Natural Selection

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The theory of natural selection was postulated by Charles Darwin (and independently by Alfred Wallace) who described it as 'survival of the fittest'

According to the theory of natural selection, organisms living today have changed over time and evolved from common ancestral organisms

Natural selection is a mechanism by which evolution is thought to occur and is based on five key assertions:


1. Genetic Variation

In order for natural selection to occur, there must first be inheritable variation in traits within a population

Genetic variation describes the differences between the genetic make-up of individual organisms and accounts for the diversity of features seen both within, and between, species

There are a number of sources of genetic variation within a population, including:

  • Gene mutations:  A change in the DNA sequence of a gene which may alter the expression of the associated trait
    • A gene mutation may only be inherited if it occurs within germline tissue (i.e. the cells responsible for gamete production)
  • Gene flow:  The introduction of new alleles into a population as a result of the immigration of new organisms into a population 
    • Gene flow can also be used to describe the removal of alleles from a population as a result of emigration
  • Sexual reproduction:  The combination of alleles to form new traits as a result of random mating
    • Variation can also occur in sexually reproducing organisms as a result of crossing over (gene recombinations) during prophase I of meiosis
    • Variation can also be created as a result of the random orientation of chromosomes (independent assortment) during metaphase I of meiosis


2. Competition for Survival

  • The Malthusian dilemma states that populations tend to multiply geometrically, while food sources multiply arithmetically
  • Hence populations tend to produce more offspring than the environment can support
  • When there is an abundance of resources, a population can achieve a J-curve maximum growth rate (biotic potential)
  • However, with more offspring there will be less resources available to other members of the population (environmental resistance)
  • This will lead to competition for available resources and a struggle for survival
  • The result of this competition will be an increase in the mortality rate, leading to an S-curve growth rate 


3. Selection Pressures

External agents which affect the ability of an organism to survive are referred to as selection pressures

Selection pressures can be negative and decrease the occurrence of a trait, or beneficial and increase its proportion within a population

Types of selection pressures include:

  • Resource availability:  Presence of sufficient food, habitat and mates
  • Environmental conditions:  Temperature, weather conditions or geographical access
  • Biological factors:  Predation and disease


4. Differential Reproduction

Adaptations are features of organisms that aid their survival by allowing them to be better suited to their environment 

These adaptations may be categorised in a number of different ways:

  • Structural:  Physical differences in biological structure (e.g. tail and muzzle length in cats and dogs)
  • Physiological:  Variations in detection and responses by vital functions (e.g. homeothermy, colour blindness)
  • Behavioural:  Differences in patterns of behaviour (e.g. certain possum species feigning death when threatened)
  • Developmental:  Variable changes that occur across the lifespan of an organism (e.g. changes in bird plumage from juvenile to adult)
  • Biochemical:  Differences in molecular composition of cells and enzyme functions (e.g. different blood types, skin pigmentation)


Organisms with beneficial adaptations will be more likely to survive long enough to reproduce and pass on their genes

Organisms without these beneficial adaptations will be less likely to survive and pass on their genes

Darwin described this differential reproduction as 'survival of the fittest' - whereby the fittest are those most capable of reproducing

 

5. Change in Allele Frequency

As a result of differential reproduction, features which confer a survival advantage are more likely to be passed on to subsequent generations

Over time, this will change the relative proportions of an allele (and hence the genetic composition) within a given population 

As the viability of a particular feature was determined by naturally-occurring selective agents, this process is described as natural selection


Mechanism of Natural Selection


 Summary of Natural Selection

  • There is genetic variation within a population (which can be inherited)
  • There is competition for survival (populations tend to produce more offspring than the environment can support)
  • Environmental selective pressures lead to differential reproduction
  • Organisms with beneficial adaptations will be more suited to their environment and more likely to survive to reproduce and pass on their genes
  • Over generations there will be a change in allele frequency within a population (evolution)