Impact of Environment on Loss of Genetic Diversity and Speciation Genetic variation describes naturally occurring genetic differences among individuals of the same species. This variation permits flexibility and survival of a population in the face of changing environmental circumstances. Consequently, genetic variation is often considered an advantage, as it is a form of preparation for the unexpected. But how does genetic variation increase or decrease? And what effect do fluctuations in genetic variation have on populations over time?

1. 1Amjad Khan Afridi, 25th
October, 2020
Impact of Environment on Loss of Genetic Diversity and Speciation
Genetic variation describes naturally occurring genetic differences among individuals of the
same species. This variation permits flexibility and survival of a population in the face of
changing environmental circumstances. Consequently, genetic variation is often considered an
advantage, as it is a form of preparation for the unexpected. But how does genetic variation
increase or decrease? And what effect do fluctuations in genetic variation have on populations
over time?
Mating patterns are important
When a population interbreeds, nonrandom mating can sometimes occur because one
organism chooses to mate with another based on certain traits. In this case, individuals in the
population make specific behavioral choices, and these choices shape the genetic combinations
that appear in successive generations. When this happens, the mating patterns of that population
are no longer random.
Nonrandom mating can occur in two forms, with different consequences. One form of
nonrandom mating is inbreeding, which occurs when individuals with similar genotypes are
more likely to mate with each other rather than with individuals with different genotypes. The
second form of nonrandom mating is called outbreeding, wherein there is an increased
probability that individuals with a particular genotype will mate with individuals of another
particular genotype. Whereas inbreeding can lead to a reduction in genetic variation, outbreeding
can lead to an increase.
Random forces lead to genetic drift
Sometimes, there can be random fluctuations in the numbers of alleles in a population.
These changes in relative allele frequency, called genetic drift, can either increase or decrease
by chance over time.
Typically, genetic drift occurs in small populations, where infrequently-occurring alleles
face a greater chance of being lost. Once it begins, genetic drift will continue until the involved
allele is either lost by a population or is the only allele present at a particular gene locus within a
population. Both possibilities decrease the genetic diversity of a population.
Genetic drift is common after a population experiences a population bottleneck. A population
bottleneck arises when a significant number of individuals in a population die or are otherwise
prevented from breeding, resulting in a drastic decrease in the size of the population. Genetic

2. 2Amjad Khan Afridi, 25th
October, 2020
drift can result in the loss of rare alleles, and can decrease the size of the gene pool. Genetic drift
can also cause a new population to be genetically distinct from its original population, which has
led to the hypothesis that genetic drift plays a role in the evolution of new species.
Distribution
How does the physical distribution of individuals affect a population? A species with a
broad distribution rarely has the same genetic makeup over its entire range. For example,
individuals in a population living at one end of the range may live at a higher altitude and
encounter different climatic conditions than others living at the opposite end at a lower altitude.
What effect does this have? At this more extreme boundary, the relative allele frequency may
differ dramatically from those at the opposite boundary. Distribution is one way that genetic
variation can be preserved in large populations over wide physical ranges, as different forces will
shift relative allele frequencies in different ways at either end.
If the individuals at either end of the range reconnect and continue mating, the resulting
genetic intermixing can contribute to more genetic variation overall. However, if the range
becomes wide enough that interbreeding between opposite ends becomes less and less likely, and
the different forces acting at either end become more and more pronounced, and the individuals
at each end of the population range may eventually become genetically distinct from one
another.
Migration
Migration is the movement of organisms from one location to another. Although it can
occur in cyclical patterns (as it does in birds), migration when used in a population genetics
context often refers to the movement of individuals into or out of a defined population. What
effect does migration have on relative allele frequencies? If the migrating individuals stay and
mate with the destination individuals, they can provide a sudden influx of alleles. After mating is
established between the migrating and destination individuals, the migrating individuals will
contribute gametes carrying alleles that can alter the existing proportion of alleles in the
destination population.
The overall effect
How do populations respond to all these forces? As relative allele frequencies change,
relative genotype frequencies may also change. Each genotype in the population usually has a
different fitness for that particular environment. In other words, some genotypes will be favored,

3. 3Amjad Khan Afridi, 25th
October, 2020
and individuals with those genotypes will continue to reproduce. Other genotypes will not be
favored: individuals with those genotypes will be less likely to reproduce. What type of genotype
would be unfavorable? Unfavorable genotypes take many forms, such as increased risk of
predation, decreased access to mates, or decreased access to resources that maintain health.
Overall, the forces that cause relative allele frequencies to change at the population level can also
influence the selection forces that shape them over successive generations.
For example, if moths with genotype aa migrate into a population composed of AA and
Aa individuals, they will increase the relative allele frequency of a. However, if the aa genotype
has a clear disadvantage to survival (e.g. vulnerability to predation), eventually the changes
brought about by the initial migration will be reversed.
Summary
Genetic variation in a population is derived from a wide assortment of genes and alleles.
The persistence of populations over time through changing environments depends on their
capacity to adapt to shifting external conditions. Sometimes the addition of a new allele to a
population makes it more able to survive; sometimes the addition of a new allele to a population
makes it less able. Still other times, the addition of a new allele to a population has no effect at
all, yet the new allele will persist over generations because its contribution to survival is neutral.