Evolutionary Biology: Essential Terms and Processes
Key Concepts in Evolutionary Biology
Founder Effect
The founder effect is the loss of genetic variation that occurs when a new population is established by a small number of individuals from a larger population.
Genetic Bottleneck
A genetic bottleneck is a sharp reduction in the size of a population due to environmental events (such as earthquakes, floods, fires, or disease).
Directional Selection
Directional selection is a mode of natural selection in which an extreme phenotype is favored over other phenotypes.
Stabilizing Selection
Stabilizing selection is a type of natural selection in which genetic diversity decreases and the population mean stabilizes on a particular trait value.
Disruptive Selection
Disruptive selection, also called diversifying selection, describes changes in population genetics in which extreme values for a trait are favored over intermediate values.
Sexual Selection
Sexual selection is a mode of natural selection where typically members of one biological sex choose mates of the other sex with whom to mate (intersexual selection) and competition between members of the same sex to sexually reproduce with members of the opposite sex.
Reproductive Isolation
Reproductive isolation is a collection of mechanisms, behaviors, and physiological processes that prevent the members of two different species that cross or mate from producing offspring.
Prezygotic Isolation
Prezygotic isolation is a type of reproductive isolation that occurs *before* the formation of a zygote can take place. In most cases mating does not even occur.
Postzygotic Isolation
Postzygotic isolation is a type of reproductive isolation that occurs after members of two different species have mated and produced a hybrid offspring.
Speciation
Speciation is the evolutionary process by which reproductively isolated biological populations evolve to become distinct species.
Allopatric Speciation
Allopatric speciation is speciation that occurs when biological populations of the same species become isolated from each other to an extent that prevents or interferes with genetic interchange.
Sympatric Speciation
Sympatric speciation is the process through which new species evolve from a single ancestral species while inhabiting the same geographic region.
Adaptive Radiation
Adaptive radiation is a process in which organisms diversify rapidly into a multitude of new forms, particularly when a change in the environment makes new resources available, creates new challenges, or opens new environmental niches.
Gradualism
Gradualism is the theory which holds that profound change is the cumulative product of slow but continuous processes.
Punctuated Equilibrium
Punctuated equilibrium is a theory in evolutionary biology which proposes that once species appear in the fossil record, they will become stable, showing little net evolutionary change for most of their geological history.
Factors Influencing Natural Selection
- More offspring are produced than can survive to reproduce.
- Individuals within a species show variation in traits.
- The differences within individuals are heritable.
- Some differences affect how well adapted an organism is to its environment.
- Differences in adaptation are reflected in the number of offspring successfully produced.
- Natural selection can only work on variations that are expressed!
Sources of Variation
- Sexual reproduction (more important)
- Recombination (mixing of traits from both parents)
- Crossing over during meiosis
- Mutations in gametes (less important)
- Point mutations
- Translocations
Evidence for Evolution
- Paleontology (fossil record)
- Biogeography (distribution of species)
- Comparative anatomy
- Molecular biology (DNA, etc.)
Theories of Evolutionary Change
- Gradualism (Darwin) – Evolution proceeds through the slow and gradual accumulation of variation at a more or less constant (slow) rate.
- Punctuated equilibrium (S.J. Gould) – Evolution proceeds with relatively short bursts of rapid change followed by long periods of near stasis (stability).