THE EVOLUTION OF POPULATIONS CHAPTER 23 THE EVOLUTION OF POPULATIONS

  I. POPULATION GENETICS A.     THE MODERN EVOLUTIONARY SYNTHESIS INTEGRATED DARWINIAN SELECTION AND MENDELIAN INHERITANCE THE DEVELOPMENT POPULATION GENETICS, WITH ITS EMPHASES ON QUANTITATIVE INHERIANCE AND VARIATION, BROUGHT DARWINIAN THEORY AND MENDELIAN PRINCIPLES OF INHERITANCE TOGETHER. THE MODERN SYNTHESIS FOCUSES POPULATIONS AS UNITS OF EVOLUTION.

  B.     THE GENETIC STRUCTURE OF A POPULATION IS DEFINED BY ITS ALLELE AND GENOTYPE FREQUENCIES A POPULATION, A LOCALIZED OF ORGANISMS BELONGING TO THE SAME SPECIES, IS UNITED BY ITS POOL, THE AGGREGATE OF ALL ALLELES IN THE POPULATION.

C. THE HARDY-WEINBERG THEOREM DESCRIBES A NONEVOLVING POPULATION ACCORDING TO THE HARDY-WEINBERG THEOREM, THE FREQUENCIES OF ALLELES IN A POPULATION WILL REMAIN CONSTANT IF SEXUAL REPRODUCTION IS THE ONLY PROCESS THAT AFFECTS THE GENE POOL. IF P AND Q REPRESENT THE RELATIVE FREQUENCIES OF THE DOMINANT RECESSIVE ALLELES OF A TWO-ALLELE LOCUS, RESPECTIVELY, THEN P^2 + 2PQ + Q^2 = 1, WHERE P^2 AND Q^2 ARE THE FREQUENCIES OF THE HOMOZYGOUS GENOTYPES, AND 2PQ IS THE FREQUENCY OF THE HETEROZYGOUS GENOTYPE.

23.3 HARDY-WEINBERG THEOREM

II. CAUSES OF MICROEVOLUTION (CHANGES IN ALLELE FREQUENCIES) A.     MICROEVOLUTION IS A GENERATION–TO–GENERATION CHANGE IN A POPULATION’S ALLELE OR GENOTYPE FREQUENCIES FOR HARDY-WEINBERG THEOREM TO APPLY, THE POPULATION MUST BE : 1. VERY LARGE 2. BE TOTALLY ISOLATED 3. HAVE NO NET MUTATIONS 4. SHOW RANDOM MATING 5. HAVE REPRODUCTIVE SUCCESS FOR ALL INDIVIDUALS MICROEVOLUTION CAN OCCUR WHEN ONE OR MORE OF THE CONDITIONS REQUIRED FOR HARDY-WEINBERG EQUILIBRIUM ARE NOT MET.

  B.     THE FIVE CAUSES OF MICROEVOLUTION ARE GENETIC DRIFT, GENE FLOW MUTATION, NONRANDOM MATING, AND NATURAL SELECTION NATURAL SELECTION: THE INCREASE IN ALLELE FREQUENCIES DUE TO THE IMPACT OF THE ENVIRONMENT MUTATIONS: RANDOM CHANGE IN DNA SEQUENCE (+/- OR 0) NON-RANDOM MATING: OCCURS WHEN INDIVIDUALS CHOOSE MATES BASED UPON THEIR PARTICULAR TRAITS. INBREEDING: INDIVIDUALS MATE WITH RELATIVES SEXUAL SELECTION: FEMALES CHOOSE MALES BASED UPON THEIR ATTRACTIVE APPEARANCE OR BEHAVIORAL PATTERNS GENE FLOW: INTRODUCTION OR REMOVAL OF ALLELES FROM THE POPULATION WHEN INDIVIDUALS LEAVE (EMIGRATION) OR ENTER (IMMIGRATION)THE POPULATION GENETIC DRIFT: RANDOM INCREASE OR DECREASE OF ALLELES FOUNDER EFFECT: WHEN ALLELE FREQ. IN A GROUP OF MIGRATING INDIVIDUALS ARE, BY CHANCE, NOT THE SAME AS THAT OF THEIR POPULATION OF ORIGIN. BOTTLENECK: WHEN THE POPULATION UNDERGOES A DRAMATIC DECREASE IN SIZE.

23.4 GENETIC DRIFT: RANDOM INCREASE OR DECREASE IN ALLELES

23.5 BOTTLENECK EFFECT

III. GENETIC VARIATION, THE SUBSTRATE FOR NATURAL SELECTION A. GENETIC VARIATION OCCURS WITHIN AND BETWEEN POPULATIONS GENETIC VARIATION INCLUDES INDIVIDUAL VARIATION IN DISCRETE AND QUANTITATIVE CHARACTERS WITHIN A POPULATION, AS WELL AS PHYSICAL VARIATION BETWEEN POPULATIONS.

