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Species Genetically Distinct Populations Natural Selection • Driving force for change • Competition for resource(s) – Winner survives or thrives more • “Best Fit” => Dominant Reproducer – Spectrum of traits exist. – Trait(s) capitalizes on procurement of resource(s) • Time tells. – Most successful feeder = best mate = most genes Seed Size (mm) vs.Beak Size (mm) 35 30 25 20 15 10 5 0 1 Island: A 2 3 B 4 5 C 6 7 D 8 9 E 10 11 12 13 14 15 F G H Gene Pool Sum of all individual genes in a population – Variations of genes = alleles • Allele frequency = # of a certain allele/ total # of alleles – Out of 10 possible alleles, 4 are A. 4/10 = 0.4 or 40% – Chance of 2 alleles combining = (freq A)(freq A) = 0.16 • Different combinations => different phenotypes – Phenotype = physical manifestation of trait • Phenotype frequency= # certain phenotype/# subjects – Catastrophe leads to limited survivors • Chance determines who gets hit • Genetic drift affects small populations more Hardy-Weinberg Genetic Equilibrium • Ideal, hypothetical, non-evolving population • Assumptions – No net mutations occur (alleles stay constant) – No one leaves or enters (population is constant) – Population is large (ideally, infinitely so) – Individuals mate randomly – Selection does not occur Hardy-Weinberg Calculations • p2 + 2pq + q2 = 1 – All allele frequencies combined must equal 1 • • • • • p = dominant allele frequency q = recessive allele frequency p2 = homozygous dominant pq = heterozygotes q2 = homozygous recessive Mutation • Occur spontaneously, constantly, normally • Accelerated with mutagens – Radiation and certain chemicals • Natural selection acts on phenotypes, not alleles individually • Recessive alleles “hidden” in heterozygotes • Beneficial mutations vital to evolution Gene Flow • • • • Immigration (moving into population) Emigration (moving out of population) When organisms move, their genes move too Typically occurs when younger males move out, individuals migrate, natural disasters hit • Plants routinely disperse their seeds/ spores Genetic Drift • When allele frequencies change due to random events • More noticeable in small populations • Essential for survival of species/ population • Diversity of alleles in gene pool gives greater nuances of traits • Natural selection works on variants – The more traits, the better the species survival Nonrandom mating • Sexual Selection – Female typically chooses male • Geographical Isolation – Original population divided • Reproductive Isolation – Barriers to successful breeding in same area – Prezygotic isolation (premating) • Breed at different times or locations – Postzygotic isolation (post-mating) • Hybrid offspring not always fertile Species Concepts • A population that can successfully interbreed but not with other groups (Biological) • Organisms that have same internal & external appearances (Morphological) • Modern definition includes parts of both – A single kind of organism – Morphologically similar – Interbreed to produce fully fertile offspring Speciation • Allopatric speciation (“different homelands”) – Geographical isolation = no gene flow – More likely in smaller populations • Smaller gene pool is more affected by genetic drift & natural selection • Sympatric speciation (“same homeland”) – Use of different niches adaptive advantage – Specialization reproductive isolation Rates of Speciation • Gradualism – Regular, gradual rate – Selective pressures are fairly constant • Punctuated Equilibrium – Occurs in “bursts” – Rapid, sudden changes due to catastrophe – Some species exist for long time, but other new forms appeared in only a thousand years or less