Chapter 13 How Populations Evolve 13 1 A Sea voyage helped Darwin frame his theory of evolution Evolution the idea that Earth s many species are descendants of ancestral species that were different from those living o Aristotle believed species were perfect and permanent which agreed with JudeoChristian culture o Jean Baptiste Lamarck believed that fossils prove that life evolves Using or not using its body parts cause change in traits o Charles Darwin believed in descent with modifications natural selection Fossils the imprints or remains of organisms that lived in the past Adaptations diverse adaptations 13 2 Darwin proposed natural selection as the mechanism for evolution Artificial selection selecting and breeding individuals with desired traits over many generations Natural selection o Members of a population often vary in their inherited traits o All species are capable of producing more offspring than the environment can support o Key Points Populations evolve over time not individuals of a particular environment Only heritable traits can be amplified or diminished Evolution is not goal directed because there is not perfect adapted organism 13 3 Scientist can observe natural selection in action Two important points o Natural selection is more an editing process than a creative mechanism o Natural selection is contingent on time and place 13 4 The study of fossils provide strong evidence for evolution Paleontologists scientist who study fossils o Fossils prove and support extinct species Fossil record sequence in which fossils appear within layers of sedimentary rocks o Rocks form in layers called strata 13 5 Many types of scientific evidence support the evolutionary view of life Biogeography geographic distribution of species Homology characteristics that result from common ancestry Homologous structures features that often have different functions but are structurally similar because of common ancestry Vestigial structures remnants of features that served important functions in the organism s ancestors 13 6 Homologies indicate patterns of descent that can be shown on an evolutionary tree Evolutionary tree chart following the patterns of descent o Hypotheses reflecting current understanding Anatomical Molecular 13 7 Evolution occurs within populations Population a group of individuals of the same species that live in the same area and interbreed o Gene pool the total collection of genes in a population at any one time 13 8 Mutation and sexual reproduction produce genetic variation that makes evolution possible 13 9 The Hardy Weinberg equation can test whether a species is evolving Hardy Weinberg principle frequency of each allele in the gene pool o p q 1 o p2 2pq q2 1 p frequency of homozygous dominant q frequency of homozygous recessives Main Conditions o Very large populations o No gene flow between populations o No mutations o Random mating o No natural selection 13 10 The Hardy Weinberg equation is useful in public health science 13 11 Natural selection genetic drift and gene flow can cause microevolution Natural selection o None all individuals in a population be equal in ability to reproduce Genetic Drift chance events can cause allele frequencies to fluctuate unpredictably from one generation to the next o Smaller the population the more impact genetic drift is likely to have Bottleneck effect drastic reduction in population size Founder effect a few individuals colonize an island or other new habitat Gene flow a population may gain or lose alleles when fertile individuals move into or out of a population 13 12 Natural selection is the only mechanism that consistently leads to adaptive evolution Relative fitness the contribution an individual makes to the gene pool of the next generation relative to the contributions of other individual 13 13 Natural selection can alter variation in a population in three ways Stabilizing selection o Favor intermediate phenotypes o Eliminate extremes o Reduces variation Directional selection o Shifts against one of the extremes o Most common during environmental change Disruptive selection o Favor extremes over intermediate o Leads to two or more contrasting phenotypes 13 14 Sexual selection may lead to phenotypic differences between males and females Sexual selection a form of natural selection in which individuals with certain traits are more likely than other to obtain mates o Intrasexual selection competing with members of the same sex to mate o intersexual selection individuals of one sex are choosy in selecting their mates Sexual dimorphism distinction in appearance o Size o Forms of adornment 13 15 The evolution of antibiotic resistance in bacteria is a serious public health concern 13 16 Diploidy and balancing selection preserve genetic variation Balancing selection occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population Heterozygote advantage a type of balancing selection in which heterozygous individuals have greater reproductive success than either type of homozygote Frequency dependent selection a type of balancing selection that maintains two different phenotypic forms in a population 13 17 Natural selection cannot fashion perfect organisms Selection can act only on existing variations o Favors only the fittest variants from the phenotypes available Evolution is limited by historical constraints o Each species has a legacy of descent with modification from ancestral forms Adaptations are often compromises o Each organism must do many different things Chance natural selection and the environment interact o Affects the genetic structure of populations to a greater extent than was once recognized Chapter 14 The Origin of Species 14 1 The origin of species is the source of biological diversity The origin of species of life o Diversity o Unity Microevolution changes in the gene pool of a population from one generation to the next Speciation the process by which one species splits into two or more species o Increase diversity of life 14 2 There are several ways to define a species Species means kind or appearance Biological species concept species is a group of populations whose members have the potential to interbreed in nature and produce fertile offspring o Reproductive isolation prevents genetic exchange and maintains the gap between species Hybrid distinct species interbreed Morphological species concept classification based on physical traits Does not require