Evolution Flashcards

Evidence for Evolution

Learning Objectives

  • Explain different types of evidence for evolution:
    • Fossil & Extant Comparative Anatomy
    • Comparative embryology
    • Molecular Biology
    • Biogeography
  • Define:
    • Homologous structures
    • Analogous structures
    • Vestigial structures
  • Distinguish between homologous and analogous structures.
  • Explain and interpret evolutionary relationships indicated in phylogenetic trees.
  • Distinguish between a monophyletic group and polyphyletic group.

Evidence Supporting Evolution

  • Comparative anatomy (morphology):
    • Similarity in structures.
    • Homology.
    • Vestigial structures.
  • Fossil record (transitional fossils).
  • Comparing embryos.
  • Biogeography.
  • Molecular Biology (Bioinformatics).

Comparative Anatomy and Morphology

  • Anatomy = internal structures of organisms (bones, organs).
  • Morphology = external body structures (shape, structure, colour, pattern, size).
  • Comparing morphology and organisation of structures between different groups.
  • Similarity or difference indicates relationships.

Homologous Structures

  • Word homologous derived from Greek.
    • "Homos…" = the same; "logos" = proportion or ratio.
  • Structures that are similar, and have the same function e.g., the spine on all vertebrates.
  • Species grouped in the same genus, family or order often have similar structures.
  • Indicate evolutionary relationships (ancestor in common).
  • Structures that are similar but have different functions.
  • Indicate evolutionary relationships (ancestor in common) but divergence has occurred from the ancestor.
  • E.g., the forelimbs of human, cats, whales, and bats share the same skeletal elements, but functions have become different since they diverged from the ancestral forelimb.
Definition of Homology in Evolution
  • Similarity in structures due to common ancestry.

Vestigial Structures

  • Definition: A structure that is small and not well developed.
  • Some vestigial structures are interesting examples of homologous structures.
  • Structures of limited importance to an organism, but which had important functions in ancestors.
  • E.g., the skeletons of some snakes and of fossil whales retain vestiges of the pelvis and leg bones of walking ancestors.
  • The whale has pelvic and leg bones which are small and not well developed or used. They would have been used by an ancestral form which walked on land.
  • Snakes have vestigial limbs.

Fossil Evidence

  • Comparison of extant organisms only tells us so much.
  • Jawless Fishes, Jawed Fishes, Amphibians, Mammals, Reptiles, Birds.

Transitional Fossils

  • A transitional fossil is any fossilised remains of a life form that shows characteristics common to both an ancestral group and its derived descendant group.
  • e.g., Evolution of horse - comparing homologous structures in different fossils.
    • Hyracotherium (55-45 MYA).
    • Miohippus (33-29 MYA).
    • Merychippus (17-11 MYA).
    • Equus (5 MYA).
  • Evolution of snakes transitional fossils.
    • Tetrapodophis amplectus.
    • Is this evidence of transition from lizards to snakes?
  • A perfectly preserved dinosaur embryo could link modern-day birds to dinosaurs (December 2021).
    • A 66-million-year old dinosaur embryo now dubbed ‘Baby Yingliang’ was recently discovered in southern China.
    • Prenatal dinosaur looks just like a baby bird curled in its egg (pre-hatching posture).
    • Fossil shows that the dinosaur embryo’s position is similar to that of modern bird embryos ready for ‘hatching’.
    • Evidence that many features characteristic of today’s birds first evolved in their dinosaur ancestors.

Comparative Embryology

  • Some homologous structures can be seen only in-embryo development.
  • All vertebrate embryos have tail structure in the early stages of development.
  • Tail structure does not necessarily appear in the fully developed organism.
  • The similar stages in embryonic development indicate a possible common ancestor.
  • Vertebrate embryos have pharyngeal gill pouches = Homologous structures.
  • Gill pouches in vertebrate embryos:
    • E.g., fish, frogs, snakes, birds, apes.
    • All have a stage where gill pouches appear on the sides of the throat.
    • Develop into different structures.
    • In fish gill pouches become gills and gill arches.
    • In other vertebrates, the Gill pouches become bones of the skull, bones supporting the tongue.
    • In Mammals, the Gill pouches develop in the Larynx or voice box.

Biogeography

  • Definition: Biogeography is the study of geographical distribution of past and present species (plants and animals).
  • Species tend to be more closely related to other species from the same area, rather than to other species with the same way of life but living in different areas.
  • This suggested to Darwin that today’s organisms evolved from ancestral forms.
  • Absence of placental mammals means marsupials evolved/radiated in isolation.
  • Marsupials have diverged to fill many niches in Australia.
  • Similar niches on other continents are filled by animals with similar adaptations.
  • Adaptations have resulted from similar selective pressure.
  • Convergent evolution is when similar selection pressures act on species in isolation resulting in characteristics that are similar.
  • Not because of shared ancestry! The species are not related.
  • Different species do NOT have a common ancestor with these characteristics.
  • Similarities are due to similar selection pressure are called analogous features.
  • All the marsupial mammals are more closely related to each other than they are to any placental mammal.

