Humans have a large impact on the environment—pollution, land use change, climate change, etc.

  • We are just beginning to realize that we also cause evolutionary change, and the consequences of this.
  • Ecology and evolution are strongly interconnected.

What Is Evolution?

Evolution can be viewed as genetic change over time or as a process of descent with modification.

Biological evolution is change in organisms over time. For natural selection to lead to evolution; traits must be heritable.


  • Horn size in bighorn sheep is a heritable trait. Trophy hunting selectively eliminates rams with large horns, thus favoring rams with genes for small horns.
  • Trophy hunting is likely causing the genetic makeup of bighorn sheep populations to change, or evolve, over time.
  • Evolution can be defined more broadly as descent with modification.
  • As a population accumulates differences over time and a new species forms, it is different from its ancestors.
  • But the new species has many of the same characteristics as its ancestors, and resembles them.

Mechanisms of Evolution

Natural selection, genetic drift, and gene flow can cause allele frequencies in a population to change over time.

Phenotype: Observable characteristics that are determined by the genotype.

Individuals differ from one another in part because they have different alleles for genes.

Different alleles arise by mutation: Change in DNA.

Mutations can result from copying errors during cell division, mechanical damage, exposure to chemicals (mutagens) or high-energy radiation.

Formation of new alleles is critical to evolution.

If mutation did not produce new alleles, all members of a population would have identical genotypes and evolution could not occur.

Recombination also produces different genotypes within a population.

Offspring have combinations of alleles that differ from those of their parents. However recombination and mutation are not responsible for the short term evolutionary changes that we observe

Three types of natural selection:

  • Directional selection: Individuals at one phenotypic extreme (e.g., large size) are favored.

Example: Drought favored large beak size in medium ground finches.

  • Stabilizing selection: Individuals with an intermediate phenotype are favored.

Example: Parasitic wasps select for small gall size of Eurosta flies; while birds select for large gall size.

  • Disruptive selection: Individuals at both phenotypic extremes are favored.

Example: African seedcrackers (birds) have two food sources—hard seeds that large beaks are needed to crack, and smaller, softer seeds that smaller beaks are more suited to.

Natural selection can result in populations in which all individuals have the favored allele: 100% and the allele is fixed.

Genetic drift occurs when chance events determine which alleles are passed to the next generation. It is significant only for small populations.

Genetic drift has four effects on small populations:

1. It acts by chance alone, thus causing allele frequencies to fluctuate at random. Some may disappear, other may reach 100% frequency (fixation).

2. Because some alleles are lost, genetic variation of the population is reduced.

3. Frequency of harmful alleles can increase, if the alleles have only mildly deleterious effects.

4. Differences between separate populations of the same species can increase.

2. and 3. can have dire consequences.

***Loss of genetic variation reduces the ability of the population to respond to changing environmental conditions.

****Increase of harmful alleles can reduce survival and reproduction. These effects are important for species that are near extinction.

Gene flow: Alleles move between populations via movement of individuals or gametes.

Gene flow has two effects:

  • Populations become more similar
  • New alleles can be introduced into a population (the stronger the selection force will have a greater affect on the frequency of the new allele)

  • Natural selection is the only evolutionary mechanism that consistently causes adaptive evolution.
  • Adaptations are features of organisms that improve their ability to survive and reproduce.
  • Natural selection is not a random process.
  • By consistently favoring individuals with certain alleles, natural selection causes adaptive evolution—traits that confer advantages tend to increase in frequency over time.
  • only mechanism in which adaption can occur.

Rapid Evolution seen in organisms:

  • short life span (i.e insects)
  • high favorable selection pressure (i.e resistance to antibiotics)
  • low variable of alternative alleles of the same gene (i.e wing colour; clear or black)

Gene flow can limit adaptation to local conditions.

Adaptive evolution can occur in a population if the natural selection forces are strong enough to overcome gene flow.

Species: Groups of organisms whose members have similar characteristics and can interbreed.

Speciation: The process by which one species splits into two or more species.

Speciation most commonly occurs when a barrier prevents gene flow between two or more populations of a species. (allopatric) (i.e mountain range, large river & lake)

Barriers can be geographic or ecological. The populations then diverge genetically over time.

The key step in speciation occurs when a population accumulates so many genetic differences that they cannot produce viable, fertile offspring if they mate with the parental species. Reproductive barriers can arise as a by-product of natural selection. (however it may not always be due to separation; it could be due to different food medium, mating rituals etc.)

Greatest impact mankind has on organisms is land use change.

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