23 Evolutionary Processes Flashcards
Natural selection
individuals with certain phenotypes (heritable traits) survive better and produce more offspring than individuals with other phenotypes; certain alleles associated with favored phenotypes increase in frequency
Genetic variation
(1) the number and relative frequency of alleles present in a particular population (2) the proportion of phenotypic variation in a trait that is due to genetic rather than environmental influences
What are the four modes or patterns of natural selection?
directional selection, stabilizing selection, disruptive selection, balancing selection
Directional selection
mode of natural selection that favours one extreme phenotype with the result that the average phenotype of a population changes in one direction; generally reduces overall genetic variation
What do you call alleles that reach a frequency of 1 and alleles that reach a frequency of 0?
fixed and list
Purifying selection
selection that lowers the frequency of or even eliminates deleterious/disadvantageous alleles
Stabilizing selection
a mode of natural selection that favors phenotypes near the middle of the range of phenotypic variation; reduces overall genetic variation but no change in average value of a trait over time
Disruptive selection
mode of natural selection that favors extreme phenotypes at both ends of the range of phenotypic variation; increases overall genetic variation
How can disruptive selection lead to speciation?
If mating only occurred between those with identical extreme phenotypes, selection would result in two distinct populations and may eventually form two new species
Speciation
the formation of new species
Balancing selection
mode of natural selection where no single allele is favored over time and across locations, on average; an overall balance in terms of fitness and frequency is maintained
When does balancing selection occur?
(1) heterozygote advantage (2) the environment of a population varies over time so the alleles favored by natural selection also varies (3) frequency-dependent selection
Heterozygote advantage
pattern of natural selection that favors heterozygous individuals (fitness advantage) compared with homozygotes; a form of balancing selection
Frequency-dependent selection
pattern of selection where certain alleles are favored when they are rare; a form of balancing selection
Most fundamental attribute of natural selection
it increases fitness and leads to adaption
Genetic drift
any change in allele frequencies due to chance (i.e. blind luck or sampling error); causes allele frequencies to drift up and down randomly over time and can eventually lead to fixation or loss
Small-scale definition of evolution
a change in allele frequencies at a genetic locus in a population from one generation to the next
Three key points about genetic drift
(1) random with respect to fitness (2) most pronounced in small populations (3) can lead to the random loss or fixation of alleles over time
What are the two special cases of genetic drift?
founder effects and bottlenecks
Founder effect
change in allele frequencies that often occurs when a new population is established from a small group of individuals (founder event) due to sampling error (the group is not a representative sample of the source population)
Founder event
when a group of individuals immigrate to a new geographic area and establishes a new population
Genetic bottleneck
reduction in the diversity of alleles in a population resulting from a sudden reduction in the size of that population (population bottleneck) due to a random event
Gene flow
the movement of alleles between populations; occurs when individuals leave one population, join another, and breed
What is the outcome of gene flow?
Gene flow homogenizes (or equalizes) allele frequencies between the source population and the recipient population
What increases genetic diversity? Where do entirely new alleles come from?
Mutation
What ways can mutation occur?
Point mutations, chromosome-level mutations, lateral gene transfer
Deleterious allele
an allele that reduces an individual’s fitness and tends to be eliminated by purifying selection
Beneficial allele
an allele that allows individuals to produce more offspring and should increase in frequency in the population due to natural selection
Neutral allele
an allele with no effect on fitness, which occurs when a point mutation is silent
What is mutation’s role in evolution?
(1) It is the ultimate source of genetic variation (2) If it didn’t occur, evolution would eventually stop (3) Mutation alone is usually inconsequential in changing allele frequencies in a particular gene
What are the three forms of non-random mating?
inbreeding, assortative mating, and sexual selection
Inbreeding
mating between closely related individuals; increases homozygosity of a population and often leads to a decline in the average fitness via selection (inbreeding depression)
Two fundamental points about inbreeding
(1) Inbreeding increases homozygosity (2) Inbreeding only changes genotype frequencies, not allele frequencies, so it is not an evolutionary mechanism
Inbreeding depression
decline in average fitness that takes place when homozygosity increases and heterozygosity decreases in a population due to inbreeding; results from the exposure of deleterious recessive alleles to selection
What are the two causes of inbreeding depression?
