English for

p
to indicate the frequency of one and the symbol 
q
to indicate the 
frequency of the other. 
Geneticists were perplexed by the persistence of recessive traits in the early twentieth century because 
they assumed that natural selection replaced recessive or rare s with dominant or common ones. In 1908, an 
English mathematician, G. H. Hardy, and a German physician, Wilhelm Weinberg, worked separately on this 
problem. The 
Hardy-Weinberg principle
, now known as their analysis, specifies the conditions under which 
a population of diploid organisms achieves 
genetic equilibrium
, defined as the point at which neither 
frequencies nor genotype frequencies change in succeeding generations. 
The Hardy-Weinberg principle describes how genotype frequencies in sexually reproducing organisms 
are established
.
 
Figure 68.
Hard-Weinberg Equilibrium
Genetic equilibrium, according to this mathematical model, is only possible if all of the following 
conditions are met: 
1. There are no mutations. 
2. There is no migration from other populations into the population. 
3. There is an infinite population. 
4. In the population, all genotypes survive and reproduce equally well. 
5. Individuals in the population mate at random in terms of genotype. 
If the genotype frequencies of a population do not match the predictions of this model, or if the 
frequencies change over time, microevolution may be taking place. Understanding how and why the gene pool 
is changing begins with determining which of the model's conditions are not met
44
. 
 
4.4 Reading Resource #2
: Macroevolution
 
Macroevolution is concerned with large-scale phenotypic changes in populations that generally 
warrant their inclusion in taxonomic groups at the species level and higher, i.e., new species, genera, families, 
orders, classes, and even phyla, kingdoms, and domains. Macroevolution attempts to explain evolutionary 
novelties, or significant phenotypic changes that occur during the evolution of arthropods, such as the 
44
 
https://www.khanacademy.org/science/ap-biology/natural-selection/hardy-weinberg-equilibrium/a/allele-frequency-
the-gene-pool
 

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development of jointed limbs, as one of its main objectives (crustaceans, insects, and spiders). These 
phenotypic changes are so important that new species exhibiting them are placed in higher taxonomic 
categories or genera. 
Adaptive radiation, when many species appear, and mass extinction, when many species vanish, are 
two examples of major changes in species diversity over time that macroevolutionary studies aim to identify 
and explain. Thus,