MCAT Content / Evolution / Natural Selection

Natural selection

Topic: Evolution

Natural selection, or the survival of the fittest, is a process of evolution whereby the gene pool for a population is changed over time in favor of traits that are more advantageous to the survival of the population.

Natural selection, also known as “survival of the fittest,” is the reproduction of individuals with favorable traits that survive environmental change because of those traits. It is an inevitable outcome of three principles:

  1. most characteristics are inherited;
  2. more offspring are produced than can survive; 
  3. offspring with more favorable characteristics will survive and have more offspring than those individuals with less desirable traits.

Fitness is a measure that gives an advantage to individuals that can adapt better to change. In return, the better-adapted group in a population make more significant contributions to the gene pool of the next generation through increased reproduction. This process is called differential reproduction. It is the process of favoring the survival and reproduction of the fittest individuals in a population. Individuals with favorable characteristics have a competitive advantage and are more likely to pass their genes on to the next generation. For example, individuals that have a trait for camouflage against predators will be more likely to survive to reproduce.

Sometimes natural selection doesn’t just favor one organism but many. This is group selection, the idea that natural selection sometimes acts on whole groups of organisms, favoring some groups over others, leading to the evolution of traits that are advantageous to the group. 

The products of evolution can be seen from many perspectives- from population change to the presence of a gene or even its specific alleles. Alleles that produce traits will increase in frequency in the gene pool, thereby slowly changing a species over time, this is evolution. 


Practice Questions

Khan Academy

Selective pressures on cacti needle length

MCAT Official Prep (AAMC)

Biology Question Pack, Vol. 1 Passage 12 Question 79

Biology Question Pack, Vol. 1 Passage 14 Question 90

Biology Question Pack, Vol. 1 Passage 14 Question 94

Key Points

• Natural selection, also known as “survival of the fittest,” states that individuals with traits that enable them to survive are more reproductively successful; this leads to those traits becoming predominant within a population.

• Natural selection is an inevitable outcome of three principles: most characteristics are inherited, more offspring are produced than can survive, and offspring with more favorable characteristics will survive and have more offspring than those individuals with less desirable traits.

• Fitness is a conceptual measure that selects for individuals with greater contributions to the gene pool of the next generation.

• Differential reproduction is a process of favoring survival and reproduction of the fittest individuals in a population; individuals with favorable characteristics have a more competitive advantage and are more likely to pass their genes on to the next generation.

• Group selection refers to the idea that natural selection sometimes acts on whole groups of organisms, favoring some groups over others, leading to the evolution of traits that are group advantageous.

• Alleles that produce traits will increase in frequency in the gene pool, thereby creating another measure of evolutionary success.


Key Terms

evolution: the change in the genetic composition of a population over successive generations

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

fitness: an individual’s ability to propagate its genes

group selection: when natural selection sometimes acts on whole groups of organisms

differential reproduction:  when organisms best adapted to a given environment will be most likely to survive to reproductive age and have offspring

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

alleles: a different form of the same gene



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