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Population Genetics and Evolution (Population Genetics (factors that cause…
Population Genetics and Evolution
Concepts
Evolution
is the gradual conversion of one species into on or in some cases, several new species.
it occurs for the most part by natural selection
is an extremely slow process that may require thousands of generations and millions of years to produce change
because it is so slow compared with the length of a human lifetime
it is easy to understand that it went undetected until recently
Mutations cause new alleles or new genes to arise that affect the fitness of the individual,
making it less or more adapted to environment than other individual without the new allele or gene
if mutation is deleterious, the individual may grow or reproduce slowly or even die early without reproducing
if the mutation is beneficial, the individual should grow and reproduce better than other individuals
producing a greater number of gametes and ultimately a greater number of seeds than the other individuals
the abundance of the new allele increases relative to the original alleles
Population Genetics
factors that cause the Gene Pool to change
although it is possible theoretcally for a gene pool to remain constant,
in reality changing allele frequencies are the rule because populations are always affected by factors other than sexual reproduction
natural selection
which is the most significant factor causing pool changes, is usually described as survival of the fittest:
those individuals that are most adapted to an environment, where as those less adapted do not .
however, natural selection is such an important factor in evolution that is must be given careful attention
two conditions must be met before natural selection can occur
(1) the population must produce more offspring than can possibly grow and survive to maturity in that habitat
this condition is almost always valid for plants anywhere on Earth.
most plants produce hundreds of seeds which often germinate near the parent plant
besides limited resources, the number of individuals that can survive in a particular habitat is affected by
predators, pathogens, and competitors.
all plants are faced with attack by herbivorous animals, ranging from microscopic mites and nematodes to
much larger beetles, reptiles, birds and mammals.
animals not only eat plants but may also lay eggs in them, and rob nectar without carrying out pollination.
pathogenic fungi and bacteria are similarly harmful.
competitors are other organisms that use the same resources the plants needs to survive.
when root systems grow together, the two plants compete for the same water and nutrients
(2)the second condition necessary for natural selection is that the progeny must differ from each other in their types of alleles
if they are all identical all are affected by adversity in the same way and to the same degree.
under crowded conditions, probably all are stunted similarly, all grow poorly, and finally, none reaches reproductive maturity
if all individuals of a species are equally susceptible to pathogenic fungus, no increase in survivability
and fitness occurs as the result of a fungal attack.
even if some survive and reproduce, they are identical genetically to those that died
so no change occurs in allele frequency; natural selection has not occurred
Natural selection refers to the differetail survival among organisms that have different phenotypes.
Natural selection can act only on preexisting alleles; it does not the mutations
Natural selection does not always result from the action of an agent outside the organism
Artificial Selection
is the process in which humans purposefully change the allele frequency of a gene pool.
when plants with beneficial qualities are found, they are collected
and used in breeding programs to produce seed for future crops
is also used to produced ornamental plants that flower more abundantly or for longer time
artificial selection has also been used to alter flower color and size and to make the plants hardy in regions
where they otherwise could not grow
artificial selection is often carried out in conjunction with artificial mutation.
plants are exposed to mutagens such as acridine dyes or irradiation with ultraviolet light or gamma rays
to increase the number of new alleles that come into existence.
the plants are allowed to grow to see how the new alleles affect the phenotype and those plants
with the desired phenotypic traits are used in selective breeding programs
Accidents
are events to which an organism can not adapt, such as the collision of a large meteorite with earth
when the meteorite of sufficient size strikes Earth, a large region of earth is destroyed, killing all life in the area
all of the organisms, along with their alleles are eliminated
if the population in the impacted area has the same gene frequencies as the general population
the alleles are eliminated in the same proportions as they exist generally, and no change in allele frequency occurs
many phenomena qualify as accidents
a volcanic eruption produces poisonous gases and molten rock that destroys everything
Infrequent floods, hailstorms or droughts can act as accidents for plants too small an delicate to become
adatped to those events, all individuals in the affected area are killed
for other species that consist of plants with larger bodies, such natural phenomena act as selective forces,
removing the weaker, less well adapted [individuals] but not affecting the more well adapted members
the continental drift of Antarctica southward from a temperate region to the South Pole was an accident
for all the plants living in it
while located in the temperate latitudes, Antarctica had a rich flora with abundant plant life as it drifted southward
entered a region too cold and severe for any plant life to survive
all alleles were eliminated
accidents can be small events as well as large ones
Mutations
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all genomes are subjected to mutagenic factors, and mutations occur continually.
