evolution ii - microevolution & natural selection Flashcards
what is the hardy-weinberg equilibrium and what is it used for?
- the hardy-weinberg equilibrium states that the frequencies of alleles and genotypes in a population’s gene pool remain constant from generation to generation, provided that only mendelian segregation and recombination of alleles are at work.
- it describes a gene pool whose various allele frequencies are at equilibrium under certain conditions
- the hardy-weinberg equilibrium is only attainable in non-evolving populations, which is rarely attained in natural populations
- hence, microevolution can be detected by noting deviations from the hardy-weinberg equilibrium of the allele and genotype frequencies in the gene pool of a population
what are the 5 conditions for non-evolving population?
i.e. at hardy-weinberg equilibrium
- the population is large - changes in gene frequencies of large populations are less likely to occur by chance (non-evolving), but in a small population, therandom loss of 1 or more genotypes can eliminate 1 or more alleles from the population (evolving)
- random mating occurs - there is no tendency for certain genotypes to mate with other specific genotypes (non-evolving). if individuals preferentially choose mates with certain genotypes, random mixing of gamete does not occur (evolving)
- no gene flow - net movement of individualsbetween populations must not be extensive enough to change gene frequencies (non-evolving)
- no mutations - mutations, that are often spontaneous, introduce or remove genes from chromosomes, or change 1 allele into another, which modifies the gene pool (evolving)
- no natural selection occurs - all genotypes have equal fitness. all genotypes are equally adaptive and reproduce equally well. the number of offspring must be independent of genotype (non-evolving)
so for microevolution, there will be deviations from the hardy weinberg equilibrium of the allele and genotype frequencies in the gene pool of a population - suggests that one or more of the 5 conditions are not occurring in a population
why is the population the smallest unit that can evolve?
- a population is a group of organisms belonging to the same speceies that live in a defined geographical area and share a common gene pool.
- individual organisms do not evolve. natural selection acts on individual organisms, affecting its survival and reproductive success compared with those of other individuals
- certain alleles may be favoured over otherss, leading to a change in the allele frequencies in the entire population, leading to microevolution
- microevolution is a change in the allele frequencies in the gene pool of a population over generations.
what are the 3 processes that contribute to genetic variation in populations?
- gene mutations that generate new alleles
- crossing over and independent assortment of homologous chromosomes during meiosis, leading to new combinations of alleles
- sexual reproduction, or the random fusion of new gametes
how do mutations, particularly gene mutations, result in genetic variation?
a mutation is a permanent change in the nucleotide sequence of an organism’s DNA
- a mutation immediately brings about changes in the gene pool of a population by substituting one allele for another -> changes the allele frequency in the gene pool of a population
- in multicellular organisms, only mutations in cells that produce gametes can be pased to offspring and affect a population’s genetic variability
- random mutations within pre-existing genes are a source of new alleles that are new heritable variations on which other evolutionary processes can act
- mutations are rare and slow, so they have little effect on the hardy-weinberg equilibrium.
- beneficial mutations are the only source of genetic variation that makes evolution possible
how do mutations, particularly chromosome aberrations, result in genetic variation?
- chromosomal changes that delete, disrupt or rearrange many loci are usually harmful, but when such large-scale changes leave genes intact, they may not affect the organisms’ phenotype
- duplication of genes due to errors in meiosis can be harmful if large chromosome segments are duplicated, but the duplication of smaller pieces of DNA may not be.
- gene duplications that do not have severe effects can persist over generations, allowing mutations to accumulate -> resulting in an expanded genome with new genes that may take on new functions
how does meiosis and sexual reproduction result in genetic variation?
- most genetic variation in sexually reproducing organisms results from the unique combination of alleles that each individual receives from its parents. sexual reproduction “shuffles existing alleles and deals them at random to produce individual genotypes”
- crossing over at prophase I of meiosis, independent assortment of chromosomes at metaphase I of meiosis, and random fertilization of gametes contributes to the shuffling of existing alleles
- in meiosis, homologous chromosomes, 1 inherited from each parent, trade some alleles by crossing over. these homologous chromosomes and the alleles they carry are then distributed at random into gametes. a myriad of possible mating combinations exist in a population, so fertilization brings together gametes that are likely to have different genetic backgrounds
- the combined effects of these 3 mechanisms ensure that sexual reproduction rearranges existing alleles into fresh combinations in each generation, providing much of the genetic variaiton that makes evolution possible
(CT2 content)
define natural selection
natural selection is the process by which certain individuals that are better adapted to an environment survive to reproduce (i.e. differential survival & reproduction).
