Microevolution and Macroevolution
Microevolution
You already know that evolution is the change in species over time, due to the change of how
often an inherited trait occurs in a population over many generations. Most evolutionary
changes are small and do not lead to the creation of a new species. These small changes are
calledmicroevolution.
An example of microevolution is the evolution of pesticide resistance in mosquitoes. Imag-
ine that you have a pesticide that kills most of the mosquitoes in your state one year. As
a result, the only remaining mosquitoes are the pesticide resistant mosquitoes. When these
mosquitoes reproduce the next year, they produce more mosquitoes with the pesticide resis-
tant trait. This is an example of microevolution because the number of mosquitoes with this
trait changed. However, this evolutionary change did not create a new species of mosquito,
because the pesticide resistant mosquitoes can still reproduce with other mosquitoes if they
were put together.
Macroevolution
Macroevolutionrefers to much bigger evolutionary changes that result in new species.
Macroevolution may happen:
- when many microevolution steps lead to the creation of a new species,
- as a result of a major environmental change, such as volcanic eruptions, earthquakes
or an asteroid hitting Earth, which changes the environment so much that natural
selection leads to large changes in the traits of a species.
After thousands of years of isolation from each other, some of Darwin’s finch population,
which was discussed in theEvolution by Natural Selectionlesson, will not or cannot breed
with other finch populations when they are brought together. Since they do not breed
together, they are classified as separate species.
Genotype or Phenotype?
Natural selection acts on the phenotype - the traits or characteristics - of an individual, not
on the underlying genotype. For many traits, the homozygous genotype,AAfor example,
has the same phenotype as the heterozygousAagenotype. If both anAAandAaindividual
have the same phenotype, the environment cannot distinguish between them. So natural
selection cannot choose a homozygous individual over a heterozygous individual. If homozy-
gous recessiveaaindividuals are selected against, that is they are not well adapted to their