IB Biology Topic 10: Macroevolution and Speciation

1. 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

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1. 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

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3. 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).

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4. 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

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1. Distinguish between allopatric and sympatric speciation.

Allopatric speciation = occurs due to a geographic barrier

Sympatric speciation = occurs w/out a geographic barrier

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6. 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

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1. 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

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