Answer is option A.
The evolution of the modern horse with a single hoof on each limb from a dog-sized ancestral species with multiple digits is an example of macro evolution.
Explanation:
In macroevolution, major evolutionary changes occur over a long time period and as a result, a species changes into another species or splits into two new species. Whales descending from a land mammal, fish descending from invertebrates, etc are examples of macroevolution.
Microevolution occurs within a population, where the evolutionary changes take place over a short period of time. This is due to the change in gene frequency by natural or artificial selection. These changes can be observed between one generation and the next generation. Antibiotic resistance shown by certain bacteria, pesticide resistance shown by certain pests in the crops, herbicide resistance shown by ants and mosquitoes, the size of the sparrow in North America (sparrows in warmer places are normally smaller than that in colder places), etc are examples of microevolution by natural selection. The production of a new breed of dog is an example of microevolution by artificial selection. Both macroevolution and microevolution are based on mutation, gene migration or gene flow, selection and genetic drift.
The types of natural selection: Directional selection, stabilizing selection and disruptive selection. The organisms that are better adapted to their environments are favored by natural selection. Stabilizing selection is the most common form of natural selection that favors intermediate phenotypes (non-extreme trait). For example, very short plants in a plant population may not get enough sunlight and very tall ones may get damaged by winds, so medium-height plants are favored. As a result, the number of very tall and very short plants decreases and the number of medium-height plants increases as they survive better.
Another example is the birth weight of humans. The babies having very high birth weight can cause life-threatening complications during delivery and the survival chance of babies having a very low birth weight is very less. Thus, the birth weight of babies is normally maintained within a medium-range by nature.
In a population, the disruptive selection favors both the extreme phenotypes and they survive better than the intermediate phenotypes. For example, the finches with large and small beaks on the Galapagos Islands (the majority of seeds on the islands were either large or small) survived better than the finches with medium-sized beaks.
The directional selection favors only one extreme phenotype over the other extreme and intermediate phenotypes. The neck length of giraffes is an example, where the giraffes with longer necks are favored (they could reach for more food in taller trees) and survive better than those with short and medium necks.
