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IB Biology Topic 10: Macroevolution and Speciation
Distinguish between stabilizing, disruptive, and directional selection in terms of the factors that cause each type of selection and the effect on trait frequencies in a population.
Stabilizing selection has the phenotypic distribution entirely concentrated/clustered in the intermediate phenotypes at the expense of the extreme phenotypes
Because of gradual changes to the environment
High concentration of intermediate phenotypes
Disruptive selection has phenotypic distribution entirely towards one phenotypic extreme
Because of an extreme change in the environment that favors one phenotype
High concentration of just one phenotype
Will often stabilize once an intermediate phenotype is determined
Directional selection has the phenotypic distributed to two extreme phenotypes, ‘two-hump’ graph
Because environment favors two separate phenotypes
High concentration of both extremes at the expense of the intermediate
Define species and gene pool.
Species = a group of organisms that can mate and reproduce and produce viable and fertile offspring
Gene pool = the combination of all alleles of a trait for a species
Explain how low genetic variation/diversity within a population can lead to extinction.
Low genetic variation/diversity implies that if one organism of such a population is vulnerable to a disease, the entire population is also vulnerable. This means that it is easier for such populations to make a disease endemic and can kill them off (seen in cheetahs right now).
Define speciation. Explain how temporal, behavioral, and geographic isolation can lead to speciation.
Speciation is the development of different species
Temporal isolation results in sympatric speciation
Temporal isolation = differences in mating times for two different populations
Results in one organism being able to mate and the other not being able to, which means that it is even more difficult to reproduce, therefore preventing exchange of genes and making the different populations adapt on their own
Behavioral isolation results in sympatric speciation
Behavioral isolation = differences in mating behaviors that for two population
Results in a population not finding the other population’s mating behavior attractive therefore not wanting to mate and preventing reproduction, therefore preventing the exchange of genes over time and causing the two populations to eventually adapt into different species
Geographic isolation results in allopatric isolation
Geographic isolation = when two populations are separated geographically
The separation of the two populations prevents the continuation of reproduction between the two populations and they gradually adapt to their own different environments an eventually grow too far apart to reproduce viable, fertile offspring
Distinguish between allopatric and sympatric speciation.
Allopatric speciation = occurs due to a geographic barrier
Sympatric speciation = occurs w/out a geographic barrier
Distinguish between the following two models of the rate of evolution–gradualism and
Gradualism = when speciation occurs gradually over time
Occurs under gradual changes to the environment
Fossil records show transitional species that prove small changes occur over time, and the changes occur gradually over time
There is also continuous variation seen that proves the small changes actually do occur
Punctuated equilibrium = when speciation occurs in short, rapid bursts with periods of no change in between
Occurs when there are extreme changes in the environment
Fossil records show abrupt changes in a species with periods of ‘extinction’ that are perceived to be periods of stasis under the theory of punctuated evolution
Explain how polyploidy can occur and can lead to reproductive isolation and speciation. Use plants of the Allium genus or vizcacha rats as an example.
Occurs when meiosis does not occur correctly and the chromosomes are unable to be divided into two haploid gametes, and instead produce one diploid gamete by hybridization
The diploid gamete can be combined with a haploid gamete and produce a triploid that is able to reproduce asexually not sexually
This prevents the exchange of genetic information between the triploid and any other populations, reproductive isolation and eventually the triploid develops to be so different from the other population that reproduction of fertile, viable offspring are not possible
The plants of allium genus have developed to be so different from each other that reproduction is no longer possible
The diploid gamete can be combines with another diploid and a tetraploid is produced that can reproduce both sexually and asexaully
