Microevolution

Microevolution refers to the gradual changes in a population's gene frequencies from one generation to the next, driven by the fundamental forces of evolution. This process, which occurs below the species level, alters the genetic equilibrium in a Mendelian population and leads to relatively small changes.

1.0What is Microevolution?

As defined by Goldschmidt, microevolution is key to changes in a population's gene pool. These mutations occur within genes. Genetic recombination, or Mendelian recombination, alters the gene frequency within the population's gene pool. Furthermore, genetic drift and natural selection also influence populations by changing gene frequencies, disrupting genetic equilibrium.

2.0Mechanism of Microevolution

Mutation

• DNA sequence changes can be caused by irradiation, transposable elements, mutagenic agents, or errors during replication.
• If an advantageous mutation occurs spontaneously, it can increase in frequency over generations, benefiting the organism.
• Neutral mutations may rise in frequency within a population due to genetic drift.
• Deleterious mutations are typically selected against and generally do not increase in frequency.
• Although recombination during meiosis can shuffle genes into new combinations, mutation is the sole source of entirely new genes.

Migration/Gene Flow

• When a group of individuals moves to a different location and integrates into a new population, it leads to shifts in gene frequencies within both the original and the new population. The migration introduces new genetic variations into the new population while reducing the presence of these variations in the original population. If this migration occurs repeatedly, it results in ongoing gene flow between the populations.

Effects of Migration on Gene Pools 

• Immigration -  Introduces new genes to an established gene pool in plant, animal, or microbial populations, increasing genetic diversity.
• Emigration - Removes genetic material from a population, potentially altering the gene pool.

Non-Migratory Gene Flow

• Gene flow can occur without physical migration. For instance, during the Vietnam War, the local population's genetic composition was altered.
• Barriers to Gene Flow 
• Natural barriers such as mountain ranges, oceans, and deserts.
• Man-made structures like the Great Wall of China.

Genetic Drift (Sewall Wright effect) 

• Genetic drift is a random change of gene/allelic frequencies in a population merely by chance. 
• It operates rapidly with a small population. 
• It is due to habitat fragmentation, isolation, natural calamities, or epidemics. The founder effect and bottleneck effect are two forms of genetic drift. 

Founder effect

• When a section of the population gets isolated or migrated or drifted from the original population, then this section becomes genetically different from the original population due to a change in allelic frequency because the gene pool of this section may contain some alleles in a very low frequency or may lack a few alleles. When the frequency of alleles undergoes significant alteration within a new population sample, it can lead to the emergence of a distinct species. The initial population that underwent this genetic drift then serves as the founders, a phenomenon termed the founder effect.

Bottleneck effect

• Bottlenecks are natural calamities like earthquakes, volcanic eruptions, floods, storms, etc. A rapid shift in the environment can lead to a significant decline in population size, causing genetic divergence from the original population. This can result in varying allele frequencies among survivors, with some becoming more common, others less so, and some disappearing entirely. If a population that has experienced such a bottleneck eventually rebounds in numbers, it may exhibit reduced genetic diversity for an extended period, potentially leading to the emergence of a distinct species.

Natural Selection

• Individuals with more adaptive variations are "better fit" than those with less adaptive variations. Hence, those who are better fit in an environment are selected by nature and leave more progeny than others. Darwin called this natural selection and implied it as a mechanism of evolution. 
• Fitness results from the ability to adapt and get selected by nature. 
• Fitness, according to Darwin, ultimately refers only to reproductive fitness. 
• It is observed that all adult individuals of a population don't have equal chances of mating; females prefer some males with better phenotypes. This is called Sexual selection. 

Microevolutionary Forces

• Microevolutionary forces acting over short periods lead to sequential evolution. However, when these forces continue over many generations, they can result in the emergence of new populations from existing ones. The formation of these new populations can occur in two distinct ways-

1. Successional microevolution—Successional microevolution refers to evolutionary changes within a single population that result in new variants replacing pre existing ones over time. This process can be observed in successive layers of the fossil record. It often leads to the formation of clines, where a population's traits gradually change across its geographical range. Clines illustrate how populations adapt to gradual environmental changes, such as climate changes.
2. Divergent microevolution—Divergent microevolution divides a parent population into two or more distinct populations, each exhibiting genetic differences. Additionally, isolation is crucial in promoting genetic divergence within these related populations.

