EVOLUTION OF POPULATIONS

YOU MUST KNOW…• HOW MUTATION AND SEXUAL

REPRODUCTION EACH PRODUCE GENETIC VARIATION

• THE CONDITIONS FOR HARDY-WEINBERG EQUILIBRIUM

• HOW TO USE THE HARDY-WEINBERG EQUATION TO CALCULATE ALLELIC FREQUENCIES AND TO TEST WHETHER A POPULATION IS EVOLVING

CONCEPT 23.1• MUTATION AND SEXUAL

REPRODUCTION PRODUCE THE GENETIC VARIATION THAT MAKES EVOLUTION POSSIBLE

• MICROEVOLUTION – CHANGE IN THE ALLELE FREQUENCIES OF A POPULATION OVER GENERATIONS (EVOLUTION ON A SMALL SCALE)

• MUTATIONS – THE ONLY SOURCE OF NEW GENES AND NEW ALLELES

• POINT MUTATIONS – CHANGES IN ONE BASE IN A GENE

• EX. SICKLE-CELL DISEASE• CHROMOSOMAL MUTATIONS – DELETE,

DISRUPT, DUPLICATE, OR REARRANGE MANY LOCI AT ONCE (USUALLY HARMFUL BUT NOT ALWAYS)

HBB: The Gene Associated with Sickle Cell Anemia

• Locus: 11p15.5 - The HBB gene is found in region 15.5 on the short (p) arm of human chromosome 11.

• Protein Function: Hemoglobin molecules, which reside in red blood cells, are responsible for carrying oxygen from the lungs to various parts of the body for use in respiration. The HBB gene codes for one of the two types of polypeptide chains found in adult hemoglobin.

• MOST GENETIC VARIATIONS ARE DUE TO THE SEXUAL RECOMBINATION OF ALLELES THAT ALREADY EXIST IN A POPULATION

• CROSSING OVER – DURING PROPHASE I OF MEIOSIS

• INDEPENDENT ASSORTMENT OF CHROMOSOMES DURING MEIOSIS

• FERTILIZATION (223 x 223 DIFFERENT COMBINATIONS FOR HUMAN SPERM AND EGG)

CONCEPT 23.2• THE HARDY-

WEINBERG EQUATION CAN BE USED TO TEST WHETHER A POPULATION IS EVOLVING

• POPULATION – A GROUP OF INDIVIDUALS OF THE SAME SPECIES

• GENE POOL – ALL THE ALLELES AT ALL LOCI IN ALL THE MEMBERS OF A POPULATION

• IF ALL MEMBERS OF A POPULATION ARE HOMOZYGOUS FOR THE SAME ALLELE, THE ALLELE IS FIXED

• ONLY ONE ALLELE EXISTS AT THAT PARTICULAR LOCUS IN THE POPULATION

• THE GREATER THE NUMBER OF FIXED ALLELES, THE LOWER THE SPECIES’ DIVERSITY

• THE HARDY-WEINBERG THEOREM DESCRIBES A POPULATION THAT IS NOT EVOLVING

• THE FREQUENCIES OF ALLELES AND GENES IN A POPULATION’S GENE POOL WILL REMAIN CONSTANT OVER THE COURSE OF GENERATIONS UNLESS THEY ARE ACTED UPON BY FORCES OTHER THAN MENDELIAN SEGREGATION AND THE RECOMBINATION OF ALLELES

5 CONDITIONS FOR HARDY-WEINBERG EQUILIBRIUM

• NO MUTATIONS• RANDOM MATING• NO NATURAL SELECTION• THE POPULATION SIZE MUST BE

EXTREMELY LARGE (NO GENETIC DRIFT)

• NO GENE FLOW (IMMIGRATION, TRANSFER OF POLLEN)

• IT IS UNLIKELY THAT ALL THE CONDITIONS FOR HARDY-WEINBERG EQUILIBRIUM WILL BE MET

• ALLELIC FREQUENCIES CHANGE AND POPULATIONS EVOLVE

• p = dominant allele (R)• q = recessive allele (r) • p2 + 2pq + q2 = 1

CONCEPT 23.3• NATURAL SELECTION, GENETIC

DRIFT, AND GENE FLOW CAN ALTER ALLELE FREQUENCIES IN A POPULATION

(ALTHOUGH MUTATIONS DO TOO)

