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Transcript
Darwin proposed natural selection as the mechanism of evolution Darwin observed that – Organisms vary in many traits – traits that increase survival or reproduction allow those organisms to leave more offspring than others – Result favorable traits accumulate in a population over generations important points – Individuals do not evolve: populations evolve – Evolution is not goal directed and does not lead to perfect adaptation; favorable traits vary as environments change Evidence for evolution 1. fossil record - shows that organisms have evolved in a historical sequence Many fossils link early extinct species with species living today Pakicetus (terrestrial) – Ex. Rhodocetus (predominantly aquatic) Pelvis and Dorudon (fully aquatic) hind limb Pelvis and hind limb Balaena (recent whale ancestor) 2. Comparative anatomy Homology = similarity in characteristics that result from common ancestry – Ex. Vertebrate forelimbs Humerus Radius Ulna Carpals Metacarpals Phalanges Human Cat Whale Bat 3. Comparative embryology = – Many vertebrates have common embryonic structures, revealing homologies – When you were an embryo, you had a tail and pharyngeal pouches (just like an embryonic fish) 4. Molecular biology: Comparisons of DNA and amino acid sequences between different organisms reveal evolutionary relationships Populations are the units of evolution A population = group of individuals of the same species living in the same place at the same time Evolution = change in heritable traits in a population over generations A gene pool Microevolution is a change in the relative frequencies of alleles in a gene pool over time Mutation and sexual reproduction produce genetic variation, making evolution possible Four sources of genetic variation: 1. Mutation 2. Independent assortment 3. Crossing over 4. Random fertilization How do we know if a population is evolving? Hardy Weinberg principle = allele and genotype frequencies within population will not change unless outside forces act to change those frequencies For a population to remain in Hardy-Weinberg equilibrium for a specific trait, it must satisfy five conditions: 1. . 2. . 3. No mutations 4. Random mating 5. No natural selection Is Hardy-Weinberg equilibrium even possible? Natural selection, genetic drift, and gene flow can alter allele frequencies in a population The three main causes of microevolutionary change are Natural selection – If individuals differ in their survival and reproductive success, natural selection will alter allele frequencies Genetic drift – Genetic drift – Ex. In a small population, chance events may lead to the loss of genetic diversity Genetic drift – The bottleneck effect – Ex. the northern elephant seal was hunted to near extinction in the 1700s and 1800s – A remnant population of fewer than 100 seals was discovered and protected; the current population of 175,000 descended from those few seals and has virtually no genetic diversity Original population Bottlenecking event Surviving population founder effect = when a few individuals colonize a new habitat – Ex. Four moose were taken from the Canadian mainland to Newfoundland in 1904. These two males and two females rapidly formed a large population of moose that now flourishes in Newfoundland. Gene flow is the movement of individuals or gametes/spores between populations and can alter allele frequencies in a population The fossil remains of pygmy (or dwarf) mammoths (1.5 m to 2 m tall) have been found on Santa Rosa and San Miguel Islands off the coast of California. This population of pygmy mammoths is descended from a population of mammoths of normal size (4 m tall). Dwarfing is common in island populations and is not the result of chance events. What mechanism do you think best accounts for the decrease in mammoth size on these islands? – Gene flow – Genetic drift – Natural selection 3 types of natural selection: 1. Stabilizing selection 2. 