Evolution of Populations

What is the Definition of Evolution of a Population?

A population is a group of individuals of the same species who inhabit the same area and can reproduce with one another to produce viable offspring. The evolution of a population is defined as follows:

The smallest scale of evolutionary change occurs as a change in allele frequencies in a population over generations. This is called microevolution. Evolutionary processes can also give rise to new species and higher taxonomic groups. This is called macroevolution.

Figure 1 below distinguishes between micro- and macroevolution.

What are Alleles? What is Genetic Variation?

A prerequisite for evolution on any scale is Genetic Variation, or the differences in the genetic composition of individuals in a population.

Each living individual contains DNA that is made up of Genes. A gene is a DNA segment that codes for a specific trait. A trait can be monomorphic or polymorphic:

• A monomorphic trait has only one form. This means that the base pair sequence for this trait is always the same.

• A polymorphic trait has multiple forms. This means that this trait has multiple versions of base pair sequences.

Variations in a gene are called Alleles.

Phenotypic traits can be discrete or continuous:

• Discrete traits occur in an “either-or” basis. Such traits are usually determined by a single gene locus (or location), with different alleles coding for distinct phenotypes.

  • For example, a person can have alleles for left-handedness or right-handedness.

• Continuous traits occur as gradations in a continuum. Such traits are usually determined by two or more genes.

  • Examples of continuous traits include human height and horse coat color.

An individual with two different alleles for a given locus is referred to as heterozygous, whereas an individual with two identical alleles for that locus is referred to as homozygous.

All individuals in a population are homozygous for an allele if there is only one allele for a given locus in that population, in which case the allele is said to be fixed in the gene pool. Individuals can, however, be homozygous or heterozygous if there are two or more alleles present for a certain locus in a population.

Each allele has a specific frequency in a population. The collection of all alleles in a population is referred to as its gene pool.

Biologists do not just study the allele frequencies of a population, but also the frequencies of the resulting genotypes, which is referred to as the population's genetic structure. By identifying the genetic structure of a population, biologists can surmise the distribution of phenotypes.

What are the Types of Evolution in Population?

The present environmental conditions determine whether a trait is advantageous or not. Such factors affecting survival and Reproduction are known as selective pressures.

Because a population can be geographically separated and subjected to different selective pressures, the two Populations can evolve in different directions, resulting in two genetically-distinct Populations. Over time, this can lead to Speciation, or the emergence of new species. This pattern of evolution is called divergent evolution.

Divergent evolution can be observed in the way reproductive organs vary among flowering Plants: while some species share basic anatomies, they have different forms because they adapt to different environments and ecological relationships.

On the other hand, organisms that are not closely related to each other can have similar phenotypic traits. These similarities emerge because they are exposed to similar selective pressures, a pattern of evolution called convergent evolution. It can be observed in birds, bats, and flying insects, which all have wings despite not being closely related.

How do we know if a Population is Evolving?

If allele and genotype frequencies remain constant from one generation to the next, it is said that the population is in a Hardy-Weinberg equilibrium, a state in which a population is not evolving.

There are five conditions that a population must meet in order to be in a Hardy-Weinberg equilibrium:

1. No mutations

2. Random mating

3. No Natural Selection

4. Extremely large Population Size

5. No Gene Flow

It is rare for all of these conditions to be met. As such, allele and genotype frequencies are usually constantly fluctuating.

What are the Mechanisms of Evolution in Populations?

The violation of any of the five conditions required for Hardy-Weinberg equilibrium can cause allele or genotype frequencies to change, driving the evolution of a population. These deviations are mutations, non-random mating, natural selection, Genetic Drift, and Gene Flow. We'll briefly discuss the first two. Later, we will delve into the last three, which are major causes of evolution in populations.

Mutations

New alleles can be formed as a result of mutation, which is a random change in the nucleotide sequence in an organism’s DNA. Mutations can positively or negatively impact the survival or reproductive success of an organism, while other mutations can have no impact at all.

Harmful mutations are selected against and are quickly eliminated from the population. On the other hand, Beneficial Mutations tend to increase in frequency over time. Neutral mutations can linger because they are unaffected by natural selection. The accumulation of mutations over time causes populations to evolve.

However, mutations are rare so a change from one generation to the next is likely to be very small.

Non-random mating

If individuals mate within a specific subset of the population, homozygous and heterozygous genotype frequencies change.

