOST offspring.

(3) The process of natural selection throughout the history of life on Earth has resulted in the success of some species and the extinction of other species. Why has natural selection NOT resulted in the existence of a SINGLE best-adapted species?

Critical Thinking

(2) Population (SMALLEST unit CAPABLE of evolving)• Interbreeding group of same species; (e.g., a population’s GENE POOL consists of the TOTAL ALLELES held by that species).

(A) Charles Darwin (1831-1836—H.M.S. Beagle)• Naturalist collected DATA on species of around the world—interested in BIODIVERSITY and GEOGRAPHY.

(B) Voyage of the Beagle (5 years, WIND-driven ship)• A book (Principles of Geology)—influenced Darwin on the principles of UNIFORMITARIANISM.

(1) Uniformitarianism (e.g., fossils of seashells in the mountains)• Earth’s HABITATS have been SHAPED (changed) through SLOW processes that operate OVER time.

NOTE: This influenced Darwin to believe modifications of ENVIRONMENTS required LONG periods of time, much like the CHANGES it would take SPECIES to ADAPT to these CHANGES (i.e., speciation).

(C) Analysis of Darwin’s Data (on geography AND biodiversity)• Supported “adaptive radiation” of FINCH and TORTOISE populations that dispersed over Galapagos Islands (West of S. America).

(D) Publication of The Origin of Species (1859)• Used data to propose 2 THEORIES supporting “HOW” the process of EVOLUTION occurs in a population.

III. Darwin’s Theories (following 5 year H.M.S. Beagle voyage)• Theory 1: DESCENT WITH MODIFICATION, occurs with EVERY species.

• Theory 2: MODIFICATION BY NATURAL SELECTION drives EVOLUTION to occur.

(A) Descent with Modification• NEWER forms are the MODIFIED DESCENDANTS of OLDER species.

(i.e., All species have DESCENDED from ONE or a FEW ORIGINAL types of life; remote, COMMON ANCESTORS are common to many species).

(B) Modification by Natural Selection (requires a DIVERSE gene pool)• NATURAL forces ACT UPON the GENETIC VARIATION within a population (resulting in VARYING degrees of FITNESS).

DARWIN’S THEORY OF EVOLUTION HOLDS FOUR TENETS (influenced by colleague Thomas Malthus)

(1) Populations will naturally grow BEYOND the environmental capacity leading to a STRUGGLE for existence (Malthus’s work with populations).(2) Individuals within a POPULATION show VARIATION due to mutations AND genetic recombination.

(3) The VARIATION is ACTED UPON (natural selection) allowing MOST FIT members to survive and YIELD MORE OFFSPRING.

(4) Over TIME (and through evolution), the gene frequencies (i.e., GENE POOL) of the population WILL CHANGE (and thus, traits WILL CHANGE).

(1) Adapt (adaptations ARE phenotypes that aid FITNESS) • Adaptation RESULTS in an INCREASED number of FAVORABLE genes.

(2) Fitness (Darwinian Fitness)• NUMBER of VIABLE offspring an individual produces—i.e., fitness IS measured by OFFSPRING COUNT.

(4) In recent years, paleontologists have claimed that in some cases the evolution of a new species occurs quite suddenly—in LESS than a thousand years. Darwin stated that evolution was a gradual process. What effect may GENERATION TIME have on evolution RATE?

Critical Thinking

(3) Adaptive Advantage (over disadvantage)

• Is defined when an INHERITED TRAIT is known to…

(1) Enhance an ability to OBTAIN NUTRITION.

(2) Enhance an ability to successfully MATE yielding offspring.

(3) Enhance an ability to EVADE predation.

I. Evidence of Evolution• MODERN organisms (homologous AND analogous structures)—we can see CHANGE occurred.

15-3 Evolution in Process

(A) Homologous and Analogous Structures (i.e., divergent & convergent evol.) • MORPHOLOGY may OR may NOT reflect a COMMON ANCESTOR.

(1) Homologous Structures (formed through DIVERGENT evolution)• Similar structures originated in a SHARED ancestor; (e.g., changed the ORIGINAL to meet changing environmental forces).

(2) Analogous Structures (formed through CONVERGENT evolution)• Structures serve similar FUNCTIONS, BUT organisms do NOT share a RECENT COMMON ANCESTOR (anatomical differences TOO great).

