MCAT Content / Evolution / Speciation


Topic: Evolution

Speciation is the formation of a new species; two populations of organisms are considered different species when they can no longer interbreed and produce viable offspring. Speciation generally happens when two populations become very genetically distinct from one another. Events which can affect the genetic diversity of a population include inbreeding, outbreeding, and genetic drift. 

Speciation occurs when some kind of barrier prevents successful interbreeding between the members of a species.  Allopatric speciation occurs when the reproductive barrier is geographic – it is literally the geographic separation of a population from their parent species for a long enough period of time that they become genetically distinct. Sympatric speciation is speciation that occurs when two populations become genetically distinct when living in the same location, generally because they have adapted to different elements of the same location or because they have developed unique mating behaviors.

Organisms who become adapted to different environments may undergo allopatric speciation. Adaptation is the process by which a population gradually develops traits that are better suited to a particular habitat.

Adaptation As the environment changes, individuals adapt to their surroundings to survive.


The genetic diversity of a population can be affected by many processes, including mutations, inbreeding, outbreeding, and genetic drift.

Inbreeding is the mating of closely related members of a species, which has the undesirable effect of increasing the frequency of harmful conditions or diseases in a population. Inbreeding decreases the genetic diversity of a population by increasing the proportion of individuals with homozygous genotypes and reducing the proportion of individuals with heterozygous genotypes, inadvertently increasing the frequency of harmful homozygous recessive phenotypes. Outbreeding, the mating of individuals of a species with distantly related individuals, has the opposite effects and increases genetic diversity. A population with greater genetic diversity has increased reproductive fitness, therefore, outbreeding tends to promote reproductive fitness.

Genetic drift is a random change in the diversity and/or number of alleles present in a population. Genetic drift can either be the result of a bottleneck or founder effect. A bottleneck effect is when a natural disaster or other catastrophe randomly kills a large portion of a population and dramatically decreases genetic diversity. A founder effect is when a few members of a population move to a new habitat and the new population has a much smaller gene pool with much less diversity.  Like inbreeding, genetic drift often results in a decrease in the reproductive fitness of a population due to decreased genetic diversity.

Practice Questions


Khan Academy

MCAT Official Prep (AAMC)

Biology Question Pack, Vol. 1 Question 43

Biology Question Pack, Vol. 1 Passage 12 Question 77

Biology Question Pack, Vol 2. Question 81

Practice Exam 3 B/B Section Question 10


Key Points

• Speciation is the process by which new species form. It occurs due to barriers in successful interbreeding within an initial species that have become reproductively isolated and diverge; it could be of two forms: allopatric and sympatric.

• Polymorphism is the occurrence of two or more different morphs or forms, also referred to as phenotypes.

• Adaptation is the genetic change in a population that exhibits a “fit” to the environment.

• Specialization is the variation of traits to fill a niche better.

• Inbreeding, outbreeding, and bottleneck are the events that can cause a change in the genetic diversity of a population.

• Inbreeding and bottleneck decrease genetic diversity while outbreeding improves the chances of genetic diversity.

Key Terms

bottleneck effect: magnification of genetic drift as a result of natural events or catastrophes

inbreeding: mating of closely related individuals

outbreeding: breed from parents not closely related

species: a group of living organisms consisting of similar individuals capable of exchanging genes or interbreeding.

speciation: the formation of new and distinct species in the course of evolution.

allopatric speciation: the geographic separation of populations from a parent species and subsequent evolution. 

sympatric speciation: speciation occurring within a parent species remaining in one location.

phenotype: the set of observable characteristics of an individual resulting from the interaction of its genotype with the environment.

natural selection: a process in which individual organisms or phenotypes that possess favorable traits are more likely to survive and reproduce

genetic drift: vrandom ariation in the relative frequency of different genotypes in a small population, owing to the chance disappearance of particular genes as individuals die or do not reproduce.

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