Evolution by Natural Selection

There’s no doubt that the most instrumental contribution to the field of biology was Darwin’s Theory of Evolution. It is important to note that when we use the term ‘Theory’, we aren’t using the general definition (which is often misquoted by opponents of evolution), but rather the specific scientific definition:

Theory (general): “a hypothesis assumed for the sake of argument or investigation, an unproved assumption – conjecture”.

Theory (scientific): “a well-substantiated explanation of some aspect of the natural world, based on a body of facts that have been repeatedly confirmed through observation and experiment”.

And evolution is one of the most well-accepted facts within the scientific community, aided by Darwin’s proposed mechanism of natural selection, which is enabled through three key concepts:

  1. Genetic variation, where members of the population vary in their phenotypic traits, which natural selection acts on.
  2. Differential reproductive success, where there is relationship between an individual’s trait value and their capability to survive and reproduce (fitness).
  3. Inheritance, where there is a positive correlation between the phenotype of the parents and their offspring – genetic factors are passed on.

However, evolution can also arise in the absence of differential reproductive success and selection through a process called genetic drift, which is favoured in populations of small effective size. Population evolution through drift occurs from stochastic changes in allele frequencies, with chance events arising from the unpredictability of nature favouring the survival and reproduction of random individuals.

Conversely, adaptation is defined as both the process of the population increasing its fitness to a given environment, as well as the end state trait that enables this fitness increase. Fitness can present itself at a number of stages in an individual’s life, in terms of survivability (viability) to reach an age where they can reproduce, finding a mate to reproduce with (sexual selection), the relative production of gametes between individuals (fecundity), the viability of these gametes and the viability of the ensuing zygote. Fitness only becomes a meaningful measure when the individual is compared with the average of the population, generating a calculation of relative fitness.

This is exemplified through a project coordinated by evolutionary biologist Hopi Hoekstra, where the adaptation of mice to their habitat substrate – whether it be brown dirt or white sand – dictated their coat colour. Following the above framework for evolution, there was (1) phenotypic variation between populations of mice living in different regions, (2) fitness differences in terms of their camouflage against owl predation and (3) a genetic basis for the trait of fur colour (mutation in the cell surface-bound receptor, blocking the uptake of alpha-MSH which darkens fur) that is passed on to their offspring.

Hopi mice
Hopi E. Hoekstra et al. (2006) A Single Amino Acid Mutation Contributes to Adaptive Beach Mouse Color Pattern, Science 313:5783, 101-104.

Humans have also been agents of selection, in situations such as the fisheries industry, where selectively keeping larger fish within the catch precludes their genetic information from passing to the next generation (if it is caught pre-reproduction) and leads to the evolution of smaller fish.

While natural selection has proved itself a masterful designer, taking the complex morphology of the eye as an example, it has also taken advantage of stochastic shortcuts through exaptation. This is the genetic concept where existing traits are co-opted for an alternative function that improves the organism’s fitness. Examples include wings on ancestral insect’s third thoracic section being co-opted to appear like the leaves of a range of different trees for camouflage (ie. treehoppers), or feathers originally present on the tails of dinosaurs (proposed to being used for the function of sexual signalling) being altered in its avian descendants for the purpose of flight (yes – birds are one of the closest ancestors of dinosaurs).

But there are natural constraints on the degree to which natural selection can optimise traits. To visualise these constraints, we can borrow the economics framework of supply and demand, where supply is the generation of variation due to mutations, and demand is the requirement for fitness increases in response to changing environmental conditions. The first constraint involves the natural physical limits that the demand for evolution can’t overcome.

  1. Physical limits in concordance with gravity and other such biomechanical forces (ie. elephants will never be able to have long, spindly legs), which contrive biologically conserved relationships such as the ratio between femur and body mass in mammals.
  2. Lack of genetic variation. When species live for millions of years in a benign environment that doesn’t necessitate the use of an ancestral gene for a trait (ie. desiccation resistance in Drosophila birchii living in the consistently humid rainforest), then mutations can accumulate at those regions and render them non-functional.
  3. Arms races between predator and prey or host and pathogens, which constantly shifts organisms down off their adaptive peaks due to changing their adaptive landscape, which dictates what it is to be fit.
  4. Natural selection works on a per generation basis, leaving it blind to what could be beneficial to the population that may encounter changing environmental conditions in the future.

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