chapter 10

Bio1100 Chapter 10

Evolution and natural selection

Charles Darwin published "The Origin of Species" in 1859, based on his observations on a five-year voyage around the world.
- Charles Darwin (1809-1882) published The Origin of Species in 1859, after a five-year (1831-1836) voyage on the HMS Beagle.
- After graduating from Cambridge University, Darwin took an unpaid position as naturalist aboard a survey ship for a 5-year (1831 - 1836) around-the-world voyage.
  He observed a wide diversity of organisms, especially in South America and the Galapagos Islands.
His observations in South America and the Galapagos Islands led Darwin to hypothesize about the evolution of species.
- Darwin observed fossils in Argentina of extinct animals that resembled living species.
  The extinct Glyptodon was about the size of a small car, much larger than modern cat-sized armadillos.
- Darwin observed 14 species of finches on the Galapagos, differing mainly in the size and shape of their beaks.
  This led to his theories of evolution, which can be summarized as "descent with modification" from a common ancestor.
Evolution occurs when the allele frequencies in a population change.
- Evolution occurs when the allele frequencies in a population change.
  Thus the differences in the Galapagos finches are due to the accumulated differences in the alleles that shape their beaks.
  Breeding tigers in a zoo to increase the ratio of white tigers there also results in allele frequency change in its population.
  - Is the U.S. human population evolving?
    - Yes - slowly as our genetic make-up changes due to migrations.
Four mechanisms can give rise to evolution .
- mutation
  - Mutation is a change in the DNA sequence of an organism, due to errors in DNA replication or to damage from physical or chemical agents called mutagens.
    A mutated gene may cause a change in the expressed protein, such as the mutant protein that causes curled ears in cats.
    Mutations are inherited and persist in the population only if they occur in gamete cells.
    Note that most mutations are harmful, but without them, alleles do not arise and evolution does not occur.
- genetic drift and bottlenecks
  - Genetic drift is a random change in allele frequencies of a population.
    White-spotting (S) in cat fur color is dominant over no-white-spot (s).
    Random events in meiosis and fertilization can cause genetic drift in small populations, possibly leading to genetic fixation.
    Examples of genetic fixation include true-bred organisms and clones.
  - Assuming white-spotting (S) is dominant over no-white-fur (s):
    - A heterozygous (Ss) pair has a 1/4 probability of producing 1 offspring who is no-white-fur (ss).
    - The probability of producing 2 offspring who are both no-white-fur (ss) is 1/16.
    - This is unlikely but not impossible, especially in a small population.
    - Now the overall allele frequency of s in the population has increased from 50% to 100%.
    If an allele frequency reaches 100%, genetic fixation occurs: no further genetic drift is possible.
  - If a population is reduced to a small size, genetic drift may lead to reduced genetic diversity; this reduction is called a bottleneck.
    The diversity remains reduced even if the population size recovers afterward.
- migration and gene flow
  - Migration is the movement of individuals - and their alleles - of from one population to another.
    - The migrating individuals may have a different proportion of alleles than the recipient population.
    - If migrants reproduce, gene flow has occurred: alleles have moved between populations.
    - Both the originating and the recipient populations have experienced change in allele frequencies.
- natural selection
- Natural selection occurs when individuals with better "reproductive fitness" pass their genes to offspring.
  This process depends on 3 conditions:
  1. Variation for a trait
  2. Heritability of that trait
  3. Differential reproductive success based on that trait
  Summary: organisms evolve over time; "descent with modification" from a common ancestor.
  Nature is the selection agent in wild populations, but humans can also cause evolution by artificial selection.
- Variation in some traits are easily observable among different individuals: size, shape, color, etc.
  Other traits, such as different alleles of enzymes, may also be variable, but less visible.
- Many traits are heritable.
  Offspring inherit traits from their parents.
  This can be seen in many similarities between Goldie Hawn and daughter Kate Hudson, as well as in other families.
- Different alleles may result in differential reproductive success (reproductive fitness).
  One trait among rabbits is running speed: there is variation in the population concerning this trait.
