Rapid adaptation and Diversification in
Contemporary History

Intrinsic to creation biology, particularly with Lower Cenozoic Flood Boundary models, is the concept of rapid and extensive diversification within baraminic kinds. Our definition of ‘rapid’ is not the same as that in the secular model, where ‘rapid’ can refer to changes occurring within a few hundred thousand years. For the young earth creation model, ‘rapid’ means occurring within several thousand years at most. This has led to the inaccurate portrayal of baraminic diversification as ‘hyper-evolution’.

There are distinct differences between divergence within the evolutionary model and divergence that occurs within baraminic diversification. 1) The many diverse baraminic lineages do not share a universal common ancestor (thus the creationist ‘orchard’ versus the evolutionary ‘tree’ or ‘bush’). 2) Baraminic diversification occurs within lineages that were originally designed for change (which may have included optimal heterozygosity, genetic front-loading, genetic architecture for non-random mutational adaptations, etc.). 3) Change is purposeful within a creation framework: it occurs to fulfill God’s mandate for creatures to fill the Earth, which includes adapting to new environmental and ecological parameters as they come in what is now a fallen world.

Creationists expect the rate of diversification within kinds to be much lower now than it was thousands of years ago, particularly after terrestrial and aerial creatures left the Ark after the Flood. Today, increased homozygosity in the terminal branches of baraminic kinds (usually determined as species) mean that changes today will usually be small-scale adaptations to localized environmental and ecological conditions.

Genetic adaptations are inheritable, but may or may not lead to reproductive isolation (Barton 2000), let alone speciation. As has been noted in one debate in the secular literature, “[p]opulation differentiation is not a sufficient condition for incipient speciation” (Howard et al. 2001), but “each new ‘species’ initially went through a stage in which it was a newly derived population, with only minor genetic differences from its colonizing source” (Hendry et al. 2001), so rapid diversification sets up a platform for potential new species trajectories.

Not all morphological change is inheritable genetic adaptation. Some change simply demonstrates phenotypic plasticity within a given species, which is the genomic response to environmental changes—that plasticity may or may not be preadaptational for long-term, inheritable change. As Martin and Richards (2019) noted, “a phenotypically plastic ancestral population can rapidly adapt to a new environment and diverge into multiple ecomorphs, potentially followed by selection against plasticity in each of these specialists.” Not all traits in an organism have the same degree of plasticity (Kristjánsson et al. 2002), and ‘non-adaptive’ plasticity may have greater bearing on population divergence than does ‘adaptive’ plasticity (Ghalambor et al. 2015). Epigenetic mechanisms such as maternal effects can also influence phenotype without being genetically inheritable.

The genetic process for rapid divergence and adaptation may be aided by founder effects (Santos et al. 2012), genetic drift (Velo-Antón et al. 2012), hybridization or genomic admixture from separate lineages (Krehenwinkel et al. 2015; Meier et al. 2017), polymorphism and ‘ancient’ adaptive alleles (Van Belleghem et al. 2018; Martin and Richards 2019), transposable elements (Schrader er al. 2014), gene duplication (Chang and Duda Jr. 2012; Assis and Bachtrog 2015), hybrid dysfunction (Cutter et al. 2019), and other mechanisms (Fondon III and Garner 2004). Creationists have specifically discussed natural processes such as founder effects (Lightner and Ahlquist 2017) and genetic drift (Jeanson 2015), while a few have even proposed unique genetic mechanisms which could play a role: Altruistic Genetic Elements (Wood 2003); Step-Down Saltational Intrabaraminic Diversification (Wise 2017); Continuous Environmental Tracking (Guliuzza and Gaskill 2018).

Significant change (up to and including speciation events) still occurs today under certain conditions. When significant changes occur ecologically, an organism can rapidly adapt within contemporary (recent) history. It doesn’t always take thousands of years. This gives us a glimpse at the rapid rates of diversification that would have occurred in the period after the Flood as animals spread around the world, entering new habitats, experiencing climatic change, and building new predator-prey, competitive, and mutualist relationships. Today, organisms may change when introduced to a new habitat (through invasion, colonization, introduction, or habitat change), when new predators are introduced, when new prey is introduced, or when humans significantly impact the environment (anthropogenic change). Even fishing and hunting are known to influence phenotypes of game animals (Law 2000; Coltman et al. 2003; Allendorf and Hard 2009).

What follows is a brief survey of recent rapid adaptation and diversification recorded in the scientific literature.

