Natural Selection
Natural Selection 17.3, 16.1, 16.3d, 16.5 Charles Darwin (1809-1882) - 1831 – travels on HMS Beagle - Contributors o Lyell’s Principle of Geology book o Malthus’s Essay on the Principles of Population Competition for resources - Galapagos Islands – why so many diff. organisms occupy one small island cluster? - Artificial Selection o Selective breeding of animals/plants to ensure that certain desirable traits appear at higher frequency in successive generations - Natural Selection o Evolutionary process by which alleles that increase the likelihood of survival and the reproduction output of the individuals that carry them become more common in subsequent generations - On the Origins of Species o Descent w/ modification Evolutionary alteration and diversification of ancestral species Variation in Natural Population - Phenotypic Variation o Differences in appearance/function – that if based on heritability – are passed from generation to generation o Possibilities Different genotype exhibit same phenotype Same genotype exhibit different phenotype o In population, cause by: 1) genetic difference 2) environmental factors 3) genetics & enviro. Only genetically based variation is subject to evolutionary change o Quantitative Variation Individuals differ in small, incremental ways E.g. mass, toe length of students in biology Bar Graphs Describe Population o Variability
-
Broad, low curve – a lot of variation High, narrow – little variation o Mean average value of character o Qualitative Variation 2 or more discrete states, NO intermediate form E.g. geese are either blue or white o Polymorphism Discrete variants of a character E.g. type A, B, AB, O, number of stripes, background colour Genetic Variation o Sources Production of new alleles Rearrangement of existing alleles o Measuring Gel Electrophoresis Seperates 2 or more forms of a given protein if they differ significantly in shape, mass, or net electrical charge o Identifying protein infer genetic variation at the locus coding for that protein
Natural Selection Shapes Genetic Variability by Favouring Some Traits over Others Natural Selection o process by which beneficial, heritable traits become more common in subsequent generations (changing allele frequency) therefore, doesn’t agree w/ HW equilibrium o no effect on traits that appear during an individual’s post-reproductive life (ex. Huntington disease) - rate of loss of lethal alleles declines as more and more lethal alleles are found in heterozygous individuals - Relative Fitness o # of surviving offspring that an individual produces compared with the number left by others in the population (by the other allele) o Individuals with the best phenotype have relatively more offspring than those with less beneficial phenotypes So, best phenotypes don’t only reproduce - Modes of Natural Selection 1) Directional Selection
-
Occur when , individual’s near one end of the phenotypic spectrum have the highest relative fitness - Shifts trait toward favoured extreme - Mean shifts - common 2) Stabilizing Selection - Occur when, individuals expressing intermediate phenotypes have the highest relative fitness - Eliminating extremes, genetic and phenotypic variation, frequency of intermediate phenotypes - Favours mean - most common 3) Disruptive Selection - Occur when, extreme phenotypes have higher relative fitness than intermediate phenotypes - Extreme phenotypes more common polymorphism - mean - Less common Adaptation and Evolutionary Constraints - Adaptive Trait o Product of natural selection that relative fitness of an organism in its environment - Adaptation o Accumulation of adaptive traits over time - Constraints of Adaptive Evolution o no organism can be perfectly adapted to its environment b/c environments change over time o historical: natural selection can only affect existing genetic variation natural selection acts on mutations and genetic variation but b/c mutations is rare, natural selection works primarily on alleles that have been present for many generations thus, adaptive changes in the morphology of an organism are based on small modifications of existing structures o e.g. 4-footed humans 2-footed humans natural selection didn’t produce new skeletal design just modified existing traits
-
Broad, low curve – a lot of variation High, narrow – little variation o Mean average value of character o Qualitative Variation 2 or more discrete states, NO intermediate form E.g. geese are either blue or white o Polymorphism Discrete variants of a character E.g. type A, B, AB, O, number of stripes, background colour Genetic Variation o Sources Production of new alleles Rearrangement of existing alleles o Measuring Gel Electrophoresis Seperates 2 or more forms of a given protein if they differ significantly in shape, mass, or net electrical charge o Identifying protein infer genetic variation at the locus coding for that protein
Natural Selection Shapes Genetic Variability by Favouring Some Traits over Others Natural Selection o process by which beneficial, heritable traits become more common in subsequent generations (changing allele frequency) therefore, doesn’t agree w/ HW equilibrium o no effect on traits that appear during an individual’s post-reproductive life (ex. Huntington disease) - rate of loss of lethal alleles declines as more and more lethal alleles are found in heterozygous individuals - Relative Fitness o # of surviving offspring that an individual produces compared with the number left by others in the population (by the other allele) o Individuals with the best phenotype have relatively more offspring than those with less beneficial phenotypes So, best phenotypes don’t only reproduce - Modes of Natural Selection 1) Directional Selection
-
Occur when , individual’s near one end of the phenotypic spectrum have the highest relative fitness - Shifts trait toward favoured extreme - Mean shifts - common 2) Stabilizing Selection - Occur when, individuals expressing intermediate phenotypes have the highest relative fitness - Eliminating extremes, genetic and phenotypic variation, frequency of intermediate phenotypes - Favours mean - most common 3) Disruptive Selection - Occur when, extreme phenotypes have higher relative fitness than intermediate phenotypes - Extreme phenotypes more common polymorphism - mean - Less common Adaptation and Evolutionary Constraints - Adaptive Trait o Product of natural selection that relative fitness of an organism in its environment - Adaptation o Accumulation of adaptive traits over time - Constraints of Adaptive Evolution o no organism can be perfectly adapted to its environment b/c environments change over time o historical: natural selection can only affect existing genetic variation natural selection acts on mutations and genetic variation but b/c mutations is rare, natural selection works primarily on alleles that have been present for many generations thus, adaptive changes in the morphology of an organism are based on small modifications of existing structures o e.g. 4-footed humans 2-footed humans natural selection didn’t produce new skeletal design just modified existing traits