B. MUTATION AND SEXUAL RECOMBINATION GENERATE GENETIC VARIATION MOST MUTATIONS HAVE NO EFFECT OR ARE HARMFUL, BUT ARE ADAPTIVE. SEXUAL RECOMBINATION PRODUCES MOST OF THE VARIATION THAT MAKES ADAPTATION POSSIBLE IN POPULATIONS OF REPRODUCING ORGANISMS. MUTATIONS: RANDOM CHANGE IN DNA SEQUENCE SEXUAL REPRODUCTION: GENETIC RECOMBINATION CROSSING OVER INDEPENDENT ASSORTMENT RANDOM JOINING OF GAMETES OUTBREEDING (MATING WITH UNRELATED PARTNERS) ALSO INCREASES VARIATION

C. DIPLOIDY AND BALANCED POLYMORPHISM PRESERVE VARIATION DIPLOIDY: PRESENCE OF TWO COPIES OF EACH CHROMOSOME (2N) DIPLOIDY MAINTAINS A RESERVOIR OF LATENT VARIATION IN HETEROZYGOTES. BALANCED POLYMORPHISM MAY MAINTAIN VARIATION AT GENE LOCI AS A RESULT OF HETEROZYGOTE ADVANTAGE OR FREQUENCY DEVELOPMENT SELECTION. HETEROZYGOUS ADVANTAGE- SOMETIMES CARRYING A TRAIT, BUT NOT EXPRESSING IT IS ADVANTAGEOUS FREQUENCY DEPENDENT SELECTION- WHEN THE LEAST COMMON PHENOTYPE HAVE A SELECTIVE ADVANTAGE. SOME GENETIC VARIATION MAY BE UNAFFECTED BY NATURAL SELECTION.

IV. NATURAL SELECTION AS THE MECHANISMS OF ADAPTIVE EVOLUTION A.     EVOLUTIONARY FITNESS IS THE RELATIVE CONTRIBUTION AN INDIVIDUAL MAKES TO THE GENE POOL OF THE NEXT GENERATION DARWINIAN IS MEASURED ONLY BY REPRODUCTIVE SUCCESS. ONE GENOTYPE HAS GREATER RELATIVE FITNESS THAN ANOTHER IF IT LEAVES MORE DESCENDANTS. SELECTION FAVORS CERTAIN GENOTYPES IN A POPULATION BY ACTING ON THE PHENOTYPE OF INDIVIDUAL ORGANISMS. THE WHOLE ORGANISM IS THE OBJECT OF SELECTION

  B.     THE EFFECT OF SELECTION ON A VARYING CHARACTERISTIC CAN BE STABILIZING, DIRECTIONAL, OR DIVERSIFYING NATURAL SELECTION CAN – ACT AGAINST EXTREME PHENOTYPES (STABILIZING SELECTION), FAVOR RELATIVELY RARE INDIVIDUALS ON ONE END OF THE PHENOTYPIC RANGE (DIRECTIONAL SELECTION), OR FAVOR INDIVIDUALS AT BOTH EXTREMES OF THE RANGE OVER INTERMEDIATE PHENOTYPES (DIVERSIFYING SELECTION).

23.11 MODES OF SELECTION

MALE COMPETITION: AWARD THE STRONGEST MALES   C.     SEXUAL SELECTION MAY LEAD TO PRONOUNCED SECONDARY DIFFERENCES BETWEEN THE SEXES SEXUAL SELECTION LEADS TO THE EVOLUTION OF SECONDARY SEX CHARACTERISTICS, WHICH CAN GIVE INDIVIDUALS AN ADVANTAGE IN MATING. MALE COMPETITION: AWARD THE STRONGEST MALES EX. EVOLUTION OF HORNS, ANTLERS, MUSCLES FEMALE CHOICE: TRAITS OR BEHAVIORS IN MALES THAT ARE ATTRACTIVE TO FEMALES EX. COLORFUL BIRD PLUMAGE, MATING SONGS, MATING RITUALS

D. NATURAL SELECTION CANNOT FASHION PERFECT ORGANISMS THE REASONS ARE: STRUCTURES RESULT FROM MODIFIED ANCESTRAL ANATOMY, ADAPTATIONS ARE OFTEN COMPROMISES THE GENE POOL CAN BE AFFECTED BY GENETIC DRIFT AND NATURAL SELECTION CAN ACT ONLY ON AVAILABLE VARIATION.