Inquiry into Life, Thirteenth Edition
CHAPTER 27: EVOLUTION OF LIFE
27.1 Origin of Life
The common ancestor for all living things was the first cell or cells. The very first living thing had to have come from nonliving chemicals. Evolution of Small Organic Molecules Most chemical reactions take place in water, and the first protocell undoubtedly arose in the ocean. The Miller-Urey experiment supports the hypothesis that small organic molecules were formed at the ocean’s surface. Macromolecules Once formed, the first small organic molecules gave rise to still larger molecules and then macromolecules. The RNA-first hypothesis, protein-first hypothesis, and Cairns-Smith hypothesis help explain this stage of life. The Protocell After macromolecules formed, something akin to a modern plasma membrane was needed to separate them from the environment. Thus, before the first true cell arose, there would likely have been a protocell. The Heterotroph Hypothesis It has been suggested that the protocell likely was a heterotroph. The True Cell A true cell is a membrane-bounded structure that can carry on protein synthesis to produce the enzymes that allow DNA replication. The three main hypotheses about the origin of life have varying explanations of how this cell arose.
27.2 Evidence of Evolution
Evolution is all the changes that have occurred in living things since the beginning of life due to differential reproductive success. Evolution is defined as “common descent.” Fossil Evidence Fossils are the remains and traces of past life or any other direct evidence of past life. Geological Timescale As a result of studying strata, scientists have divided Earth’s history into eras, and then periods and epochs. The absolute dating method relies on radioactive dating techniques to assign an actual date to a fossil. Biogeographical Evidence Biogeography is the study of the distribution of species throughout the world. Mass Extinctions Extinction is the death of every member of a species. During mass extinctions, a large percentage of species become extinct within a relatively short period of time. So far, there have been five major mass extinctions. Anatomical Evidence The fact that anatomical similarities exist among organisms provides further support for evolution via descent with modification. Biochemical Evidence When the degree of similarity in DNA nucleotide sequences or the degree of similarity in amino acid sequences of proteins is examined, the more similar the sequences are, generally the more closely related the organisms are.
27.3 The Process of Evolution
Evolution occurs at the population level. Change in gene frequencies within a population over time is defined as microevolution. Population Genetics A population is all the members of a single species that occupy a particular area at the same time and that interbreed and exchange genes. The total number of alleles at all the gene loci in all the members make up a gene pool for the population. The Hardy-Weinberg Principle The Hardy-Weinberg principle states that allele frequencies in a gene pool will remain at equilibrium after one generation of random mating in a large, sexually reproducing population as long as five conditions are met: 1) no mutations, 2) no genetic drift, 3) no gene flow, 4) random mating, and 5) no selection. Five Agents of Evolutionary Change Mutations Mutations are genetic changes that provide the raw material for evolutionary change. Genetic Drift Genetic drift refers to changes in the allele frequencies of gene pool due to chance. The founder effect and the bottleneck effect are both examples of genetic drift. Gene Flow Gene flow is the movement of alleles between populations, as occurs when individuals migrate from one population to another. Nonrandom Mating