• 3 MAJOR FACTORS ALTER ALLELIC FREQUENCIES AND BRING EVOLUTIONARY CHANGES

1. NATURAL SELECTION

2. GENETIC DRIFT

3. GENE FLOW

NATURAL SELECTION• RESULTS IN ALLELES BEING

PASSED TO THE NEXT GENERATION IN PROPORTIONS DIFFERENT FROM THEIR RELATIVE FREQUENCIES IN THE PRESENT GENERATION

• STRONGER INDIVIDUALS TEND TO PRODUCE MORE OFFSPRING

GENETIC DRIFT• UNPREDICTABLE FLUCTUATION IN

ALLELIC FREQUENCIES FROM ONE GENERATION TO THE NEXT

• THE SMALLER THE POPULATION, THE GREATER THE CHANCE IS FOR GENETIC DRIFT

• IS A RANDOM, NONADAPTIVE CHANGE IN ALLELIC FREQUENCIES

• FOUNDER EFFECT – A FEW INDIVIDUALS BECOME ISOLATED FROM A LARGER POPULATION AND ESTABLISH A NEW POPULATION WHOSE GENE POOL IS NOT REFLECTIVE OF THE SOURCE POPULATION

• BOTTLENECK EFFECT – A SUDDEN CHANGE IN THE ENVT. DRASTICALLY REDUCES THE SIZE OF A POPULATION

• THE FEW SURVIVORS MAY HAVE A GENE POOL THAT NO LONGER REFLECTS THE ORIGINAL POPULATION’S GENE POOL

GENE FLOW• OCCURS WHEN A POPULATION GAINS

OR LOSES ALLELES BY GENETIC ADDITIONS TO AND/OR SUBTRACTIONS FROM THE POPULATION (aka MIGRATION)

• A RESULT OF THE MOVEMENT OF FERTILE INDIVIDUALS OR GAMETES

• TENDS TO REDUCE THE GENETIC DIFFERENCES BETWEEN POPULATIONS

CONCEPT 23.4• NATURAL SELECTION IS THE

ONLY MECHANISM THAT CONSISTENTLY CAUSES ADAPTIVE EVOLUTION

RELATIVE FITNESS• REFERS TO THE CONTRIBUTION

AN ORGANISM MAKES TO THE GENE POOL OF THE NEXT GENERATION RELATIVE TO THE CONTRIBUTIONS OF OTHER MEMBERS

• IT IS MEASURED ONLY BY REPRODUCTIVE SUCCESS

• NATURAL SELECTION ACTS MORE DIRECTLY ON PHENOTYPE AND INDIRECTLY ON THE GENOTYPE AND CAN ALTER THE FREQUENCY DISTRIBUTION OF HERITABLE TRAITS IN 3 WAYS

• DIRECTIONAL SELECTION• DISRUPTIVE SELECTION• STABILIZING SELECTION

DIRECTIONAL SELECTION

• INDIVIDUALS WITH ONE EXTREME OF A PHENOTYPIC RANGE ARE FAVORED, SHIFTING THE CURVE TOWARD THIS EXTREME

DISRUPTIVE SELECTION

• OCCURS WHEN CONDITIONS FAVOR INDIVIDUALS ON BOTH EXTREMES OF A PHENOTYPIC RANGE RATHER THAN INDIVIDUALS WITH INTERMEDIATE PHENOTYPES

STABILIZING SELECTION

• ACTS AGAINST BOTH EXTREME PHENOTYPES AND FAVORS INTERMEDIATE VARIANTS

• GENETIC VARIATION IS PRESERVED IN A POPULATION BY:

• DIPLOIDY – ORGANISMS ARE CAPABLE OF HIDING GENETIC VARIATION FROM SELECTION

• HETEROZYGOTE ADVANTAGE – INDIVIDUALS HAVE AN ADVANTAGE FOR SURVIVAL

• EX. SICKLE-CELL DISEASE