Topic 5 - Evolution

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• IB Biology DP
• 5. Evolution and Biodiversity
• CONTENTS


  • 5.1 Evolution & Natural Selection
  • 5.2 Classification & Cladistics
• Evolution


  Changes in the heritable characteristics of organisms over generations
• Heritable characteristics


  Those that can be inherited by, or passed on to, the next generation
• Changes in characteristics that are not inherited, e.g. a plant having its leaves eaten, do not lead to evolution
• Heritable characteristics


  Determined by the alleles of genes that are present in an individual
• Alleles


  May change as a result of random mutation, causing them to become more or less advantageous
• Natural selection


  Heritable characteristics that are advantageous are more likely to be passed on to offspring, leading to a gradual change in a species over time

  
• Changes in the heritable characteristics of organisms can also lead to the development of completely new species
• Theoretically, at the origin of life on Earth, there would have been just one single species
• Speciation


  1. Species evolve into separate new species

  2. These species would then have divided again, each forming new species once again

  3. Over millions of years, evolution has led to countless numbers of these speciation events, resulting in the millions of species now present on Earth

  
• Speciation


  The development of new species from pre-existing species over time

  
• Speciation can result from geographical isolation


  • A species is separated into two populations by e.g. being on different islands or different sides of a mountain range
  • The ocean and the mountains in these examples are geographical barriers
  • This creates two populations of the same species who cannot interbreed due to being in different places; as a result, no exchange of genes, or gene flow, will occur between them
  • The different environmental conditions for the two populations might mean that different alleles are advantageous, so different alleles are more likely to be passed on and become more frequent in each population; this is the process of natural selection
  • Over time, the two populations may begin to differ physiologically, behaviourally and morphologically to such an extent that they can no longer interbreed to produce fertile offspring; speciation has occurred

  
• Speciation in trees


  • A population of trees exists in a mountainous habitat
  • A new mountain range forms that divides the species into two populations
  • The geographical barrier prevents the two populations from interbreeding, so there is no gene flow between them
  • The two populations experience different environments, so different alleles become advantageous
  • Different alleles are therefore more likely to be passed on in each population
  • Over thousands of years, the divided populations form two distinct species that can no longer interbreed to produce fertile offspring

  
• Speciation commonly occurs after a species expands its range to new geographical areas. This phenomenon is evidenced by the large number of endemic species found on islands such as the Hawaiian honeycreepers; a group of more than 50 bird species found in the Hawaiian archipelago!
• Fossils


  The preserved remains of organisms, or the traces left by organisms, such as footprints, burrows and faeces
• Fossils


  • They can be preserved, e.g. in rocks, by the process of petrification, during which the tissues of organisms are replaced with minerals
  • The fossil record is small in relation to the number of organisms that have ever lived, due to the conditions for fossilisation being so rare
  • We can tell from fossils that organisms have changed significantly over millions of years
  • Fossils, as well as the rocks they are found in, can be dated, allowing us to accurately put fossil organisms into a sequence from oldest to youngest, and therefore see how organisms have changed through evolutionary time
  • The fossil record shows the kind of progression that the theory of evolution would lead us to expect, with older fossils showing simpler life forms and complexity increasing with time
  • The sequence of fossils aligns with ecology groups: plant fossils appear earlier in the fossil record than animals, and plants with the ability to be pollinated by insects appear before insect pollinators
  • Fossils can show evidence for transitional species, showing how one species could evolve into another e.g. Ambulocetus is a fossil that links amphibians with early whale-like organisms, and Archaeopteryx appears to link reptiles with birds
• Selective breeding


  A process in which humans choose organisms with desirable characteristics and breed them together to increase the expression of these characteristics over many generations
• Desirable crop varieties and livestock with exaggerated characteristics from wild varieties and species

  • Desirable crop varieties include those with a high yield and disease resistance
  • Exaggerated characteristics in livestock include thick, heavy wool in sheep, and large volumes of milk produced in dairy cattle
• Artificial selection


  Also known as selective breeding, it makes use of the principles of natural selection, but is carried out by humans
• Selective breeding process


  1. The population shows variation; there are individuals with different characteristics

  2. Breeders select individuals with the desired characteristics; selected individuals should not be closely related to each other

  3. Two selected individuals are bred together

  4. The offspring produced reach maturity and are then tested for the desirable characteristics; those that display the desired characteristics to the greatest extent are selected for further breeding

  5. The process is repeated over many generations; the best individuals from the offspring are continually chosen for breeding until all offspring display the desirable characteristics
• Homologous structures


  Body parts that may look and function very differently but share structural similarities
• Homologous structures


  • The limbs of animals are a good example; animals have many different mechanisms of motion and limb use, but the basic arrangement of bones in many different types of limbs is very similar
  • One explanation for the similarities of these different limbs is that of adaptive radiation; the idea that organisms with homologous structures have all evolved from a shared, common ancestor but have adapted to different environments in the process

  
• Pentadactyl limb


  • A limb that has five digits i.e. five fingers or toes
  • Pentadactyl limbs are present in many species from many groups of organisms, including mammals, birds, amphibians, and reptiles
  • In different species, the pentadactyl limb has a similar bone structure but can enable an animal to move in a very different way
• Vestigial structures


  Structures that no longer have a function in an organism
• Vestigial structures suggest a shared ancestry with those species that possess a fully functioning equivalent of the same structure</b>
• Vestigial structures are considered to be 'evolutionary leftovers'; they would have had a function in an ancestral organism, but a change in the environment led to loss of use
• The presence of vestigial structures does not harm the species in which they are found, so there is no advantage to be gained by losing them completely; hence their persistence
• When patterns and trends are observed in nature, scientists seek to find explanations that fit with these observations
• The explanation that best fits the pattern of homologous structures is that all animals evolved from a common ancestor that itself had a pentadactyl limb, in the process of adaptive radiation
• Continuous variation


  Small amounts of variation between populations of a species e.g. a few mm in beak length between bird populations
• Divergence


  The species becoming separate; this is the process of speciation

  
• It can sometimes be difficult to make decisions about the point at which populations showing continuous variation become separate species

  
• Adaptive radiation


  • Organisms evolved from a common ancestor that itself had a pentadactyl limb
• This is the only explanation so far that makes sense of the pattern of homologous structures seen in nature, and it supports the theory that organisms evolve over time

  
• It can sometimes be difficult to make decisions about the point at which populations showing continuous variation have diverged into different species, and biologists sometimes disagree over whether separate populations are the same species, different subspecies, or separate species