Natural Selection stands at the core of the mechanisms of evolution beside genetic drift and gene flow. However it is the most misundersood concept in biology. Due to prevailing usage of this concept in explaining biological diversity, people use it blindly and also fewer can dare to look at it differently under fear of standing wrong. In this knol, I am gathering the evidences that could help us looking at this concept from a different perspective.
Natural Selection is used for adaptive traits that species have evolved to be compatible with their environments. Hence we notice the usage of a range of adjectives with natural selection. Just consider the life cycle of an organism and usage of the natural selection to it. As an organism's genotype seldom changes during its developmental stages, so the natural selection works on phenotypic changes during the life cycle of the organism.
A. Parent: a. Fecundity Selection, b. Gametic Selection
B. Gamets: Compatibility Selection
C. Zygot: Viability Selection
D. Adult: a. Sexual Selection, b. Survival Selection
1. Ntive VS Invasive Species
One of the most debated topic in ecology is the problem of the native and invasive species. Often invasive species successfully spread in the new environment at the cost of the native species.
Discussion:
Native species are well adapted to their environment and have long lived successfully in their environment but as invasive species, invade to their environment they compete much better than native species. The main reasons behind this success is not adaptation (as native species is better adapted to their environments/they have lived for a long time in this environment) but the absence of the Predators and parasites for the invasive species in this new environment which favors their population. In meantime the native species - though well adapted - suffer from predators and parasites.
Conclusion: Invasive species is better favored in new environment over native species. Favorability is more appropriate usage the adaptability.
2. Prey and Predator Relationships:
A large part of the natural selection experiments are based on the prey and predator relationships, however most of these experiments are oversimplication of the relationship.
Observations:
a. Population of the predators are smaller than population of thier preys.
b. Population of the predators follow the oscilations in the population of their preys. The famous example is the coupled oscilaitons in population of Lynx (predator) and Hare (Prey).
c. Predators usually prefer to prey on young, weak, sick and old (easy preys).
d. They can't kill all their preys.
e. Preys have evolved some sort of defense mechanims against the predators.
d. Prey and predators are part of a larger food web.
Discussion:
Let's simplify the problem by generalizing it. We assume that the prey is a herbivore and its predator is a carnivore. Usually in natural selection experiments the focus is on how population of preys are limited by predators or how some preys are suffer more mortality and others less. However, more important than this factor is the limitation of the preys by resources available in the environment. If one species of prey is more vulnerable to predator either because of visibility, sedantary life style or risk taking strategy, etc and another one less, it is taken as an evidence of the natural selection. This could be true only if we could exclude all other ecological factors in natural environment (as it is usually done in N.S experiments). But population of prey is not limited only by predators but more important is resource limitation. In our exmaple, the hebivore is limited by availability of herbs it feed on. If the herb is overgrazed or declined by other factors the population of the prey will decline, irrespective of, how successful were they in escaping the predators. Beside this, the predators prefer easy preys like young, weak, sick and old as predators are also limited by energy and time they invest in preying and handling of the prey.
Conclusion:
Based on observation and discussion (very limited one, it could be much expanded) we can say the adaptations of preys play in role in the controlling the population but it is only one part of the story and other factors also determine the population, (IT IS NOT only Commons that have ADAVANTAGES but also RARE that have ADVANTAGE (though we have not discussed it here) such as resources of preys and weather. So, adaptation or natural selection is a very narrow and limited usage for this whole phenomenon but Natural favorability could incorporate all, from resources limitations, to changes in weather, to adaptations of preys in escaping or evolving some sort of defense mechanisms and behaviors of predators in choosing easy preys.
3. Ecotones:
The boundaries between, Grassland: Shrubland, Grassland:forest, Tundra:forest, Broad-leaved forest:Needle-leaved forest are a few example of the ecotones. Two factors cause such boundaries between the two communities,
1. Physical environment changes abruptly such as slope, aquatic-terresterial, wet:dry, eastward:westward direction or northward:southward facing, warm:cold, acidic:alkaline (soil for example) etc.
2. Dominance of a community inhibit the spread of other community, e.g, grass inhibit the spread of trees by absorbing the moisture in soil that is key for germination of seeds and trees inhibit the spread of grass by shadding.
Though, each community is well adopted to their environment but the ecotone is not a stable boundary. The environmental changes is constantly changing it. For example the climate warming favors the forests to invade into tundra and it is also favors the forests the invade on the grassland. As this shift is better explained in favorability of nature rather than adaptability as it changes with changes in environment and changes in environment is sharp and much quicker than the time requires for adaptations.
No comments:
Post a Comment