Evolution: Evidence (DP IB Biology: SL)

• Fossils are the preserved remains of organisms, or the traces left by organisms, such as footprints, burrows and faeces
  • These remains 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
    • Plants with the ability to be pollinated by insects appear before insect pollinators

• Selective breeding is a process in which humans choose organisms with desirable characteristics and breed them together to increase the expression of these characteristics over many generations
  • The process of selective breeding has enabled humans to produce desirable crop varieties and livestock with exaggerated characteristics from wild varieties and species, e.g.
    • 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

The process of selective breeding

The wolf was bred selectively over thousands of years to produce a wide variety of domesticated dog breeds.

• Homologous structures 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
  • E.g. 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 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
  • 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

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

NOS: Looking for patterns, trends and discrepancies; there are common features in the bone structure of vertebrate limbs despite their varied use

• When patterns and trends are observed in nature, scientists seek to find explanations that fit with these observations
  • Here, scientists have observed a pattern in the limb structure of animals; despite differences in appearance and function, the general structure of the pentadactyl limb is repeated throughout the animal kingdom
  • The explanation that best fits this observation is that all animals evolved from a common ancestor that itself had a pentadactyl limb, in the process of adaptive radiation
    • 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

• 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

Evidence for gradual divergence

• 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, e.g.
  • 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

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

• Because evolution generally happens over millions of years, it is difficult to see it taking place, and we often have to rely on evidence from the fossil record, and evidence of common ancestry such as homologous structures and continuous variation between species
• There are, however, some examples of evolution, on a small scale, that show changes in heritable characteristics in a short time frame
  • E.g. in insects and bacteria
  • Examples like this rely on short generation times

• A famous example of evolution taking place in insects is that of the peppered moth and industrial melanism
  • Melanin is a dark pigment produced in the cells; the more melanin is produced, the more melanistic an individual is said to be, and the darker it will be in colour
• It has been noted that melanistic peppered moths have become more common than non-melanistic individuals in industrialised parts of the UK where air pollution has increased
  • Air pollution kills organisms called lichens that grow on the bark of trees
  • In areas with clean air, lichens grow on tree bark, causing tree trunks and branches to appear paler in colour
    • In these areas, non-melanistic moths are well camouflaged against the trees, and therefore more likely to survive and pass on the alleles for non-melanism

In areas with higher levels of air pollution the frequency of melanistic moths increases