4.5: Mechanisms of Evolution and Their Impact on Populations

Mechanisms of Evolution and Their Effect on Populations

Recall from earlier in the unit when you learned that individual organisms do not evolve?  Populations evolve, not individuals.  In order to study evolution, it is important to study changes within populations.  The gene pool of a population consists of all of the alleles of all genes of each individual in that population.  The percentage of each allele in any given gene present in the population determines the genetic characteristics of that population.  For example, the coat colour of the grey wolf in many populations has a grey appearance, however, in very far north populations, a white coat colour predominates. 

5A: Factors that Change Allele Frequencies in Populations

• Mutation

• Gene Flow

• Non-Random Mating

• Genetic Drift

• Natural Selection

Remember that changing frequencies of alleles within populations are small events that can lead to evolution within a population (microevolution).  Allele frequencies are the number of copies of an allele compared to the total number of alleles in the population.   Natural selection is the most significant factor in the formation of new species.  When a new species is formed, it is called "speciation". 

Mutations

A mutation is a change that occurs in the DNA of an individual.  If a mutation is heritable, it has the potential to impact an entire gene pool.  The more genetic variation there is in a population, the greater the diversity of the population and the greater the chance of a selective advantage to some individuals in a changing environment. 

Gene Flow

Gene flow is the net movement of alleles from one population to another as a result of the migration of individuals.  For example, grey wolves have large territories, and a lone grey wolf may travel over 800 km in search of a new territory or breeding partner.  It is common for a grey wolf from one population to mate with a member from a nearby population, bringing new alleles into the gene pool of the nearby population, increasing the genetic diversity.  Increased genetic diversity may help the population survive because natural selection acts on diversity.  

The wolves in the image above are grey wolves.  Grey wolves have large territories, and can travel great distances to find new mating partners, resulting in gene flow between populations.

Non-Random Mating

Non-random mating is mating among individuals on the basis of mate selection for a particular phenotype or due to inbreeding.  In contrast, random mating is much like a draw in which breeding partners are randomly selected by drawing names out of a hat.  If mating is random, there is no way to predict which males will mate with which females, or which females will mate with which males.  The likelihood  of any individual with a specific genotype mating with another individual with a specific genotype depends on the distribution in the population.

Non-Random Mating - Preferred Phenotypes

In animal populations, individuals may choose mates based on their physical and behavioural traits.  Male caribou compete with other males for mates by using their antlers.  This is a form of non-random mating because it prevents individuals with certain phenotypes from breeding.  Only the individuals that mate will contribute to the gene pool of the next generation.

The male caribou in the image above have locked antlers in a fight, to show who is the best.  The winner of the fight will be able to mate with females, and contribute to the gene pool of the next generation.

Non-Random Mating - Inbreeding

Inbreeding occurs when closely related individuals breed together.  An extreme example of this is the self-pollination of some flowers.  Some close relatives share similar genotypes, so inbreeding increases the frequency of homozygous genotypes.  Inbreeding does not directly affect the distribution of alleles, however, it does increase the amount of homozygous genotypes, which makes harmful recessive alleles more likely to be expressed.

The flower above has both male and female sex organs, and is capable of fertilizing it's own eggs.  This is the most extreme form of inbreeding.

The negative impacts of inbreeding are sometimes seen in purebred farm animals and pets.  Purebred animals tend to have a higher incidence of deformities and health problems compared with non-purebred animals.  For some purebred animals, fertility rates are very low, and offspring die at a young age. 

Genetic Drift

There are two mechanisms of genetic drift:

1. The Founder Effect

2. The Bottleneck Effect

In small populations, the frequencies of certain alleles can be changed by chance alone.  This is called genetic drift.  For example, imagine flipping a coin.  Each time you flip the coin, you have a 50:50 chance of the coin landing on heads or tails.  In a large sample size involving 1000 flips of the coin, you would expect the number of heads and tails to be fairly close.  You would not expect widely different results.  However, in a small sample size, like 10 flips of the coin, getting a ratio of 7 heads to 3 tails rather than something closer to the 50:50 ratio would not be surprising. The smaller the sample size, the greater the uncertainty of your results.

