Evidence for Evolution


There are now numerous established pieces of evidence which support the theory of evolution by natural selection

Something provides evidence for evolution when it demonstrates a change in characteristics from an ancestral form


Fossil Record

A fossil is the preserved remains or traces of any organism from the remote past 

Fossil evidence may be either: 

  • Direct (body fossils):  Bones, teeth, shells, leaves, etc. 
  • Indirect (trace fossils):  Footprints, tooth marks, tracks, burrows, etc.

Types of Fossils


The totality of fossils (both discovered and undiscovered) is known as the fossil record

  • The fossil record reveals that, over time, changes have occurred in features of organisms living on the planet (evolution)
  • Moreover, different kinds of organisms do not occur randomly but are found in rocks of particular ages in a consistent order (law of fossil succession)
  • This suggests that changes to an ancestral species was likely responsible for the appearance of subsequent species (speciation via evolution)
  • Furthermore, the occurrence of transitional fossils demonstrate the intermediary forms that occurred over the evolutionary pathway taken within a single genus

Law of Fossil Succession


While fossils may provide clues regarding evolutionary processes and ancestral relationships, it is important to realise that the fossil record is incomplete

  • Fossilization requires a unusual combination of specific circumstances to occur, specifically:
    • Rapid burial (high pressure)
    • Lack of oxygen / no decomposition by bacteria
    • Preservation of remains (i.e. not consumed or removed by scavengers)
  • As these conditions are not common, this means that there are many gaps in the fossil record
  • Only the hard parts of an organism are preserved and often only fragments of fossilized remains are discovered
  • Additionally, there may be contention between scientists concerning evolutionary pathways and debate as to whether a fossil is representative of the past species

Fossilization


Biogeography

Biogeography describes the distribution of lifeforms over geographical areas, both in past and present times

Biogeographical distribution supports the theory of evolution as it is found that closely related species are usually found in close physical proximity to one another, and that fossils from these regions resemble modern organisms

This suggests that these species share a common lineage (if speciation was random, distribution would be expected to be scattered)

Examples of biogeographical distribution indicating shared ancestry can be seen in the fact that:

  • Most modern marsupials are found almost exclusively in Australia (~ 70% of the extant species)
  • Australian has few placental mammals compared to South America, even though environmental conditions are similar between the two regions


Exceptions to this correlation between biogeographical distribution and common ancestry can be explained due to continental drift

  • Over 250 million years ago, there was one continental landmass (Pangaea) which split into the six current biogeographical regions
  • Closely related species that were separated by the breaking up of the continental landmasses can often be found localised to the regions where the continents were once connected
    • Shrubs of the family Proteaceae (e.g. waratahs) can be found across Australia, as well as South America, Indo-China and parts of Africa


Bieogeographical Distribution of Proteaceae can be Explained via Continental Drift


Comparative Anatomy

A comparison of the anatomic features of different species provides further evidence of evolution

  • The presence of homologous structures and shared embryonic development between species indicates descent from common ancestors
  • The presence of analogous structures and vestigial organs highlight the role of environmental influences in the process of natural selection


Homologous Structures

Homologous structures possess a similar underlying anatomy as a result of a shared evolutionary origin, but have evolved into a variety of distinct forms due to the presence of different selective pressures

  • An example is the pentadactyl limb structure in vertebrates, whereby many animals show a common bone composition, despite the limb being used for different forms of locomotion (e.g. whale fin for swimming, bat wing for flying, human hand for manipulating tools, horse hoof for galloping, etc.)
  • This illustrates adaptive radiation (via divergent evolution) as a similar basic plan has been adapted to suit various environmental niches
  • The more similar the homologous structures between two species are, the more closely related they are likely to be


Homologous Structures (Pentadactyl Limb)


Analogous Structures

Analogous structures are adaptations that possess similar features and functionality as a result of exposure to a common selective pressure, but have different underlying anatomies due to having unrelated evolutionary origins

  • An example is the formation of a streamlined body shape in aquatic animals, regardless of ancestral origin (e.g. sharks are fish, dolphins are mammals, penguins are birds, etc.)
  • This illustrates convergent evolution as unrelated species have become structurally more alike due to exposure to shared selection pressures


Analogous Structures (Aquatic Forelimbs)


Vestigial Organs

Some organisms show the presence of functionless and reduced remnants of organs that were once present and functional in their ancestors

Changes to the environment have rendered these organs redundant and so over time they have lost their functionality 

These structures are called vestigial organs and demonstrate the evolutionary divergence of a species from a past behaviour or activity

  • An example of a vestigial organ is the pelvic bone in a whale - this bone serves no current purpose and is a remnant of a time when whales were terrestrial mammals


Comparative Embryology

Studying the growing embryo in animals and plants shows that closely related organisms go through similar stages of development

  • All terrestrial animals have non-functional gill slits (pharyngeal slits) as early embryos (indicating an aquatic origin)
  • Many vertebrates (including humans) demonstrate a primitive tail at certain stages of embryonic development


Comparative Embryology


Molecular Evidence

Molecular evidence involves identifying conservation in DNA and protein sequences as a basis for determining evolutionary relationships

One technique used to enable comparison is DNA-DNA hybridisation:

  • DNA is double-stranded, but the strands can be separated with sufficient heat and will reform (re-anneal) as the temperature falls
  • Single-stranded DNA from different species can be mixed together to identify the degree of similarity (as measured by complementary base pairs)
  • Closely related sequences will join together (hybridise) more strongly as they share more complementary base pairs
  • The strength of the hybrid molecule (and degree of similarity) can be measured by how much heat is required to separate the strands
  • Closely related (conserved) sequences will have a higher melting temperature (TM) than distantly related sequences


DNA-DNA Hybridisation