AP biology: 23 The evolution of populations Flashcards
Natural selection acts on……..
individuals but populations evolve
generic variations in populations….
contribute to evolution
Microevolution
Is change in the genetic makeup of a population from generation to generation
Population genetics
Study of changes within gene pool/ allele distribution over time
Reconciled Darwin’s and Mendel’s ideas
A population
Is a localized group of individuals that are capable of interbreeding and producing fertile offspring
The gene pool
Is the total of genes in a population at any one time ( all alleles for that trait)
Consists of all gene loci in all individuals of the population
The Hardy-Weinberg theorem
Describes a population that is not evolving
Frequencies of alleles and genotypes in a population’s gene pool stay the same over time.
Assumes that segregation and recombination of alleles are at work
Mendelian inheritance
Preserves genetic variation in a population
Hardy-Weinberg equilibrium
Describes a population where allele frequencies do not change
The Hardy-Weinberg theorem
Describes a hypothetical population
In real populations
Allele and genotype frequencies do change over time
If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then
p2 + 2pq + q2 = 1
And p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype
The five conditions for non-evolving populations
are rarely met in nature Extremely large population size No gene flow No mutations Random mating No natural selection
We can use the Hardy-Weinberg equation
To estimate the percentage of the human population carrying the allele for an inherited disease
Mutation and sexual recombination
produce the variation that makes evolution possible
Two processes
mutation and sexual recombination
Produce the variation in gene pools that contributes to differences among individuals
A point mutation
Is a change in one base in a gene
Can have a significant impact on phenotype
Is usually harmless, but may have an adaptive impact
Chromosomal mutations that affect many loci
Are almost certain to be harmful
May be neutral and even beneficial
Mutation rates
Tend to be low in animals and plants
Average about one mutation in every 100,000 genes per generation
Are more rapid in microorganisms
In sexually reproducing populations
sexual recombination
Is far more important than mutation in producing the genetic differences that make adaptation possible
Three major factors alter allele frequencies and bring about most evolutionary change
Natural selection
Genetic drift
Gene flow
Differential success in reproduction
Results in certain alleles being passed to the next generation in greater proportions
Genetic Drift
Due to random changes
Statistically, the smaller a sample
The greater the chance of deviation from a predicted result
Genetic drift Describes
how allele frequencies can fluctuate unpredictably from one generation to the next
Tends to reduce genetic variation
In the bottleneck effect
A sudden change in the environment may drastically reduce the size of a population
The gene pool may no longer be reflective of the original population’s gene pool
Understanding the bottleneck effect
Can increase understanding of how human activity affects other species
The founder effect
Occurs when a few individuals become isolated from a larger population
Can affect allele frequencies in a population
Hemophilia is a result of this ( son of Czar Nicholas)
Gene flow
Causes a population to gain or lose alleles
Results from the movement of fertile individuals or gametes ( Migration)
Tends to reduce differences between populations over time
Natural selection
Accumulates and maintains favorable genotypes in a population
Genetic variation
Occurs in individuals in populations of all species
Is not always heritable
Phenotypic polymorphism
Two or more distinct forms for a character are present in high enough frequencies to be readily noticeable
Genetic polymorphisms
Are the heritable components of characters that occur along a continuum in a population
Population geneticists
Measure the number of polymorphisms in a population by determining the amount of heterozygosity at the gene level and the molecular level
Average heterozygosity
Measures the average percent of loci that are heterozygous in a population
Most species exhibit
geographic variation
Differences between gene pools of separate populations or population subgroups
Some examples of geographic variation occur as a
cline, which is a graded change in a trait along a geographic axis
Natural selection increases the
frequencies of certain genotypes, fitting organisms to their environment over generations
Fitness= baby making potential
Depends on genetic makeup
The phrases “struggle for existence” and “survival of the fittest”
Are commonly used to describe natural selection
Can be misleading
Reproductive success
Is generally more subtle and depends on many factors
Selection
Favors certain genotypes by acting on the phenotypes of certain organisms Three modes of selection are Directional Disruptive Stabilizing
Directional selection
Favors individuals at one end of the phenotypic range
Disruptive selection
Favors individuals at both extremes of the phenotypic range
Stabilizing selection
Favors intermediate variants and acts against extreme phenotypes
Various mechanisms help to preserve
genetic variation in a population
Diploidy
Maintains genetic variation in the form of hidden recessive alleles
Balancing selection
Occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population
Leads to a state called balanced polymorphism
Heterozygote Advantage
Some individuals who are heterozygous at a particular locus
Have greater fitness than homozygotes
The sickle-cell allele
Causes mutations in hemoglobin but also confers malaria resistance
Exemplifies the heterozygote advantage
In frequency-dependent selection
The fitness of any morph declines if it becomes too common in the population
Sexual selection
Is natural selection for mating success
Can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics
Intrasexual selection
Is a direct competition among individuals of one sex for mates of the opposite sex
Intersexual selection
Occurs
when individuals of one sex (usually females) are choosy in selecting their mates from individuals of the other sex
May depend on the showiness of the male’s appearance
Sexual reproduction
Produces
fewer reproductive offspring than asexual reproduction, a so-called reproductive handicap
If sexual reproduction is a handicap, why has it persisted?
It produces genetic variation that may aid in disease resistance
Why Natural Selection Cannot Fashion Perfect Organisms
Evolution is limited by historical constraints
Adaptations are often compromises
