Natural Selection & Evolution (SL IB Biology)

• Species do not stay the same over time; the species that we see around us today have developed over millions of years
  • This process of species change is known as evolution
• Evolution can be defined as:

Changes in the heritable characteristics of organisms over generations

• Scientist Charles Darwin came up with the theory of evolution by natural selection after a five-year expedition around the world during which he observed an incredible variety of organisms 
• Natural selection is the process that drives evolution. It can be defined as:
  • The process by which organisms that are better adapted to their environment survive, reproduce, and pass on their advantageous alleles, causing advantageous characteristics to increase in frequency within a population

On the Origin of Species

• Darwin published his book "On the Origin of Species" in 1859
• It contained the following statements and deductions
  • The increased survival chances of individuals with advantageous alleles mean that advantageous characteristics are more likely to be passed down through the generations because those individuals reproduce more
  • The number of individuals in a population with a particular favourable characteristic will increase over time; the characteristic is said to increase in frequency
  • Eventually, this favourable characteristic will become the most common of its kind in the population; the population can be said to have adapted to its environment through the process of natural selection
  • While favourable characteristics increase in frequency by natural selection, unfavourable characteristics decrease in frequency by the same process
    • Individuals with unfavourable characteristics are less likely to survive, reproduce, and pass on the alleles for their characteristics, so unfavourable characteristics are eventually lost from the population

NOS: Darwin’s theory provided a convincing mechanism and replaced Lamarckism. Students should understand the meaning of the term “paradigm shift”

• Darwin's theory was seen as very controversial at the time, it is said to have caused a paradigm shift
  • Paradigm shifts occur when scientific research contradicts previous assumptions
  • Darwin's theory replaced Lamarckism
    • This was the idea that an organism could pass on physical characteristics they acquired during their lifetime to their offspring
• Nearly 200 years of genetic research backs up Darwin's theory of evolution by natural selection

An example of natural selection in rabbits

• Variation in fur colour exists within a rabbit population
  • One allele codes for brown fur and another for white fur
• Rabbits have natural predators such as foxes, which act as a selection pressure
• The brown rabbits are more likely to survive and reproduce due to having more effective camouflage
• When the brown rabbits reproduce they pass on their alleles to their offspring
• The frequency of brown fur alleles in the population will increase
• Over many generations, the frequency of brown fur will increase and the frequency of white fur will decrease

Selection pressures in a rabbit population diagram

Selection pressures act on a rabbit population for one generation; predation by foxes causes the frequency of brown fur in rabbits to increase and the frequency of white fur in rabbits to decrease

• Differences exist between organisms of the same species
• These differences are known as variation
• Examples of variation include:
  • Coat colour in mammals
  • Body length in fish
  • Flower colour in flowering plants
• The process of natural selection can only take place when there is variation in a population
  • If every organism in a population is identical then no individual will be favoured over another
  • There will be no advantageous characteristics leading to increased survival and chances of reproduction, and so there would be no increased likelihood of passing on those advantageous alleles
  • In this situation, a population's characteristics would remain the same over time and it would be unable to adapt to any environmental changes
• Variation results from small differences in DNA base sequences between individual organisms within a population
• There are several sources of these differences in DNA base sequences:
  • Mutation
  • Meiosis
  • Random fertilisation during sexual reproduction

Mutation

• The original source of genetic variation is mutation
  • A mutation is a change in the DNA base sequence that results from a copying error during DNA replication
• Mutation results in the generation of new alleles
• Mutations that take place in the dividing cells of the sex organs lead to changes in the alleles of the gametes that are passed on to the next generation
• A new allele may be advantageous, disadvantageous or have no apparent effect
  • An advantageous allele is more likely to be passed on to the next generation because it increases the chance that an organism will survive and reproduce
  • A disadvantageous mutation is more likely to die out because an organism with such a mutation is less likely to survive and reproduce
• Note that a mutation taking place in a body, or somatic, cell will not be passed on to successive generations, and so will have no impact on natural selection
• Mutation is the only source of variation in asexually reproducing species

