Chapter 9 Flashcards
Which idea is central to the biological species concept?
Reproductive isolation
The biological species concept has been widely adopted, but it also has some practical difficulties. Which of the following is not one of these difficulties?
The concept is poorly defined
According to the biological species concept, speciation consists of the evolution of biological barriers to gene flow. The most important distinction is between _______ and _______ barriers.
Prezygotic; postzygotic
Which of the following is an example of allopatric speciation?
Evolution of genetic reproductive barriers between populations of angelfish isolated on either side of the Isthmus of Panama
Most models of sympatric speciation postulate the existence of
Disruptive selection and assortative mating based on resource use
Which assumption does not underlie Kimura’s neutral theory of molecular evolution?
Advantageous mutations occur often, and many fixation events are due to positive selection
What class of barrier to gene flow would we be observing if we noted that the offspring of a lion and a tiger (two different species) are awkward, gangly beasts that have poor survival in the wild?
Postzygotic barrier
Reproductive isolation as a result of Dobzhansky-Muller incompatibility is caused by
The inability of two or more loci from different lineages to function well together
You collect individuals from different allopatric populations of a leaf-eating beetle. In the laboratory, you conduct mate-choice experiments to assess levels of reproductive isolation among beetles from different populations. Assuming that ecological speciation occurred in the wild, which pattern should you expect?
Less reproductive isolation among populations that share similar habitats
Why does reinforcement involve a strengthening of prezygotic, rather than postzygotic, isolation?
Alleles that prevent interbreeding are more likely to be inherited by viable, nonhybrid offspring than alleles that permit random mating
Which observation would be evidence for allopatric speciation among finch species of the Galápagos Islands?
No pairs of sister species formed on the same island
In North America, hawthorn trees were the ancestral hosts of the apple maggot fly (Rhagoletis pomonella). In the last 150 years, however, cultivated apples have become a host for some populations of R. pomonella. What factor may have allowed the colonization of apples as a new host plant, and thus subsequent disruptive selection based on resource use?
The timing of reproduction in Mexican hawthorn feeding populations overlaps with apple fruiting in the Northeast
How might the fate of two hybridizing populations—whether or not they persist as distinct populations following hybridization—depend on the kinds of isolating barriers that reduce gene exchange between them (i.e. pre-mating vs. postmating mechanisms of reproductive isolation)?
The fate of two hybridizing populations are formed as a result of pre-zygotic barriers and/or post-zygotic barriers of reproductive isolation. Pre-zygotic barriers reduce the probability in which two different populations will form a hybrid prior to successful mating via zygote formation. This can occur from habitat separation, different mating seasons, and failure to unite gametes. Thus, these barriers inhibit the likelihood that gene exchange will occur between individuals of separate populations and have a larger opportunity for gene flow restriction. On the other hand, post-zygotic barriers reduce the probability in which a hybrid will be viable and successfully reproduce. The gene exchange between populations is, therefore, even further reduced such that the hybridized individual will not be able to contribute to the gene pool.
The fate of hybridizing populations differs when the mechanisms of reproductive isolation are premating, as opposed to postmating. Premating isolation mechanisms can preserve genetic diversity, but this need not be the case for postmating barriers. If hybridizing populations have different sizes and only postmating barriers exist, genes from the larger population can “swamp” the smaller population. Premating isolation can be environment-dependent, and a change to the environment can cause breakdown of isolation and interbreeding. For examples, turbid water prevents African rift lake cichlids from recognizing mating displays, leading to interbreeding. Postzygotic isolation is often independent of the environment and can persist if environment changes.
Suppose the phylogenetic species concept (PSC) were preferred over other species concepts, such as the biological species concept (BSC). What would be the implications for (a) discourse on the evolutionary mechanisms of speciation (i.e. what does each concept emphasize, and how do they differ?); and (b) estimates of species diversity on a worldwide basis (e.g., would estimates increase or decrease if you used one concept versus another?)
a) Implications associated with the phylogenetic species concept include principles underlined by reproductive isolation. The phylogenetic species concept is bound to the idea that species are the outcome of evolution whereas the biological species concept is based on the idea of reproductive isolation as a major determinant in diverse lineage production. Therefore, in the phylogenetic species concept, discussion would primarily regard to divergence of lineages into species rather than the reproductive isolation mechanism that could have led to the rise of speciation. Allosteric populations would, however, be better analyzed through the phylogenetic species concept since classification can still be applied unlike the biological species concept with indistinguishable reproductive barriers between neighboring populations.
b) In the phylogenetic species concept, estimates of species diversity would increase in comparison to the biological species concept in which would decrease. A phylogenetic species consists of clusters of organisms that are different from other clusters and that arose from ancestral descent patterns. Genealogical relationships are more easily distinguished in comparison to reproductive barriers in certain populations ( example: allopatric). Due to the allopatric population and asexual reproduction implications in the biological species concept, it would restrict the determination on whether certain populations are capable of interbreeding with another. Since most organisms are allopatric and many organisms are asexually produced, this would decrease estimates on species diversity.
Some degree of genetic exchange occurs in bacteria, which reproduce mostly asexually. What evolutionary factors should be considered in debating whether or not the biological species concept (BSC) can be applied to bacteria?
The biological species concept typically does not consider organisms that reproduce asexually. Yet, bacteria are asexual organisms that have been debated upon on whether to apply the concept to or not. Evolutionary factors that are needed to be considered is the method of gene transfer in bacteria and whether it is enough to define it as a potential barrier for reproductive isolation. While meiosis has been the primary method in genetic exchange, bacteria also participates in exchange through horizontal gene transfer. This shows that genetic exchange does occur, but not all the time. Since the biological species concept is based on reproductive isolation to recognize species, the exchange processes of bacteria need to be considered to determine if the method of exchange is a reproductive barrier.
Complications arise when one tries to apply the biological species concept to bacteria. This is because the idea of “reproductive isolation” is central to the BSC, and bacterial reproduction tends to be asexual. An exception is lateral gene transfer. If lateral gene transfer occurs frequently between different strains of bacteria, they are no longer reproductively isolated and can be viewed as members of the same species via the BSC. The magnitude and frequency of gene exchange should be considered. In cases where lateral gene transfer is rare, we may still consider the strains reproductively isolated. Note that detection of lateral gene transfer often requires a genomic approach (thankfully, bacteria have small genomes).