2. Directional selection acts against individuals at one of the phenotypic extremes- common during periods of environmental change 3. Disruptive selection favors individuals at both extremes of the phenotypic range- may occur in patchy habitats Frequency of individuals Types of selection Original population Phenotypes (fur color) Original Evolved population population Stabilizing selection Directional selection Disruptive selection Why doesn’t natural selection act to eliminate genetic variation in populations, retaining only the most favorable alleles? – A recessive allele is only subject to natural selection when it influences the phenotype in homozygous recessive individuals – Ex. cystic fibrosis Endangered species often have reduced variation Low genetic variability Natural selection cannot fashion perfect organisms 1. Selection can only act on existing variation 2. Evolution is limited by historical constraints – Birds arose as the forelimb of a small dinosaur evolved into a wing Wing claw (like dinosaur) Long tail with many vertebrae (like dinosaur) Teeth (like dinosaur) Feathers Natural selection cannot fashion perfect organisms 3. Adaptations are often compromises 4. Chance, natural selection and the environment interact Ch. 16: Evolution of Single-celled organisms have inhabited Earth for billions of years are the oldest life-forms and remain the most numerous and widespread organisms How did life arise on earth? Water vapor CH4 “Atmosphere” Electrode Condenser Simulations of such conditions have produced amino acids, sugars, lipids, and the nitrogenous bases found Figure 16.3B in DNA and RNA Cold water H2O “Sea” Cooled water containing organic molecules Sample for chemical analysis The first genes may have been RNA molecules that catalyzed their own replication C A G U G G G G C U C A U U G C A U U A C C A C G U U G A A U A A G U C G A Monomers G G C U U U 1 Formation of short RNA polymers: simple “genes” 2 Assembly of a complementary RNA chain, the first step in replication of the original “gene” RNA might have acted as templates for the formation of polypeptides which in turn assisted in RNA replication Self-replication of RNA RNA Self-replicating RNA acts as template on which polypeptide forms. Polypeptide Polypeptide acts as primitive enzyme that aids RNA replication. Figure 16.6A Membranes may have separated various aggregates of selfreplicating molecules which could be acted on by natural selection Membrane LM 650 RNA Figure 16.6B, C Polypeptide Prokaryote fun facts Prokaryotes lived alone on Earth for over 1 billion years They remain the most numerous and widespread organisms on Earth. The total biomass of prokaryotes is ten times that of eukaryotes There are ten times as many prokaryotes living in and on your body as the number of cells in your body Bacteria and Archaea are the two main branches of prokaryotic evolution Domains Bacteria and Archaea are distinguished on the basis of nucleotide sequences and other molecular and cellular features Types of Nutrition Autotrophs make their own organic compounds from inorganic sources Photoautotrophs Chemoautotrophs Heterotrophs obtain their carbon atoms from organic compounds Photoheterotrophs obtain energy from sunlight Chemoheterotrophs almost any organic molecule can serve as food for some species Energy source CO2 Light Chemical Photoautotrophs Chemoautotrophs Carbon source Organic Photoheterotrophs Chemoheterotrophs compounds Archaea thrive in extreme environments— and in other habitats Figure 16.12A, B SEM 12,000 Spirochete that causes Lyme disease Figure 16.14A, B “Bull’s-eye”rash SEM 2,800 Some bacteria cause disease Tick that carries the Lyme disease bacterium Bacteria can be used as biological weapons Bacteria, such as the species that causes anthrax can be used as biological weapons Figure 16.15 Prokaryotes help recycle chemicals and clean up the environment Bioremediation Prokaryotes are decomposers in Sewage treatment and can clean up oil spills and toxic mine wastes Rotating spray arm Rock bed coated with aerobic bacteria and fungi Figure 16.16A, B Liquid wastes Outflow A model of the origin of eukaryotes Cytoplasm Plasma membrane Ancestral prokaryote Endoplasmic reticulum Nucleus Membrane infolding Aerobic heterotrophic prokaryote Cell with nucleus and endomembrane system Some cells Ancestral host cell Photosynthetic prokaryote Endosymbiosis Mitochondrion Chloroplast Mitochondrion Figure 16.17 Photosynthetic eukaryotic cell Nuclear envelope Protists are an extremely diverse assortment of mostly unicellular eukaryotes The parasitic Giardia SEM 2,300 Single celled algae Apex Red blood cell TEM 26,000 Colorized SEM 4,000 Plasmodium causes malaria Multicellularity evolved in several different lineages probably by specialization of the cells of colonial protists Gamete Locomotor cells 2 1 Somatic cells 3 Foodsynthesizing cells Unicellular protist Colony Early multicellular organism with specialized, interdependent cells Later organism that produces gametes