Evolution

• Haemoglobin molecule


  Similar in all animals that possess it, but there are differences
• Haemoglobin of the lamprey has only one polypeptide chain, not four
• Most animals have haemoglobin with four chains, but the chains do vary
• Hemoglobin composition

  • Four peptide chains made up of alpha, beta, and gamma (Y) polypeptides
  • + indicates no differences in amino acid sequence from that of human
• Evolutionary biologists suggest that selective breeding is a good indicator that nature (environmental conditions) can also select desirable characteristics and eliminate unfit characters from the population
• The accumulation of changes in the population would gradually bring about evolution
• Evidence from plant and animal breeding supports the concept of natural selection
• Types of natural selection

  • Stabilizing or normalizing selection
  • Directional selection
• Stabilizing or normalizing selection


  Operates in response to gradual changes in environmental conditions, favors one extreme phenotype, reduces variation, and leads to a shift in the population mean for the selected character
• Directional selection

  For example, thicker fur in foxes is an advantage in a cold climate, thinner fur in a hot climate
• Directional selection examples

  • Thicker fur in foxes in a cold climate
  • Thinner fur in foxes in a hot climate
  • Light and dark moth variations
• Antibiotic resistance is a modern example of natural selection in action
• As long as two populations are able to interbreed, they are unlikely to evolve into distinct species
• Both allopatric and sympatric speciation involve isolating mechanisms that prevent different populations from interbreeding for a period of time
• Genetic differences between populations increase due to different selection pressures in different environments
• Eventually, populations become so different that they are unable to interbreed
• Gametic isolation occurs when gametes of similar species are chemically incompatible, preventing reproduction
• Figure 4.35 Sympatric speciation in the palms of Lord Howe Island
• Gametic isolation

  1. Gametes of similar species are chemically incompatible

  2. If a sperm cannot fertilize an egg cell, then no reproduction will occur

  3. Many marine organisms, such as sea urchins, release sperm and egg cells into the water

  4. Gametes display unique surface molecules that enable an egg to recognize sperm of the same species

  5. In the absence of a “match,” fertilization will not occur, and the gene pools will remain separate
• Figure 4.38 Convergent evolution in anteaters
• Parallel evolution

  1. Occurs when two species evolve independently of each other, maintaining the same level of similarity

  2. It occurs when independent species acquire similar characteristics while evolving together at the same time in the same Eco-space

  3. Implies that two or more lineages have changed in similar ways, so that the evolved descendants are as similar to each other as their ancestors were

  4. Usually occurs between unrelated species that do not occupy the same or similar niches in a given habitat
• Figure 4.25 Types of evolution; A) Divergent B) Convergent C) Parallel
• Genetic Drift

  1. The average expected change in allele frequency is zero

  2. A small percentage of alleles may continually change frequency in a single direction for several generations

  3. A very few new mutant alleles can drift to fixation

  4. The variance in the rate of change of allele frequencies is greater in small populations than in large populations

  5. Example of genetic drift in a rabbit population
• From this second generation, what if only two of the BB offspring survive and reproduce to yield the third generation?
• In this series of events, by the third generation, the b allele is completely lost from the population
• The overall rate of genetic drift (measured in substitutions per generation) is independent of population size
• If the mutation rate is constant, large and small populations lose alleles to drift at the same rate
• This is because large populations will have more alleles in the gene pool, but they will lose them more slowly
• Smaller populations will have fewer alleles, but these will quickly cycle through
• This assumes that mutation is cons
• Mutation rate is constant, large and small populations lose alleles to drift at the same rate
• Large populations


  Have more alleles in the gene pool, but lose them more slowly
• Smaller populations

  Have fewer alleles, but these cycle through quickly
• This assumes that mutation is constantly adding new alleles to the gene pool and selection is not operating on any of these alleles
• Examples of genetic drift

  • The founder effect: Occurs when a small group splits off from the main population to found a colony. Reduced genetic variation from the original population. A random sample of the genes in the original populations. The founder effect is similar in concept to the bottleneck effect, but it occurs via a different mechanism (colonization rather than catastrophe)
• Hardy-Weinberg equilibrium: At Hardy–Weinberg equilibrium, allele frequencies remain constant from one generation to the next; evolution does not occur. It occurs only in populations that meet the following assumptions: Gene flow/immigration and emigration
• Gene flow is any movement of individuals, and/or the genetic material they carry, from one population to another. Gene transfer is the flow of alleles from one species to another. Migration: The permanent movement of genes into (immigration) or out of (emigration) a population, causing a change in allele frequency. Immigration and emigration can change the overall balance of the gene pool of the populations. Gene flow/admixture/ is can be an important source of genetic variation. Gene flow can take place between two populations of the same species through migration, and is mediated by reproduction and vertical gene transfer from parent to offspring. Alternatively gene flow can be take place between to different species through horizontal gene transfer, such as gene transfer from bacteria or viruses to higher organism, or gene from an endosymbiotic to the host. Horizontal gene transfer is especially common in bacteria
• Modern humans and other primates have evolved from a common primate ancestor that lived before the dinosaurs became extinct
• Human evolution
• Lucy had a brain about the same size as that of a chimpanzee, so her discovery was able to settle a debate amongst biologists at the time– which came first, large brain or bipedalism? The fossil Lucy was significant because it showed that bipedalism evolved before large brains. Lucy was discovered by Donald Johanson and Tom Gray in 1974 at Hadar in Ethiopia. Lucy is a fossil dated at about 3.2 million years. She was an adult female of about 25 years, belonged to the species Australopithecus afarensis. Her skeleton was about 40% completed, she was bipedal (could walk upright on two legs). There is also evidence that Lucy was also pa