The Founder Effect

Often, new populations are formed by only a few individuals, or founders.  For example, b winds may carry a single, pregnant fruit fly to a previously unpopulated island, where the fruit fly and her offspring may form a new colony.  These founders will carry some, but not all of the alleles from their original population's gene pool.  Therefore, the diversity in the new gene pool will be limited.  Furthermore, the founders may not be typical of the population they came from, so previously rare alleles may increase in frequency.  The gene pool change that occurs when a few individuals start a new, isolated population is called the founder effect.  The founder effect occurs frequently on islands, and likely occurred when various plants, animals and insects  first colonized the Hawaiian and Galapagos islands. 

The founder effect also occurs in human populations, and the lack of genetic diversity in these populations can be a medical concern. Due to the founder effect, the incidence of inherited health conditions in these populations is much higher than average. 

The Bottleneck Effect

Starvation, disease, human activities and natural disasters such as extreme weather, can quickly reduce the size of a population.  Since the survivors likely have only a fraction of the alleles that were present before the population declined, the gene pool has lost diversity.  Gene pool change that results from a rapid decrease in population size is known as the bottleneck effect.  

The image above demonstrates the bottleneck effect, showing how a natural disaster can limit a population, and change allele frequencies in the gene pool.

The bottleneck effect is often seen in species who are about to go extinct.  For example, by the 1890's overhunting had reduced the number of northern elephant seals (Mirounga angustirostris) to as few as 20.  Today, there are tens of thousands of these seals.  However, due to the bottleneck effect followed by genetic drift, their genetic diversity is very low. 

The bottleneck effect has put extra pressure on the population of northern elephant seals, pictured above.  Their gene pool is less diverse than before.

Natural Selection

Populations have a wide range of phenotypes and genotypes, and some individuals in a population produce more offspring than others.  Selective forces such as competition and predation affect populations.  As a result, some individuals are more likely to survive and reproduce than others.  If having a single allele gives even a slight, yet consistent selective advantage, the frequency of the allele will increase from one generation to the next, at the expense of less favourable alleles.  There is a greater chance of an individual with the slightly favourable allele surviving, producing and passing on this allele to offspring.  Thus, natural selection causes changes in the allele frequencies of a population, which can lead to evolutionary change.

		1. Stabilizing Selection2. Directional Selection3. Disruptive Selection
Stabilizing selection, pictured above,  favours an intermediate phenotype and acts against extreme phenotypes.  The most common (intermediate) phenotype becomes more common in the population as the extreme phenotypes are reduced or eliminated.  This type of selection reduces variation and improves the adaptation of the population to adapt to relatively constant environmental conditions.	Directional  (above) favours the phenotypes at one extreme over the other.  This type of selection is common during times of environmental change or when a population migrates to a new habitat that has different environmental conditions and niches to exploit.  The changes in the colouration of peppered moths you read about earlier in this unit is an example of directional selection.	Disruptive selection (also referred to as diversifying selection) takes place when the extreme phenotypes are favoured over the intermediate phenotypes.  When this type of selection occurs, intermediate phenotypes can be eliminated from the population.  An example is the extreme size difference in mature coho salmon.  The smallest phenotype averages around 500 g, whereas the larger phenotype averages around 4500 g.  The small salmon specialize in sneaking in to fertilize the eggs of females, while the larger salmon fight for access to a female's eggs.

Sexual Selection

The different phenotypes of the male coho salmon are also an example of another type of natural selection - sexual selection.  Sexual selection involves the choices females make for mates.  Males and females of many species often have different physical characteristics, such as colourful feathers in male birds, or antlers in male deer.  This difference between males and females is called sexual dimorphism.  Courtship displays and other mating behavour are also aspects of sexual selection.

Sexual dimorphism is evident (above) in the difference in plumage of male mallard ducks (top), and female mallard ducks (bottom).