Marjorie Angelica Balaguer
Baccalaureate Biology HL 11-1/2
Option D: Evolution
Species and Speciation I-III (3.18 – 4.14 2010) three note templates in one
Species and Speciation I
Allele frequency: the measure of the proportion of a specific variation of a gene (allele) in a population,
expressed as a fraction, decimal or percentage.
Ex: In a population, 25 individuals are AA, 14 are Aa and 10 are aa. What is the frequency of the two
alleles?
25A, 25A; 14A, 14a; 10a, 10a: total 98
A=64, a=34
A=64/98: 65% frequency
a= 34/98: 35% frequency
Gene pool: all of the genetic information present in the reproducing members of a population at a given
time.
+ State the relationship between evolution and allele frequency
Evolution involves a change in allele frequency in a population’s gene pool over a number of
generations.
+ Explain how the Hardy-Weinberg equation is derived (aka Hardy-Weinberg equilibrium)
**Based on a population that is NOT evolving.
To account for two alleles per genotype this equation is squared: (p + q)2 = (1)2 – Thus yielding the HardyWeinberg equation: p2 +2pq + q2 =1
(p + q)2 = p2 +2pq +q2
p + q = 1 — 100%
If p = frequency of dominant allele
and
If q = frequency of recessive allele
p2 – Dominant homozygous -- AA
q2 – Recessive homozygous -- aa
pq – Dominant heterozygous – Aa
Hardy-Weinberg equation: p2 +2pq +q2 = 1
Example: In a population of pea plants, 25% of the individuals have white flowers (pp). What is the frequency
of P, p – the frequency of the homozygous dominant and heterozygous?
pp – 25%, 0.25 = q2
q = √q2 = √0.25 = 0.5
p=1–q
p = 1 – 0.5 = 0.5
p2 = (0.5)2 = 0.25
2pq = 2 (0.5) (0.5) = 0.5
1.
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Asexual species – do not mate; they are identical to mother
check: q2 + p2 + 2pq = 0.25 + 0.25 + 0.5 = 1 = 100%
Marjorie Angelica Balaguer
Baccalaureate Biology HL 11-1/2
+ State the 5 assumptions made when the Hardy- Weinberg equation is used
**The Hardy Weinberg equation is used with populations that are in equilibrium.
1.
2.
3.
4.
5.
When using the Hardy Weinberg equation it must be assumed that a population is large.
There is random mating
There are no allele-specific mortality (natural selection)
There are no mutations
There is no emigration/immigration
Species and Speciation II
+ Describe three examples of barriers between gene pools
Geographic isolation – alleles from different gene pools do not mix to a physical barrier such as mountains
or a body of water separating two populations.
Temporal (time) isolation – alleles from different gene pools do not mix due to differing periods of activity
(ex: diurnal and nocturnal) between two populations [being active/asleep at different times].
Behavioral isolation – alleles from different gene pools do not mix due to mating behaviors between
members of two populations.
+ Discuss the definition of the term species
Many definitions of “species” exist. The most commonly accepted definition is that which states that a
species is a group of individuals that can mate and reproduce fertile offspring.
There are acceptions to this; asexual1 species are comprised of individuals that do not mate at all;
bacteria of a different species can pass genetic information between each other; different plant species have
produced fertile offspring through inter-species pollination. Other definitions do exist including ones involving
mate recognition, roles played in the ecosystem, DNA sequence similarity, and decent from a common
ancestor.
+ Compare allopathic and sympatric speciation
Sympatric speciation is the formation of two species when populations of the original species are in the
same geographical areas SAME COUNTRY
Allopathic speciation is the formation of two species when populations of the original species are in
different geographical areas. DIFFERENT COUNTRY
1.
Asexual species – do not mate; they are identical to mother
Marjorie Angelica Balaguer
Baccalaureate Biology HL 11-1/2
Haploid: (n) – gametes – one chromosome [NORMAL]
Diploid: (2n) – gametes – two chromosomes [ABNORMAL PASS]
Polyploid: (2 + n) (n) – extra chromosomes (multiple) – plants [ABNORMAL PASS]
*Species 1  2n [normal individual] + 2n [normal individual]
*Species 2  3n [polyploid abnormality] (different species than 1)
+ Explain how polyploidy can contribute to speciation
Due to errors during meiosis, some offspring, particularly among plant species, are polyploid,
having more sets of chromosomes than their parents. In these cases, the offspring can produce fertile
offspring among themselves, but not when crossed with members of the previous (paternal) generation.
Thus the offspring are a different species than the parents. (See above*)
+ Outline the process of adaptive radiation
Adaptive radiation – diversification (both structural and ecological) among the decedents of a single
ancestral group to occupy different niches
Niches – the role of an organism in an ecosystem (ways of life)
Prerequisite (what is needed): When niches are filled, species diverse greatly. Adaptations to different
niches arise in different species.
+ Compare convergent and divergent evolution
Divergent evolution occurs when two similar populations become increasingly different over time.
Divergent evolution is the basis of biodiversity, yielding the cast array of life forms on the planet.
Convergent evolution occurs when two populations become increasingly similar.
Convergent evolution is due to organism in similar environments evolving in reaction the similar selection
pressure.
1.
Asexual species – do not mate; they are identical to mother
Marjorie Angelica Balaguer
Baccalaureate Biology HL 11-1/2
Species and Speciation III
+ Discuss ideas on the pace of evolution, including gradualism and punctuated equilibrium
The rate at which biological evolution occurs has been studies and debated. Darwin and his
contemporaries favored gradualism, a relatively slow and (importantly) steady rate of change. In the
second half of the 20th century the model of punctuated equilibrium was proposed in which long periods of
time without appreciable change and short periods of rapid evolution occur.
+ Describe one example of transient polymorphism (many forms - alleles).
A transient polymorphism is one that involves temporary change. One example of that of the
industrial melanism and the peppered moth in England. Originally the vast majority of peppered moths had
light coloration which effectively camouflaged them against the light colored trees and lichens which they
rested upon. However, due to widespread pollution during the industrial revolution in England, many of the
lichens dried out and the tree which peppered moths rested upon became blackened by soot causing most of the
light colored moths to die off due to predation. Ah the same time the colored or melanic moths flourishing
because of their ability to hide on the darkened trees since then with improved environmental standards, light
colored moths have again become common. Thus the state of dark coloration as the most common phenotype
was transient.
+ Describe sickle-cell anemia as an example of balanced polymorphism.
Balanced polymorphism refers to forms of natural selection that work to stabilize genetic
polymorphisms (multiple alleles) within a population. One ex of this is among populations where the sickle cell
allele and malaria are both found.
*** Homozygous dominant (HbHb) individuals are normal but susceptible to malaria.
*** Homozygous recessive (HbsHbs)
*** Heterozygous (HbHbs)
Thus, there is no advantage to either the Hb or Hbs allele, and both remain at high frequency in these
populations.
Polymorphism: is the condition when two or more formals of either a genre or phonotype are represented in
high enough frequencies to be readily noticeable.
Transient Polymorphism— a phenotype temporarily is most common in a population
Balanced Polymorphism— two alleles at the same frequency in a population
1.
Asexual species – do not mate; they are identical to mother