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Learning Goals
By the end of this reading you should be able to:
- Describe how hybrid fitness can influence speciation in a hybrid zone
- Differentiate between gradual and punctuated equilibrium
- Explain how population size and environmental variables can impact speciation rates
Introduction
In some cases, after being geographically separated and undergoing adaptive evolution two populations can reconnect. What happens next depends on many factors, including the types of adaptations that have occurred as well as underlying genetic changes. Sympatric speciation, in which there is no geographic separation also occurs over a span of evolutionary time, so when a new species arises, there is a transition period during which the closely related species continue to interact. What happens when these populations reconnect?
Reconnection
In some cases, after two populations have been separated and evolved in different ways possibly even becoming different species, the two populations may come into contact again. It is possible that if they are still similar enough that they may recombine or just interact and remain separate indefinitely. In some instances, individual organisms within a population can and will mate with any nearby individual with whom they are capable of breeding. The area where two closely related species continue to interact and reproduce, forming hybrids is referred to as a hybrid zone. Over time, the hybrid zone may change depending on the fitness of the hybrids and the presence/absence of any reproductive barriers.
Hybrids can be either less fit than the parents, more fit, or about the same. Usually, hybrids tend to be less fit; therefore, such reproduction diminishes over time, nudging the two species to diverge further in a process we call reinforcement. The hybrids’ reduced fitness and thus lower reproductive success reinforce the original speciation and overtime the species will continue to diverge until they can no longer hybridize. If the hybrids are as fit or more fit than the parents with which they will be competing then the two species may fuse back into one species. The weakening of reproductive barriers between the two species will reinforce this reconnection. Scientists have also observed that sometimes two species will remain separate but also continue to interact to produce some hybrid individuals. Scientists classify this as stability because no real net change is taking place. For a hybrid zone to be stable, the offspring produced by the hybrids have to be less fit than members of the parent species. (Fig. 1)
A final possibility that has been observed is the eventual development of the hybrids as a species of their own. In this case, the hybrids are fit but only in relation to other hybrids, meaning that they will be most likely to reproduce with other hybrids. Over time this can result in reproductive barriers between the hybrids and the original parental species and lead to the formation of a separate species. The