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(ch 22)1A1 B. Natural Selection and Adaptation 1- All species produce more offspring than can possibly survive - Natural Selection can only amplify or diminish heritable variations. - If the environment changes, what is favorable also changes. 2- Resources are limited, so organisms must compete 3- There is variation within a population (Darwin did not know the source.) 4- Some variations make individuals more successful in the environment 5- The individuals w/ the best variations will survive and reproduce (Natural Selection) ch23 -gene pool: total aggregate of genes in a population at any one time. Consists of all the alleles at all gene loci in all individuals of a population. -An allele is said to fixed if all members are homozygous for that allele -Hardy-Weinberg equilibrium If the gene frequencies are not altered over succeeding generations then the population is said to be in Hardy-Weinburg equilibrium. -Hardy-Weinburg equation p2 + 2pq + q2 = 1 frequency of AA Aa aa p + q = 1 - At genetic equilibrium there is no evolution (Barron’s p187) 1. No natural selection 2. No mutations 3. No gene flow 4. No genetic drift 5. Random mating Problems: 1. If 9% of the population has blue eyes, what percent of the population is hybrid for brown eyes? Homozygous for brown eyes? 2. Determine the percent of the population that is homozygous dominant if the percent of the population that is homozygous recessive is 16%. 3. Determine the percent of the population that is hybrid if the allelic frequency of the recessive trait is 0.5. Genetic Drift ch23 1A3 Genetic Drift: a random increase or decrease of alleles. When populations are small the effect of genetic drift can be very strong and dramatically influence evolution. a. Founder effect: occurs when allele frequencies in a group of migrating individuals are, by chance, not the same as that of their population of origin. ex: Amish - polydactylism Polydactyly -- extra fingers or sometimes toes -- is one symptom of Ellis-van Creveld syndrome. The syndrome is commonly found among the Old Order Amish of Pennsylvania, a population that experiences the "founder effect." (http://www.pbs.org/wgbh/evolution/library/06/3/l_063_03.html) b. Bottle neck: Occurs when the population undergoes a dramatic decrease in size (natural disaster, predation, disease) Evidence of Evolution 1A4 1. Biogeography (p448) : geographical distribution of species -Islands have many native species which are closely related to species on nearest mainland or neighboring island. 2. Fossil Record: is incomplete Shows chronological appearance of vertebrates beginning with fish, amphibians, reptiles, then birds and mammals. 3. Comparative Anatomy: Anatomical similarities among species are a reflections of their common descent. a. homologous structures: structures that are similar because of common ancestry ex) mammalian forelimbs: bat wing, cat leg, human arm, whale flipper - Vestigial organs: structures of marginal or no use to an organism - had important functions in ancestral forms ex) appendix in humans, pointed canine teeth, tail bone - remnants of leg bones in snakes - eyes in bats and moles 4. Comparative embryology : Closely related organisms go through similar stages in their embryonic development. All vertebrates embryos go through a stage in which they possess gill slits on the sides of their throats. 5. Molecular biology: An organism's hereditary background is reflected in its genes and their protein products. The more closely related two species are, the higher the percentage of common DNA. - Closely related species also have proteins of similar amino acid sequence. -best evidence -common genetic code ch 24 p474 (EK 1C2) -Maintaining Reproductive Isolation: If species are not physically separated by a geographic barrier, various mechanisms exist to maintain reproductive isolation and prevent gene flow. A. Pre-zygotic Isolating Mechanisms: prevent fertilization 1) (Ecological) Habitat isolation: occurs when species don't encounter one another. 2) Temporal isolation: occurs when species mate or flower during different seasons or different times of day 3) Behavior isolation: occurs when a species doesn't recognize another species as a mating partner because it doesn't perform the correct courting rituals, release proper chemicals, etc. 4) Mechanical isolation: occurs when male and female reproductive structures are incompatible or flower structures select for different pollinators. 5) Gametic isolation: occurs when male gametes don't survive in the environment of the female gamete or when female gametes don't recognize male gametes. B. Post-zygotic isolating mechanisms: Consists of mechanisms that prevent the formation of fertile progeny. A zebroid (also zedonk, zorse, zebra mule, zonkey, and zebrule) is the offspring of any cross between a zebra and any other equine: essentially, a zebra hybrid 1) Hybrid inviability: occurs when the zygote fails to develop properly and aborts or dies before reaching reproductive maturity. 2) Hybrid sterility: occurs when hybrids become functional adults but are reproductively sterile ex) mule 3) Hybrid breakdown: occurs when hybrids produce offspring that have reduced viability or fertility. Polyploidy: possession of more than the normal 2 sets of chromosomes. Often occurs in plants. A tetraploid zygote can be established when a diploid sperm fertilizes a diploid egg. Since meiosis will continue to produce diploid gametes reproductive isolation and speciation occurs in a single generation.