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Sexual reproduction and speciation
Natural selection
Natural selection and evolution
Modern examples of Evolution
His book On the Origin of Species by means of
Natural Selection Nov. 24, 1859
– This book presented the first convincing case for
– Covered many aspects of living organisms including
diversity, origins and relationships, similarities and
differences, geographical distribution, and
adaptations to their environment
Darwin made two major points in his book
– Species evolved from ancestral species and were not
specially created
– Natural Selection is a mechanism that could result in
this evolutionary change
Five Agents of Evolutionary
Gene Flow
Nonrandom Mating
Genetic Drift
Mutation rates are generally slow
A single gene may mutate 1 to 10 times
per 100,000 cell divisions (although some
genes may mutate more frequently)
Nonetheless, mutation is the ultimate
source of genetic variation
Gene Flow
Movement of alleles from one population
to another = movement and reproduction
– Powerful mechanism of change because
members of two different populations may
exchange genetic material
– Occur if, for example, one organism moves
from one place to another. If the
characteristics of the newcomer differ from the
native organisms and it adapts well and mate
successfully, the genetic composition of the
population will be altered
Nonrandom Mating
– Does not change the frequency of the alleles ,
but the proportion of homozygous individuals
because relatives are likely to be genetically
similar and thus produce offspring with two
copies of the same allele
Genetic Drift
In small populations, frequencies of
particular alleles may change drastically
by chance alone.
These changes occur randomly, as if the
genes were drifting
If the gametes of only a few individuals
form the next generation, the alleles they
carry may by chance not be representative
of the parent population from which they
were drawn (Bottleneck effect)
The Bottleneck Effect
See the following site
– Bottleneck effect (BIOdotEDU)
As Darwin pointed out, some individuals
leave behind more progeny than others,
and the rate at which they do so is
affected by phenotype and behavior
– Artificial Selection –breeder selects
– Natural Selection -in natural selection,
environmental conditions determine which
individuals in a population produce the most
For Natural Selection to occur and produce
evolutionary changes three conditions must
be met:
1. Variation must exist among individuals
in a population. Natural selection favors
individuals with some traits over individuals with
alternative traits
2. Variation among individuals results in
differences in number of offspring
surviving in the next generation. Because
of their phenotype or behavior, some
individuals are more successful than
3. Variation must be genetically inherited.
For natural selection to result in
evolutionary change, the selected
differences must be inherited.
Darwin, Wallace and Evolution by
Natural Selection
Evolution –
– Accumulation of changes in the heritable
characteristics of a population
– Processes that have transformed life on Earth
– Charles Darwin developed the theory that
evolution occurs as a result of natural
selection (explained in The Origin of Species,
1859). What was Wallace’s contribution?