5 C H A P T E R EVOLUTION AND GENETICS 5-2

EVOLUTION AND GENETICS Biochemical, or Molecular, Genetics Population Genetics and Mechanisms of Genetic Evolution The Modern Synthesis 3

EVOLUTION AND GENETICS What is evolution, and how does it occur? How does heredity work, and how is it studied? What forces contribute to genetic evolution?

EVOLUTION Humans have uniquely varied ways—cultural and biological—of adapting to environmental stresses Many scholars became interested in biological diversity and our position within the classification of plants and animals during the 18th century

EVOLUTION Creationism: biological similarities and differences originated at the Creation Linnaeus (1707–1778) developed the first comprehensive and still influential classification, or taxonomy, of plants and animals Fossil discoveries during the 18th and 19th centuries raised doubts about creationism

EVOLUTION Catastrophism: modified version of creationism that accounts for the fossil record by positing divinely authored worldwide disasters that wiped out creatures represented in the fossil record Both theories assert that similarities and differences from creation have not changed

THEORY AND FACT Evolution: transformation of species; descent with modification Alternative to creationism and catastrophism Darwin best known of evolutionists

THEORY AND FACT Darwin influenced by: Theory of evolution – belief that species arise from others through a long and gradual process of transformation; all life forms are related and the number of species has increased over time Grandfather, Erasmus Darwin, who proclaimed a common ancestry of all animals Lyell’s principle of uniformitarianism: the present is the key to the past; - past geological events can best be explained by observing ongoing events of the present and generalizing backward through time Darwin applied uniformitarianism to living things; contributed a theory of evolution through natural selection – how evolution occurred

THEORY AND FACT Darwin proposed natural selection to explain the origin of the species, biological diversity, and similarities among related life forms Reached the conclusion along with Alfred Wallace Natural selection: the process by which nature selects the forms most suited to survive and reproduce in a given environment Variety within that population Competition for strategic resources Giraffes – long versus short necks Argues that organisms that have a better fit within their environment, will reproduce more frequently than those less fit Reproduction is the key Natural selection continues today Peppered moth

Theory and Fact

GENETICS Genetic science helps explain causes/origin of biological variation Mendelian genetics: ways in which chromosomes transmit genes across generations Biochemical genetics: examines structure, function, and changes in DNA Population genetics: investigates natural selection and the causes of genetic variation, stability, and change

MENDEL’S EXPERIMENTS Austrian monk Gregor Mendel began series of experiments that revealed basic principle of genetics in 1856 Studied inheritance of seven contrasting traits in pea plants Concluded that heredity is determined by discrete particles or units (genes) that may disappear in one generation and reappear in the next

MENDEL’S EXPERIMENTS Observed two traits: dominant and recessive Dominant forms appear in each generation/recessive forms are masked when paired with dominant form of same trait Concluded that a dominant form could mask another form in hybrid individuals, without destroying the recessive trait Basic genetic units Mendel described were factors (now called genes or alleles) located on chromosomes

Figure 5.1: Mendel’s Second Set of Experiments with Pea Plants 15

MENDEL’S EXPERIMENTS Chromosome: a paired length of DNA, composed of multiple genes Gene: a place (locus) on a chromosome that determines a particular trait Allele: a variant to a particular gene

Figure 5.2: Simplified Representation of a Normal Chromosome Pair 17

MENDEL’S EXPERIMENTS Heterozygous: dissimilar alleles of a gene in an offspring Homozygous: two identical alleles of a gene in an offspring Genotype: organism’s hereditary makeup Phenotype: evident biological traits Dominance produces a distinction between genotype and phenotype

Figure 5.3: Punnett Squares of a Homozygous Cross and a Heterozygous Cross 19

Figure 5.4: Determinants of Phenotypes (Blood Groups) in the ABO System 20

INDEPENDENT ASSORTMENT AND RECOMBINATION Independent assortment: traits are inherited independently of one another Recombination: traits may appear in new combinations with other traits; new types in an offspring on which natural selection can operate; two main ways produces variety

BIOCHEMICAL OR MOLECULAR GENETICS Mutation: changes in the DNA molecules of which genes and chromosomes are built; produces variety (source of new forms on which natural selection may operate) BIOCHEMICAL OR MOLECULAR GENETICS

CROSSING OVER Crossing over: Chromosomes temporarily intertwine in the course of reduplication and exchange lengths of their DNA the process wherein homologous chromosomes exchange segments by breakage and recombination Can occur with any chromosome pair An important source of variety on which natural selection operates

Figure 5.6: Crossing Over 24

MUTATION Mutations: the most important source of variety upon which natural selection acts Chromosomal rearrangement: pieces of a chromosome break off and reattach someplace else on that chromosome A mismatch of chromosomes resulting from arrangement can lead to congenital disorders, cancer, and possibly to speciation Chromosomes may also fuse: When ancestors of humans split off from those of chimpanzees around six milion years ago, two ancestral chromosomes fused together in the humans line Humans have 23 chromosome pair versus 24 for chimps

MUTATION Approximately three mutations will occur in every sex cell Most mutations are neutral Evolution depends on mutations Mutations are major source of genetically transmitted variety Mutations may be neutral, harmful, or acquire an adaptive advantage through changing selective forces

NATURAL SELECTION Genotype: the genetic makeup of an organism Phenotype: organism’s biological traits (outward physical appearance as well as internal organs, tissues, and cells and physiological processes and systems) Natural selection acts only on phenotypes Human biology has considerable plasticity The environment works on a genotype to build a phenotype Diet and altitude affect how a person grows

DIRECTIONAL SELECTION After several generations of selection, gene frequencies change Adaptive traits (favored by natural selection) will be selected from generation to generation Directional selection (long-term selection of same traits) continues as long as environmental sources stay the same Maladaptive genes removed from gene pool If environment changes, new selective forces start working, favoring different phenotypes Selection operates only on traits that are present in a population Favorable mutation may occur but doesn’t usually happen just because one is needed or desirable; many species are extinct because they couldn’t adapt to environmental shifts Humans adapt rapidly to environmental variation by modifying biological responses and learned behavior

SEXUAL SELECTION Selection also operates through competition for mates Sexual selection: based on differential success in mating; a selection of traits that enhances mating success

GENE FLOW Gene flow: exchange of genetic material between populations of the same species Direct or indirect interbreeding Allele that isn’t advantageous in one environment might reach an environment in which it has selective advantage Alleles spread through gene flow even when selection not operating on the allele Species: group of related organisms whose members can interbreed to produce offspring that live and reproduce Gene flow tends to prevent speciation: the formation of new species

Figure 5.8: Gene Flow between Local Populations 31

THE MODERN SYNTHESIS Currently accepted view of evolution: Modern Synthesis: combination of Darwin’s theory of evolution by natural selection and Mendel’s genetic discoveries Speciation occurs when related populations become reproductively isolated from one another Microevolution: small-scale changes in allele frequencies over just a few generations, but without speciation Macroevolution: large-scale changes in allele frequencies in a population over a longer time period, which result in speciation

PUNCTUATED EQUILIBRIUM Punctuated equilibrium: long periods of stasis (stability) may be interrupted by evolutionary leaps (revealed in fossil record) Occurs through: Extinction of one species followed by invasion of closely related species Replacement of one species by a more fit related group in particular environment Period of sudden environmental change that permits survival of radically altered species with significant mutations or a combination of genetic changes Sudden environmental change offers possibility for the pace of evolutions to speed up Species can survive radical environmental shifts, but extinction is more common Extinction of dinosaurs was accompanied by rapid spread of mammals and birds

Darwin Cartoons