(1) Many recessive alleles represent loss-of-function mutations (2) Many genes, especially those involved in fighting disease, are under intense selection for heterozygote advantage, a selection process that favors genetic diversity
Assortative mating
mating that is nonrandom with respect to specific traits
Positive assortment
individuals choose mates that share a particular phenotypic trait with them; like mates with like
Negative assortment or Disassortative mating
individuals choose mates that differ in a specific phenotypic trait; opposites mate
Big-picture definition of evolution
all organisms on earth are related by common ancestry and they have changed over time and continue to change via the major evolutionary forces
Population Genetics
combines genetics and evolutionary biology to make predictions/inferences about populations
Population
a group of interbreeding individuals from the same species, living in the same place at the same time
Gene Pool
pool of all possible genes and their alleles in a population
What assumptions are we aking when predicting offspring genotype frequencies using the HWE model?
(1) random fusion of gametes or random mating so sex doesn’t matter with respect to the gene (3) a very large population so there is no change to allele frequency if someone doesn’t contribute (4) no other changes to allele frequencies between generation or no evolution
How do we know if a population is changing over time?
HWE gives us a baseline or null model that describes the scenario where the population is not changing over time, meaning no evolution is occurring
What are the assumptions of the HWE model that can be violated by evolutionary mechanisms?
(1) No new alleles are generated (2) Population is so large that if someone doesn’t reproduce, it doesn’t affect allele frequencies much (3) No genotype is more successful than the others at surviving and reproducing (4) No new alleles are added by new individuals coming into the population (5) Gametes combine at random in the gene pool to create the next generation
How can you tell if a population is in HWE?
A population with observed genotype frequencies that have the same or very similar expected genotype frequencies
Hardy-Weinberg Equilibrium
A mathematical null model describing genotype frequencies in a population that is not evolving, that is when all gametes go into a single group (gene pool) and combine at random to form offspring
IF the observed genotype frequencies are different from the expected genotype frequencies, does this mean the population has evolved?
It MAY have evolved IF the allele frequencies also changed and that difference persists into the next generation. We can say that it is not in HWE, meaning one of the assumptions is not met.
How can you tell if a population is NOT in HWE?
A population with a 50% or more difference between at least one genotype’s observed frequency and expected frequency.
Mutations are unpredictable (random) with respect to…
an individual’s environment, effect on the individual’s phenotype, effect on the individual’s fitness
Fitness
how successful an individual is at passing alleles to offspring relative to other members of its population i.e. lifetime reproductive success
What is the impact of mutations?
can have a negative, positive, or neutral effect (but very rarely beneficial) and usually minor but it depends on the location and the environmental context
How can you determine the size of the effect of a mutation?
generally, the more base pairs affected by mutation, the larger the effect, the more likely it will be harmful
Prerequisites of natural selection
(1) the trait has to have variation (2) the trait is heritable at least in part (3) fitness has to vary because of the trait
Sexual selection
non-random variation in reproductive success due to variation in mating and/or fertilization success; occurs when individuals within a population differ in their ability to attract mates and favors individuals with heritable traits that enhance their ability to obtain mates
Asymmetry of sex
female fitness is not usually limited by ability to find a mate while male fitness is limited by the number of females they can mate with; features that increase male fitness may decrease fitness of some females, vice versa
Fitness of an individual vs. a trait vs. an allele
Individual: the number of offspring she/he produces in a lifetime; Trait: the number of individuals in the next generation with that trait; Allele: the number of copies of that allele that successfully make it into the next generation
Adaption
a heritable trait that is associated with higher fitness in the current environment; trait is heritable, has a function, and affects fitness
