Important Terms and Definitions

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Lecture 19
๎‚™ African elephants are poached for ivory; the proportion of the population that have tusks
is decreasing
๎‚™ By targeting older, larger fish, commercial fishing for cod has led to a reduction in the
age and size at which these fish mature
๎‚™ Evolution can be viewed as genetic change over time or as a process of descent with
๎‚™ Evolution:
o A change in the genetic characteristics of a population over time
o Descent with modification-organisms gradually accumulate differences from their
๎‚™ Natural selection: Individuals with particular adaptations tend to survive and reproduce at
a higher rate than other individuals
๎‚™ The genotype is the genetic makeup, and is represented by letters, one for each allele
๎‚™ Natural selection, genetic drift, and gene floe can cause allele frequencies in a population
to change over time
๎‚™ Phenotype- the observable characteristics of individuals that are determines by the
๎‚™ Directional selection: Individuals with one extreme of a heritable phenotypic trait (for
example, large size) are favoured
๎‚™ Stabilizing selection: Individuals with an intermediate phenotype are favoured
๎‚™ Disruptive selection: Individuals at both phenotypic extremes are favoured
๎‚™ Genetic drift occurs when chance events determine which alleles are passed to the next
๎‚™ Some alleles may disappear, other may reach 100% frequency- fixation
๎‚™ Gene flow occurs when alleles are transferred from one population to another via
movement of individual or gametes
๎‚™ Natural selection is the only evolutionary mechanism that consistently causes adaptive
๎‚™ Adaptations are features of organisms that improve their ability to survive and reproduce
in their environments
๎‚™ Adaptive evolution-traits that confer advantages tend to increase in frequency over time
๎‚™ There are several constraints on evolution:
o Lack of genetic variation
o Evolutionary history
o Ecological trade-offs
๎‚™ Lack of genetic variation: If there is no beneficial allele, adaptive evolution at the gene
cannot occur
๎‚™ Evolutionary history: Natural selection works on the traits already existing in organisms
๎‚™ Ecological trade-offs: All organisms face trade-offs in allocation of energy and resources
to growth, reproduction and survival
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