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Lecture 2

HLTB20H3 Lecture Notes - Lecture 2: Genetic Drift, Consanguinity, Point Mutation


Department
Health Studies
Course Code
HLTB20H3
Professor
Lianne Tripp
Lecture
2

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Lecture 2 Notes
Contemporary Human Evolution & Variation
Week 3
LECTURE 2 — THE FORCES OF EVOLUTION AND MATING SYSTEMS: THE
FUNDAMENTALS OF HUMAN POPULATION STRUCTURE AND COMPOSITION
-which genes are present in a population (forces): natural selection, mutation, genetic drift,
gene flow
-how genes are assembled into genotypes (mating systems): random mating, assortative
mating, inbreeding
Evolution
-a process — a dynamic series of events that ultimately lead to change
-macroevolution: large scale change (fossils), typically associated with speciation
-microevolution: smaller scale changes (gene frequencies), typically over generations and
accumulation of changes
-human populations are the by product of both biology and culture and as a result:
-genotype + environment = phenotype
-gene composition — which genes are present in a population
-evolutionary forces: mutation, natural selection, gene flow, random genetic drift
Evolutionary Forces
-many mechanisms through which population variation and change can arise: mutation, natural
selection, gene flow and random genetic drift
-evolution is not the product of an individual, but a population changing over time
Genetic Inheritance
-allele - alternative form of a gene (one member of a pair) that is located at a specific position
(locus) on a specific chromosome
-each allele of the pair is inherited from parents (one from each parent)
Factors that Produce and Redistribute Variation
Mutation
-mutation - change in genetic code resulting in change in an allele
-the basic creative force in evolution — only way to produce new variation
-mutations are rare and would have little effect on populations by themselves
-mutations are more powerful and become more common in population when paired with
natural selection
-some mutations are beneficial
-typically a rare event
-stochastic (random) force
-can involve: chromosomal mutations (observed), gene mutations
-most mutations are minor
-many mutations are harmful
-some mutations are lethal
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Lecture 2 Notes
Contemporary Human Evolution & Variation
Week 3
-few mutations are helpful in a specific environment
-overall effect can range from none to lethal
-mutation example: trisomy 21 (downs syndrome)
-chromosomal mutations: nondisjunction, failure of chromosomes to segregate during meiosis
-gene mutations: 4 organic bases (T,A,C,G), pairs (T-A and C-G), point mutation can change in
the nucleotide sequence of DNA
-implications of the location mutation: !
-germ line mutation involves cells which give rise to gametes, gamete carries mutation !
to next generation, therefore a heritable mutation
-this must be in gamete (sex) cells and during reproductive period !
- acquired mutation occurs in an individuals lifetime !
- germ line or somatic is if germ line cells are exposed, mutation could be passed to !
offspring
Gene Flow
-movement of alleles from one population to another, increases genetic exchange
-migration, trade, warfare result in mating and reproducing
-few breeding isolates remain today
-cultural factors (ex. marriage practices and transportation) and length of time of contact can
play a role
-gene flow reduces variation among populations
-breeding population - smallest unit of a population (deme)
Random Genetic Drift (sampling error)
-stochastic factor in evolution due to sampling phenomenon in small populations
-if an allele is rare to begin with, chance factors may prevent its transmission to the next
generation
-generally, the smaller the population the larger the potential effect of genetic drift
-genes become more or less frequent by chance, can not predict direction of change of allele
frequencies
-product of small population size — allele frequencies susceptible to small changes in
population
-increases variation among population
-founder effect: migration, very small group of individuals contribute to the gene pool of the
next generation, founding group and descendants only carry a small fraction of the alleles and
variation present in the original population
-bottleneck: small portion of a population survives famine, war, plague, etc.
Drift — Sampling Error — S. Wright (Random Genetic Drift)
-increases the probability that over time one of the alleles in the system becomes fixed and the
other becomes extinct
-over the long course the result is intra-population homogeneity
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