(B) Vestigial Structures• Useful to an ANCESTRAL organism but are NO longer useful in the MODERN-day descendant.

(1) Vestigial (preserved REMNANT of evolution)• Evolution tends to be CONSERVATIVE, eliminating ONLY features that REDUCE fitness. (e.g., tailbone, appendix, pelvic bones in snakes).

(5) Many vestigial traits, such as the human tailbone, seem to be largely NEUTRAL, that is, neither beneficial nor harmful. The APPENDIX is an example of a vestigial structure in humans. How might having an appendix be HARMFUL to humans?

Critical Thinking

(2) Conserved (Structures)• Remain UNCHANGED throughout MUCH of evolutionary history of a species, (for not completely understood reasons).

(C) Similarities in Embryology (developmental similarity)• ALL vertebrate embryos INITIALLY appear THE SAME (limb buds, backbone, eyes), BUT the similarities FADE.

(D) Similarities in Macromolecules (MOLECULAR evidence for evolution)• Shared DNA, RNA, AND AA sequences MAY confirm evolutionary histories suggested by FOSSILS, MORPHOLOGY, AND EMBRYOLOGY.

II. Patterns of Evolution (3 PATTERNS)• Change RESULTS from changing demands of ENVIRONMENTS.

(1) Co-evolution

(2) Convergent Evolution

(3) Divergent Evolution

(6) Some MONARCH butterflies contain chemicals that are TOXIC to birds. Another species of butterfly, the VICEROY, has some protection from predation because it closely RESEMBLES the monarch. What pattern of evolution is illustrated by THIS example?

Critical Thinking

(A) Coevolution• Change of 2 or MORE species in CLOSE ASSOCIATION with EACH OTHER.

Ex: (1) Predators AND prey sometimes co-evolve, (2) Parasites AND their hosts often co-evolve, (3) Herbivorous animals AND plants they feed on also co-evolve, and lastly (4) Pollinators AND pollinated flowering plants often co-evolve.

(B) Convergent Evolution (result in ANALOGOUS structures)• Occurs when environment selects FOR similar phenotypes, even though ANCESTRAL types were QUITE different from each other.

Ex: Great-White Shark AND Bottlenose Dolphin (similar morphology for SWIMMING, but DIFFERENT ANCESTRIES).

(C) Divergent Evolution (result in HOMOLOGOUS structures)• 2 populations become more DISSIMILAR a response to geographical isolation (and ultimately, SPECIATION).

Ex: American Porcupine AND Old World Porcupine display HOMOLOGOUS features derived from a COMMON ANCESTOR estimated ~ 70 m.y.a.

(1) Adaptive Radiation (i.e., DIVERGENT evolution)• Related species EVOLVED from a single ancestral POPULATION.

(Ex: Darwin’s FINCHES of the Galapagos Islands have DIVERGED due to geographical isolation).

(2) Artificial Selection (we can SPEED UP divergent evolution)• HUMAN influence on EVOLUTION of other species for various reasons (i.e., Selective Breeding).

• Examples include DOMESTICATION of dog breeds, LIVESTOCK, and agricultural CROPS.

Extra Slides AND Answers for Critical Thinking Questions

(1) You can conclude that the organism lived, relatively unchanged, during the period during which the two strata were deposited.(2) A favorable trait that does not enhance the reproductive success does not contribute to evolution.

(3) Coevolution

(4) Organisms with a short generation time could be modified more rapidly than those with a long generation time. For example, a bacterium with a generation time of one hour would have almost 9 million generations in 1,000 years, whereas humans would have about 40 generations in the same time span.

(5) There are many different environments and many different demands on species. A species adapted to one environment might fare poorly in a different environment. Moreover, different types of organisms are interrelated in complex ways. For example, animals depend on plants for their existence, and plants may depend on animals for pollination.

(6) The appendix can become infected, and if left untreated, this condition is life-threatening.

Revisiting Evolution

• The theory of evolution states that species change over time; the mechanism for this change is natural selection.

Assessing Prior Knowledge

• How does the DNA in an organism determine the structure and function of the proteins an organism produces?• How could a change in DNA result in a change in the protein it manufactures?

• Fossils are one line of evidence for evolution, including the use of morphology, embryology, and DNA sequencing between species.