because of mutation, existing alleles decrease in frequency, and new alleles increase
whether or not mutations is significant depends in part on the populations size
Situation in which Natural Selection Does not operate
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Competition does not occur in a habitat that can support the growth and reproduction of all individuals
if survival is universal, natural section does not occur
situations like this occur in newly opened habitats such ass a plowed field
all seeds present may germinate and grow vigorously, even the ones not well adapted for competition
becauses all survive, no natural selection ha occured.
other examples are the sides of road cut ,a recently burned area or a recently flooded plain covered with rich sediments
if the road cut passes through a heavy, dark forest, the newly exposed sides may be too bright and exposed for seedlings
from plants adapted to the forrest shade.
the environment favors those plants that can tolerate full sunlight and suppresses those plants that can need shade
many selections selctions pressures but not all have been eliminated, and some natural selection can still occur
Multiple Selection pressures
the loss of individuals and reduced reproduction are not caused by a single factor such as pathogenic fungus
the plants are also affected by insects attack, drought,need for pollinators, the need for mechanism to spread seeds
as well as the efficiency of their own metabolism, such as the ability of their membranes to pump ions
the capacity to reduce nitrogen, or efficiency of producing just enough P-protein in the phloem without a wasteful
a mutation that produces an allele that would result in improved fitness potentially advantageous selectively,
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but it may never have the opportunity to improve the fitness of the plant or the species.
a mutation that results in improves cold hardiness may be eliminated from the gene pool if the plant carrying
the new allele is killed by fungus or drought or cannot reproduce because of poor competition for pollinators
such a loss of this allele is simply an accident
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however if the new allele for cold hardiness does survive it may be able to improve the species
if the cold winters are common this allele greatly improves fitness, and its frequency may increase rapidly
if the cold winters are infrequent, they do not exert a strong slection pressure, the allele does not improve fitness much
and its frequency may remain low for years until a harsh winter does occur
until that time the alleles frequency is determined by several factors
it may be tightly linked to an allele hat is strongly advantageous for important condition
deals with the abundance of different alleles within a population
and the manner in which the abundance of particular allele increases, decreases or remains the same with time
the genetic recombination that occurs during sexual reproduction is important only if the two sexual partners have differing phenotype.
a crossing between two plants that have A1,A1,B1,B1 X A2,A2,B2,B2 produces offspring that have the
genotype A1,A2,B1,B2 which is different from that of either parent.
crossing over also increases genetic diversity in populations
the total number of alleles in all sex cells of all individuals of a population constitutes the
gene pool
of the population
Rates of Evolution
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most populations are relatively well adapted to their habitat or they would not exist,
very few mutations produce a new phenotype so superior that it immediately out competes all other members of the population
at the extremes , there are many species of seedless plants
that have persisted of ten of millions of years without diverging into new species
in contrast, very special speciaton is occurring in a group of aster in Haw ii
shortly after Kauai formed 5.2 million years ago , an aster seed arrived, thrived, reproduced and spread rapidly in the unpopulated island
is descendant have diversified into three distinct genera, each with many species and on average a new species arises in this group once every 500,000 years
it is difficult to identify the presence of particular alleles in a population unless they result in an easily identifiable effect
on the phenotype
most studies of evolution concern the changes in gross structures such as flowers, leaves, fruits, shoots and trichomes
but theses complex structures are the product of the developmental interaction of many genes.