  This is a heritable trait: fast rabbits tend to have fast offspring; slow rabbits tend to have slow offspring.
  A slow-running rabbit is more likely to be eaten by a fox, and less likely to leave slow offspring.
  Over time, the population evolves by natural selection: rabbits are fast.
  Thus speed confers greater reproductive fitness for rabbits.
  - Are there other traits that allow a slow-running rabbit to have reproductive success?
    - Some other possible traits: foraging ability, camouflage, breeding capacity
- Artificial selection by humans can also change allele frequencies in species.
  Turkeys have been selected for large breasts due to market demand.
  Commercial turkeys have such large breasts that they can no longer mate, and require artificial insemination to breed.
  Most crops are products of artificial selection; one ancestral species of wild cabbage has yielded many of today's vegetables.
Over time, populations evolve and become adapted to their environments.
- Inherited traits that enhance an organism's survival and reproduction result in adaptation to its environment.
  Porcupines with quills are less likely to be eaten by predators, and have greater reproductive fitness than those without quills.
  Quills are an adaptation that reduces the risk of predation for porcupines.
Evidence for evolution includes:
- Biogeography
  - Biogeography
    "Descent with modification" from a common ancestor implies descendants are usually found geographically close together.
    Australia is filled with unusual organisms that are not closely related to organisms elsewhere.
    Their marsupial species bear superficial resemblances to placental mammals, but
    - Marsupials have a pouch where the fetus finishes development
    - Placental mammals have a full placenta where the fetus completes gestation
- The fossil record
  - Dinosaurs became extinct 65 million years ago, but we know these ancient reptiles existed from fossil records.
    The fossils of Archaeopteryx and other dinosaurs such as Sinosauropteryx provide early evidence that modern birds evolved from reptilian ancestors that had feathers.
    - What predictions can we make to support the hypothesis that birds descended from reptilian ancestors?
      - Modern birds should share more traits with reptiles than with other vertebrates.
  - The 375 million-year-old Tiktaalik fossils represent a transitional phase between lobe-finned fishes and terrestrial vertebrates.
    They had gills, scales, and fins; the arm-like joints in their fins could support weight on land, eventually evolving into 4 limbs.
    Most modern fishes are ray-finned and do not possess these joints.
- The anatomical record.
  - vestigial structures
    - Vampire bats consume a completely liquid diet, yet retain molars that other mammals use for grinding solid food.
      These useless teeth are vestigial structures inherited from a common ancestor of all mammals.
  - homologous structures
    - Homologous structures of related species are similar: they are inherited from a common ancestor.
      Mammalian forelimbs share bones derived from a common ancestor that have been adapted for different functions.
      It is hard to have other explanations for forelimb bones of porpoises and whales.
  - analogous structures
    - Analogous structures that resemble each other due to parallel adaptations to similar environments are results of convergent evolution.
      The wings of these flying animals are superficially similar but have different anatomical bases, inherited from unrelated ancestors: insect and mammal.
  - comparative embryology
  - Comparative embryology
    During early development, all vertebrates (such as fish, reptiles, humans, and birds) share a basic set of features such as gill pouches and tails.
    Presumably all vertebrates descend from a common ancestor that possessed these traits.
- The molecular record.
- Molecular record
  Evolutionary changes involve accumulation of mutations.
  Distantly-related organisms accumulate a greater number of mutations than closely-related ones.
  This divergence is seen among vertebrates in the amino acids of a hemoglobin protein.
  Humans are closely related to rhesus monkeys, with 8 differences out of 146; the distantly related lamprey has 125 differences.
  Mutations are random and accumulate at a constant rate, so can be used as a molecular clock to date a common ancestor among related species.
Evolution is a random process and does not yield perfect organisms.
- Evolution is based on random mutations and does not guarantee perfect traits.
  The beak sizes of Galapagos finches of a given species can vary within a few short years:
  - Dry years selected for bigger beaks when their food is restricted to large, hard seeds.
  - Wet years selected for smaller beaks when small, soft seed are plentiful.
  Traits are not perfect, but are results of adaptation.