Speciation, the process by which new species arise, is a fundamental concept in evolutionary biology. While the process typically occurs over long periods, there have been instances of observed speciation in both laboratory and natural settings. This page explores the evidence for and against observed speciation in evolution.
Arguments Against Observed Speciation
1. Definition and Criteria of Speciation
Critics argue that many claimed instances of observed speciation do not meet strict criteria for new species formation:
- The biological species concept, which defines species based on reproductive isolation, may not be applicable to all organisms, especially those that reproduce asexually (Coyne & Orr, 2004). For example, bdelloid rotifers have evolved for millions of years without sexual reproduction (Welch et al., 2009).
- Some argue that observed changes represent microevolution rather than true speciation, as they often do not result in complete reproductive isolation (Futuyma & Kirkpatrick, 2017). The apple maggot fly (Rhagoletis pomonella) case, while showing host race formation, still allows for some gene flow between populations (Feder et al., 2003).
- The time scale of human observation may be insufficient to witness complete speciation in many organisms (Hendry, 2009). For instance, speciation in mammals is estimated to take on average 1-2 million years (Avise et al., 1998).
- There is ongoing debate about whether polyploid plant lineages should be considered new species, as they can sometimes still interbreed with their parent species (Soltis et al., 2007).
2. Laboratory vs. Natural Conditions
Some researchers question the relevance of laboratory-observed speciation to natural processes:
- Laboratory conditions may impose artificial selection pressures that do not reflect natural evolutionary processes (Mallet, 2008). For example, Dodd's (1989) experiment on Drosophila pseudoobscura used extreme environmental conditions not typically found in nature.
- The controlled environment of laboratories may not account for the complex interactions and gene flow that occur in natural populations (Coyne & Orr, 2004). Studies on wild Heliconius butterflies show that hybridization and introgression play important roles in their evolution, which may be overlooked in lab settings (The Heliconius Genome Consortium, 2012).
- Critics argue that laboratory-induced speciation may not be stable if organisms were reintroduced to natural environments (Rice & Hostert, 1993). This is supported by cases where artificially selected traits in crops revert when cultivation ceases (Gepts, 2004).
- The founder effect and genetic drift in small laboratory populations may lead to rapid changes that are not representative of larger natural populations (Templeton, 1980).
3. Incomplete Reproductive Isolation
In many cases of claimed speciation, reproductive isolation is not complete:
- Some observed "species" can still interbreed under certain conditions, challenging their status as distinct species (Mallet, 2008). For instance, many plant species considered distinct can form hybrids, such as in the genus Helianthus (sunflowers) (Rieseberg, 1997).
- The process of speciation is often gradual, making it difficult to determine when two populations have truly become separate species (Nosil et al., 2009). Studies on Timema stick insects show a continuum of divergence rather than discrete species boundaries (Nosil et al., 2012).
- Hybridization between closely related species is common in nature, complicating the definition of species boundaries (Abbott et al., 2013). For example, extensive hybridization occurs between polar bears and brown bears, despite their distinct adaptations (Cahill et al., 2013).
- In some cases, reproductive barriers may be environment-dependent and break down under certain conditions, as seen in cichlid fish in African lakes during periods of turbid water (Seehausen et al., 1997).
4. Reversibility of Observed Changes
Some argue that observed changes may be reversible and thus not true speciation:
- Adaptations observed in short-term studies may not persist over longer evolutionary timescales (Hendry et al., 2007). For example, rapid beak evolution in Galápagos finches can reverse with changing environmental conditions (Grant & Grant, 2002).
- Some populations that appear to be diverging may reconverge if environmental conditions change (Taylor et al., 2006). This has been observed in threespine stickleback fish, where marine and freshwater forms can re-evolve from each other (Bell & Foster, 1994).
- The genetic basis of observed changes may not be sufficiently stable to result in permanent speciation (Welch, 2004). Studies on Drosophila show that reproductive isolation can sometimes be lost after multiple generations (Rundle et al., 2000).
- Phenotypic plasticity may account for some observed differences between populations, rather than genetic changes leading to speciation (West-Eberhard, 2003).
5. Limited Taxonomic Scope
Critics argue that observed speciation is limited to certain types of organisms:
- Many examples of observed speciation come from organisms with short generation times, which may not be representative of all life forms (Coyne & Orr, 2004). Most cases involve insects, plants, or microorganisms, with fewer examples from vertebrates or long-lived organisms.
- Speciation in more complex organisms with longer generation times is more difficult to observe directly (Hendry, 2009). For instance, estimated speciation rates for mammals are much slower than for insects or plants (Hedges et al., 2015).
- The mechanisms of speciation may vary across different taxonomic groups, making generalizations difficult (Seehausen et al., 2014). For example, polyploid speciation is common in plants but rare in animals (Otto & Whitton, 2000).
- Some argue that observed cases of rapid evolution, such as in Darwin's finches, represent adaptive radiations within a species complex rather than true speciation (Zink, 2002).