Important Terms and Definitions

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Published on 26 Jul 2010
Biological Sciences
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
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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