Evolution is changes in allele frequencies, but allele frequencies change independently in each species.
Morphospecies are identified on the basis of phenotypic differences; biological species are identified by failure to produce viable hybrid offspring; phylogenetic species are the smallest monophyletic groups on a tree of populations. The morphospecies concept is widely applicable but misses cryptic species and can become arbitrary when experts disagree. The BSC is sound theoretically but cannot be applied to extinct forms or the many species that reproduce asexually. The PSC is sound theoretically and widely applicable but the required data are only available for a relatively small number of species.
The PSC led to the recognition of additional species in both cases. In copepods, species that appear morphologically identical are actually reproductively isolated and well-differentiated genetically. In elephants, populations that appear somewhat different morphologically are also well-differentiated genetically.
Genetic exchange or "sex" in bacteria is one-way instead of reciprocal, involves just one or a few genes, and may take place between widely divergent species.
Dispersal occurs when individuals leave an area and physically move to a new habitat and colonize it, forming a new populations. Vicariance occurs when an existing population is fragmented into 2 or more isolated populations by changes in the habitat. Dispersal and vicariance produce geographic isolation, which reduces or eliminates gene flow between populations. Stated another way, geographic isolation leads to reproductive isolation.
The opening of the land bridge allowed terrestrial species to disperse between North and South America. The opening of the Panamal Canal is a vicariance event for terrestrial organisms but allowed dispersal for marine organisms from the Atlantic to the Pacific Ocean and vice versa.
The presence of Caribbean-Pacific sister species pairs is predicted by vicariance, because a continuous population should have been split into Caribbean and Pacific populations by the rise of the isthmus.
Because the islands appeared one by one over time, with the oldest islands to the west and the youngest to the east, the dispersal hypothesis predicts that flies would move from older islands to younger islands, forming a new species each time.
Yes, because glacial sheets could split habitats into fragments separated by ice, or because climate change associated with glaciation could fragment large areas of forest or grassland. Fragmentation should reduce gene flow most in species that do not readily disperse long distances, such as salamanders, snails, and trees with large seeds.
Tetraploid individuals produce diploid gametes; diploid individuals produce haploid gametes. Hybrid, triploid offspring cannot undergo meiosis normally and rarely produce viable gametes.
If hybrid offspring have low fitness, then reinforcement should evolve and the populations should be come separate species. If hybrid offspring have equal fitness to the parental forms, then the parental populations should coalesce over time. If hybrid offspring have higher fitness than the parental forms in a certain habitat, then a new species or a stable hybrid zone may form in that habitat.
Reinforcement is the evolution of traits that reduce matings between divergent populations, due to natural selection against production of low-fitness hybrid offspring.
Speciation requires reproductive isolation and divergence. If divergent sexual selection leads changes in the traits that certain individuals use to choose mates, then sexual selection will cause those traits to diverge rapidly, and the individuals using the diverged traits as mate-choice criteria will be reproductively isolated from the original population.
Most biologists agree with the hypothesis, because selection is favoring individuals that are adapted to different habitats: clover vs. peas and benthic vs. limnetic habitats.
See the analyses and data on soapberry bugs in Chapter 2.
If crossbill populations are specialized for feeding on certain types of trees, then members of different populations should not breed together routinely. The ability to fly would slow down divergence, as birds from different populations might mix. If cones are in distinct patches, populations would tend to be restricted to those patches, which would reduce mixing.
Capture the colonizing iguanas and make morphological and genetic measurements that would quantify their characteristics. Do the same for species on nearby islands that may have served as the source population. Then repeat the measurements after 10, 20, or more years have passed.
They are distinct morphospecies and phylogenetic species. The best way to determine whether each species pollinates just one species of yucca is to mark individual moths and follow their movements.
They could be considered morphospecies based on differences in male coloration; the question provides no information relevant to the question of whether they are biological species. The best way to test whether divergence in male coloration is due to sexual selection is to offer females from different populations a choice of males with contrasting coloration, and document which males they respond to.