Non-random mating can occur by mate choice; for example, female peahens tend to choose mates with larger, more colorful tails. There are also species whose individuals prefer to mate with partners with phenotypic similarities to themselves, a form of mate choice called assortative mating.

• Physical location can also cause non-random mating because members may not have equal access to one another.

While non-random mating can affect genotype frequencies, it does not have a direct effect on allele frequencies.

The three major causes of evolution of a population

Because new mutations are relatively rare and non-random mating does not have a direct effect on allele frequencies, violations of the remaining three conditions are considered to cause most Evolutionary Changes.

Natural selection

When individuals of a population have different heritable traits, those whose traits confer better survival and reproductive success tend to produce more offspring. This means that beneficial traits will be passed on to the next generation at a higher rate than traits that are harmful.

In other words, beneficial traits are selected for, while harmful traits are selected against. By favoring beneficial alleles, natural selection can lead to adaptive evolution—a process in which features that improve survival or reproductive success tend to become more prevalent over time.

Genetic Drift

Genetic Drift is the term for the phenomenon where the gene pool and genetic structure of a population change randomly from generation to generation as a result of chance occurrences, particularly in small populations.

Genetic drift typically occurs as a result of founder or bottleneck effect.

• Founder effect: a small number of individuals that are separated from a larger population may go on to create a new population with a gene pool that is distinct from the original population. Let's say a few individuals from a population were blown by a storm to an unpopulated island. Because the storm displaces only a few individuals (and their alleles) from the main population, the gene pool of the new population will be represented only by the genes and alleles of the “founders”.

• Bottleneck effect: a population may be substantially reduced as a result of an abrupt environmental event like a fire or flood. The bottleneck effect can be brought on by a sharp decline in Population Size. By pure chance, certain alleles may be overrepresented, others underrepresented, and others may even be completely missing among the survivors.

Genetic drift can result in a loss of alleles (and therefore Genetic Variation) within a population, which can then influence how effectively a population adapts to changes in the environment. It can also cause harmful alleles to become fixed (or reach a frequency of 100%) in very small populations. Some alleles can also completely replace other alleles over time.

As such, even without external selective forces, genetic drift can cause a population to become two divergent populations.

Gene flow

Gene flow–the transfer of alleles from one population to another due to the movement of fertile individuals or their gametes–can also change allele frequencies.

For example, a population that migrates into a different area and mates with individuals from the local population introduces their alleles into the local population.

By transferring alleles between populations, gene flow can reduce genetic differences between the two populations, and if extensive enough, can lead to the combination of the two into a single population with a shared gene pool.

Does the Environment have an Impact on the Evolution of a Population?

In addition to the mechanisms stated earlier, environmental variance can also be an evolutionary force. Environmental conditions have an influence on phenotypes such as coloration and even sex.

Geographic separation can also lead to phenotypic variation between populations. Gradual differences among populations of a species can be observed across an ecological gradient; such geographic variation is called a cline.

For example, the blooming period of flowering Plants can differ according to their location along the slope of a mountain; this is called an altitudinal cline (Fig. 2).

If there is gene flow between populations, individuals from both populations will likely show gradual phenotypic changes along the cline. On the contrary, if gene flow is restricted, individuals may exhibit more drastic changes and even result in Speciation.

What are Examples of Evolution of a Population?

Let’s look at examples of evolution of population in humans and fruit flies.

Evolution of a human population

In 1814, fifteen British colonists established a settlement on Tristan da Cunha, a cluster of small islands located halfway between Africa and South America. Turns out, one of the colonists had a recessive allele for retinitis pigmentosa, a type of blindness that affects homozygous individuals.

By the late-1960s, the fifteen colonists had 240 descendants, four of which had retinitis pigmentosa. In this example, we can see how the founder effect led to a higher frequency of a disease-causing allele on Tristan da Cunha compared to the populations where the founders originated from.

Evolution of a fruit fly population

In the early 1930s, fruit flies collected from the wild had an allele frequency of 0% for an allele that confers resistance to insecticides like DDT. However, in flies collected after 1960–after over 20 years of DDT use–the allele frequency rose to 37%.

This allele either developed between 1930 and 1960 as a result of mutation or was present in 1930 but was extremely uncommon. In either scenario, the increased frequency of this gene was most likely caused by DDT, a powerful poison that exerts significant selection pressure on fly populations exposed to it.