Invertebrates

The intertidal snail Littorina obtusata, found in northern New England, collected prior to 1900, had high-spired shells with thin walls. After the European green crab invaded the region, intertidal snail populations rapidly developed low-spired, thick-walled shells which were less vulnerable to the crab. Areas where crabs are rare still have good numbers of high-spired shells, and areas where crabs are moderately present may have intermediate-spired shells. Seeley (1986) noted that changes in the shell morphology “could result from changes at a few gene loci and need not represent a speciation event”.

Weinberg et al. (1992) compared two natural populations of a marine polychaete worm, Nereis acuminata, from the Los Angeles-Long Beach area, to a laboratory population started in 1964 (collected in the same region in California) that underwent two separate bottleneck events followed by exponential population growth. They determined there was strong evidence for rapid laboratory speciation—for example, the laboratory polychaetes were unable to produce viable offspring with either natural population.

An example of plasticity can be seen in Daphnia, where some species experience morphological and life history changes through developmental responses to chemical cues in the water indicating the presence of invertebrate and vertebrate predators (Black 1993; Weider and Pijanowska 1993). Often these responses are seasonal, so there is no long-term basis. However, when new predators are introduced to the environment, plasticity may end up anchoring adaptational responses. As salmonid fish have been introduced to previously fishless alpine lakes in the Sierra Nevadas, Daphnia’s developmental mechanisms that regulate plasticity have been co-opted into broader, more expansive adaptational responses to these new predators (Fisk et al. 2007; Latta IV et al. 2007; Scoville and Pfrender 2010).

Examples of predator-induced phenotypic plasticity in Daphnia

"Inducible morphological defenses are manifold in the genus Daphnia. The listed examples show helmet expression in D. cucullata (A); crest expression in D. longicephala (C); head- and tail-spine formation D. lumholtzi (D), and neckteeth expression in D. pulex (B). Undefended morphotypes are displayed on the left side, and the defended morphotype on the right side. Images by Becker & Weiss." [Weiss 2019, CC-BY 4.0]

Fish

The threespine stickleback (Gasterosteus aculeatus, usually considered a species complex) is a Northern Hemisphere marine coastal fish that has colonized freshwater habitats around the world, leading to numerous isolated freshwater populations. Trajectories in morphological variation may be influenced by resource polymorphism (with different morphotypes focusing on different prey). Even in the same freshwater area, different morphotypes may develop in separate habitats, becoming reproductively isolated (Kristjánsson et al. 2002). Differences may occur in body size, body scale armor, fin structure, egg size, etc.

Kristjánsson et al. (2002) demonstrated that a freshwater lagoon stickleback population, no more than twelve generations separated from their marine source, “showed different morphotypes” from the ancestral morphotype. In an experiment, Kristjánsson (2005) determined that sticklebacks from marine tidal pools transplanted to two separate freshwater ponds of different sizes showed distinctive morphological changes in a single year. The rapid changes could have been due to strong selective pressures and/or phenotypic plasticity; the author suggested it was likely a combination of both.

When sticklebacks in an altered Alaskan lake habitat (bioproductivity growth via increased phosphorus intake) were examined over a sixteen-year period, they demonstrated rapid shifts in various life history traits such as egg and clutch size changes (Baker et al. 2011). In some cases the changes were so great they may have led to detrimental results (e.g. a shift to extremely small egg sizes during one year).

Freshwater sticklebacks are known to have greater tolerance to colder temperatures than marine sticklebacks have. Barrett et al. (2011) experimentally demonstrated rapid adaptation to cold tolerance by transplanting marine sticklebacks to freshwater ponds, then after three generations measuring their cold tolerance. The results showed that the ‘evolved’ cold tolerance matched that of wild freshwater populations.

Sticklebacks were introduced to Lake Constance and the upper Danube in Europe from the Baltic Sea approximately 150 years ago (Marques et al. 2016). These have diverged into a resident non-migratory ‘stream’ ecotype and a migrating ‘lake’ ecotype, that differ in size, morphology, male nuptial coloration, longevity, and reproductive behavior. Roy et al. (2015) suggested that the hybridization of multiple genetic lineages of sticklebacks introduced into the Lake Constance region may have acted as a catalyst for “rapid onset of adaptive diversification” within contemporary history.

Kitano et al. (2008) noted that a freshwater population of sticklebacks in an urban lake in Seattle, Washington, demonstrated ‘reverse evolution’ by increasing the frequency of completely armored sticklebacks over a forty-year period. The authors suggest this may have been a response to higher levels of predation on sticklebacks when water clarity increased during the 1970s.

