AGENTS OF EVOLUTIONARY
CHANGE
Today, scientists view evolution as occurring when the genetic
equilibrium of a species is disrupted. Genetic equilibrium is the
condition in which the allele frequencies in a population do not
change from generation to generation. An allele is an alternate form
or different variations of a gene. Evolution occurs when genetic
equilibrium is disrupted. Change in gene or more specifically allele
frequencies can occur due to agents of evolutionary change. The five
agents of evolutionary change which disrupt genetic equilibrium
discussed below
Populations:
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Members of a sexually-reproducing species are able to interbreed,
produce fertile offspring, and have a shared gene pool
Gene pool refers to the collective group of alleles of all the
individuals in a population
Different species do not exchange genes with each other by
interbreeding
A population is a group of organisms of the same species occupying
a certain area
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Musk Ox Population
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Members of a population vary from one another
Variation is the raw material for evolutionary change
Features that make an organism suited for its environment so it can
survive, reproduce, & pass its alleles onto its offspring are called
adaptations
Speciation is the splitting of one species into two or more species or
the transformation of one species into a new species over time
Speciation is the final result of changes in gene pool allelic and
genotypic frequencies
Micro- & Macro- Evolution:
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Macroevolution refers to large scale evolutionary changes such as
the formation of new groups above the species level
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Microevolution refers to smaller scale changes such as changes
within species
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Microevolution of May beetles
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In studying evolution at the population level, geneticists focus on the
gene pool
When the relative frequency of alleles in a population changes over a
number of generations, it is called microevolution
Five Causes of Change in Gene Pools:
1. Mutations
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Mutations result in the introduction of new genes (new genetic
information) into a gene pool
Mutations can be changes in genes (DNA sequences) or changes to
chromosomes (additions, deletions, substitutions, translocations)
Gene mutations provide new alleles, and therefore are the ultimate
source of variation
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A gene mutation is an alteration in the DNA (deoxyribonucleic acid)
nucleotide sequence of an allele
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Mutations occur at random
Mutations can be beneficial, neutral, or harmful
Some chromosomal mutations are alterations in the number of
chromosomes inherited
Others are alterations in arrangement of alleles on chromosomes
due to inversions and translocations
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2. Gene Flow
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Gene flow is the movement of genes into (immigration) & out of a
population (emigration)
Migration of breeding individuals moves alleles among populations
through interbreeding
Gene flow may be agent of microevolution (e.g. isolated populations
with limited gene flow result in genetic distinctions among groups
living in different locations)
Continued gene flow tends to decrease the diversity among
populations, causing gene pools to become similar
3. Genetic Drift
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Genetic drift refers to changes in allele frequencies of a gene pool
due to chance (random) events
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Genetic drift occurs in both large and small populations
Genetic drift causes gene pools of two isolated populations to become
dissimilar as some alleles are lost and other are fixed
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Genetic drift occurs when founders start a new population, or after
a genetic bottleneck with interbreeding
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Founder effect is a case of genetic drift in which rare alleles, or
combinations of alleles, occur in higher frequency in a population
isolated from the general population
Example: Polydactyly -- extra fingers or sometimes toes -is one symptom of Ellis-van Creveld syndrome. The
syndrome is commonly found among the Old Order
QuickTime™ and a
(Uncomp resse d) de com press or
Amish of Pennsylvania, a population that experiences the TIFF
are nee ded to s ee this picture.
"founder effect." Genetically inherited diseases like Ellisvan Creveld are more concentrated among the Amish
because they marry within their own community, which
prevents new genetic variation from entering the
population. Children are therefore more likely to inherit two copies of
the particular recessive genes that lead to genetic disease.
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Bottleneck effect is genetic drift in which a severe reduction in
population size due to natural disaster, predation, or habitat
reduction, causes severe reduction in total genetic diversity of the
original gene pool (e.g. Intense interbreeding in cheetahs has
prevented most genotypes from being passed to the next generation)
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4. Non-random Mating
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Inbreeding (mating with close neighbors instead of more distant
members of a population) can effect the frequency of some
genotypes
Causes a reduction in heterozygous genotypes & an increase in
homozygous genotypes
5. Natural Selection:
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Populations must adapt to their environment
Natural selection produces adaptive evolution
Natural selection requires variation (heritable genetic differences) in
the members of a population
Some differences affect how well an organism is adapted to its
environment & make them more fit or more likely to reproduce
Fitness is the extent to which an individual contributes fertile
offspring to the next generation & is measured against the
reproductive success of other genotypes in the same environment
Isolation Factors for Species:
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A biological species is a category whose members are reproductively
isolated from all other such groups
Reproductive isolation occurs when members of one species can only
breed successfully with each other
Reproductive isolating mechanisms are any structural, functional,
or behavioral characteristic that prevents successful reproduction
from occurring
Habitat isolation occurs when two species occupy different habitats,
even within the same geographic range, so that they are less likely to
meet and to attempt to reproduce
Grand Canyon Isolates Rodent Populations
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Temporal isolation occurs when two species live in the same
location, but each reproduces at a different time of year, and so they
do not attempt to mate
Behavioral isolation results from differences in mating behavior
between two species
Mechanical isolation is the result of differences between two species
in reproductive structures or other body parts, so that mating is
prevented
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Gamete isolation is physical or chemical incompatibility of gametes
of two different species so that they cannot fuse to form a zygote; an
egg may have receptors only for the sperm of its own species
Zygote mortality is when hybrids (offspring of parents of two
different species) do not live to reproduce
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