Chapter Outline
INTRODUCTION Evolution Well Known, But Poorly Understood by Public Many feel evolution challenges their religious beliefs fig 21.1 Trends to teach religious dogma as scientific creationism Evolutionary Change in Natural Populations Is Adaptive Microevolution: changes in gene frequency within populations of species Macroevolution: replacement of one species by another THE EVIDENCE THAT NATURAL SELECTION EXPLAINS MICROEVOLUTION Some Genetic Variation Maintained by Natural Selection tbl 21.1 Sickle-Cell Anemia Causes red blood cells to assume irregular, elongated shapes Disease affects shape of hemoglobin molecule Hydrophobic valine substituted for polar glutamic acid Creates "sticky" patch on surface of hemoglobin Oxygen shields patch, no unusual interactions Without oxygen "sticky" patches bind to other patches Molecules form long, fibrous clumps that deform blood cell Sickle-cell trait Heterozygous, Ss individuals Produce few sickle-shaped cells Frequency of recessive allele in various populations African-Americans = 0.045 White Americans = 0.001 Central Africans = 0.12 1 per 5 are heterozygous 1 per 100 heterozygous recessive and die before reproducing Recessive allele maintained at unusually high levels Heterozygotes less susceptible to malaria Heterozygous women more fertile than homozygotes Environment acts to maintain allele frequency Selective force in Africa is presence of malaria Maintenance of allele has adaptive value in Africa fig 21.2 No such selective force in US black population Selection acts to eliminate allele in US Peppered Moths and Industrial Melanism fig 21.3 European moth that rests on trees during daytime Prior to 1850 most had light-colored wings After 1850 most had dark-colored wings Possess dominant allele Allele rare in populations until then Observed dark tree trunks in industrial areas Dark moths less conspicuous on their surfaces Air pollution killed light-colored lichens Kettlewell hypothesis: birds ate moths on trees More dark moths survived in polluted areas More light moths survived in unpolluted areas Industrial melanism Evolutionary process affecting light-colored organisms Populations become dark-colored by natural selection Trends reversing due to pollution controls Lead Tolerance Bent grasses grow on lead mine refuse Soils contain toxic chemicals Few plants survive conditions Comparison of plants in pasture and mine refuse areas Mine plants in pasture soil survived but grew slowly Mine plants in mine soil grew well Most pasture plants in mine soil grew poorly if at all Few exceptions that grew well Were of same ancestral stock as mine plants Genetic predisposition to lead tolerance Population change is rapid when environment demands it An Overview of Adaptation Documented cases of adaptation exist as indicated above Environment dictates direction and extent of change THE EVIDENCE FOR MACROEVOLUTION Support for Darwin`s Evolutionary Theory Summarization of Darwin`s evidence for macroevolution tbl 1-1 Adaptation provides strong evidence for microevolution tbl 21.2 The Fossil Record More evidence available than in Darwin`s time Formation of fossils Organisms buried in sediment Calcium in bone and hard tissue is mineralized Sediment converted to rock Date of rocks reflects age of fossils Dating in Darwin`s day solely by relative position Recent dating uses more accurate techniques Measure rate of radioisotope decay Rate constant over time, not affected by temperature or pressure Fossils arrayed from oldest to youngest Provide evidence of progressive evolutionary change Examples Hoofed mammals fig 21.4 Horse evolution fig 1.15 Oyster shell shape fig 21.5 The Molecular Record Progressive evolutionary change implies a change within DNA Result from accumulation of genetic changes Distant relatives have greater number of differences Comparison of DNA sequences between organisms Greater time since divergence associated with more nucleotide changes Example: cytochrome c fig 21.6 Example: hemoglobin fig 21.7 Phylogenetic tree Pattern of genetic descent Determined by comparing nucleotide sequences Often similar to relationships predicted by anatomy Homology Structures derived from common form, but functions are variable Example: forelimbs of mammals fig 1.14 Development Evolutionary history reflected in development of embryo Embryo exhibits characteristics of its ancestors` embryos Example: human development Possess fish-like gill slits early in development Exhibit tail, its vestige becomes coccyx Possess fine fur during fifth month Examples Vertebrate embryo comparisons fig 21.8 Compare larval form of slug and giant squid Vestigial Structures Structures with no apparent function resembling those of presumed ancestors Examples Human ear muscles Whale pelvic bone fig 21.9 Four-footed "missing link" whales fig 21.10 Human vermiform appendix Indicate presumed common ancestry of related organisms Convergent Evolution Different areas may possess very distantly related communities with similar appearance Unlikely that similarities result from coincidence Example: forms of Australian marsupials fig 21.11 Similarities in groups of species peculiar to one habitat Examples: albinism and blindness in cave-dwelling organisms Patterns of Distribution Organisms on islands most closely resemble forms on nearest continent Forms not identical, but diverged over time Example: Galapagos finches fig 1.10 THE TEMPO AND MODE OF EVOLUTION Evolution of Different Groups Proceed at Different Rates Most mammal species evolved fairly recently, genera are older Lungfish and other animals exhibit little change Groups of Organisms Have Fast and Slow Periods of Change Evidence in fossil record Punctuated equilibria: evolution proceeds in spurts fig 21.12 Occurs when populations are small Different from parent population by founder effect Rapid adaptation to novel ecological circumstances Stasis: lack of evolutionary change Large populations Diverse and conflicting selective pressures Gaps may also be due to incomplete fossil record Contrasted with gradualism: gradual evolutionary change SCIENTIFIC CREATIONISM Acceptance of Evolution Universally supported by biologists to explain diversity Supported by observations and experiments Theory as readily accepted as laws of gravity Scientific Creationism Attempts to Explain Diversity Literal interpretation of the Bible Religious, non-scientific perspective Earth much younger than scientists believe All organisms created as they exist today Arguments to present as theory comparable to evolution Acceptance of premise that it is truly scientific Lacks empirical scientific evidence Does not infer principles from observation Assumptions do not lead to testable hypotheses Denies scientific facts assembled over centuries Implies deceptive creator Evolution provides scientific explanation Controversy about how evolution operates, not that it operates