chapter twenty-three/twenty-four Flashcards
3 major factors that alter allele frequency
- natural selection
- genetic drift
- gene flow
genetic drift
- allele frequencies fluctuate unpredictably/randomly from one gen to next
- reduces genetic variation
- sig in small populations
gene flow
- movement of alleles among populations
- transferred through fertile ind. or gametes
- reduces variation
- decrease/increase fitness
founder effect
few individuals become isolated from larger population
- allele frequencies can be dif
bottleneck effect
sever drop in pop size due to environmental change
- can be affect by genetic drift
what does evolution by natural selection involve?
change and sorting
- new genetic variations arise by chance
- beneficial alleles sorted
what results in adaptive evolution?
natural selection
- increases frequencies of alleles that provide reproductive advantage
- acts on organism’s phenotype
why are the phrases “struggle for existence” and “survival of the fittest” misleading?
- imply direct competition
- reproductive success more subtle
relative fitness
contribution an ind makes to the gene pool of the next generation relative to contributions of other ind
3 modes of selection
- directional
- disruptive
- stabilizing
directional selection
favors one extreme end of phenotypic range
disruptive selection
favors individuals at both ends of phenotypic range
stabilizing selection
favors intermediate variants and acts against extreme phenotypes
what increases the frequencies of alleles that enhance survival and reproduction?
natural selection
what occurs as the match between an organism and its environment
adaptive evolution
why is adaptive evolution a continuous process?
the environment can change
why don’t genetic drift and gene flow lead to adaptive evolution
they can increase/decrease match between organism and its environment
sexual selection
natural selection for mating success
sexual dimorphism
marked differences between the sexes in secondary sexual characteristics
intrasexual selection
competition among ind of one sex (often males) for mates of opposite sex
intersexual selection
- mate choice
- occurs when ind of one sex (usually females) are choosing in selecting mates
how to female preferences evolve?
good genes hypothesis - if trait is related to male health, both male trait and female preference for trait should increase in frequency
neutral variation
genetic variation that doesn’t confer a selective advantage/disadvantage
diploidy
maintains genetic variation in form of recessive alleles
- heterozygotes can carry recessive alleles that are hidden from effects of selection
balancing selection
occurs when natural selection maintains stable frequencies of 2+ phenotypic forms in a pop
- includes heterozygote advantage and frequency-dependent selection
heterozygote advantage
occurs when heterozygous have greater fitness than homozygotes
- natural selection will maintain 2 alleles at that locus
- sickle-cell allele causes mutations in hemoglobin but also confers malaria resistance
frequency-dependent selection
fitness of phenotype declines if it becomes to common in pop
- selection can favor less common phenotype in pop
- equal numbers of right and left mouth fish
why can natural selection not fashion perfect organisms?
- can only act on existing variations
- evolution limited by historical constraints
- adaptations often compromises
- chance, natural selection, and environment interact
what preserves genetic variation?
diploidy and balancing selection
speciation
origin of new species
microevolution
changes in allele frequency in pop over time
macroevolution
broad patterns of evolutionary change above species level
“species” word
Latin word meaning “kind” or “appearance”
what is compared when grouping organisms?
morphology, physiology, biochemistry, DNA sequences
biological species concept
- states species is a group of populations whose members have the potential to interbreed and produce viable/fertile offspring and don’t breed successfully w/ other populations
what holds the phenotype of a population together?
gene flow between populations
reproductive isolation
existence of biological factors/barriers that impeded 2 species from producing viable/fertile offspring
- prezygotic and postzygotic
hybrids
offspring of crosses between dif species
prezygotic barriers
block fertilization from occurring by:
1. impeding dif species from attempting to mate
2. preventing successfully completion of mating
3. hindering fertilization if mating is successful
types of prezygotic barriers
- habitat
- temporal
- behavioral
- mechanical
- gametic
habitat isolation
2 species encounter each other rarely/not at all because they occupy dif habitats
temporal isolation
species that breed at dif times of day, dif seasons, or dif years can’t mix gametes
behavioral isolation
courtship rituals and other behaviors unique to species are effective barriers
mechanical isolation
morphological dif can prevent mating
gametic isolation
sperm of one species may not be able to fertilize eggs of another species
post zygotic barriers
prevent hybrid zygote from developing into a viable, fertile adult
1. reduced hybrid viability
2. reduced hybrid fertility
3. hybrid breakdown
reduced hybrid viability
genes of different parent species may interact and impair hybrid’s development and survival
reduced hybrid fertility
even if hybrids are vigorous, they may be sterile
hybrid breakdown
some first-get hybrids fertile, but when they mate w/ another hybrid or w/ either parent species, offspring of next gen are feeble/sterile
limitations of biological species concept
- can’t be applied to fossils/asexual organisms/prokaryotes
- emphasizes absence of gene flow
can gene flow occur between distinct species?
yes, like grizzly and polar bears
morphological species concept
species defined by body shape and structural features
- applies to sexual/asexual species but relies on subjective criteria
ecological species concept
views species in terms of its ecological niche
- applies to sexual/asexual, emphasizes role of disruptive selection
phylogenetic species concept
defines species as smallest group of ind on phylogenetic tree
- applies to sexual/asexual, but difficult to determine degree of difference required for separate species
