Natural Selection A Summary Individuals with certain heritable
Natural Selection: A Summary • Individuals with certain heritable characteristics survive and reproduce at a higher rate than other individuals • Natural selection increases the adaptation of organisms to their environment over time • If an environment changes over time, natural selection may result in adaptation to these new conditions and may give rise to new species Video: Seahorse Camouflage Copyright © 2008 Pearson Education, Inc. , publishing as Pearson Benjamin Cummings
Fig. 22 -12 (a) A flower mantid in Malaysia (b) A stick mantid in Africa
Anatomical and Molecular Homologies • Homologous structures are anatomical resemblances that represent variations on a structural theme present in a common ancestor Copyright © 2008 Pearson Education, Inc. , publishing as Pearson Benjamin Cummings
Fig. 22 -17 Humerus Radius Ulna Carpals Metacarpals Phalanges Human Cat Whale Bat
Convergent Evolution • Convergent evolution is the evolution of similar, or analogous, features in distantly related groups • Analogous traits arise when groups independently adapt to similar environments in similar ways • Convergent evolution does not provide information about ancestry Copyright © 2008 Pearson Education, Inc. , publishing as Pearson Benjamin Cummings
Fig. 22 -20 Sugar glider NORTH AMERICA AUSTRALIA Flying squirrel
Genetic Variation • Variation in individual genotype leads to variation in individual phenotype • Not all phenotypic variation is heritable • Natural selection can only act on variation with a genetic component
Fig. 23 -2 (a) (b)
Variation Between Populations • Most species exhibit geographic variation, differences between gene pools of separate populations or population subgroups
Fig. 23 -3 1 2. 4 8. 11 9. 12 3. 14 5. 18 10. 16 13. 17 6 7. 15 19 XX 1 2. 19 3. 8 4. 16 5. 14 9. 10 11. 12 13. 17 15. 18 6. 7 XX
• Hardy-Weinberg equilibrium describes the constant frequency of alleles in such a gene pool • If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then – p 2 + 2 pq + q 2 = 1 – where p 2 and q 2 represent the frequencies of the homozygous genotypes and 2 pq represents the frequency of the heterozygous genotype
80% CR ( p = 0. 8) 20% CW (q = 0. 2) Sperm CR (80%) CW (20%) 64% ( p 2) CRCR 16% ( pq) CRCW CR (80%) CW (20%) Eggs Fig. 23 -7 -1 16% (qp) CRCW 4% (q 2) CW CW
Genetic Drift • The smaller a sample, the greater the chance of deviation from a predicted result • Genetic drift describes how allele frequencies fluctuate unpredictably from one generation to the next • Genetic drift tends to reduce genetic variation through losses of alleles Animation: Causes of Evolutionary Change
Fig. 23 -8 -2 CR CR CW CW CR CR CW CW CR CR CR CR CW Generation 1 p (frequency of CR) = 0. 7 q (frequency of CW ) = 0. 3 CW CW CR CR CR CW Generation 2 p = 0. 5 q = 0. 5
Fig. 23 -9 Original population Bottlenecking event Surviving population
Case Study: Impact of Genetic Drift on the Greater Prairie Chicken • Loss of prairie habitat caused a severe reduction in the population of greater prairie chickens in Illinois • The surviving birds had low levels of genetic variation, and only 50% of their eggs hatched
Gene Flow • Gene flow consists of the movement of alleles among populations • Alleles can be transferred through the movement of fertile individuals or gametes (for example, pollen) • Gene flow tends to reduce differences between populations over time • Gene flow is more likely than mutation to alter allele frequencies directly
• Gene flow can decrease the fitness of a population • In bent grass, alleles for copper tolerance are beneficial in populations near copper mines, but harmful to populations in other soils • Windblown pollen moves these alleles between populations • The movement of unfavorable alleles into a population results in a decrease in fit between organism and environment
Directional, Disruptive, and Stabilizing Selection • Three modes of selection: – Directional selection favors individuals at one end of the phenotypic range – Disruptive selection favors individuals at both extremes of the phenotypic range – Stabilizing selection favors intermediate variants and acts against extreme phenotypes
Frequency of individuals Fig. 23 -13 Original Evolved population (a) Directional selection Original population Phenotypes (fur color) (b) Disruptive selection (c) Stabilizing selection
Heterozygote Advantage • Heterozygote advantage occurs when heterozygotes have a higher fitness than do both homozygotes • Natural selection will tend to maintain two or more alleles at that locus • The sickle-cell allele causes mutations in hemoglobin but also confers malaria resistance
Fig. 23 -17 Frequencies of the sickle-cell allele 0– 2. 5% Distribution of malaria caused by Plasmodium falciparum (a parasitic unicellular eukaryote) 2. 5– 5. 0% 5. 0– 7. 5% 7. 5– 10. 0% 10. 0– 12. 5% >12. 5%
Conditions on early Earth made the origin of life possible • Chemical and physical processes on early Earth may have produced very simple cells through a sequence of stages: 1. Abiotic synthesis of small organic molecules 2. Joining of these small molecules into macromolecules 3. Packaging of molecules into “protobionts” 4. Origin of self-replicating molecules
Protobionts • Replication and metabolism are key properties of life • Protobionts are aggregates of abiotically produced molecules surrounded by a membrane or membrane-like structure • Protobionts exhibit simple reproduction and metabolism and maintain an internal chemical environment
The First Eukaryotes • The oldest fossils of eukaryotic cells date back 2. 1 billion years • The hypothesis of endosymbiosis proposes that mitochondria and plastids (chloroplasts and related organelles) were formerly small prokaryotes living within larger host cells • An endosymbiont is a cell that lives within a host cell
Fig. 25 -9 -4 Plasma membrane Cytoplasm Ancestral prokaryote DNA Endoplasmic reticulum Nucleus Nuclear envelope Aerobic heterotrophic prokaryote Photosynthetic prokaryote Mitochondrion Ancestral heterotrophic eukaryote Mitochondrion Plastid Ancestral photosynthetic eukaryote
• Unit 4 • C 22 45&46, 70&71, 79&80 C 23 5&6, 11&12, 31&32, 44&46, 50&52, 58&60, 69&70, 89&90 C 24 14&18 C 25 5&12, 54&60, 93&screen shot
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