3.0Examples of Microevolution

The size of a sparrow

• House sparrows were introduced to North America in 1852. Since then, they have developed distinct characteristics in various locations. 
• Sparrow populations in northern regions tend to have larger bodies than those in the south. 
• This divergence is partly due to natural selection, as larger-bodied birds can survive lower temperatures than their smaller-bodied counterparts. 
• The colder climate in the north likely favors larger birds, so sparrows in cold areas are generally larger than those in warmer regions.

Drug resistance

• Drugs that eliminate pathogens become ineffective over time because individuals of pathogenic species that can tolerate them survive and flourish to produce tolerant/resistant populations.
• The widespread application of herbicides, pesticides, and similar chemicals has accelerated the emergence of resistant strains at a much faster pace. This phenomenon is not limited to plants and pests but extends to microbes targeted by antibiotics and eukaryotic cells targeted by drugs. Consequently, resistant organisms develop within months or years rather than over centuries, showcasing how human activities drive evolutionary change. This underscores that evolution isn't guided by a predetermined path; rather, it's a stochastic process shaped by random mutations and natural chance events.

4.0Macroevolution Definition

Macroevolution, also called adaptive radiation, is the evolutionary process that creates new adaptive types through population fragmentation and genetic divergence. This process occurs above the species level, dividing a species' population into multiple subgroups, each undergoing changes in specific adaptive directions. These directional changes are known as adaptive trends, and the overall phenomenon is termed adaptive radiation or macroevolution. Macroevolution is synonymous with adaptive radiation.

5.0Mechanism of Macroevolution

• Macroevolution operates above the species level, forming new genera, families, and orders. This process is driven by significant mutations, referred to as macroevolutions or systematic mutations by Goldschmidt. Macroevolution occurs within groups of individuals that have entered a new adaptive zone with little to no competition. The mechanism can be outlined as follows:
• The population is small in a new adaptive zone, and abundant opportunities exist to explore new habitats. This results in minimal intraspecific competition. Additionally, the new zone is typically free from predators. Consequently, the newly arrived population disperses into all available habitats within the adaptive zone and begins to adapt to the varying conditions and needs of these environments. 
• As a result, the original population splits into several subpopulations, each accumulating mutations and evolving independently but concurrently in different directions. Due to the diverse environmental conditions, natural selection pressures vary, leading to different adaptive modifications in each subpopulation. These modifications accumulate over time, resulting in directional evolution within each group.

6.0Mega Evolution Definition

• Mega evolution is a phenomenon in which new forms of biological organization emerge from existing ones, creating novel classes, groups, or phyla. 
• These transformative shifts are rare occurrences in the history of evolution, happening only sporadically. What's particularly fascinating is that these new biological structures persist over time without facing extinction. 
• Across microorganisms, plants, and animals, we see evidence of mega-evolution giving rise to distinct phyla and most classes. 
• Examples like the transition from fishes to amphibians, amphibians to reptiles, and reptiles to birds and mammals showcase the profound impact of mega evolution on shaping the diversity of life on Earth.

7.0Similarities between Microevolution and Macroevolution

• Embracing novel general adaptations to thrive in a fresh adaptive environment.
• Expanding into uncharted territories or niches within the newly adapted ecosystem through the evolution of specialized traits.
• Diminished evolutionary versatility leading to heightened specialization tailored to specific ecological subzones.
• Reoccupying zones and subzones were left partially vacant due to the specialized nature of the original inhabitants.
• Mega evolution paves the way for subsequent macroevolutionary changes.

8.0Solved Questions

Q.1: What are the unique features of mega evolution?

Answer:

1. Mega evolution encompasses the exploration and experimentation of a new environment by ancestral members, leading to the emergence of novel traits potentially suited for the new habitat.

2. Mega evolution targets individuals with pre-existing adaptations that offer some suitability for the new environment.

3. A group of pre-adapted individuals successfully breaches the ecological barrier, allowing entrance into the new habitat.

4. The transition and adaptation occur swiftly, as prolonged delays result in significant negative selection pressures, leading to failure.

Q.2: Explain the features of macroevolution.

Answer:

1. Large-scale evolutionary changes arise from significant mutations, known as macromutations.

2. Macroevolution occurs within populations transitioning into or occupying a novel adaptive niche.

3. Macroevolution leads to the emergence of evolutionary divergence, where the original population branches into multiple distinct descendant populations by acquiring unique adaptations.

4. Macroevolution gives rise to similar specialized adaptations across diverging lineages.

Q.3 Discuss the differences between microevolution and macroevolution.

Answer: