MCAT Content / Evolution / Natural Selection

Natural selection

Topic: Evolution

Evolution is the process whereby the gene pool of a species is gradually changed over time because individuals with more desirable genes are selected for by the environment. A species is a group of organisms which are capable of reproducing with one another and producing viable offspring.

The mechanism of evolution, natural selection (also known as “survival of the fittest”), is the relative increase in the reproduction of organisms which have traits that are better suited for their environment. Natural selection occurs due to 3 guiding principles:

  1. variation: individuals within a population will have different traits;
  2. inheritance: offspring inherit traits from their parents;
  3. competition: more offspring are produced than can survive, so offspring with traits better suited to the environment will survive and have more offspring than individuals with less desirable traits.

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. 

Point 1 of natural selection – the variation of traits within a species – depends on the number and types of alleles which are present in a population. Alleles are the alternative forms of a gene which code for different versions of a trait. Different alleles in a population – and by extension, variation – arise through processes which either produce new alleles or bring in copies of alleles from other populations. Random mutations to DNA can produce new types of alleles encoding for unique traits. Processes such as immigration and emigration can cause the outflow or inflow of alleles to/from different populations of the same species, respectively.

 


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

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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