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Transcript


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


Mutations
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Genetic Drift
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

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
population

Stabilizing Selection

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Directional Selection
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

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
spectrum
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
impossible
 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
chromosomes
 results in immediate reproductive isolation

2 Views
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
between


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
distinct
Occurs when populations adapt to very different
environments
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
environments


Ex: dolphins and sharks or euphorbs and cacti