Ch21 : microevolution: genetic change within populations Flashcards
microevolution
small-scale genetic changes within populations, often in response to shifting environmental circumstances or chance events
population
a group of organisms of the same kind that live together in the same place
phenotypic variation
differences in appearance/ function between individual organisms
quantitative variation
variation that is measure on a continuum rather than categories . Data vary continuously over an range of measurements. Data exists in discrete(5) or continuous form (0.5)
qualitative variation
variation that exists in 2 or more discrete states, with intermediate forms often being absent. Data varies discontinuously
polymorphism
the existence of discrete variants of a character among individuals in a population
gene pool
the sum of all alleles at all gene loci in all individuals in a population
genotype frequency
the percentage of individuals in a population possessing a particular genotype
allele frequency
the abundance of one allele relative to others at the same gene locus in individuals of a population
null model
a conceptual model that predicts what one would see if a particular factor had no effect
Hardy weinberg principle
an evolutionary rule of thumb that specifies the conditions under which a population of diploid organisms achieves genetic equilibrium
genetic equilibrium
the point at which neither the allele frequencies not the genotype frequencies in a population change in succeeding generations
mutation
a spontaneous and heritable change in DNA
gene flow
the transfer of genes from one population to another through the movement of individuals or their gametes
genetic drift
random fluctuations in allele frequencies as a result of chance events; usually reducing genetic variation in a population
founder effect
an evolutionary phenomenon in which a population that was established by just a few colonizing individuals has only a fraction of genetic diversity seen in the population from which it is derived
natural selection
the evolutionary process by which alleles that increase the likelihood of survival and the reproductive output of the individuals that carry them become more common in subsequent generations
relative fitness
the number of surviving offspring that an individual produces compared with the number left by others in the population
directional selection
a type of natural selection in which individuals near one end of the phenotypic spectrum have the highest relative fitness
stabilizing selection
a type of natural selection in which individuals expressing intermediate phenotypes have the highest relative fitness
disruptive selection
a type of natural selection which extreme phenotypes have higher relative fitness than intermediate phenotypes
sexual selection
a form of natural selection established by male competition for access to females and by females’ choice of mates
sexual dimorphism
differences in the size or appearance of males and females
inbreeding
a special (haha) form of non random mating in which genetically related individuals mate with each other
balanced polymorphism
the maintenance of 2 or more phenotypes in fairly stable proportions over many generations
heterozygote advantage
an evolutionary circumstance in which individuals that are heterozygous at a particular locus have higher relative fitness than either homozygote
frequency dependent selection
a form of natural selection in which rare phenotypes have a selective advantage simply because they are rare
neutral variation hypothesis
an evolutionary hypothesis that some variations at gene loci coding for enzymes and other soluble proteins is neither favored nor eliminated by natural selection
adaptive trait
a genetically based characteristic, preserved by natural selection, that increases an organism’s likelihood of survival or its reproductive output
adaptation
characteristic that helps an organism survive longer or reproduce more under a particular set of environmental conditions
within a given population there is phenotypic variation. what is the shape if you had to quantify it (number of individuals on the y axis and the measurement of the trait on the x)?
what does a broad , low curve entail?
what does a high, narrow curve entail?
- bell shaped curve
- broad, low curve= lots of variation among individuals
- narrow, high curve= little variation
example of polymorphism
peppered moth
phenotypic variation within a population is caused by 3 things and ex
- genetic differences between individuals (differences in parts of genotype)
- differences in environmental factors that individuals experience e.g acidity in soil influences flower colour in some plants
- interactions between genetics and the environment
2 main processes that generate genetic variation
- production of new alleles
- arise from small scale mutations - rearrangement of existing alleles into new combinations can result from :
- crossing over between homologous pairs during meiosis
- independent assortment of non-homologous chromosomes during meiosis
- random fertilization between genetically different sperm and eggs
2 of Hardy- Weinberg’s formula
p+q=1
p^2 +2pq +q^2 = 1
genetic equilibrium is possible if and only if all 4 of these conditions are met
- no mutations are occurring
- the population is closed to migration from other populations
- the population is infinitely large
- all genotypes in the population survive and reproduce equally well
what are the 5 agents of microevolution
- mutation
- gene flow
- genetic drift
- natural selection
- non-random mating
4 types of mutation
- deleterious
- lethal
- neutral
- advantage
how does gene flow take place in plants since they are stationary + 2 ex
through dispersal agents e.g the wind , seed-carrying animals. ex: birds carry acorns from one oak population to another
genetic drift reduces variability within populations. Does it have a greater effect on large/ small populations and why?
small populations because only a few individuals contribute to the gene pool and because any given allel may be present in very few individuals
what are 2 circumstances that foster genetic drift
- population bottle necks
2. founder effect
what is a population bottleneck
a drastic reduction in population size due to a stressful factor e.g. disease or a drought. This cause of genetic drift reduces variation and rare genes are inevitably lost
what form of genetic drift do endangered species experience +ex
severe population bottlenecks, which result in loss of genetic variability
e.g cheetahs today are pretty uniform in their genetic make up making them highly susceptible to disease , etc
3 modes of natural selection
- directional
2, stabilizing - disruptive
what is the shape of a disruptive mode of natural selection
bimodal
sexual selection often exaggerates showy features in males examples
brightly coloured feathers, tails, as well as courtship behaviour (behaviour designed to attract females)
sexual selection encompasses 2 related processes
- intersexual selection : selection based on the interations between males and females, e.g. showy structures
- intrasexual selection: based on interactions between members of the same sex, males use for example their large size to intimidate or injure competing males
how can much variation persist under the action of stabilizing selection and genetic drift
3 ways
- diploidy can mask recessive traits from the action of natural selection
- natural selection can maintain balanced polymorphisms
- some genetic variations may be selectively neutral
what is the most probable cause of sexual polymorphism and an ex
cause= sexual selection
e.x. female peacocks have short, dull, brown tails while males have long colourful tails
natural selction can maintain balance polymorphisms through 3 things
- heterozygote advantage
- different alleles are favored in different environments
- frequency dependent selection
example of heterozygote advantage
Hbs (sickle) allele that codes for a defective form of haemoglobin.
Homozygous Hbs-Hbs individuals often die of sickle cell anemia before reproducing,
Heterozygous individuals often survive malaria because the shape of the blood cells offers difficulties for the protist.
heterozygous Hbs-HbA(normal) have higher relative fitness in malaria prone areas than homozygous HbA-HbA. And so the allele is preserved.
example of how different alleles are favored in different environments
European garden snails exist in a range of colours. This polymorphism is conserved by the selection for camouflage in different habitats. Thrushes are visual predators,well camouflaged snails survive. Success varies with habitat e.g. the brown unstripped predominate in woods covered with leaves
an example of frequency dependent selection
the Elderflower orchid exists as yellow and purple flowers. Bumble bees prefer the more rare flowers to pollinate. Therefore the reproductive success of the plant is higher when its colour is rare. When the yellow =rare they get pollinated more and are no longer rare and so then the bumble bees prefer the purple. its continuously shifting.
