GED Science Practice Test: The Role of Genetic Diversity in Evolution

In order for a species to be able to evolve through natural selection, there must be genetic diversity in the population.  Genetic diversity means that not all individuals in a population are the same.  In the case of Darwin’s finches, they had a variety of beak shapes.  A lack of genetic diversity would mean that all individuals in a population were the same, or that there was very little variation. If Darwin’s finches showed a lack of genetic diversity, there would only be one or two beak types.  This was not the case.

Genetic diversity, if you remember from previous lessons, can arise from cellular processes, such as meiosis (both crossing over, and the splitting up of the parent genes into haploid gametes), and genetic mutation.  Each of these processes makes offspring genetically different from their parents.  This creates a population with genetic diversity, in which the individual organisms differ from each other.

Genetic diversity does not just occur at the level of cellular processes, but it can also be affected by changes at the population level.  Random changes to a population are referred to as genetic drift, and include the founder effect, the bottleneck effect, and immigration.  Genetic drift can be a source of decreased or increased genetic diversity in a population.

Founder Effect & Bottleneck Effect:  These two random changes are similar in that a small portion of an existing population separates from the larger, more diverse parent population.  As a result, the newer, smaller population can have different frequencies of alleles (or versions of a gene).  In both the founder effect and the bottleneck effect, a critical event causes this small portion of the population to separate. The founder effect occurs when a small population of existing organisms in a population migrates to a new location.  In the diagram below, you can see that the new population has a high “blue” to “yellow” ratio, while the original population had a more equal mix of blue and yellow.  The bottleneck effect occurs when the original population is reduced drastically in number by a critical event.  While the effects of both effects are the same (new population with potentially different frequencies of alleles), the two effects differ in what happens to the original population.  In the founder effect, the original population continues to exist.  In the bottleneck effect, the original population ceases to exist.  Northern elephant seals experienced a bottleneck effect in the 1890’s, due to hunting.  Some estimates are that the population was reduced to fewer than 100 individual elephant seals.  The Northern elephant seal population has a much lower genetic variability than the Southern elephant seal population for this reason.

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In cases of isolation of the new population, speciation can occur.  Speciation is a process through which species are formed.  In biology, a species is one of the basic units of biological classification, and is often defined as the largest group of organisms capable of interbreeding and producing fertile offspring. At some point in the evolution of such a group, some members may diverge from the main population and evolve into a subspecies, a process that may eventually lead to the formation of a new species if isolation (geographical or ecological) is maintained. There are four types of speciation:

Allopatric Speciation:  Occurs when a physical barrier separates two populations.  The two populations become uniquely suited to their environments.  An example of allopatric speciation is two different squirrel species that live on the northern and southern sides of the Grand Canyon.  The Grand Canyon served as the physical barrier to separate an existing single species of squirrels.

Peripatric Speciation: Also occurs when a physical barrier separates two populations.  However, peripatric speciation differs because one group is substantially smaller than the other, meaning that different genetic characteristics are likely to get passed to offspring.

Parapatric Speciation:  Occurs when a species is spread out over a large area.  Organisms on the far ends of the range do not reproduce with each other, and form separate species over time.  Instead of a physical barrier creating speciation (like in peripatric and allopatric speciation), there are differences in habitat occurring among organisms in the population.

Sympatric Speciation: Occurs when some organisms in close proximity begin to take advantage of a new food source or species when others do not.  The existence of apple maggots is an example of sympatric speciation.  A fly population used to lay eggs in the fruits of hawthorn trees.  When apples were introduced to America, the fly population diverged, with some flies laying eggs in apples, instead.

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Immigration:  Immigration is a random change to a population that is almost the opposite of the founder effect.  Instead of a small population of organisms leaving the original population, a group of organisms comes into an existing population.  This can serve to increase the genetic variability in the existing population.

Genetic diversity allows a population of organisms to better cope with evolutionary pressures.  An evolutionary pressure is a change in the habitat, food source, or other characteristic that causes some organisms of a population to be better suited, and some to be less suited, to survive.  If there are not a variety of organisms in an environment, an evolutionary pressure could eliminate all of the organisms in a population.  A good example of how genetic diversity in a population is necessary for evolution is the evolution of the peppered moth during the industrial revolution.  As the industrial revolution started, more and more pollution darkened buildings and other structures.  This darkening represents the evolutionary pressure to the moth population.  Peppered moths come in dark and light variations, as shown in the photo below:

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Notice that on a black background, the black moth is well-camouflaged, and difficult to see.  The lighter moth is more visible.  So predators of the peppered moth were better able to prey on the lighter colored moths.  Over time, the darker colored moths experienced natural selection, and the population of moths evolved to have greater numbers of dark colored moths.  Without the genetic variability (the moth population included light and dark moths), the population of moths might not have been able to deal with this evolutionary pressure.  If you doubt this, imagine a non-genetically diverse population made up of only light colored moths!

From the reading in this lesson, you may notice that populations can change due to reasons other than natural selection.  For example, populations can change through genetic drift or mutation.  It is important to distinguish changes in populations that happen by chance (founder effect, bottleneck effect, immigration, and genetic drift) from those that happen due to natural selection.  Natural selection is the specific change in a population (in the frequency of its alleles) due to an evolutionary pressure.  Random changes in the population are due to a critical event.

 

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