10.1 Essential ideas

10.1.3 Gene pools and speciation

Gene pools

Figure 10.1.3a – Gene pools change over time in response to different factorsFigure 10.1.3a – Gene pools change over time in response to different factors

Factor

Definition

Effect

Mutation

Random, rare, and spontaneous, heritable change to the structure of DNA

Introduces new alleles to the gene pool

Natural selection

Some individuals produce more offspring because of better suitability to the environment

Gradual changes to frequency of existing alleles, increasing some, decreasing others

Gene flow

Immigration or emigration of individuals

Keeps neighbouring populations similar in allele frequency

Genetic drift

i) Bottleneck effect – disaster reduces the original population variation

ii) Founder effect – a small population inhabits a new microhabitat

Instant isolation of population; some alleles of original population lost; greatest effect in small populations

 

Figure 10.1.3b – Genetic drift has a great effect on small populationsFigure 10.1.3b – Genetic drift has a great effect on small populations

Evolution by natural selection

  • Natural selection acts on individuals via their phenotypes. Phenotypes are the result of interactions between genes and the environment. The genetic basis of favourable phenotypes will be passed on to future generations. In this way, natural selection can alter allele frequencies in a gene pool.
  • There are three patterns of natural selection:

Figure 10.1.3c – Patterns of selectionFigure 10.1.3c – Patterns of selection
Changes in frequency of phenotypes as a result of directional, stabilising and disruptive selection.

 

Directional

Stabilising

Disruptive

Effect of environment

Favours one extreme of the phenotype

Removes extreme phenotypes

Removes intermediate forms/favours extremes

Example

Antibiotic resistance

Mammalian birth mass, and clutch size

Sexual dimorphism; polymorphism in the peppered moth

 

Speciation by isolation

Figure 10.1.3d – Principle of reproductive isolationFigure 10.1.3d – Principle of reproductive isolation

  •  Reproductive isolation can be:
    • Behavioural – individuals within a population develop different behaviour routines, especially in courtship or mating, which exclude other members of the same population.
    • Temporal – individuals within a population produce gametes, or mate, during different seasons, or at different times of the day.
    • Geographical – a physical barrier separates individuals, or individuals living in different microhabitats do not interbreed.

Try it!

Outline how isolation led to speciation for the organisms below.

Figure 10.1.3e – The Eastern and Western meadowlarks, Sturnela magna and Sturnela neglecta.Figure 10.1.3e – The Eastern and Western meadowlarks, Sturnela magna and Sturnela neglecta

Figure 10.1.3f – Nocturnal and diurnal lizardsFigure 10.1.3f – Nocturnal and diurnal lizards

Figure 10.1.3g – Kaibib squirrel, Sciurus aberti kaibabensis (left) and Abert’s squirrel, Scirus aberti (right).Figure 10.1.3g – Kaibib squirrel, Sciurus aberti kaibabensis (left) and Abert’s squirrel, Scirus aberti (right).

Gradualism vs Punctuated equilibrium

  • The mechanisms of evolution may operate gradually or abruptly.
  • Gradualism predicts a steady pace of evolution, in which species are replaced at a slow rate, through a series of intermediate forms.
  • The fossil record for certain groups does not support the idea that evolution and speciation occur gradually. Instead, the record indicates that some species have tended to remain unchanged for millions of years, and that there are periods of mass extinction and speciation.

Figure 10.1.3h – Periods of mass extinction have been followed by speciation in geological historyFigure 10.1.3h – Periods of mass extinction have been followed by speciation in geological history

  • To account for the difference, the theory of punctuated equilibrium was proposed.

Figure 10.1.3i – Gradualism vs Punctuated equilibriumFigure 10.1.3i – Gradualism vs Punctuated equilibrium

  • In punctuated equilibrium:
    • When environments undergo violent sudden changes (e.g. volcanic eruption), there are mass extinctions. However, populations at the edge of the disturbance may be saved from extinction.
    • These populations may occupy new niches, undergo rapid population growth and evolution through the founder effect.
    • Both gradualism and punctuated equilibrium have contributed to the evolution of life.

Essential idea

Gene pools change over time. Speciation can occur gradually or abruptly.

Figure 10.1.3j – Ground nestersFigure 10.1.3j – Ground nesters

Figure 10.1.3k – Tree nestersFigure 10.1.3k – Tree nesters

Double-breasted cormorants, Phalacrocrorax auritus, nest in trees (above) and on the ground (below). Eventually the two nesting populations may become reproductively isolated by these behaviours.

Figure 10.1.3l – Siamese fighting fishFigure 10.1.3l – Siamese fighting fish
Sexual dimorphism. Male (left) and female (right) Siamese fighting fish, Micracanthus marchei, look very different as a result of disruptive selection.

Figure 10.1.3m – Stephen Jay GouldFigure 10.1.3m – Stephen Jay Gould
Stephen Jay Gould (1941–2002) with bust of Charles Darwin. Gould proposed the theory of punctuated equilibrium, with Niles Elderedge, in 1972.

TOK

Punctuated equilibrium used to be considered a competing theory of evolution, challenging the Darwinian paradigm. How do paradigm shifts proceed in science?

Nature of Science

Looking for trends and patterns: Patterns of chromosome number in some genera can be explained by speciation due to polyploidy.

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