Meiosis

• There are two main events during the process of meiosis that generate variation
  • Crossing over
  • Random orientation
• Crossing over is the process by which homologous chromosomes exchange alleles
  • During meiosis I homologous chromosomes pair up
  • The non-sister chromatids can cross over and get entangled
  • As a result of this, a section of chromatid from one homologous chromosome may break and rejoin with the chromatid from the other chromosome
• This swapping of alleles is significant as it can result in a new combination of alleles on the two homologous chromosomes

Chromosomes crossing over diagram

The process of crossing over can result in new combinations of alleles

• Random orientation occurs due to the independent arrangement of homologous pairs along the equator of the cell during metaphase I
  • Each pair can be arranged with either chromosome on either side of the cell; this is completely random
  • The orientation of one homologous pair is independent, or unaffected by the orientation of any other pair
  • This is sometimes described as independent assortment
• The homologous chromosomes on the equator of the cell are pulled apart to different poles, and will each end up in a separate daughter cell
• The combination of alleles that end up in each daughter cell depends on how the pairs of homologous chromosomes were lined up
• To work out the number of different possible chromosome combinations the formula 2ⁿ can be used, where n corresponds to the number of chromosomes in a haploid cell
  • E.g. for humans this is 2²³ which calculates as 8,324,608 different combinations

Random orientation of chromosomes diagram

Random orientation of chromosomes

Random fertilisation during sexual reproduction

• Meiosis creates genetic variation between the gametes through crossing over and independent assortment
• This means each gamete carries substantially different alleles
• During fertilisation any male gamete can fuse with any female gamete to form a zygote
• This random fusion of gametes at fertilisation creates genetic variation between zygotes as each will have a unique combination of alleles
• There is an almost zero chance of individual organisms resulting from successive sexual reproduction being genetically identical

Fusion of gametes during fertilisation diagram

The random fusion of gametes during fertilisation

Sources of genetic variation table

• The number of offspring, or young, produced in each breeding event differs between species
  • Some species produce small numbers of young, e.g. elephants usually give birth to just one baby per pregnancy
  • Some species produce many offspring e.g. some species of ant can lay 3-4 million eggs in one go
• It is more usual for organisms to produce multiple offspring
• There are often more offspring produced than can be supported by the surrounding environment
  • Darwin noticed this, and named the phenomenon 'overproduction of offspring'
• The overproduction of offspring within a population leads to competition for resources as population size is naturally limited by environmental factors
  • E.g. availability of food, space and light
• These environmental factors limit the carrying capacity of a species' population
  • An insufficient amount of resources means that a large number of offspring fail to survive and reproduce
• Overproduction of offspring and competition for resources promote natural selection

• Habitats have limited resources
• When individuals within a habitat fight over or try to obtain a limited resource they are said to be "competing"
  
• Competition can occur between individuals of different species (interspecific competition) and between individuals of the same species (intraspecific competition)
• Intraspecific competition plays a greater role in evolution because:
  • Individuals are more likely to interact with members of their own species
  • Individuals of the same species share the same niche
  • They are affected by the same abiotic and biotic factors

How does intraspecific competition promote natural selection?

•  Variation is present within the species population
• Some individuals have characteristics that make them better adapted for survival
  • For example, lions that are stronger and faster are more likely to be able to catch prey and therefore more likely to survive
  • This is sometimes described as 'survival of the fittest'
• Individuals that are well adapted and survive into adulthood are more likely to find a mate and reproduce, producing many offspring
• Individuals that are less well adapted, do not survive long into adulthood and are likely to reproduce less often than those that survive for longer, so producing fewer offspring
  • These individuals may not reach adulthood and so do not get the chance to reproduce at all

• Many of the characteristics that affect an individual's chances of survival are determined by the alleles of genes present in their DNA
• Characteristics that are determined by alleles are heritable
  • Heritable characteristics can be physical e.g. the length of a giraffe's neck, or behavioural e.g. the innate behaviour of a woodlouse moving towards a dark hiding place
• Individuals with characteristics that increase their chances of survival are likely to produce more offspring
• This means that they are more likely to pass on the alleles that code for these advantageous characteristics to their offspring
• Note that non-heritable characteristics are not passed on to offspring
  • Non-heritable characteristics are those acquired during the lifetime of an organism e.g. gaining weight after eating lots of nuts and berries in autumn, or being injured by a predator