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Selection & Genetic drift working on variation in the population cause. EVOLUITON. SELECTION.
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Selection & Genetic drift working on variation in the population cause EVOLUITON
Artificial Selection = genetic changes within a population which occur because human beings intentionally select for certain characteristics either by choosing which organisms to breed or eliminating undesirable individuals. Natural Selection = genetic changes within a natural (wild) population which occur because of differences in the reproductive success of some individuals over other individuals.
Why doesn’t selection work anymore on running speed? • There are probably diseases involved. • Too much inbreeding causes genetic defects. • There is no more genetic variation in the alleles determining running speed. • Epistatic effects are now in play. • Artificial insemination programs inadvertently select for defective (weaker) sperm.
In order for Evolution to Occur there must be variation in the population
Which is not a cause of allele variation in a population? • Homology • Mutation • Genetic recombination (crossover) • Sexual reproduction • Migration (gene flow) 30
Artificial Selection • Farm animals (e.g. chicken egg production) • 126 eggs/hen/year Today 365 eggs/hen/year
Dogs mtDNA indicates that all dogs are derived from a species of wolf ~10,000 BC in Eurasia. The Romans (2,000 yrs. ago had 6 only breeds In 1837 the book British Quadrupeds listed < 20 breeds Now, ~200 breeds Most developed in the last 150 yrs.
EVOLUTION CAN BE FAST ! Over 200 breeds
Domestication of silver foxes • Dmitri Belyaev • Artificial Selection
NATURAL SELECTION • Differences in mortality & reproduction lead to differences in the proportion of alleles passing to the next generation. • Positive selection= a selective advantage for one allele and that favored allele increases in the population. • Negative selection= a selective disadvantage for a allele and that tends to disappear from the population.
NATURAL SELECTION IN BACTERIA • Development of antibiotic resistance in bacteria
The Plague Deaths
e.g. NATURAL SELECTION IN INSECTSResistance to pesticides • Foliate insecticide
Please look at the video. NATURAL SELECTION IN PLANTS FOR COPPER TOLERENCE
WHAT PRODUCES SELECTION? • Internal Environment • e.g. genetic environment (society of genes) • e.g. physiological/developmental environment • External Environment • Physical Environment (water, temp., 02,) • Biological Environment • Competition • Predation • Parasitism • Food supply • Mate selection (Sexual Selection)
Success must be judged in 2 ways: • Absolute Terms= Will the genome work? • Relative Terms= Is the genome the best available model?
Small selective advantagesSuppose there were a selective advantage to having a long nose and suppose that advantage produced a trend so the average nose length increased 0.001 inch/ year. How many inches will the nose will have grown in 100,000 years? • One • Ten • 100 • 1,000 • None of the above
Slight advantages over long time = Enormous changes!
SELECTION IN 2 ALLELE TRAITS • Selection against a dominant trait Genotypes: AA Aa aa Rapid elimination of the trait should occur. Speed depends on how deleterious the allele is.
Fitness & Natural Selection • How to calculate relative fitness Compare the success of the favored allele to that of the unfavored allele. W= number of children produced by 100 unfavored genotype number of children produced by 100 favored genotype
e.g. Achondroplastic Dwarfism Due to a dominant allele DD and Dd = dwarf condition dd = normal Number of children per 100 dwarf parents = 20 Number of children per normal parents = 100 W= 20/100= 0.20 = fitness coefficient
If the w= 20/100 = 0.2 the fittness value of the unfavored allele, what is the fittness value for the “favored allele”? • Zero • 0.2 • 0.5 • 1.0 • None of the above
Selection in two allele systems Continued • Selection against the recessive Genotypes: AA Aa aa Selection will be slow to remove the “a” allele in complete dominance because it is hidden in the heterozygous condition “Aa.” These are carriers.
Cystic Fibrosis • Caused by recessive trait, cc • Normal allele C causes channels in the cell membranes (lung, gut) to allow Cl- out of cells and H20 follows. If both alleles are cc, then this doesn’t happen & thick sticky mucus clogs lungs & gut & these tissues are breeding ground for bacteria
Cystic Fibrosis • Most common genetic disorder in Caucasians; affects 1 out of 2,500 white babies • 5% of Caucasians are carriers • CC is normal • Cc is normal, but carrier • cc has cystic fibrosis
If two carriers have children, what are the chances that their child will have cystic fibrosis? • 100% • 75% • 50% • 25% • Zero %
Cystic Fibrosis • Cc x Cc = 25% of kids are cc & have cystic fibrosis • Why aren’t they eliminated from the pop? • Because Cc have protection against diarrhea ?
Selection in two allele systems Continued • Selection favoring heterozygotes “Aa” Can only occur if incomplete or co-dominance because the trait is expressed. e.g. Sickle-cell anemia
Sickle Cell Co-dominance Normal Mild Anemia Severe Anemia HbN HbN HbN HbS HbS HbS In the USA: W = 1.0 W = 0.9 W = 0.14 In Africa: W = 0.88 W = 1.0 W = 0.14 Heterozygotes are favored in Africa because they are more resistant to malaria! The environment determines what is fit!
What kind of selection is this? • Directional • Stabilizing • Disruptive • Eliminative • No way to tell
This is negative selection—elimination of “unfit” genotypes.
