MCAT Content / Evolution / Speciation

Speciation

Topic: Evolution

Speciation is the formation of two species from one original species; it can be the result of polymorphism, adaptation, and specialization; the events that affect the genetic diversity of a population include interbreeding, outbreeding, and bottleneck. 

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. For speciation to occur, two new populations must be formed from one original population, and they must evolve in such a way that it becomes impossible for individuals from the two new populations to interbreed and produce fertile offspring, becoming separate species. Speciation could be of two broad categories. Allopatric speciation involves the geographic separation of populations from a parent species and subsequent evolution or sympatric speciation involves speciation occurring within a parent species remaining in one location.

Speciation can be the result of polymorphism, adaptation, and specialization. 

Genetic polymorphism refers to the occurrence of two or more different morphs or forms, also referred to as phenotypes. It often displays different phenotypes or expresses different alleles of a gene. One morph may confer a higher fitness than another but may not increase in frequency because the intermediate morph is detrimental. 

Polymorphism. Color and pattern morphs of Grove Snail, Cepaea nemoralis. The polymorphism seems to have several causes, including predation by birds.

Adaptation is the genetic change in a population caused by natural selection. Organisms exhibiting a “fit” to the environment are referred to as adaptation. All adaptations enhance the reproductive potential of the individuals exhibiting them, including their ability to survive to reproduce. Adaptations are not constant. As an environment changes, natural selection causes the characteristics of the individuals in a population to track those changes.

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

The variation of traits to better fill a niche (e.g. specialized anole lizards) is called specialization. For example, two variants of anole lizards can be observed. Ground anole lizard prey on the ground while tree anole lizard prey on trees. This specialization allows them to live in the same area without competing.

Specialization. Ground anole lizard (a) and tree anole lizard (b) prey on the ground and trees, respectively, to live in the same area without competing for food. Species that are not closely related evolutionarily can develop specialized features that are similar – this is called convergent evolution.

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

Inbreeding is the mating of closely related individuals which can have an undesirable effect of bringing together harmful recessive mutations that can cause abnormalities and susceptibility to disease. It increases the frequency of homozygotes, decreases heterozygotes, and decreases genetic diversity. Outbreeding, the opposite of inbreeding, is the reproduction among distantly or unrelated individuals and improves chances of genetic diversity. It protects against mutations and the widespread increase in recessive disorders within a population.

A natural disaster or catastrophe that randomly kills a large portion of the population is known as the bottleneck effect. A bottleneck is a severe reduction in population size. It increases the impact of genetic drift that can reduce the genetic diversity within a population. 


Practice Questions

 

Khan Academy

https://jackwestin.com/khan-academy-mcat/human-impact-on-animal-populations

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

polymorphism: the regular existence of two or more different genotypes within a given species or population

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.

alleles: a different form of the same gene

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

convergent evolution: the process by which distantly-related species develop similar characteristics over time

genetic drift: variation 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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