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Chapter 17: Pop. Genetics & Evolution (Population Genetics (Factors…
Chapter 17: Pop. Genetics & Evolution
Population Genetics
deals with the different alleles within a population
& the manner in which the abundance of a particular allele increases, decreases, or stays the same w/ time
gene pool:
total number of alleles in all sex cells of all individuals of a population
Factors that Change the Gene Pool
mutation:
all genomes subject to mutagenic factors
causes existing alleles to decrease in frequency, new alleles to increase
accidents:
events to which an organism cannot adapt
ex. fire
gene pool altered if area of accident included new mutated allele, or higher frequency of an allele
artificial selection:
process in which humans purposefully change allele frequency of a gene pool
ex. selective breeding: organisms with desired trait intentionally bred together
natural selection:
usually described as “survival of the fittest”
really means survival of the best adapted
#when well adapted allele frequencies remain relatively stable
2 conditions must be met:
population must produce more offspring than can possible grow & survive to maturity in the habitat
progeny must differ from each other in their types of alleles
not always external forces
multiple selection pressures
in many cases one single factor not the cause of loss of individuals, but
multiple
factors
Rates of Evolution
most populations adapted to their environment well & do not change allele frequencies often
rapid speciation occurring on Kauai
#due to lack of competition Adaptive Radiation occurs
aster seed populated the island rapidly due to lack on competition
descendants of first seed have diversified into
3
distinct genera, each w/ many species
as systems become more intricate, probability decreases that any random change is beneficial
disruptive mutations outnumber constructive mutations, loss can occur rapidly
ex. Cacti
mutations that disrupted formation of lamina advantageous
cacti lost their leaves in as a little as 10 million years
Speciation
when so much change has occurred to a population that they become a new species
distinct from species it was at beginning
Phyletic Speciation
gradual change in allele frequency in single population
gene flow:
movement of alleles physically through space
pollen transfer: alleles carried by pollen can travel great distances
seed dispersal: seeds/fruit spread by wind, floods, & stream flow
vegetative propagation: possible if species produces mobile pieces that reproduce vegetatively
Divergent Speciation
two separate species evolve differently from a common ancestor
reproductive isolation: alleles that arise in one part of range do not reach individuals in other part
Abiological Reproductive Barriers:
physical nonliving feature that prevents 2 populations from exchanging genes
speciation resulting from this called
allopatric/geographic speciation
Biological Reproductive Barriers:
any living/biological phenomenon that prevents successful gene exchange
two groups become reproductively isolated while growing together called
sympatric speciation
can be caused by different flowering times
prezygotic isolation mechanisms:
pollen prevented from moving from plant to plant, pollination/fertilization do not occur
acts before zygote can be formed
postzygotic internal isolation:
populations so genetically distinct they cannot interbreed
hybrid sterility: spore mother cells in anthers & ovules in sterile hybrids unable to complete meiosis
hybrid inviability: alleles of two parents so incompatible zygote/embryo dies early in dev.
Adaptive Radiation:
species rapidly diverges into many new species over extremely short time (few million years)
occurs when species enters new habitat where little stress/competition exists
ex. Galápagos Islands
Convergent Evolution
two distinct, unrelated species occupy the same/similar habitats, nat. selection happens to favor same phenotypes
two species then evolve to the point they resemble each other
Evolution/Origin of Life
Conditions Before Origin of Life on Earth
Chemicals in Atmosphere:
early atmosphere mostly
hydrogen
most of first atmosphere lost into space
second atmosphere
produced by released gasses from rock matrix & bombardment by meteorites
also known as
reducing atmosphere
due to lack of molecular oxygen & presence of powerful reducing agents
Energy Sources:
UV & gamma radiation from sun
heat from the coalescence of gas & dust to form earth
radioactive decay of heavy elements (
uranium & radium
)
Time Available for the Origin of Life
no time limit due to lack of free molecular oxygen
Chemosynthesis:
life originated from spontaneously reacting chemicals that become more complex with time
Early Metabolism:
aggregates complete heterotrophs
scarcity of resources encouraged natural selection
Formation of Polymers:
monomers in early ocean had to polymerize if life is to arise
polymerization requires high concentration of monomers
tide pools considered possible concentration spots
frozen pools/ponds possibly concentration site as ice is almost purely water
clay newly considered concentration spot as binding to clay similar to binding to enzyme
Aggregation & Organization:
aggregation of chemical components into masses that have some organization & metabolism
aggregates not considered alive due to lack of ability to story genetic info
one eventually develops heritable information molecule useful to aggregate
#natural selection favors this trait & so it gets passed on
glycolysis likely evolved early due to presence in virtually all organisms
Oxygen:
evolution of chlorophyll
a
& photosynthesis caused
2
things
allowed the world to rust by liberating oxygen
created conditions that selected for evolution of aerobic respiration
oxygen oxidizes all iron in ocean then accumulated in atmosphere
results in
oxidizing atmosphere
