A4.1 Evolution and speciation

• Species do not stay the same over time; the species that we see around us today have developed over millions of years
• 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 do not lead to evolution
• • Heritable characteristics are 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
• Heritable characteristics that are advantageous are more likely to be passed on to offspring, leading to a gradual change in a species over time - natural selection
• • Changes in the heritable characteristics of organisms can also lead to the development of completely new species
  • The formation of new species via the process of evolution has resulted in a great diversity of species on Earth
• • Theoretically, at the origin of life on Earth, there would have been just one single species
  • This species evolved into separate new species
  • These species would then have divided again, each forming new species once again
  • 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 evolution of a new species from an existing one
• how does speciation occur?


  Reproductive isolation occurs between 2 populations, and a combination of natural selection and genetic drift leads to the development of two separate species with different alleles.
• who proposed the theory of evolution by natural selection

  charles darwin
•  theory of evolution by natural selection


  - Individuals in a species show a wide range of variation due to random mutations in their DNA

  • Individuals within a population must compete for survival due to selection pressures
  • Individuals with characteristics most suited to the environment have a higher chance of survival and so are more likely to reproduce
  • Advantageous alleles are passed down to offspring
  • Over many generations the advantageous alleles become more frequent in a population
• Darwinian evolution by natural selection requires that characteristics are heritable
• Lamarck’s theory

  • A characteristic that is used frequently by an organism becomes better and stronger, whereas a characteristic that isn't used gradually disappears
  • The beneficial characteristics that are used frequently are passed to offspring
• Lamarck’s ideas were incorrect because they lack the component of heritability; acquired characteristics are not passed on to offspring
•  The theory of evolution by natural selection predicts and explains a broad range of observations and is unlikely to ever be falsified
• why can't The theory of evolution by natural selection predicts and explains a broad range of observations and is unlikely to ever be falsified

  • Scientists can gather information about the world by observing events
  • They formulate theories that seek to explain observed events
  • The theory of natural selection explains many observations, and is widely accepted as a correct explanation of observed events; no other reasonable theories have ever been proposed, and so this theory is likely to remain as the scientific explanation for species change over time
• where can 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
• sequencing DNA

  • 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
• why is it important The sequences for comparison must come from the same part of the DNA

  • 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

  • those that code for essential proteins-
  • haemoglobin
  • enzymes involved in respiration
• how to compare 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
  • he data gained from comparing sequence data can be used to build an evolutionary tree
• Data from multiple sources are compared to increase the level of certainty
• Similarities and differences between the DNA of two species provide information about their divergence from a common ancestor
• selective breeding

  • 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
• why selective breed

  • 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
• artificial selection

  • Humans have been able to develop desirable crop and domestic animal varieties from individuals with desirable characteristics
  • 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
• consequences of selective breeding

  • 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
• why is selective breeding an example of evolution in action

  • because it involves changes to heritable characteristics over many generations
• process of selective breeding

  • The population shows variation; there are individuals with different characteristics
  • Breeders select individuals with the desired characteristics
  • Two selected individuals are bred together
  • 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
  • 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
• example of selective breeding

  Variation in wild brassica plants allowed humans to selectively breed many of the crop plants that we eat today
• what are homologous structures

  body parts that may look and function very differently but share structural similarities
• examples of homologous structures

  • limbs of animals
  • 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
  • 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
• explanation of the similarities of different limbs

  • adaptive radiation
• adaptive radiation

  •  evolved from a shared, common ancestor but have adapted to different environments in the process the idea that organisms with homologous structures have all 
  • 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
• example of a homologous structure

  pentadactyl limb
• pentadactyl limb


  any limb that has five digits
• Pentadactyl limbs are present in many species from many groups of organisms, including mammals, birds, amphibians, and reptiles
• 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