Homology vs. Analogy

  • Homology is the similarity in structures due to common ancestry. The structures are homologous structures.
  • Analogy: Analogous structures are similar BUT are not related in an evolutionary sense. They do not have a common ancestor with the structure.

Molecular Biology and Bioinformatics

  • More recent evidence for evolution has come from molecular biology.
  • By comparing gene sequences (DNA) and proteins of different organisms.
  • Related individuals have greater similarity in their gene sequences and proteins than do unrelated individuals.
  • Certain fossils are preserved in a way that ancient DNA can be extracted.
  • Bioinformatics uses computers to analyse data (e.g., DNA and amino acid sequences).
  • Genetic relatedness among some primates:
    • Chimps and Humans < 2% different in DNA sequence (or better than a 98% match).

Evidence for Evolution Example

  • Using evidence from different areas of biology has helped build a more comprehensive understanding…..
    • Fossils (including transitional).
    • Comparative anatomy.
    • Molecular biology.

Evolution of Whales (Cetaceans)

  • Cetaceans are a mammalian order that includes whales, dolphins and porpoises.
  • Evidence from comparative anatomy that they have ‘four feet/limbs’ – vestigial structures.
  • Included in the tetrapods.
  • Comparing anatomy:
    • Backbone.
    • Forelimb (bones).
    • Vestigial structures (unattached pelvis & hindlimb).
  • Whales (aquatic mammals) linked to their terrestrial ancestors.
  • Whales are descended from hoofed wolf-like carnivore.
  • E.g., Pakicetus (~50mya) were land animals, with long skulls, and carnivorous teeth, ear region similar to living whales.
  • Indicates that whales are most closely related to even-toed ungulates.
  • Molecular evidence: DNA analysis supported hypothesis that whales and Hippos are both descendants of a cloven-hoofed ancestor.
Evolutionary Adaptations of Whales
  • Adaptations for:
    • Hearing under water.
    • Movement in water.
    • Echolocation for hunting.
    • Feeding.

Evolutionary Trees

  • Central idea of evolution is that life has a history.
  • Darwin compared history of life to structure of tree.
  • First forms found on trunk.
  • New branches represent new forms of life.
  • Common ancestor at fork of branch.

Phylogeny or Family Tree

  • Diagrams that show hypotheses regarding the evolutionary history of species.
  • Hypothesis is a possible explanation based on some evidence.
  • Phylogeny shows the relationships for all life on Earth.
  • The three domains: Archaea, Bacteria, and Eukaryota.

Understanding Phylogenies

  • Use homologous characteristics to infer relationships (homology reflects evolutionary history).
  • Homologous characteristics are used to group organisms together.
  • A Shared character is a character that 2 or more groups (lineages) have in common.
  • A derived character is a character that has appeared in the most recent ancestor.
  • A clade:
    • The group of organisms that includes all the descendants of a common ancestor and that ancestor is also called a CLADE.
    • Described as monophyletic (Greek for single tribe).

Building the Tree or Phylogeny

  • Phylogenetic trees represent patterns of ancestry.
  • Based on information about Characters – which are inherited.
  • Can have characters that are same as ancestor or are different.
  • A Character state is the presence or absence of these changes.

Characters and Character States

  • Characters can be morphological, behavioural or molecular and need to have a shared ancestry – homologous.
  • Shared ancestral (primitive) characteristics are inherited with little / no change from an ancestors.
  • Shared derived characters are those that have undergone recent change and may be found only in closely related species.

Constructing a Phylogenetic Tree for Vertebrates

  • The outgroup is used as a comparison.
  • An outgroup is a species or group known to have diverged before the group being studied.
  • Compare character for each group to determine branch points (present or absent state).
  • A potential source of confusion when constructing a phylogenetic tree is characters that show similarity between organisms are not from a common ancestor.

Homology and Analogy

  • Homologous characters - Inherited from a common ancestor. Human eyes and mouse eyes are homologous structures because we each inherited them from our common ancestor that also had the same sort of eyes.
  • Analogous characters - Similar because of convergent evolution, and not because of common ancestry. Two characters are analogous if the two lineages evolved them independently.
  • Which characters do we use to build a tree?
  • Bird and bat wings are analogous.
    • They have separate evolutionary origins.
    • But are superficially similar because they evolved to serve the same function.
    • Analogies are the result of convergent evolution.
      But bird and bat wings are homologous as forelimbs(bone structure).

Key Concepts

  • Evolution
    • Descent with modification.
  • Microevolution
    • Mechanism: Natural selection.
    • Favorable variations enable survival and reproduction; less favorable do not.
    • Results in adaptation.
  • Macroevolution
    • Evidence from fossils, comparative anatomy, molecular biology.
    • Fossil record shows ancestral and transitional forms.
    • Homology (common ancestry) vs. Analogy (convergent evolution).
  • Phylogenies
    • Hypotheses for evolutionary relationships.
    • Based on homologous characters.
  • Biogeography
    • Species distribution influenced by geography.

Review Questions

  • What evidence supports the theory of Evolution?
  • Explain homology and give examples.
  • How do analogous features arise?
  • What are phylogenetic trees and what information is used to construct them?