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Chapter 18

Biology 1201A Chapter Notes - Chapter 18: Adaptation, Probability Distribution, Sickle-Cell Disease

Course Code
BIOL 1201A
Jennifer Waugh

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Chapter 18- Microevolution: changes within a population
-Humpback whales declined from 125 000 to 5000 individuals because of hunting. They are
now back up to 80 000 because of anti poaching laws but biologists wonder if their genetic
variability have been lost, which would lead to decreased resistance to disease, and ability to
survive environmental changes.
-A situation in which a population regrows from a small number is called a bottleneck.
-When a population is started by a few organisms with limited genetic variability it is an example
of the founder effect.
-The evolution of the humpback whales is an example of microevolution, which is a change in
frequencies of alleles or heritable phenotypic variants in a population over time.
-Population: includes all the individuals of a single species that live together in the same place
and time.
18.1- Variation in Natural Populations
Phenotypic variation: differences in appearance or function among individuals of a population. If
a difference is heritable it is passed from generation to generation.
Quantitative Variation: Most individuals differ in small, incremental ways. example in humans is
weight, height, length of toes.
Qualitative variation: they exist in two or more discrete states, and intermediate forms are
absent. ex. snow geese have either blue or white feathers, no pale blue.
Polymorphism: the existence of discrete variants of a character (these traits are called
-We describe the phenotypic polymorphisms quantitatively by calculating the percentage of
frequency of each trait.
-Phenotypic variation within populations may be caused by genetic differences between
individuals, by differences in the environmental factors that individuals experience, or by an
interaction between an individuals genetics and the environment.
-Genetic and phenotypic variations may not be perfectly correlated. Organisms with different
genotypes often exhibit the same phenotype. (ex. homozygous dominant & heterozygous)
-Organisms with the same genotype sometimes exhibit different phenotypes, for example the
acidity of soil influences flower colour in some plants.
*Only genetically based variation is subject to evolutionary change. Natural selection operates
on the phenotype, not the genotype.
-Because environmental factors can influence the expression of genes, an organisms
phenotype is frequently the product of an interaction between its genotype and its environment.
-We can test if a phenotypic variation is caused by environmental factors by changing one
variable and measuring the effuses on genetically similar objects.
18.1b- Genetic Variation
-An allele is one member of a gene pair that occupies a single location on a chromosome.
-Genetic variation, the raw material moulded by microevolutionary processes, has two potential
sources: the production of new alleles and the rearrangement of existing alleles.

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-Most new alleles probably arise from small scale mutations in DNA. The rearrangement of
existing alleles into new combinations can result from larger scale changes in chromosome
structure or number, as well as from several forms of genetic recombination.
18.1c- Natural Selection and Phenotypic Variation
Directional Selection: traits undergo directional selection when individuals near one end of the
phenotypic spectrum have the highest relative fitness.
-Shifts a trait away from the existing mean and toward the favoured extreme.
-After selection, the traits mean value is higher or lower than before, the variability in the trait
may be reduced.
-Directional selection is very common and is used in many human uses such as when producing
domestic animals and crops
Stabilizing Selection: traits undergo this when individuals expressing intermediate phenotypes
have the highest relative fitness.
-eliminates phenotypic extremes, increasing the frequency of intermediate phenotypes
-Can result from multiple selective forces acting on the same trait but in opposite directions.
-decreases genetic diversity
Disruptive Selection: traits undergo this when extreme phenotypes have higher relative fitness
than intermediate phenotypes.
-Alleles producing extreme phenotypes become more common, promoting polymorphism.
- Does not alter mean phenotype, variance increases
- Selection against intermediate phenotype, reproductive chances are less frequent
- Greater variation among population
18.2a- Genetic Structure of Populations
-populations are made up of individuals of the same species, each with its own genotype.
-Individuals genotype has two alleles (two of the same of different) at each gene locus.
-Gene Pool: the sum of alleles at all gene loci in all individuals of a population
-Genotype frequencies: the percentage of individuals possessing each genotype
-allele frequencies: the relative abundances of different alleles
-For a gene locus with two alleles, there are 3 genotype frequencies but only two allele
frequencies (p and q).
-The sum of the 3 genotype frequencies must equal 1, same with the allele frequencies
18.2b- The Hardy-Weinberg Principle
Null models: conceptual models that predict what would be seen if a particular factor had no
-The H-W principle specifies the conditions under which a population of diploid organisms
achieves genetic equilibrium, the point at which neither allele frequencies nor genotype
frequencies change in succeeding generations.
-Their work showed that dominant alleles don't need to replace recessive ones, and that the
shuffling of genes in sexual reproduction does not in itself cause allele or genotype frequencies
to change.
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