Evidence of Evolution (SL IB Biology)

• Sequence data can be obtained from:
  • DNA
    • The base sequence of DNA found in the nucleus, mitochondria and chloroplasts of cells can be determined
  • RNA
    • RNA is the product of transcription, and the RNA base sequence provides information about the DNA base sequences of genes that are expressed in a cell
  • Proteins
    • The amino acid sequence of expressed proteins can be determined
• Similarities between sequence data in different species suggest that all species share a common ancestor
• The sequences for comparison must come from the same part of the DNA, and are often taken from regions of DNA that are highly conserved, meaning that they have changed very little over time; this is important for several reasons: 
  • Like needs to be compared with like; comparing two completely different regions of DNA will not yield useful information
  • There are likely to be relatively few differences, so similarities and differences can be easily identified
  • Conserved sequences are also more likely to exist in a wide range of species
• Examples of conserved sequences are those that code for essential proteins, e.g. haemoglobin, or enzymes involved in respiration

Comparing DNA sequences

• DNA is extracted from cells
  • DNA can be extracted from blood or skin samples from living organisms or from fossilised remains
• The extracted DNA is processed, analysed and the base sequence is obtained
• The base sequence is compared to that of other organisms to determine evolutionary relationship
  • The more similarities there are in the DNA base sequence, the more closely related members of different species are
  • E.g. in 2005 the chimpanzee genome was sequenced, and when compared to the human genome it was discovered that humans and chimpanzees share almost 99% of their DNA sequences, making them our closest living relatives
• Data from multiple sources, e.g. several different genes, are compared to increase the level of certainty
• The data gained from comparing sequence data can be used to build an evolutionary tree

Comparing DNA sequences diagram

Similarities and differences between the DNA of two species provide information about their divergence from a common ancestor

• Selective breeding is a process in which humans choose organisms with desirable characteristics and breed them together repeatedly to increase the expression of these characteristics over many generations
  • The process of selective breeding has enabled humans to take advantage of naturally occurring variation, e.g.
    • Variation between individuals in plants means that some individuals may have a higher food yield or disease resistance
    • Variation between individuals in domestic animal varieties means that some individuals may have thick, heavy wool, or large volumes of milk production
  • Humans have been able to develop desirable crop and domestic animal varieties from individuals with desirable characteristics
• This practice is also known as artificial selection
  • It makes use of the principles of natural selection, but is carried out by humans
    • In natural selection, advantageous alleles are more likely to be passed on because they increase an organism's chances of survival
    • In artificial selection, or selective breeding, desirable alleles are more likely to be passed on because humans decide which individuals will be used for breeding
• Selective breeding involves changes to heritable characteristics over many generations, and so it is an example of evolution in action
  • Selective breeding leads to faster change than natural selection; this is because only the selected individuals are allowed to breed together, while in natural selection there will still be some breeding between individuals with less favourable alleles
• Selective breeding provides evidence that evolution occurs due to the accumulation of small changes to the DNA of organisms over time

The process of selective breeding

1. The population shows variation; there are individuals with different characteristics
2. Breeders select individuals with the desired characteristics
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

Selective breeding diagram

Variation in wild brassica plants allowed humans to selectively breed many of the crop plants that we eat today

• 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

Homologous structures diagram

The pentadactyl limbs of humans, whales, birds, frogs, and alligators all have the same basic layout despite having evolved for different functions