Biology 2 /SL Review Book Topic D.2: Species and Speciation
Name _________________________
Review Book Topic D: Evolution
D.2 Species and Speciation
Key facts
1. The sum total of all alleles and their frequencies within a population constitute the gene pool of a
population of organisms.
2. A large gene pool exists in a population that shows a high variety of traits; a small gene pool exists in
a population where members show little variation.
3. Evolution is a change in the frequency of alleles in a population when measured over many
generations.
4. Allele frequency is expressed as a proportion or percent, but does mean that the same percentage
of individuals in the population have the allele.
5. Allele frequencies may change due to natural selection or to populations mixing due to immigration,
or when part of a population emigrates.
6. The concept of a species cannot be appreciated in a simple definition.
7. Species is made up of organisms which:
a. have similar physiological and morphological characteristics which can be observed and
measured;
b. have the ability to interbreed and produce fertile offspring;
c. are genetically distinct from other species;
d. have a common phylogeny.
8. Speciation is the formation of new species and is initiated when a single gene pool is separated by
various kinds of barrier.
9. Barriers between gene pools include:
a. geographic isolation, e.g., separation on sides of a mountain range, stream, or clearing;
b. temporal isolation, e.g., the development of timing patterns for migration, hibernation, etc.
lead to organisms no longer mating and sharing the same gene pool;
c. behavioral isolation, i.e., development of behaviors which are not favorable to mating
chances and thus a gene pool is no longer shared.
d. hybrid infertility, i.e., offspring of the mating of two closely related species are most often
physiologically incapable of reproduction;
10. Allopatric speciation occurs when two or more subsets of a population are geographically separated.
An example might be sea-level rise that isolates populations on separate shores.
11. Sympatric speciation happens in the same geographic area.
12. Adaptive radiation is a term that describes the development of many species, all from a single
common ancestor (Galapagos finches; lemurs in Madagascar, etc.)
13. Divergent evolution is a form of adaptive radiation where many species share a common ancestor.
14. Convergent evolution occurs when two or more organisms undergo similar natural selection
pressures and thus evolve to become more and more similar to each other. Convergent evolution
will never lead to two species becoming one.
15. Two main theories have been developed to explain the relative pace of evolution:
a. gradualism, i.e., evolutionary changes are small, continuous, and slow.
b. punctuated equilibrium, i.e, evolutionary changes are relatively quick and then followed by
long periods of little or no change.
Biology 2 /SL Review Book Topic D.2: Species and Speciation
Name _________________________
16. Transient polymorphism is a term that describes a population of organisms that has two or more
forms and that change their “fitness” depending on the environmental selective pressures at work
at a given time (e.g., peppered moths).
17. Balanced polymorphism occurs when two or more alleles within a population are not transient or
changing but are stabilized by natural selection.
18. Sickle cell anemia is an example of balanced polymorphism: different forms of hemoglobin are
selected for because if different positive advantages of each hemoglobin form. Each form of
hemoglobin has some positive advantages and some negative aspects and thus each allele type
stays relatively balanced in frequency.
Further notes
Complete the following.
1. Define allele frequency and gene pool.
2. Describe the relationship between evolution, allele frequency, and gene pool.
3. Contrast geographical isolation and temporal isolation.
4. Give two examples of geographical isolation.
Biology 2 /SL Review Book Topic D.2: Species and Speciation
Name _________________________
5. Explain why a zebroid is not a new species.
6. Describe an example of behavioral isolation as a barrier between gene pools.
7. Compare and contrast allopatric speciation and sympatric speciation.
8. What is adaptive radiation?
9. How are lemurs on Madagascar an example of adaptive radiation?
10. Compare and contrast convergent and divergent evolution