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Ch 23 Part II, Ch 24 Evolution : Mechanisms Natural Selection (only one that consistently leads to adaptive evolution) Genetic Drift Genetic Flow Alleles drift but how does that look? • Survival of the fittest? • Adaptive advantage can lead to greater relative fitness • Relative fitness: contribution an individual makes to the gene pool of the next generation • PHENOTYPE DIRECTLY Fig. 23-13 Original population Original Evolved population population (a) Directional selection Phenotypes (fur color) (b) Disruptive selection (c) Stabilizing selection Sexual Selection • Likelihood of mating • Can result in sexual dimorphism – Intrasexual selection : mostly males competing physically with each other – Intersexual selection: usually females choose Fig. 23-16 EXPERIMENT Female gray tree frog SC male gray tree frog LC male gray tree frog SC sperm Eggs LC sperm Offspring of Offspring of SC father LC father Fitness of these half-sibling offspring compared RESULTS Fitness Measure 1995 1996 Larval growth NSD LC better Larval survival LC better NSD Time to metamorphosis LC better (shorter) LC better (shorter) NSD = no significant difference; LC better = offspring of LC males superior to offspring of SC males. What preserves genetic variation? • (Tendency of directional and stabilizing selection is to reduce variation) • Mechanisms to preserve variety: • Diploidy (“hide” recessive) • Balancing Selection – Heterozygote advantage – Frequency dependent selection: fitness of a phenotype decreases if it becomes too common • Neutral variation: no affect on protein fxn Why Can’t We Be Perfect? • 1. Selection can only act on existing variations (may not be ideal) • 2. Evolution is limited by historical constraints (bats, birds from walking) • 3. Adaptations are often compromises • 4. Chance, natural (founders effect does not ensure fit alleles in new pop) selection,and the environment interact Fig. 23-19 Ch 24 Origin of Species • Biological Species concept – Populations – Reproductively compatible – Gene flow even over long distances can hold gene pool together ( if NS or drift = divergence) – *emphasizes separateness of species Types of Isolation • Reproductive isolation: barriers will isolate a gene pool – Prezygotic barriers – Postzygotic barriers Fig. 24-4 Prezygotic barriers Habitat Isolation Temporal Isolation Individuals of different species (a) Postzygotic barriers Behavioral Isolation Mechanical Isolation Gametic Isolation Mating attempt (c) (d) (e) (f) Reduced Hybrid Viability Reduced Hybrid Fertility Hybrid Breakdown Viable, fertile offspring Fertilization (g) (h) (i) (j) (b) (k) (l) 3 types of species concepts unity within a species • morphological species concept: body shape and other structural features (sexual and asexual, most used method) • Ecological species concept: niche, interaction with enviro. Disruptive selection • Phylogenetic species concept: smallest group that share a common ancestor Sympatric vs Allopatric speciation • Allopatric: gene flow is interrupted by a geographic barrier cuts a population off from the main • Colonists • Evidence: frogs, squirrels, highly subdivided regions tend to have more species than areas with fewer barriers • Reproductive isolation increases with distance • Sympatric: speciation occurs in populations that live in same geography • Less common • Gene flow is reduced by – polyploidy, (plants) – habitat differentiation – sexual selection Fig. 24-7 Mantellinae (Madagascar only): 100 species Rhacophorinae (India/Southeast Asia): 310 species Other Indian/ Southeast Asian frogs 100 60 80 1 2 40 20 0 3 Millions of years ago (mya) 1 3 2 India Madagascar 88 mya 65 mya 56 mya Fig. 24-5 (a) Allopatric speciation (b) Sympatric speciation Polyploidy • A species may originate from an accident during cell division that results in extra sets of chromosomes • Autopolyploid: extra sets of chromosomes derived from a single species – Ex, failure in cell division – Tetraploid offspring tend to be less fertile – Can produce fertile offspring with self fertilization or with other tetrapods – One generation can be reproductively isolated Fig. 24-10-3 2n = 6 4n = 12 Failure of cell division after chromosome duplication gives rise to tetraploid tissue. 2n Gametes produced are diploid.. 4n Offspring with tetraploid karyotypes may be viable and fertile. Allopolyploid • Two different species interbreed and have infertile offspring • Propagate asexually • Plants more tolerant of meiotic and mitotic errors • After several generations a sterile hybrid can become fertile with each other not the parent species • Frog (occasionally in animals) • 80% plant species may have formed this way) Fig. 24-11-4 Species B 2n = 4 Unreduced gamete with 4 chromosomes Meiotic error Species A 2n = 6 Normal gamete n=3 Hybrid with 7 chromosomes Unreduced gamete with 7 chromosomes Normal gamete n=3 Viable fertile hybrid (allopolyploid) 2n = 10 Habitat Differentiation & Sexual Selection • Apple maggot fly(slow hawthorn tree vs apple tree) – Temporal isolation and post zygotic (helpful alleles in one tree, harmful in the other) – Habitat or food source not used by parent population • Female driven selection of male coloration patterns in cichlids Fig. 24-12 EXPERIMENT Normal light P. pundamilia P. nyererei Monochromatic orange light Speciation can occur rapidly or slowly and can result from changes in many or a few genes • Punctuated Equilibria: periods of apparent stasis punctuated by sudden change – Relatively rapidly • Gradualism • Adaptive Radiation Fig. 24-17 (a) Punctuated pattern Time (b) Gradual pattern Adaptive Radiation • Over the last 250 my diversity of life has increased in the fossil record • Periods of evolutionary change in which groups of organisms form many new species whose adaptations allow them to fill different ecological roles or niches • Large scale after each of the 5 mass extinctions • Seed plants, mammals etc.. Predator genera (percentage of marine genera) Fig. 25-16 50 40 30 20 10 0 Paleozoic Mesozoic Era D C P C E O S J Tr Period 359 488 444 416 542 299 251 200 145 Time (millions of years ago) Cenozoic P 65.5 N 0 Fig. 25-17 Ancestral mammal Monotremes (5 species) ANCESTRAL CYNODONT Marsupials (324 species) Eutherians (placental mammals; 5,010 species) 250 200 100 150 Millions of years ago 50 0 Fig. 25-18 Close North American relative, the tarweed Carlquistia muirii Dubautia laxa KAUAI 5.1 million years MOLOKAI OAHU 3.7 LANAI million years 1.3 MAUI million years Argyroxiphium sandwicense HAWAII 0.4 million years Dubautia waialealae Dubautia scabra Dubautia linearis New roles in community lead to radiations • Rise of photosynthetic prokaryotes • Evolution of large predators in the Cambrian explosion • Colonization of land by – plants, (stems, waxy coat) – insects – Tetrapods The radiation of plants stimulated radiation of insects