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Mangrove Finch
The biosphere: a classic example of a dog-eat-dog world where only the toughest prosper, and the weak perish. The victors grow stronger and stronger at each turn; changing, adapting, evolving, guided by the forces of natural selection. However, Mother Nature does not always make the right choices.
The phenomenon of evolutionary “dead ends” has been brought to light fairly recently. The term refers to a state where a species cannot undergo any further evolution. This does not mean the organism is evolutionarily perfect; on the contrary, such situations often lead to the inevitable decline of its kind. Highly specialised organisms (referred to as ‘specialists’) often have reduced potency to persist and diversify compared to their relatively underspecialised ‘generalist’ counterparts.
The detection of dead ends in organisms that are not yet extinct is quite difficult, due to the ambiguity in the meaning of “specialisation”, and difficulty in the detection of extinction trends. Hence, the concept majorly remains as a speculation to explain the influences of specialisation on a species’ macroevolutionary potential. Fossil records, too, are spotty and incomplete. However, quite a few examples exist to illustrate this intriguing phenomenon.
Sexual dimorphism refers to distinct differences in the appearance between the sexes of an organism. Studies have been performed on sexual dimorphism in organisms like ostracods and male birds, and its possible influence on evolution. Investing heavily in reproductive fitness may hamper a species’ overall ability to survive. The idea may initially sound far-fetched; after all, sexual dimorphism exists in almost all living organisms we see around us.
However, the bright colours that male birds evolved to attract mates also serve to attract predators. It should be noted that it is the type of sexual dimorphism that affects survival ability, and not sexual dimorphism itself. Comparative studies on extinct and existing ostracod species revealed that the extinct males had bigger carapaces, which housed bigger male sex organs.
Sexual dimorphism in ostracods
On the other hand, self-fertilising organisms are also thought to potentially lead to dead ends. Only around 20-25% of existing plant species are selfers. Repeated selfing leads to loss of evolutionary flexibility as well as accumulation of deleterious characteristics. Both of these problems can, however, be overcome if the species is capable of reverting to outcrossing.
Although specialisation implies that a species is highly adapted to its environment, it need not make it adaptable to varying external conditions. This applies not only to morphological characteristics like sexual dimorphism, but specialization in general: in habitat, geographic range, maturation age, behaviour, and environmental tolerance. Darwin’s finches from the Galapagos Islands have long served as model organisms for understanding evolution and specialisation; and one of these finches, the Mangrove finch, may currently be encountering its very own dead end. The mangrove finch is one of the rarest birds in the world, with less than 100 surviving individuals in two different populations on the island of Isabela, geographically isolated by miles of volcanic terrain. Studies on these finches have shown that these two tiny populations, of around 80 and 20 individuals each, are also becoming reproductively isolated from one another. Birds from one population do not respond as strongly to the birdsongs of the other population as to their own. This could have drastic repercussions on the survival of the species.
It is implicit that niche specialists have the ability to grow faster than generalists in one or two habitats. The generalist, a metaphorical “jack of all trades”, has a growth rate that is much more consistent over a wide range of habitats. They are also more likely to establish themselves as invasive species in non-indigenous habitats, due to their abundance and flexibility. This grants the generalist an increased overall ability to survive. In fact, over geological time, most mass extinction events are largely associated with the extinction of specialist species.
But does the phenomenon of dead ends make specialisation undesirable? Does it mean a large number of generalist species is better for nature than an assortment of specialists? The answer to these questions is muddled by our incomplete understanding of the concept of dead ends. Rich biodiversity is undoubtedly advantageous to an ecosystem, and helps stabilise it. A reduction in the number of specialists in an ecosystem is indicative of ongoing degradation. Hence more questions arise: should an increase in the number of generalists be promoted or discouraged? This would lead to a phenomenon termed as biotic homogenisation, which can be seen as the opposite of differentiation: ecosystems become increasingly similar and simplified. Since natural selection acts on diversity, both between organisms and between species, if that diversity is hampered, the possible future evolutionary paths are limited. Biotic homogenisation is a relatively new area of study as well, and our understanding of it is largely half-baked, nevertheless, it is regarded as one of the key components of the ongoing biodiversity crisis.
An interesting philosophical take on evolutionary dead ends examines the example of our human race. Is it possible that humans, the human intellect and consciousness are heading towards a dead end? It depends on the perspective from which the question is looked at. It would do well to keep in mind that a dead end need not imply that the organism is evolutionarily perfect, nor that there is no further room for improvement. Humans have evolved intellects that are undoubtedly superior to that of other organisms on earth. We have used it to communicate effectively amongst ourselves, build societies, and develop technologies that are characteristic of our species. We have even been regarded as the pinnacle of evolution. This could not be further removed from the truth. We have divided ourselves into societies that are bent on destroying one another. The amount of impact that our technological advents continue to pose on the rest of the ecosystem and its organisms is more than evident. Will we end up paving the way for our own extinction, and putting an end to the rest of life as we know it in the process? Only time will tell.
- written by Aparna R
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