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Evolutionary trends
By Martin Samaj
Rates of evolution
• Gradualism – claims that evolution proceeds very
slowly, large changes can occur over long periods of
time
• This theory does not fit with fossil record, which show
periods where the evolutionary rate was stable,
followed by a period of rapid changes
• These changes in evolutionary rates may be due to the
environmental changes which would require the
species to adapt to these new conditions
• This view of the pace of evolution is called punctuated
equilibrium
Adaptive radiation
• When each species evolves in a
different way, we call it divergent
evolution
• Adaptation to different ecological
roles avoids competition among
species
• If species in a group deviate rapidly
it is called adaptive radiation
• Example: when a group has a
characteristics that gives it a
competitive advantage over existing
species
• Mammals also demonstrate
adaptive radiation – the pentadactyl
limb
Transient polymorphism
• Population in which there are
two alleles of a gene in the
gene pool is polymorphic
• If one allele is gradually
replacing the other the
population shows transient
polymorphism
• The peppered moth is an
example of this
Convergent evolution
• Living organisms often find the
same solutions to particular
physiological problems
• If natural selection acts the same
also in other parts of the world,
species can become remarkably
different despite not being
closely related at all
• This is called convergent
evolution
• Instead of closely related species
showing striking differences,
unrelated species show striking
similarities
Balanced Polymorphism
• Sometimes alleles of gene
can persist indefinitely in the
gene pool of a population
• This is called balanced
polymorphism
• The most researched
example of this would be
sickle anemia
• Heterozygous individuals do
not develop sickle cell
anemia and are resistant to
sickle cell anemia
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