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Lecture 20, part 5 of 5
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V. EVOLUTION AND SPECIATION

A. The distinction between Evolution and Speciation

The terms "evolution" and "speciation" are often misused, despite the fact that these terms are critical to our understanding of almost every area of biology. Evolution can be thought of as a broad umbrella term that encompasses all processes by which genetic changes occur at the population level over time. Speciation is one aspect of evolution that is both a result of evolution, and an engine for further evolution.

evolution - A change in allele frequency within a population over time.
This is the definition favoured by population geneticists. In the broad sense, if we define the term "allele" to include any
  heritable genetic change, including mutations in non-coding DNA, and changes in chromosome organization of structure, including translocations, deletions, insertions, inversions, polyploidies and aneuploidies.

speciation - The division of a population into two subpopulations, as a result of heritable reproductive barriers.
Reproductive barriers include any heritable mutation or change in chromosomal structure that would prevent mating or result in infertile offspring produced by a cross between members of the two populations.


Based on what we have discussed in this section, it should be obvious that any mechanism, such as translocations or inversions,  that can interfere with meiotic pairing would establish a powerful reproductive barrier, leading to speciation.

When a mutation such as a translocation occurs, it begins as a single mutational event in a single individual. When this individual mates with so-called "wild type" members of the population, some progeny may carry that mutation. In the second generation, mutant progeny may mate with each other, yielding some progeny that are homozygous for the mutation. In subsequent generations, homozygotes for the mutation will be more fertile, and thus have greater reproductive fitness than hybrid progeny. The result will be two populations between which there is minimal gene flow. As further mutations occur within each population, gene flow between the populations will cease. Once reproductive barriers arise, the two species will accumulate more mutations, thus diverging over time.


B. The Components of Evolution

It is almost as important to explain what evolution is NOT, as it is to define the term.  Evolution is best thought of as the observation that allele frequencies have changed in a population, making no judgement as to the mechanisms of the change. This is a critical nuance, because many people assume that evolution is synonymous with selection. But selection is only one of the processes that can drive evolution ie. that can cause a change in allele frequency.

In this section we will define the mechanisms that can change allele frequencies in a population. These mechanisms are usually categorized to include mutation, migration, selection, random drift, non-random mating, and linkage disequilibrium.  In order to describe these mechanisms, we must first provide a context in which to describe these changes, the Hardy-Weinberg Equilibrium.

1. The Hardy-Weinberg Equilibrium: The baseline against which evolution is measured

Newton's First Law of Motion

Objects in motion remain in motion, and objects at rest remain at rest, unless acted upon by external forces

Hardy-Weinberg law (HW)
 
Allele frequencies in a population will stay the same from one generation to the next, unless other mechanisms, such as mutation, migration, selection, random drift, non-random mating or linkage disequilibrium act to change them.

One might be inclined to dismiss Hardy-Weinberg as being unrealistic and therefore irrelevant, because one or more of these mechanisms will always be acting in any wild population. However, the point of Hardy-Weinberg is to provide the theoretical framework that allows us to measure changes in allele frequency, as a deviation from the ideal behavior expected under HW. That is to say, HW predicts that if
the frequencies of alleles A and a in generation 1 are p1 and q1, then the respective frequencies in generation 2 should be p2 = p1 and q2 =   q1 .  If we observe in generation 2 that p2 = p1 and q2 =   q1, then by definition, evolution has taken place.  Once again, this makes no judgement as to which mechanisms contributed to the change in allele frequencies. Evolution means simply that they have changed.

2. The Components of Evolution

The mechanisms underlying evolution can be broken down into a number of categories, each of which can act independently and simultaneously to influence the transmission of alleles from one generation to the next.

mutation

A mutation is any heritable change in the genome of an organism. This would include point mutations such as transitions and transversions, insertions, deletions, and various sorts of rearrangements such as inversions, duplications or transpositions. Any change in structure or number of a chromosome, such as a translocation or polyploidization, counts as a mutation.

from http://ghr.nlm.nih.gov/handbook/illustrations/chromosomalduplication
migration (gene flow)

Individuals or gametes can flow between populations. In an extreme case, gene flow can introduce a new allele into a population that previously was not found in the population.

Of course, gene flow can only occur when progeny can produce fertile offspring. For example, pollen from polyploid plant fertilize a diploid plant, the hybrid progeny will probably be sterile.

from http://www.slideshare.net/klemmistry101/hardy-weinberg-populations

selection

Selection occurs when progeny exhibiting different phenotypes have different survival rates. Alleles that confer a selective advantage would be said to have a greater fitness than those that confer a selective disadvantage. Some alleles confer neither an advantage nor a disadvantage, and are said to be selectively neutral.

Generally, chromosomal abnormalities, unless in the homozygous state, will result in progeny that do not have balanced numbers of genes at each locus, most likely resulting in a decrease in fitness.

from http://darwin.wcupa.edu/beneski/projects/webplots/applets/HW-model/hw.html
random drift (sampling error)

While it is true that HW predicts that allele frequencies will stay the same from one generation to the next, in practice, they are never precisely the same. For example, if the initial population had a perfect 50:50 mix of two alleles, there are a lot more ways to get a slightly different ratio in the next generation (51:49, 49:51, 52:48 etc.) than there are to get exactly 50:50. Once a difference exists, it can be magnified over time.

Ramdom drift is most important when population bottlenecks occur, such as a limited migration of a small number of individuals to a new habitat. For example, if an initial habitat has both brown and gray mice, and small group of predominantly brown mice move to a new habitat, the brown mice will probably be the only ones found after several generations of random mating. This is not due to any selective advantage of brown over gray. Rather it is due to the luck of the draw.


from http://uvmgg.wikia.com/wiki/Genetic_drift
non-random mating

Anything that biases the choice of mates in sexual reproduction can have a strong effect on gene flow between two subpopulations. Given enough time, this can lead to reproductive barriers that can result in speciation.


displayed from
http://www.phschool.com/science/biology_place/labbench/lab8/images/selmat.gif

linkage disequilibrium (genetic hitchiking)

When an allele confers a strong selective advantage, alleles nearby that are tightly-linked to the first gene will also increase in frequency, even though they themselves confer no selective advantage. This is because in a few generations of intense selection, there is not enough time for significant amounts of crossing over to occur between the selected gene and loci nearby.


from http://openi.nlm.nih.gov/detailedresult.php?img=3530541_pone.0051546.g004&req=4

3. Evolution is the result of the different evolutionary forces acting on the gene pool.

When we observe changes in allele frequencies within a population, we are observing evolution. The results that we observe are most likely due to a number of the components of evolution acting in concert.

To return to our physics analogy, the components of evolution described above can be likened to the components of a vector. In the example at right, the apparent ground speed that we measure for an airplane is the resultant vector whose components are the airspeed of the plane and the velocity of the wind that alters its path.

from http://hyperphysics.phy-astr.gsu.edu/hbase/airpw.html

The most important point to be made is that as we look at the numerous ways in which chromosomes and genomes can change, it is important to consider the implications that these changes have in the evolution of a species, and in the process of speciation.



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previous page PLNT3140 Introductory Cytogenetics
Lecture 20, part 5 of 5
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