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change in allele frequencies in a population over generations
characters that can be classified on an either-or-basis; determined by a single locus with different alleles that produce distinct phenotypes
heritable variation that varies along a continuum within a population; usually results from the influence of two or more genes on a single phenotypic character
average percent of loci that are heterozygous
differences in the genetic composition of separate populations
a graded change in a character along a geographic axis
What are the forms of genetic variation within a population?
- discrete characters: either or basis on one allele
- quantitative characters: vary along a continuum within a population; more than one gene acting on a single phenotypic character
- gene variability: avg. heterozygosity
- nucleotide variability: DNA sequences of two ind. in a population, avg.ing data
- mutation: ultimate source of new alleles
- sexual reproduction: crossing over, sexual recombination of alleles, independent assortment and fertilization
Explain how mutation is a source of genetic variation
- It can have a significant impact on phenotype, although it isn't very common, becuase they usually occur in somatic cells and they are usually lethal; however, types are point mutations- where one base is switched and may be lethal for the pop. or they may be silent; deletions- in which a fragment of the chromosome is lost resulting in mutations; duplications- fragment is repeated; inversions- it is still in the chromosome but backward; translocations- unequal crossing over (may be beneficial or harmful)
- they can also be neutral leaving genes intact and allowing for different traits that may be beneficial if the environment changes.
Explain how sexual reproduction is a result of genetic variation.
- - crossing over: produces recombinant alleles, giving a combination that may be better suited in an organism
- - independent assortment: shuffling of alleles into gametes provides different combos that may be better for organism
- - random fertilization: more than 223 different unique combos
What does the Hardy-Weinberg principle describe?
a population that is not evolving
What are the five conditions for Hardy-Weinberg equilibrium?
- 1) No mutations: by altering alleles or deleting or duplicating entire genes, they modify the gene pool
- 2) random mating: if individuals mate preferentially within a subset of the pop, like close relatives, random mixing of gametes doesn't occur and genotypic frequencies change
- 3) No natural selection: differences in the survival and reproductive success of individuals carrying different genotypes can alter allele frequencies
- 4) extremely large population size: the smaller the pop., the more likely it is that allele frequencies will fluctuate by chance from one generation to the next (genetic drift)
- 5) no gene flow: by moving alleles into or out of populations, gene flow alters allele frequencies
How does natural selection alter allele frequencies?
natural selection selects against certain alleles by choosing which organisms survive within a population; the survivors produce more of the favorable traits in the environment, while other alleles decrease.
How does genetic drift alter allele frequencies?
- it is the unpredictable fluctuation in allelic frequencies over generations; the smaller the population, the greater the chance for genetic drift
- founder effect: a small amt of individuals become isolated from the larger population and differ from the larger population in its gene pool, which no longer reflects the larger population's gene pool. they establish their own population
- 2) bottleneck effect: a sudden change in hte environment that drastically reduces the size of a population. The few survivors that pass through the bottleneck may have a gene poool that no longer reflects the original population's gene pool. Certain alleles may be over-, under-, or no longer represented in the population
How does gene flow alter allelic frequencies?
it occurs when a pop gains or loses alleles by genetic additions to and/ or subtractions from the population from movement of individuals or gametes; tends to reduce the genetic differences between populations (explain bent grass)
What are the four effects of genetic drift?
- genetic drift:
- - is significant in small populations: chance events can cause an allele to be disproportionately over- or underrepresented in the next generaiton and have a significant effect on small populations
- - can cause allelic frequencies to change at random: an allele may increase one year, and decrease the neaxt; its not predictable
- - can lead to a loss of genetic variation within populations: by causing allele frequencies to fluctuate randomly over time, it can eliminate alleles from a population
- - can cause harmful alleles to become fixed: they can be lost or become fixed by chance
the contribution an individual make sto the gene pool of the next generation, relative to the contributions of other individuals
a form of natural selection in which individuals with certain inherited characteristics are more likely than others to obtain mates
What preserves natural selection from reducing genetic variation by culling all unfavorable genotypes?
- diploidy: because we are diploid, a large amount of genetic variation is hidden in the recessive alleles that we carry; in order to be exposed to natural selection, we must inherit two from our parents
- balancing selection: natural selection maintains two or more forms in a population
- heterozygote advantage: heterozygous individuals at a particular locus have greater fitness than do both kinds of homozygotes
- frequency-dependent selection: fitness of a phenotype declines if it becomes too common in the population
- neutral variation: much of DNA variation has little or no impact on reproductive success, and so natural selection doesnt affect this DNA; confer no selective advantage or disadvantage are are neutral
Why aren't organisms perfect due to natural selection?
- 1) selectin can act only on existing variations: favors only the fittest phenotypes among thouse currently in the population
- 2) evolution is limited by historical constraints: each species has a legacy of descent with modification from ancestral forms; does not scrap ancestral anatomy and build new comple structures; it coops existing structures and adapts them to new situations
- 3) adaptations are often compromises
- 4) chance, natural selection, and environment interact: chance events can affect the subsequent evolutionary history of populations by causing relocation of some animals, causing natural selection to produce adaptions to the environment; and this environment may change unpredictably from year to year
Explain the historical setting that led Darwin to his idea of natural selection?
- 1) aristotle: organisms are fixed and are lcoated on a ladder of increasing complexity, each to an alloted rung: scala naturae
- o Old Testament: match between organism and environment shows the Creator created all organisms perfect
- 2) linnaues: grouped similar species in classified groups, called binomial nomenclature, reflecting their pattern of creation
- 3) Cuvier: studied stratum and discovered that older strata held evidence of different species no longer living; some species appeared and others disappeared; catastrophism
- 4) Hutton: gradual changes in earth
- 5) Lyell: uniformitarianism- earth must be very old
- 6) Lamarck: use and disuse: idea tht parts of hte body that are used extensively become alrger and stronger, while thouse that are not used deteriorate; inheritance of acquired characteristics: modifications passed on; recognized that species evolved, but incorrect about way
characteristics that enhance organisms' ability to survive and reproduce in specific environments
In his book, what did Darwin state?
- descent with modificaiton expalins life's unity and diversity
- natural selection brings about hte match between organism and environment
What are Darwin's four observatiosn?
- members of a population often vary greatly in their traits
- traits are inherited from parents to offspring
- all species are capable of producing more offspring than their environment can support
- owing to lack of food or other resources, many of these offspring do not survive
What are Darwin's inferences?
- individuals whose inherted traits give them a higher probability of surviving and reproducing in a given environment tend ot leave more offspring than other individuals
- this unequal ability of individuals to surve and reproduce will lead to the accumulation of favorable traits int he population over generations
What are the main ideas of natural selection?
- natural selection is a process in which individuals that have certain heritable chanracteristics survive and reproduce at a higher rate than other indivciduals
- over time, natural selection can increase the match between organisms and their environment
- if an environment changes, or if individuals move to a new environment, natural selection may result in adaptation to these new conditions, someitmes giving rise to new species in the process
What are three subtle key points in evolution?
- although it occurs through interactions between individual organisms and their environments, indidividuals do not evolve; the population does
- natural selection can amplify or dimisnish only heritable traits
- environmental factors vary from palce to place and over time
What are the four types of datat that document the pattern of evolution and illuminate the processes by whichit occurs?
- direct obervations of evolution
- fossil record
evolution that resutls in a better match between organimss and their environment