there is hence reproductive success of fitter individuals over those that are less fit.
it increases the frequency of favourable genotypes/alleles in the gene pool, and the resultant population becomes adapted to its particular environment
the environment acts as the selection pressure to determine the direction and results of selection.
natural selection does NOT cause genetic changes, but rather acts on existing alleles in the population’s gene pool which were generated by mutations prior to the selection event
(CT2 content)
what is fitness?
fitness is the relative reproductive success of individuals, within a population, in leaving offspring in the next generation.
relative fitness refers to the contribution an individual makes to the gene pool of the next generation relative to contributions of other individuals
(CT2 content)
what are the 3 ways natural selection acts on the phenotypes in a population?
- directional selection
- disruptive selection
- stabilising selection
(CT2 content)
what is directional selection?
directional selection favours one extreme of the phenotype range, and shifts the population mean for the selected character.
directional selection acts to eliminate the other extreme phenotype and the alleles promoting this extreme become less frequent in the population. this type of selection usually follows a changing environment
eg: peppered moths (industrial melanism)
(CT2 content)
what is stabilizing selection?
stabilizing selection favours the existing mean and both extremes are selected against/eliminated, leading to a reduction in variance and favouring intermediate phenotypes.
this type of selection is typical of an unchanging environment where competition is not severe.
stabilizing selection operates to prevent deviations away from the middle range of values. it does not change the most common phenotype of the population, but rather makes it even more common by eliminating extremes.
eg sickle cell anemia in areas endemic to malaria
(CT2 content)
what is disruptive/destablizing selection?
disruptive selection has the opposite effect of stabilizing selection; instead of favouring the intermediate phenotypes and eliminating extreme phenotypes, it eliminates intermediate phenotypes and favours extreme phenotypes.
eg black bellied seedcrackers
there is a possibility that the gene pool may become split into 2 distinct gene pools, which may result in 2 distinct populations. under certain conditions over time, the population may eventually form 2 new species
(CT2 content)
why do some phenotypes reproduce more successfully than others?
- successful phenotypes are those that have the best adaptations, which are characteristics that help an individual survive and reproduce in a particular environment
- a major source is natural selection is competition with other organisms for scarce resources. competition is most intense among members of the same species as they frequent the same districts, require the same food and are exposed to the same dangers. no two competing organisms have such similar requirements for survival as do two members of the same species.
- both predators and prey act as agents of selection. the process in which species mutually affect one another’s evolution is called coevolution. when one evolves a new feature or modifies an old one, the other typically evolves new adaptations in response
what is genetic drift?
genetic drift is the random change of allele and genotype frequencies, as a result of chance alone. this can differ from generation to generation in a small gene pool
what are the 4 effects of genetic drift?
1 - genetic drift is significant in small populations. chance events can cause an allele to be disproportionately over- or under-represented in the next generation. although chance events occur in populations of all sizes, they alter allele frequencies substantially only in small populations
2 - genetic drift causes random chance of allele frequencies. because of genetic drift, an allele may increase in frequency one year, then decrease the next. this change from one year to the next is not predictable, causing allele frequencies to change at random over time, unlike in natural seelction where a given environment consistently favours some alleles
3 - genetic drift can lead to a loss of genetic variation within population and creates genetic divergence between populations. by causing allele frequency to fluctuate randomly over time, genetic drift can eliminate alleles from a population. because evolution depends on genetic variation, such losses can influence how effectively a population can adapt to a change in the environment.
4 - genetic drift can cause harmful alleles to become fixed. alleles can be lost or become fixed entirely by chance through genetic drift. alleles fixed by genetic drift are as likely to be harmful to the orgabism as to be beneficial or of no effect. when drift leads to the fixation of harmful alleles, the survival of a small population can be threatened
what are the 2 forms of genetic drift?
- founder effect
- bottleneck effect
what is founder effect?
founder effect occurs when one or few individuals colonize a habitat isolated from their place of origin or new to that species. the alleles they carry will be a significant fraction of the gene pool.
the gounder effect may occur if a few members of a population are blown by a storm to a new island
eg amish people, galapagos finches
what is bottleneck effect?
bottleneck effect occurs when natural disasters, diseases, or predators (by chance) may kill large numbers of individuals, causing drastic short-term reductions of a population size. this will result in the survivors representing a small random portion of the original gene pool -> alleles may be under- or over-represented, or even eliminated.
even when the population increases to its original size, a portion of its genetic diversity remains lost -> leads to reduction and restriction in genetic variability
eg greater prairie chicken
