Chapter 22 Summary

Core Concepts Summary

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22.1 Reproductive isolation is the key to the biological species concept.

The biological species concept (BSC) states that species are groups of actually or potentially interbreeding populations that are reproductively isolated from other such groups. page 446

We cannot apply the BSC to asexual or extinct organisms. page 447

Ring species and hybridization further demonstrate that the BSC is not a comprehensive definition of species. page 448

The BSC is nevertheless especially useful because it emphasizes reproductive isolation. page 449

22.2 Reproductive isolation is caused by barriers to reproduction before or after egg fertilization.

Reproductive barriers can be pre-zygotic, occurring before egg fertilization, or post-zygotic, occurring after egg fertilization. page 449

Pre-zygotic isolation may be behavioral, gametic, temporal, or ecological. page 450

In post-zygotic isolation, mating occurs but genetic incompatibilities prevent the development of a viable, fertile offspring. page 450

22.3 Speciation underlies the diversity of life on Earth.

Speciation is typically a by-product of genetic divergence that occurs as a result of the fixation of different mutations in two populations that are not regularly exchanging genes. page 451

If divergence continues long enough, chance differences will arise that result in reproductive barriers between the two populations. page 451

Most speciation is thought to be allopatric, involving two geographically separated populations. page 451

Geographic separation may be caused by dispersal, resulting in the establishment of a new and distant population, or by vicariance, in which the range of a species is split by a change in the environment. page 451

A special case of allopatric speciation by dispersal is peripatric speciation, in which the new population is small and outside the species’ original range. page 453

Adaptive radiation, in which speciation occurs rapidly to generate a variety of ecologically diverse forms, is best documented on oceanic islands after the arrival of a single ancestral species. page 455

Co-speciation occurs when one species undergoes speciation in response to speciation in another. In parasites and their hosts, co-speciation can result in host and parasite phylogenies that have the same branching patterns. page 455

Speciation may be sympatric, meaning that there is no geographic separation between the diverging populations. For this type of speciation to occur, natural selection for two or more different types within the population must act so strongly that it overcomes the homogenizing effect of gene flow. page 456

22.4 Speciation can occur with or without natural selection.

Separated populations can diverge as a result of genetic drift, natural selection, or both. page 459

Natural selection may act on mutations that allow individuals to identify and mate with individuals that are more like themselves. This process is called reinforcement of reproductive isolation. page 459

Self-Assessment

  1. Define the term “species.”

    Self-Assessment 1 Answer

    Although no one definition is able to perfectly describe the genetic boundaries between different organisms, the biological species concept defines a species as a group of organisms that can share genetic material by interbreeding with one another to generate viable, fertile offspring. By this definition, organisms that cannot successfully reproduce with one another are therefore considered different species.

  2. Given a group of organisms, describe how you would test whether they all belong to one species or whether they belong to two separate species.

    Self-Assessment 2 Answer

    According to the morphospecies concept, organisms of the same species often have very similar appearances, so careful observation of the phenotypic characteristics of the organisms could provide insight into how many species were represented in the group. If possible, a more rigorous test would be to determine whether the organisms could breed with one another to produce viable, fertile offspring.

  3. Name two types of organism that do not fit easily into the biological species concept. What species concept would work best for these organisms?

    Self-Assessment 3 Answer

    Since the biological species concept defines species based on their ability to interbreed to generate viable, fertile offspring, those organisms that reproduce asexually (such as bacteria) and those that are extinct cannot be easily characterized by this definition. A combination of phylogenetic and ecological species concepts would work best for these organisms.

  4. Explain how ecological and phylogenetic considerations can help inform whether or not a group of organisms represents a single species.

    Self-Assessment 4 Answer

    Since resources are limited in every habitat, each species must have a unique niche within its environment in order to avoid competition with other species. Thus, organisms with overlapping ecological requirements are likely members of a single species. In addition, organisms of a single species will have a high degree of genetic identity due to a shared ancestral lineage, so information about the evolutionary history of an organism can suggest whether a group of organisms is indeed a single species or whether they have genetically diverged from one another to form two separate species.

  5. Name four reproductive barriers and indicate whether each is pre- or post-zygotic.

    Self-Assessment 5 Answer

    Reproductive barriers that prevent fertilization (pre-zygotic) include differences in mating behaviors like courtship rituals, mechanical blocks to fertilization such as differences in the genitalia required for mating, and biochemical incompatibilities in the gametes that can limit the fusion of sperm and egg. After fertilization has occurred, post-zygotic barriers limit the ability of the zygotes formed from gametes of different species to develop into fertile adults because of differences at the genomic level, such as an improper number of chromosomes.

  6. Describe how genetic divergence and reproductive isolation are related to each other.

    Self-Assessment 6 Answer

    In order for populations to genetically diverge from one another, they must have limited gene flow between them. Reproductive isolation limits the production of hybrid offspring, limiting genetic exchange between populations, and divergence continues as each population is subjected to the selective pressures of its environment and genetic drift. In addition, if populations begin to diverge through genetic drift or disruptive selection, these differences can result in establishment of a reproductive barrier, which would then work to further limit gene flow and promote reproductive isolation.

  7. Differentiate between allopatric and sympatric speciation, and state which is thought to be more common and why.

    Self-Assessment 7 Answer

    Allopatric speciation occurs after a geographic barrier physically separates two populations from one another, which limits gene flow and promotes genetic divergence of the two populations as they adapt to their different environments. Sympatric speciation is the divergence of one group of organisms into two distinct species within the same geographical area, so that although the organisms inhabit the same area and come in contact with one another, gene flow is limited because of strong selection against hybrid offspring. Allopatric speciation is thought to happen more often than sympatric speciation because genetic divergence between geographically isolated populations is inevitable, whereas sympatric populations will only diverge when strong disruptive selection is operating.

  8. Differentiate between allopatric speciation by dispersal and by vicariance, and give an example of each.

    Self-Assessment 8 Answer

    Allopatric speciation can occur if a subset of a population moves to a new location far from the original population (dispersal), much like what would happen if a hurricane blew a group of beetles to a distant island off the coast of the mainland. Allopatric speciation can also occur by vicariance, which is when a change in the environment effectively separates a population into two distinct groups. An example of allopatric speciation by vicariance would be if a lake receded and began to dry out leaving only smaller, isolated ponds behind. Fish from a single species might be split among several different ponds, each of which would then be subject to genetic divergence based on the conditions in their respective pond and genetic drift, which over time could lead to speciation.

  9. Describe how genetic drift can result in speciation.

    Self-Assessment 9 Answer

    Since genetic drift acts randomly to alter allele frequencies, the alleles that become fixed or extinct in a given population will be different in each population. These differences, accumulated over time, can result in genetic incompatibility that will limit gene flow and lead to the establishment of two different species.

  10. Describe how natural selection can result in speciation.

    Self-Assessment 10 Answer

    Speciation is the result of genetic divergence between two populations to the point where they are no longer capable of producing fertile offspring. Natural selection can drive genetic divergence of populations based on the characteristics that are favorable in each population’s environment, which may also influence their reproductive compatibility with other populations. The characteristics that allow for optimal survival and reproduction in one population will be specific for its environment, so two populations that have lived in different locations for a long time, both being adapted to their environments by natural selection, will likely encounter reproductive barriers, limiting gene flow between the populations and reinforcing speciation. In some cases, natural selection contributes directly to the process of speciation, when individuals better at choosing appropriate mates (i.e., from their own group) are selectively favored over those that frequently mate with members of the wrong group.