Many different species of salmonids (salmon, trout, char, etc.) have been introduced into various bodies of water outside their native range, usually for fishing purposes. Source stock may be wild or produced via aquaculture. Hendry et al. (2000) examined specimens of sockeye salmon that had been introduced into Lake Washington’s major tributary (Cedar River) between 1937 and 1945, as well as a breeding population within Lake Washington (along a beach several miles distant from the river) discovered in 1957. They discovered that while river residents and recent immigrants to the beach were genetically indistinguishable, beach resident salmon were genetically distinct and demonstrate morphological differences from river forms.

Chinook salmon were introduced into a New Zealand river in the early 1900s and spread rapidly. Divergence from origin stock (California) has been noted in a number of important life history and physiological traits. While phenotypic plasticity may have aided early colonization to the different river systems, Quinn et al. (2001) argued that rapid genetic adaptations allowed for the development of separate river populations (which exhibited significant divergence in traits).

Brown trout from two separate genetic lineages (wild Polish stock and a domestic commercial stock) were separately introduced into the rivers of the subantarctic Kerguelen Islands starting in the 1950s. Ayllon et al. (2006) determined that genetic intermixing led to four distinguishable genetic units within the accessible river systems, suggesting that selection between the two founding genetic stocks provided the material for reproductive isolation.

Westley et al. (2013) experimentally demonstrated that brown trout, introduced to Newfoundland waters in the late 1800s, had increased local (fine-scale) adaptive survivability even when phenotypic differences were unremarkable.

Anadromous steelhead trout (which migrate from rivers to the sea to spawn), from Sashin Creek, Alaska, were introduced into the fishless Sashin Lake in 1926. The lake is isolated by waterfalls that prevent fish from migrating upriver. The introduced trout formed a large resident (e.g. non-migrating rainbow trout) population despite a small founding population. Experimental hybridization between the river and landlocked populations determined ‘rapid transcriptional evolution’ in the lake population, suggesting that gene transcription “provides a single mechanism for both the rapid evolution of adaptive life history characters as well as the well known physiological plasticity associated with gene expression” (Aykanat et al. 2011).

Pearse et al. (2014) examined several populations of anadromous, natural resident, and ‘recent’ introduced resident steelhead trout, in California, showing that genetic differences between anadromous and resident populations appeared to be associated with parallel adaptations in the chromosome Omy5 genomic region. The researchers suggest that “a common genetic mechanism is involved in the life-history transition from anadromy to residency in O. mykiss through independent adaptation based on standing genetic variation”. Seeb et al. (2018) noted that the region on Omy5 of interest is an inversion, and that genomic inversions may be associated with adaptive variation. They also pointed to Willoughby et al.’s (2018) study of steelhead trout introduced into Lake Michigan, which showed that the introduction led to consistent reduction of genetic diversity across the genome, but rapid genetic adaptations still occurred, associated with three specific chromosomal regions. Thus, “genetic adaptations can still occur . . . if adaptively important alleles are retained in the standing genetic variation” (Seeb et al. 2018).

Combrink et al. (2023) explored how both cutthroat and golden trout stocked in 18 high-elevation lakes in the mountains of Wyoming adapted rapidly in gill raker morphology. When zooplankton-feeding trout are introduced to a lake, the zooplankton is driven (by size-selective predation) to decrease in body size. In response to decreasing prey size, trout gill rakers (processes on the gill arch which help trap and guide suspended zooplankton towards the mouth) adapt morphologically to capture smaller prey. The researchers were able to demonstrate rapid parallel adaptations towards greater feeding efficiency.

Sparks et al. (2024) examined the introduction of pink salmon into the Great Lakes, when approximately 20,000 fry from British Columbia, destined for the Canadian Arctic, were accidentally released into Lake Superior in 1956. Elsewhere, pink salmon are obligate anadromous fish, while in the Great Lakes they are forced to live year-round in a freshwater environment. One significant change is that Great Lakes pink salmon are no longer bound by the 2-year life cycle of their natural range, “with fish maturing at 1, 2 and 3 years old” and gene flow crossing odd- and even-year spawning groups. The researchers located several genetic regions which may be under selective pressure.

Cichlid fishes are well known for their extensive radiation in the African Great Lakes. While most of these species flocks (made up of hundreds of species) have been in place since before recent history—over 12,000 years within the secular model (Martens 1997)—lake level fluctuations in Lake Malawi within recent history set the stage for repopulation and rapid divergence towards novel endemism (Owens et al. 1990). Interestingly, an undergraduate research thesis (Russell 2021) has documented rapid morphological divergence in three cichlid species introduced into Florida in the last ~50 years.

