evolution Flashcards
evolution
change in the genetic composition of a population during successive generations, which may result in the development of new species
microevolution
small-scale allele frequency variation within a species, where the descendant is in the same taxonomic group to the ancestor
e.g. antibiotic-resistance bacteria
macroevolution
allele frequency variation at or above species level over time, resulting in the divergence of taxonomic groups, where the descendant is in a different taxonomic group to the ancestor
e.g. African cichlids
evolutionary radiation
increase in taxonomic diversity through speciation
(rapid or slow)
mass extinction
loss of ≥75% of all species
(rapid)
natural selection
variation between offspring; heritable traits; more offspring produced than can survive
individuals best suited to the environmental selection pressures have greater viability (survivability) & fecundity (fertility)
allele frequency expression
[2(# of homozygotes) + (# of heterzygotes)] ÷ 2(# of individuals)
positive & negative selection pressures of allele frequency
positive : favours a particular allele; increasing its frequency
negative : opposes a particular allele; decreasing its frequency
gene flow
exchange of alleles between populations from migrating individuals who then reproduce
new alleles change the genetic composition (gene pool) of the population
genetic drift
change in allele frequency of a population over time due to random events
greater impact on small, isolated populations
e.g. bottleneck & founder effect
bottleneck effect
random natural or anthropogenic event reduces the population size & gene pool
∴ ↓allele frequency & ↓genetic diversity
why are small populations with reduced genetic diversity (i.e. those affected by the bottleneck effect) more susceptible to genetic drift & face an increased risk of extinction ?
low genetic diversity → decreased fitness (viability & fecundity)
low genetic diversity → decreased genetic ability to evolve
small gene pool & low gene flow cannot buffer against these chance events
(less resilient & less adaptable)
founder effect
small group of individuals from a population separate & colonize new territory
∴ ↓allele frequency & ↓genetic diversity
(new, small & less diverse population)
stabilizing selection
intermediate phenotype is favoured & extreme phenotypes are selected against
occurs from stable environments
directional selection
one extreme phenotype is favoured & other extreme phenotype is selected against
population distribution shifts towards the former
occurs from gradual or sustained changes in environmental conditions
disruptive selection
both extreme phenotypes are favoured & intermediate phenotype is selected against
occurs from fluctuating environmental conditions (e.g. seasonal changes)
bimodal spread may lead to reproductive isolation & speciation
divergent evolution
two species evolve from a common ancestor
homologous structures (derived from a common ancestor; evolved to have different functions)
∵ different selective environmental pressures
e.g. Darwin’s finches
convergent evolution
two unrelated species independently evolve to become similar
analogous structures (evolved to have similar functions; derived from unrelated ancestors)
∵ similar selective environmental pressures
e.g. sharks & dolphins
parallel evolution
two species with a common ancestor independently evolve to become similar
∵ similar selective environmental pressures post separation
e.g. polydactyly chickens
co-evolution
two species evolve by exerting selection pressures on each other
e.g. lions & zebras
speciation
(macroevolutionary changes)
accumulation of microevolutionary changes over time
∵ mutation, gene flow, genetic drift
allopatric speciation
reproductive isolation (no gene flow) caused by geographical isolation (physical barrier)
sympatric speciation
reproductive isolation (non-random mating) caused by temporal, mechanical or behavioral isolation (genetic polymorphism)
(continuous but reduced gene flow)
parapatric speciation
reproductive isolation (non-random mating) caused by ecological isolation (separate niches)
(continuous but reduced gene flow)
reproductive isolation
barriers that prevent gene flow (interbreeding) between individuals leading to speciation
