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Chapter 17: Population Genetics and Evolution (Rates of Evolution (Most…
Chapter 17: Population Genetics and Evolution
Population Genetics
Factors That Cause the Gene Pool To Change
Accidents
Many phenomena qualify for accidents
Infrequent floods
Hailstorms
A volcanic eruption produces poisonous gases and molten rock that destroy everything within a limited area
Droughts
Can all act as accidents for plants too small and delicate to become adapted to those events
Accidents can be small events as well as large ones
Accidents are events to which an organism cannot adapt
Such as the collision of a large meteorite with Earth
Could destroy life in the area
Could destroy all plant life and vegetation
Could destroy a large region of the Earth's surface
Artificial Selection
The most obvious examples are the selective breeding of crop plants and domestic animals
Artificial selection is also used to produce ornamental plants that flower more abundantly or for a longer time
Artificial selection is the process in which humans purposefully change the allele frequency of a gene pool
Has been used to alter flower color and size
Plants are exposed to mutagens
Acridine dyes
Irradiation with UV light or gamma rays
:
Mutation
Because of mutations
And new alleles increase
Existing alleles decrease in frequency
Depends in part on the population's size
All genomes are subjected to mutagenic factors
Alteration in nucleotide sequence
Of the genome of the organism
DNA molecules of the organism
Natural Selection
Described as survival of the fittest
Two conditions must be met before natural selection can occur
The population must produce more offspring that can possibly grow and survive to maturity in that habitat
The second condition is that the progeny must differ from each other in their types of alleles
The most significant factor causing gene pool change
When genetic diversity exists among individuals, differential survival can occur
Populations are always affected by factors other than sexual reproduction
Gene Pool
Ex) Gene A has four alleles
A1
A2
A3
A4
The gene pool contains 10 billion haploid sex cells
Sexual reproduction alone does not change the gene pool of a population
The alleles are probably not in equal numbers
A2 = 20%
A3 = 15%
A1 = 60%
A4 = 5%
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The total number of alleles in all the sex cells of all individuals of a population constitutes the gene pool of the population
Population genetics deals with the abundance of
different alleles with a popluation
And the manner in which the abundance of a particular allele
Decreases
Or remains the same with time
Increases
Rates of Evolution
Few mutations produce a new phenotype so superior that it immediately outcompetes all other members of the population
At the extremes, there are many species of seedless plants that have persisted for tens of millions of years without diverging into new species
It seems that the allelic composition of a population could change rapidly
Within a few generations, but that is not typically the case
Most populations are relatively well adapted to their habitat
It is difficult to identify the presence of particular alleles
In a population unless they result in an easily identifiable effect of the phenotype
Most studies of evolution concern the changes in gross structures such as
Leaves
Fruits
Flowers
Trichomes
But these complex structures are the product of the developmental interaction of many genes
Any new mutation results
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As systems become more intricate, the probability decreases that any random change is beneficial
Evolutionary changes that result in the loss of a structure or metabolism
in a more adaptive structure only if the effects of the new allele morphogenesis without causing serious disruptive effects
If a feature becomes selectively disadvantageous, many of the mutations that disrupt its development become selectively advantageous
Speciation
Divergent Speciation
Biological Reproductive Barriers
Sympatric speciation
Prezygotic Isolation Mechanisms
Any biological phenomenon that prevents successful gene flow
Adaptive Radiation
Species rapidly diverges into many new species over an extremely short time, just a few million years
Founder Individuals
If just one seed is the founder
The original gene pool consists of its two sets of alleles
Adaptive radiation
Can occur in mainland populations
All offspring greatly resemble the first, founder individuals because the initial gene pool is extremely small
Is a special case of divergent speciation
Abiological Reproductive Barriers
Allopatric (Geographic Speciation)
Any physical, nonliving feature that prevents two populations from exchanging genes
Convergent Evolution
As a consequence, the two may evolve to the point that they resemble each other strongly and are said to have undergone convergent evolution
The most striking example is the evolutionary convergence of cacti and euphorbias
If two distinct, unrelated species occupy the same or similar habitats, natural selection may favor the same phenotype in each
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Cacti evolved from leafy trees in the Americas
Cactus spines are modified leaves, whereas euphorbia spines are modified shoots
Phyletic Speciation
Seed Disposal
Vegetative Propagation
Pollen Transfer
This movement of alleles physically through space is called gene flow
