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5 C H A P T E R EVOLUTION AND GENETICS 5-2
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EVOLUTION AND GENETICS
Biochemical, or Molecular, Genetics Population Genetics and Mechanisms of Genetic Evolution The Modern Synthesis 3
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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?
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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
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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
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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
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THEORY AND FACT Evolution: transformation of species; descent with modification Alternative to creationism and catastrophism Darwin best known of evolutionists
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THEORY AND FACT Darwin influenced by:
Grandfather, Erasmus Darwin, who proclaimed a common ancestry of all animal species Lyell’s principle of uniformitarianism: the present is the key to the past Theory of evolution
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THEORY AND FACT Natural selection: the process by which nature selects the forms most suited to survive and reproduce in a given environment Competition for strategic resources Variety within that population Natural selection continues today
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GENETICS Genetic science helps explain causes 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
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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 Heredity is determined by discrete particles or units
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MENDEL’S EXPERIMENTS 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
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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
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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
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INDEPENDENT ASSORTMENT AND RECOMBINATION
Independent assortment: chromosomes are inherited independently of one another Recombination: new types in an offspring on which natural selection can operate
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Figure 5.1: Mendel’s Second Set of Experiments with Pea Plants
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Figure 5.2: Simplified Representation of a Normal Chromosome Pair
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Figure 5.3: Punnett Squares of a Homozygous Cross and a Heterozygous Cross
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Figure 5.4: Determinants of Phenotypes (Blood Groups) in the ABO System
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BIOCHEMICAL OR MOLECULAR GENETICS
Mutation: changes in the DNA molecules of which genes and chromosomes are built Gametes: sex cells that make new generations BIOCHEMICAL OR MOLECULAR GENETICS
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BIOCHEMICAL, OR MOLECULAR, GENETICS
DNA molecule is a double helix RNA carries DNA’s message to cytoplasm (outer area) Structure of RNA, with paired bases, matches DNA DNA, with RNA’s assistance, initiates and guides the construction of proteins necessary for bodily growth, maintenance, and repair
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Figure 5.5: DNA Replication
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CELL DIVISION Mitosis: ordinary cell division; one cell splits to form two identical cells Meiosis: process that produces sex cells Four cells produced from one Each cell carries half genetic material of original cell Products of meiosis from one parent combine with those from the other parent Chromosomes sort independently
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CROSSING OVER Crossing over: the process wherein homologous chromosomes exchange segments by breakage and recombination Can occur with any chromosome pair An important source of variety
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Figure 5.6: Crossing Over 26
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MUTATION Base substitution mutation: substitution of one base in a triplet by another If mutation occurs in sex cell, new organism will carry mutation in every cell Chromosomal rearrangement: pieces of a chromosome break off and reattach someplace else on that chromosome
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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
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POPULATION GENETICS AND MECHANISMS OF GENETIC EVOLUTION
Population genetics studies stable and changing populations Gene pool: alleles and genotypes within breeding population Genetic evolution: change in allele frequency in a breeding population
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NATURAL SELECTION Genotype: the genetic makeup of an organism
Phenotype: organism’s biological traits Natural selection acts only on phenotypes Human biology has considerable plasticity The environment works on a genotype to build a phenotype
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DIRECTIONAL SELECTION
After several generations of selection, gene frequencies change Adaptive: favored by natural selection Directional selection continues as long as environmental sources stay the same Selection operates only on traits that are present in a population Humans adapt rapidly by modifying biological responses and learned behavior
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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
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STABILIZING SELECTION
Balanced polymorphism: frequencies of two or more alleles of a gene remain constant from generation to generation
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Figure 5.7:Distribution of Sickle-Cell Allele and Falciparum Malaria in the Old World
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APPRECIATING ANTHROPOLOGY
Scientists linked the first European explorers of the New World to the origin of sexually transmitted syphilis Researchers examined 26 geographically disparate strains in the family of Treponema bacteria Infections in the New World have been traced back at least 7,000 years by studying scars on bones
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RANDOM GENETIC DRIFT Random genetic drift: change in allele frequency that results from chance Lost alleles can reappear in gene pool only through mutation Fixation—the replacement, for example, of blue eyes by brown eyes—is more rapid in small populations
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GENE FLOW Gene flow: exchange of genetic material between populations of the same species 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
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Figure 5.8: Gene Flow between Local Populations
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THE MODERN SYNTHESIS Currently accepted view of evolution:
Microevolution: small-scale changes in allele frequencies over just a few generations Macroevolution: large-scale changes in allele frequencies in a population over a longer time period
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PUNCTUATED EQUILIBRIUM
Punctuated equilibrium: long periods of stasis may be interrupted by evolutionary leaps Sudden environmental change offers possibility for the pace of evolutions to speed up Species can survive radical environmental shifts, but extinction is more common
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