any new mutation results ina more adaptive structure only if the effects of the new allele fit into already existing highly integrated mechanism of morphogensis
without causing causing serious disruptive effects as systems become more intricate
the probability decreases that any random change is beneficial
changes that results in the loss of a structure or metabolism can come out about quiclky, however and for the same reason
complexity
if a feature becomes selectively disadvantageous, many of the mutations that disrupt its development becomes selectively advantageous
because disruptive mutations outnumber constructive mutations, loss can occur relatively rapidly
Speciation
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As Natural selection operates on a population for many generations,
the frequencies of various alleles and the phenotype of the population change
at some point, so much change has occurred that current population must be considered a new species,
distinct from species that existed at the beginning
Speciation
Natural selection has caused to evolve
at what point can we conclude that a new species exists ?
it is not possible to give an exact definition of species that always valid, but generally, to organisms are considered to
be members of distinct species if they do not produce fertile offspring when crossed
many exceptions exist, the individuals of maples in the western US cannot naturally reproduce sexually with those in the East
can occur in two Fundamental ways
(1)
Phyletic
, in which one species gradually becomes so changed that is must be a new species
(2)
Divergent
, in which some populations of a species evolve into a new second species while other populations either
continue unchanged as the original, parental species or evolve into a new this species
Phyletic Speciation
Millions of years are often require for a new species to evolve into a new one
the critical feature is that as new beneficial alleles arise and are selected for, they become spread throughout the entire population
this movement of alleles physically though space called
Gene flow
occurs in many ways such as by pollen transfer, seed dispersal and vegetative propagation
Pollen transfer
pollen grains each carry one full haploid genome and all alleles of the a plant are present in its pollen grains
wind-distributed, can travel great distances
if a new allele is carried by so,e of pollen organism, it can move to very distant plants, if the pollen grans sperm cells fertilize an egg
a new seed is formed whose embryo contains the new allele
animal- mediated pollination also contribution to gene flow, both bird and insects tend to sped most of their time in a small area;
seed dispersal
the fruits and seeds of some plants fall close to the parent but many species have long-distance dispersal mechanisms
seeds and fruits can be carried by wind, floods, and stream flow.
they can be carried to islands by rafting, in which they are trapped above water on a tangle mat of floating debris
seeds or fruits that are spiny or gummy stick to the fur or feathers of animals; migratory animals can be important in dispersing seeds
vegetative propagation
if a species produces small, mobile pieces that produce vegatatively, these too contribute to gene flow
if these various mechanisms are sufficient to enable alleles that arise in one part of the species range to travel to all other parts
the species remains relatively homogeneous even as the entire species evolves into a new species
Alleles that arise at carious geographic sites come together by gene flow; then meiosis, crossing over and genetic
recombination rear-range them into thousands of combinations
Divergent Specation
if gene flow does not keep the species homogeneous throughout the entire range, Divergent S. may occur
if alleles that arise in on part, the two regions are
Reproductively isolated
can occur in many ways but the two fundamental ways are Abiological and biological reproductive
Abiological Reproductive Barriers
any physical, nonliving feature that prevents two populations from exchanging genes
Allopatric
the original species is physically divided into two or more populations that cannot interbreeds, if speculation results
Biological reproductive Barriers
any biological phenomenon that prevents successful gene flow
Sympatric Speciation
when two groups become reproductivly isolated even thought they grow together
Postzygotic internal isolation barriers
are in place and two subpopulations must be considered separate species
Hybrid sterility
The two populations occasionally interbreed or are artificially cross-pollinated and produced viable seeds
hybrid inviability
the zygote and embryo dies early in development
Convergent evolution
if the distinct unrelated species occupy the same or similar habitats, natural selection may favor the same phenotype
the two may evolve to the point that they resemble each other strongly and are said to have undergone
Evolution and the origin of life
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the species present today have evolved from those that existed in the past, which evolved from those that existed before them
consider hypothesis about he origin about the origin of life on earth is that of Chemosynthesis
attempts to model the origin of life using only known chemical and physical processes, rejecting all traces of divine intervention.
before the origin of life, the surface of earth was different from way it is now
and the chemical present then could react spontaneously, producing more complex chemicals that could in turn continue to react
over millions of years reactions might produce all the molecules necessary for life and these might aggregate into primitive protocells
from protocells natural selection would guide the evolution of true, living cells
four conditions necessary for Chemosynthesis
an absence of oxygen in its destructive molecular form
a great deal of time
appropriate energy sources
the right inorganic chemicals
oxygen
