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Darwin developed his theory of natural
selection before knowledge of genetics
Populations evolve, not individuals!
An organism is born with its phenotype, and it never
changes during its lifetime
 Evolution occurs as a population’s genes and their
frequencies change over time
 Gene Pool: all of the alleles of the population’s genes
 Allelic Frequency: percentage of any specific allele in
the gene pool
 A population of organisms in which the allelic
frequency remains the same over generations is said
to have reached genetic equilibrium
Genetic Drift
can occur by environmental factors or chance.
most are lethal
some may be beneficial
change in allelic frequencies due to chance events
affects smaller, more isolated populations
is responsible for the species Darwin found on the
Galapagos Islands
Movement of individuals into or out of the
Stabilizing Selection
Directional Selection
favors the average individual in a population
reduces variation of the population
nature favors one extreme of the population
ex: long necks in giraffes
Disruptive Selection
nature favors individuals at both extremes of the
complete opposite of stabilizing selection
Speciation: evolution of a new species when members
of similar populations are no longer able to interbreed
and produce fertile offspring
Can be caused by:
Geographic Isolation
 when a physical barrier divides a population (islands)
 over time, the isolated gene pool may become so different that a
new species would evolve
Reproductive Isolation (two types)
 genetic material becomes so different that fertilization is
 behavioral changes (one mates in fall, one in spring)
Change in Chromosomal Number
 caused by nondisjuntion
 polyploid: an individual with a multiple of the normal set of
 results in immediate reproductive isolation
Gradualism: idea that speciation occurs
slowly and steadily over time
 Punctuated Equilibrium: speciation occurs
quickly (~10,000 years), in rapid bursts, with
long periods of genetic equilibrium in
Biologists tend to agree that both occur
depending on the circumstances
Divergent Evolution
Pattern of evolution in which species that once were
similar to an ancestral species become increasingly
Occurs when populations adapt to very different
Convergent Evolution
Pattern of evolution in which distantly related or
seemingly unrelated organisms began to resemble
each other.
 Occurs due to organisms adapting to similar
Ex: dolphins and sharks or euphorbs and cacti