Evidence for Change in Organisms Over Time (College Board AP Biology)

Morphological Differences

• The longest established way to catalogue differences in organisms is to look at their morphological differences
• This is the basis on which all the earliest forms of taxonomic classification were carried out
  • For example, animals were classified into vertebrates and invertebrates on the basis of whether they have a distinguishable backbone

Homologous Structures

• Homologous structures (homologies) are body parts that may look and function very differently but share structural similarities
• The limbs of animals are a good example of this; 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
  • Eg. The limbs of birds, bats, crocodiles, whales, horses, and monkeys are used very differently and are visually very different, but are structurally very similar to each other
• One explanation for the surprising 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
  • Note that adaptive radiation does not provide proof that these organisms have evolved from a common ancestor, but it is a good explanation for the existence of homologous structures

A homologous structure: the pentadactyl limb

• A pentadactyl limb is any limb that has five digits ie. 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
  • The human foot evolved for upright walking and running
  • Whale flippers enable them to propel themselves through a marine environment
  • Bird wings are usually highly adapted for flight
  • The limbs of frogs allow them to walk, jump and swim
  • Alligator limbs enable them to walk and swim
• Although the individual bones of the pentadactyl limb in these example animals are very different shapes and sizes due to their different mechanisms of locomotion, their layout is almost exactly the same

Pentadactyl Limb Structures Comparison Diagram

The bone structure of the pentadactyl limb of a human, whale, bird, frog, and alligator; they all have the same basic layout despite having evolved for different functions

Vestigial structures

Note that vestigial structures, while different in nature from homologous structures, can also be explained by common ancestry

• Vestigial structures are those that no longer have a function in an organism
  • E.g. pelvis bones in snakes and whales and wings in flightless birds
• These structures tend to be homologous to structures that perform a function in other species
• The presence of vestigial structures suggests a shared ancestry with those species that possess a fully functioning equivalent of the same structure
• 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 e.g. a group of fish trapped in a dark cave would have no use for eyes
  • 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

Molecular evidence

• Analysis of biological molecules such as DNA and proteins show similarities between species that indicate evolution of species from a common ancestor
  • A common ancestor is an ancestor shared by more than one species or individual
• The theory of evolution states that all species on Earth have descended from a single common ancestor, meaning that we would expect to find similarities between organisms

Evidence from Protein Sequences

• The study of proteins is known as proteomics
• In proteomics it is possible to sequence proteins; this determines the order of amino acids in the primary structure of a protein
• Because the order of amino acids is determined by the DNA base sequence of the gene that codes for a protein, amino acid sequences can be used to determine evolutionary relationships in the same way as DNA sequences (see below)
  • The amino acid sequences of some proteins are very similar across many different species; this indicates common ancestry
  • The level of similarity between amino acid sequences of equivalent proteins can give a measure of how long ago two species diverged from each other
  • Knowing how long ago a species diverged from a common ancestor enables scientists to establish the relationships between organisms

Evidence from Gene Sequences

• It is possible to carry out DNA sequencing to determine the sequence of bases that make up a gene
  • This branch of science is known as genomics
• Comparing the sequences of equivalent genes in different species can show
  • The base sequences of some genes are very similar across many different species; this indicates common ancestry
  • The level of similarity between base sequences of equivalent genes can give a measure of how long ago two species diverged from each other during speciation
    • Genes with very similar base sequences are likely to have diverged recently
    • Genes with very different base sequences are likely to have diverged a long time ago
  • Knowing how long ago a species diverged from a common ancestor enables scientists to establish the relationships between organisms
    • Species that share a recent common ancestor are more closely related than species that share a common ancestor far back in evolutionary history

Sequencing to Establish Ancestry in Primates Diagram

The results of DNA and protein sequencing enable evolutionary trees to be built that show the evolutionary relationships between species

The Fossil Record

• The fossil record
  • We can tell from fossils that organisms have changed significantly over millions of years
  • Fossils can show evidence for transitional species, showing how one species could evolve into another
  • The age of a fossil can be estimated from a knowledge of the age of a particular stratum (band) of rock
  • Carbon dating of fossils can be done; naturally occurring carbon-14 in an organism to decays at a known rate, based on the half life of the carbon 14 isotope
  • Scientists use particle accelerators to measure the level of carbon-14 in fossilized organisms to estimate how long ago that organism died

An ammonite fossil

CC BY-SA 4.0, Michael Rowe, via Wikimedia Commons

Geographical evidence

• Different populations of a species may show small amounts of variation between each population e.g. a few mm in beak length between bird populations
  • Beak length is an example of continuous variation
• The presence of continuous variation between populations across their geographical range can lead to gradual divergence
  • The term divergence refers to 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 have diverged into different species, and biologists sometimes disagree over whether separate populations are the same species, different subspecies, or separate species
  • E.g. Orca, or killer whale, populations can show different body shapes and markings, and there is debate among scientists around whether there is only one species of orca, several subspecies, or several species

Evolution in Darwin's Finches

• There are several examples around the world of groups of species found in a particular geographical location where the differences between those species are small, eg.
  • Darwin's finches; many species of small bird observed by Darwin in the Galapagos islands
  • Hawaiian honeycreepers; a group of more than 50 bird species found in the Hawaiian archipelago
• The presence of continuous variation like this, between species, and across their geographical range, suggests that these species evolved by gradual divergence as a result of continuous variation between historical populations
• For example, Hawaiian honeycreepers show continuous variation across their geographical range; because of this, they are thought to have evolved from a series of ancestral populations, from which gradual divergence gave rise to many different species

Geological Evidence of Ancestry

The Hawaiian honeycreepers show continuous variation across their geographical range, suggesting that they diverged gradually from a common ancestor