Hardy-Weinberg Equilibrium

by on January 30th, 2011
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The Hardy-Weinberg equilibrium is defined as p 2 +2pq+q 2 = 1 where p and q are allele frequencies, typically ‘p’ is homozygous dominant (AA), pq is heterozygous (Aa) and q is homozygous. This set ratio is said to be passed on from generation to generation (Mertens 1992). (p+q) 2 is the binomial expression of this ideal situation for Mendelian genetics, as it may be expanded to add additional variables (Mertens 1992).

The equilibrium only operates under specific circumstances infrequently achieved in nature, because environmental insults may cause mutations and so on. Given the Hardy-Weinberg equilibrium is only viable under a set of conditions, a few of these conditions were delineated even during the idea’s infancy, per the lectures: lacking mutation or selection with a static population with a stochastic mating process.

It’s of note these conditions to which the theorem does not apply are major drivers of adaptation. Genetic drift, mutation, migration and natural selection are frequent contributors to evolution. The previously described evolutionary catalysts are shown phenotypically in artificial selection and natural selection.

But the side effects of the aforementioned may lead to traits that are either “neutral” (spandrels) or perhaps even maladaptive. Exaptation is defined as when traits are exhibited but have a different effect than their original evolutionary intent. This is common when traits are co-expressed in the genome. When a neutral trait is positively selected for, it said to undergo genetic hitchhiking (Barton 2000).

That is not to say experimental adaptation has not been shown. It has: crabs eat snails without thick shells, so snails are developing thicker shells over the period of only a century.

In accordance with the neutral drift theory, molecular changes are linear without

abject selection pressure (excluding gene transfer) predicated on simple time scale. This theory is controversial, however; and has been modified by its original proponent, Ohta, due

to critique (2002).

Molecular drive is a component of evolution that non-Mendelian genetics, an aspect of

which is gene copying. Beneficial mutations may arise from certain mechanisms such as chromosomal crossover, or the gene will remain static subject to the same deletions and mutations as its copy, becoming redundant (Force et al. 1999). Purifying selection in mosquitos is a common result of molecular drive (Chen et al. 2010).

Consider the interesting statement from Thornhill and Thornhill, 1992 — “…adaptation underlies all human behavior”. This particular example refers to sexual coercion but does not test its hypothesis, because that’s an impossible ethical situation. So how could that possibly be known? Lynch (2007) argues the formation of multi-cellular organisms is not even adaptive. A more likely factor for adaptive radiation is random drift, discussed later (Yi 2006). It may seem like comparing apples and oranges, but psychology, behavior and morphology are all aspects of the Neo-Darwinian synthesis.

Natural selection and artificial selection are two basic types of selection, and the former may be further refined into sexual selection. Sexual selection (increased reproductive capacity of more fit organisms) is described in detail in this lecture, in particular its advantages despite high energy cost. The result of male-male competition (intersexual selection) and the female’s choice of said victor (intrasexual selection) improves survivability of the offspring at the expense of each parent, and is common in sapient beings. The desirable features of a mate have variability based on species, but dominance and likeability is wanted by females while waist-to-hip ratio is sought after by males (Singh and Young 1995) ( Rozmus-Wrzesinska and Pawlowski 2005).

Natural selection is further divided into stabilizing selection, directional selection and disruptive selection. Stabilizing selection may be exhibited in the reproductive success of humans, with average faces allegedly being more attractive than the extremes. Directional selection is bimodal, with average traits being selected against (Campbell et al. 2008). Assortive mating (trait-based mating) as sexual selection may cause organism to speciate sympatrically, albeit slowly (de Cara et al. 2008).

The saga of natural selection continues with genetic change based on somewhat new aspects of the modern synthesis like horizontal gene transfer between species. Gene flow is humans This process is incipient in microbes. Given A and B mutation being equal phenotypically, a rate of expression imbalance will favor the more frequent mutation in a population, called bias (Mendez et al. 2010). Recombination is said to be one of the reasons for sexual reproduction because of its modulating effect on linkage disequilibrium, a rate-limiting step in evolution. Despite the cursory glance at random genetic change described in this paragraph, Lynch describes it as a major — if not the greatest — contributor to evolution as we know it (2004).

Now, onto speciation. In short, the fastest way to speciation is sympatric (geographically uninhibited speciation) in asexual organisms, like autopolyploidy in plants. It does not require external events, but rather a simple mutation. Triticale is an example of hybridization after one generation, called allopolyploidy (Ma and Gustafson 2008). Polyploidy is not a viable route for most animal evolution because of both pre- and post-zygotic barriers to reproduction. Sympatric speciation is uncommon (5%) compared to allopatric (Phillimore et al. 2004).

Peripatric and parapatric speciation are additional routes of speciation less expedient than asexual reproduction. The first is similar allopatry, and often dependent on an ecological

event and small population, and can be subject to the founder effect. This was recently attempted when anole lizards were placed on an island after a hurricane had wiped out the original population (Losos and Miles 2002). Genetic change did occur, but further research indicates the change was similar to a traditional anole lizard population.

Parapatric speciation can occur in as little as 100 generations, according to Gavrilets et al. 1998. This mode of development does not depend on isolation, but instead a freely flowing pass between two regions (or rings) still resulting in a new species. Like almost all of the systems in this paper, refutations to the paradigm have yet to catch up to the dogma (Phillimore et al. 2004). Birds like the gull are cited as parapatric or sympatric speciation in some textbooks, yet exhibit traits more reminiscent of allopatric speciation.

Darwin’s finches exhibited slower speciation via both natural and artificial selection

over a period of 100,000 years. Their geographic isolation is referred to as allopatry.

Speciation tends to occur in situations where a great deal of biodiversity exists, but species

are relatively restrained ecologically. This describes the Galapagos Islands. Isolation —

whether it be climate induced, behavioral isolation, or temporal isolation — are effective at

causing genetic change.


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