Genetic diversity + Natural selection

• There can be different verisons of a single gene - these are called alleles

  Alleles code for different versions of characteristics
• Genetic diversity is the number of different alleles of genes in a species or population

  A large number of different alleles in a population means a large variety of different characteristics and a high genetic diversity
• Genetic diversity is important - if a population has low genetic diversity it might not be able to adapt to change in the enviroment and the whole population could be wiped out by a single event
• Genetic diversity within a population is increased by:

  • Mutations in the DNA forming new alleles. Some of these are advantageous while some lead to problems
  • Different alleles being introduced into a population when individuals from another population migrate into it and reproduce. This is known as gene flow
• A genetic bottleneck is an event that causes a big reduction in a population, when a large number of organisms within a population die before reproduction

  This reduces the number of different alleles in the gene pool and so reduces genetic diversity.

  The survivors reproduce and a larger population is created from a few individuals
• The founder effect describes what happens when just a few organisms from a population start a new colony and there are only a small number of different alleles in the initial gene pool
• The frequency of each allele in the new colony might be very different to the

  frequency of those alleles in the original population — for example, an allele

  that was rare in the original population might be more common in the new

  colony. This may lead to a higher incidence of genetic disease.
• • Randomly-occurring mutations sometimes result in a new allele being formed.
  • This can be harmful, which usually means that the mutated allele quickly dies out.
  • However, some mutations can produce alleles that are beneficial to an organism helping the organism to survive in certain environments.
• When the allele codes for a characteristic that increases the chances for an organism surviving, its frequency within the population can increase and is known as natural selection
• Natural selection

  1. Not all individuals are as likely to reproduce as each other - differential reproductive success within a population - individuals that have an allele that increases their chance of survival are more likely to survive, reproduce and pass on their genes, than individuals with less advantageous alleles
  2. This means that a greater proportion of the next generation inherits the beneficial allele
  3. They, in turn, are more likely to survive, reproduce and pass on their genes
  4. So the frequency of the beneficial allele in the population increases from generation to generation
  5. Over generations this leads to evolution as the advantageous alleles become more common in the population
• Directional selection is where individuals with alleles for characteristics of an extreme type are more likely to survive and reproduce.

  This could be in response to an environmental change.
• Some individuals in a bacterial population have alleles that give them resistance to an antibiotic. The population is exposed to the antibiotic, killing bacteria without the resistance allele.

  • Example — Bacteria evolving antibiotic resistance

  The resistant bacteria survive and reproduce without competition, passing on the allele that gives antibiotic resistance to their offspring. After some time, most organisms in the population will carry the antibiotic resistance allele —
• Natural selection leads to organisms becoming better adapted to their environment.

  Adaptations are features that help organisms to survive in their environment. They can be behavioural, physiological or anatomical.
• 1. Behavioural adaptations

  These are ways an organism acts that increase its chance of survival.
• Physiological adaptations

  These are processes inside an organism’s body

  that increase its chance of survival.
• Anatomical (structural) adaptations

  These are structural features of an organism’s body

  that increase its chance of survival
• A species is defined as a group of similar organisms that can reproduce to give fertile offspring
• A population is a group of organisms of the same species living in a particular area at a particular time - so they have the potential to interbreed. Species can exist as one or more populations
• • The gene pool is the complete range of alleles present in a population. How often an allele occurs in a population is called the allele frequency. It’s usually given as a percentage of the total population
• Variation is the differences that exist between individuals. Variation can be caused by genetic and/or environmental factors
• Although individuals of the same species have the same genes, they have different alleles - this causes genetic variation within a species

  The main source of this genetic variation within a species is mutation - when changes in the DNA base sequence lead to the production of new alleles

  But genetic variation is also introduced during meiosis and because of the random fertilisation of gametes during sexual reproduction
• Variation within a species can also be caused by differences in the environment like food, climate, lifestyle

  Most variation within a species is caused by a combination of genetic and environmental factors, but only genetic variation results in evolution
• The effect of natural selection on allele frequencies depends on the selection pressures acting on the population
• Directional selection

  This is where individuals with alleles for a single extreme phenotype are more likely to survive and reproduce. This could be in response to an environmental change.
• • Speciation is the development of a new species from an existing species. It occurs when populations of the same species become reproductively isolated — changes in allele frequency cause changes in phenotype, which mean they can no longer interbreed to produce fertile offspring
• • Reproductive isolation can occur when a physical barrier, e.g. a flood or an earthquake, divides a population of a species, causing some individuals to become separated from the main population. This is known as geographical isolation. There is no gene flow (transfer of genes) between the two populations, which can lead to allopatric speciation
• In allopatric speciation:

  Populations that are geographically separated will experience slightly different conditions. For example, there might be a different climate on each side

  of the physical barrier.
• The populations will experience different selection pressures and so different changes in allele frequencies could occur

  • Different alleles will be more advantageous in the different populations, so natural selection occurs. For example, if geographical separation places one population in a colder climate than before, longer fur length will be beneficial. Directional selection will then act on the alleles for fur length in this population, increasing the frequency of the allele for longer fur length.
  • Allele frequencies will also change as mutations will occur independently in each population.
  • Genetic drift may also affect the allele frequencies in one or both populations
• Over time, this can lead to speciation.

  The changes in allele frequency will lead to differences accumulating in the gene pools of the separated populations, causing changes in phenotype frequencies.

  • Eventually, individuals from the different populations will have changed so much that they won’t be able to breed with one another to produce fertile offspring — they’ll have become reproductively isolated.
  • The two groups will have become separate species