A Japanese cyprinid fish, the three-lips, Opsariichthys uncirostris, native to Biwa Lake, was inadvertently introduced to the Futatsu River of Kyushu Island in the 1980s, leading to changes within the new population in body size (decreasing), body shape, and growth rates (Iguchi et al. 2019).

Sea lampreys from the northern Atlantic colonized Lake Champlain and Lake Ontario in the mid-1800s. By the 1900s, shipping lanes allowed them to invade the other Great Lakes. The new freshwater-only populations are characterized by reduced genetic diversity (14-24%) and significant changes in physical and life history traits. Yin et al. (2021) noted several candidate genes in the sea lamprey populations’ genomes that may be responsible for rapid adaptation within the Great Lakes.

Icefish (Neosalanx taihuensis, family Salangidae) from the Yangtze River in China were introduced into the alpine Lake Erhai in the late 1980s. Population growth across the lake’s natural thermal gradient provided opportunity to examine coldwater, warmwater, and intermediate variants. Zhu et al. (2013) noted that distinctive spatially structured life-history patterns suggested rapid life-history adaptation within the populations, with temperature being the likely selective agent.

Threespine stickleback in a freshwater lake.
(CC-BY 2.0 Jason Ching / University of Washington News)

Amphibians

Cane toads are native to Central and South America, but have been introduced (usually as pest control) to other parts of the world. They were introduced to Australia in the 1930s, and have expanded their range there extensively, with individual toads traveling up to 1.8 kilometers per night. Phillips et al. (2006) showed through historical analysis that invading toads shift morphologically in leg length (longer legs aiding longer and faster dispersal), with new populations along the invasion front having longer legs, while older resident populations develop shorter legs.

Cane toads first introduced into Florida in the 1930s-1940s did not survive winter temperatures, but a group introduced into southern Florida in the 1950s established a population and expanded their range into central Florida areas where they initially failed to survive. Mittan and Zamudio (2019) determined that plasticity in cane toad’s ability to acclimate to cooler temperatures continues to play a role, but at the same time there is persistent adaptive divergence in cold temperature tolerance in the northern part of the toad’s range.

African clawed frogs were introduced into western France in the early 1980s. Araspin et al. (2020) determined that there was a distinct shift in the thermal physiology of the introduced population in comparison to source South African populations. They did note that because the introduced population derived from two separate genetic lineages in South Africa, the genetic intermixing may have proven advantageous for the rapid adaptation.

Reptiles

The common wall lizard, Podarcis muralis, has been introduced in various parts of Europe and North America. An introduced population in Passau, Germany, originating from the Bologna-Modena region of Italy in the 1930s/1940s, was examined after decades of population growth and geographic dispersal along a river. The researchers noted rapid formation of a genetic population structure, with stronger genetic differentiation at the margins, likely due to genetic drift and allele loss (Schulte et al. 2013).

Stuart et al. (2014) explored the interaction between native green anoles and invasive brown anoles in Florida. The perch height of resident green anoles was evaluated on six small Florida islands, then brown anoles were introduced to three of the islands. The brown anole populations grew quickly, and green anoles adapted rapidly with higher perch height preferences. A rapid morphological shift accompanied the green anole behavior, with the anoles exhibiting larger toepads with increased lamellae (per surveys fifteen years after the introduction). Eggs from gravid females on invaded and uninvaded islands were hatched together, offspring raised in common, and those from invaded islands continued to demonstrate the morphological changes. This strongly suggests a genetic adaptive response.

What happens when different species from similar niches invade the same territory? Three lizards (green anole, brown anole, and gold dust day gecko) were introduced into Hawaii from the 1940s to 1980s and each now have large populations there. All three are “diurnal, arboreal, insectivorous lizard[s]” (Phillips et al. 2024). Direct competition with their co-invaders is one of several factors which may have influenced distinct morphological changes within each of the three species (including both species-specific and sex-specific changes). Many of the changes measured are adaptational for running, jumping, or perching on different surfaces.

The introduction of toxic ‘prey’ is exemplified by the introduction of cane toads to Australia in the 1930s. Predators that included frogs in their diet were faced with a new amphibian with a lethal toxin, and many animals met their demise after trying to tackle the toads. Some amphibian-eating snakes have had to adapt rapidly. The red-bellied black snake is a frog-eating elapid which in its ‘toad-naïve’ populations has low resistance to cane toad toxins and no significant restraint from eating the toads (Phillips and Shine 2006). Those snakes from ‘toad-exposed’ populations had higher resistance to cane toad toxins and deliberately refrained from eating offered cane toads. The researchers noted that this rapid adaptive response likely occurred within 23 generations of the cane toad’s introduction.

Birds

The common myna, native to southeast Asia, has been introduced to numerous countries and islands in the last century or so. All founding birds were sourced from India. Baker and Moeed (1987) examined the genetics of mynas from India and several introduced populations, noting the loss of 18% of alleles determined from the India population, within the introduced populations (all 18% being rare alleles in the source population). They noted that isolation has “induced genetic changes that are substantially larger than those that have evolved in the ancestral Indian population over many millennia.” They also suggested that “bottlenecks and random drift have promoted rapid genetic shifts in isolated populations of common mynas equal to those among different subspecies of birds.”

In 1967, the U.S. Fish and Wildlife Service introduced a small group (just over 100 birds) of the endangered Laysan finch to a far distant atoll in the Pearl and Hermes Reef. The population there grew, and spread to three nearby islands within the reef. Conant (1988) (see also Pimm 1988) noted that significant morphological changes were apparent in the new population by 1984, with some differences (particularly bill shape) between groups on the different reef atolls. Later research determined that while genetic variation was lower in the reef birds, heterozygosity in alleles averaged a higher frequency than in the source population on Laysan (Fleischer et al. 1991; Tarr et al. 1998).

The snail kite is a prey-specialist raptor found in South America, the Caribbean, and southern Florida. In Florida, the bird preyed on the small native apple snail, until a much larger exotic apple snail was introduced to the state in 2004, spreading rapidly. Over the next decade, snail kites in Florida exhibited a distinct increase in fledgling body mass and bill length (Cattau et al. 2018). Survival of first year kites increased with the larger kites. The authors noted that the immediate response were likely due to phenotypic plasticity, but also suggested there was evidence of ‘additive genetic variation’ that increased throughout the decade, pointing to ‘cryptic genetic variation’ (“genetic variation expressed in response to novel environmental conditions”).

Dark-eyed juncos established a suburban campus population at the University of California of San Diego in the 1980s. There is a resident year-round population that is joined between September and May by visiting wintering juncos from other populations. Friis et al. (2022) noted that the UCSD juncos had diverged to an extent “comparable to those of long-established, geographically isolated subspecies such as townsendi and pontilis from Baja California sky islands, and in sharp contrast to the low genetic structure found among more northern subspecies.” The divergence (from a small number of initial birds) likely arose from founder effects and genetic drift.

Mammals

Rodents colonizing islands have provided ample opportunity to document rapid morphological change. ‘Dramatic’ rapid changes have also been noted with rodents invading urban mainland environments (Pergams and Lawler 2009). Deer mice populations on three different California Channel islands demonstrate distinctive traits characterizing endemic subspecies on each island—each island population likely founded by separate dispersal events from the mainland population (Pergams and Ashley 1999). Even after the development of island endemism, these three subspecies continued to change morphologically, with high rates of ‘phenotypic change’ noted from museum specimens collected over the previous century. This was despite reduced mtDNA diversity, “suggesting small numbers of founders and low historical gene flow” (Pergans and Ashley 1999). The authors suggested a few faunal and floral introductions in the 1800s-1900s that might have contributed to selective pressures, though no specific causal factors were identified.

Black rats were introduced to the Channel Island of Anacapa approximately 1850, and eradicated in the early 2000s. Museum specimens collected from 1940 to 2000 demonstrated an increase in size of all cranial traits measured (Pergams et al. 2015): “All rat cranial traits changed, some dramatically, and all became larger. When considered in the more accurate haldanes, some of these changes are among the fastest on record.”

Li et al. (2021) investigated Chinese white-bellied rat populations on islands created when a hydroelectric dam was constructed in 1959, which flooded a mountain forest and created an anthropogenic archipelago. All island populations of the white-bellied rats were significantly larger in body length and mass than the mainland population, while those on smaller islands were larger than those on bigger islands. The authors suggest that a greater abundance of predators may negatively influence rodent size. They were not able to definitely point to either plasticity or genetic adaptation as responsible for the rapid shift in body size.

Domestication is known to create rapid phenotypic changes, but feral domestic animals are under different natural selective pressures. Van Vuren and Bakker (2009) noted that feral sheep on Santa Cruz Island in California, originating from flocks brought to the island from the 1850s to 1890s. Feralization increased as annual roundups died off, with an influx of Arizona stock in the 1930s not adding to the manageability of the sheep as was hoped. Eradication began in the 1980s until all sheep were removed from the island. The feralization was accompanied by rapid changes, including reduced body size (commonly seen in other feral island sheep) and increased wool loss (self-shedding).

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