Comprehensive Evolution Notes (Macro & Micro Evolution)

MACROEVOLUTION

  • A change among species over a long time span with some lineages dying out and new species emerging.

  • Major biological changes are evident in the fossil record.

  • Major lines of evidence include:

    • Paleontology (Fossil Records)

    • Morphology and Comparative Anatomy

    • Comparative Embryology

    • Comparative Biochemistry

    • Geographical Distribution

PALEONTOLOGY

  • The study of fossils provides strong evidence for evolution.

  • What is a fossil?

    • Any trace or remains of an organism preserved by natural processes.

  • Fossils allow comparisons between ancient organisms and living ones.

  • Example: Pterodactylus kochi fossil preserved in limestone; many pterodactyl fossils found in Bavaria, Germany.

  • Source: NatGeo (National Geographic)

FOSSIL RECORDS

  • Coral fossils dating back more than 450 million years indicate that some land areas (e.g., Kansas, Mongolia) were once inland seas.

  • Homo erectus skull: Turkana Boy, 1.54 million years old, found near Lake Turkana, Kenya (1984 discovery). The skeleton is among the most complete early human fossils; brain cast and vertebrae shown for comparison with modern humans.

DINOSAUR AND MEGALODON FOSSILS

  • Utah’s Dinosaur National Monument preserves hundreds of fossils from about 150 million years ago, deposited by ancient floods carrying dinosaur and other remains and burying them under sediment.

    • Fossils include allosaurus, stegosaurus, diplodocus.

  • Megalodon (Carcharocles megalodon) tooth fossils indicate a giant prehistoric shark; estimates suggest lengths over 20 meters (≈65 feet); Megalodon went extinct about 1.5 million years ago.

GEOLOGIC TIME SCALE

  • Geologic time spans large changes in Earth’s history and life.

  • Key eras and notes:

    • Precambrian / Archaeozoic: very long, before complex life; earliest invertebrates appear late in this interval.

    • Paleozoic: early complex life and many major animal groups appear.

    • Mesozoic: age of dinosaurs; first birds and mammals appear.

    • Cenozoic: age of mammals; true modern groups diversify.

  • Notable markers from the provided scale (approximate, in millions of years ago):

    • 0 Ma: Cenozoic, Quaternary — Age of mammals

    • 65 Ma: End of the dinosaurs (Cretaceous–Paleogene boundary)

    • 100–65 Ma: Cretaceous period preceding the end of dinosaurs

    • 144 Ma: Jurassic period

    • 213–200 Ma: Triassic period — first dinosaurs, mammals, and birds

    • 248 Ma: Permian period

    • 286–300 Ma: Pennsylvanian/Carboniferous; first reptiles

    • 320 Ma: Mississippian; first fishes

    • 360 Ma: Devonian; first amphibians

    • 408–438 Ma: Silurian/Ordovician; earliest land plants and early fishes

    • 500–505 Ma: Ordovician; first fishes

    • 590–600 Ma: Cambrian; diversification of many major animal phyla

    • ~4.6 billion years ago: Precambrian; origin of life and early invertebrates

  • This table underlines the sequence: Precambrian → Paleozoic → Mesozoic → Cenozoic, with major life-appearance landmarks in each period.

EVOLUTION OF HORSE

  • Eohippus (Hyracotherium): earliest horse ancestor; lived in North America during the Eocene; small, about the size of a small dog; had five toes.

  • Mesohippus and Miohippus: lived in the Oligocene; intermediate forms.

  • Merychippus: Miocene; size of a pony; three toes in the forelimb and three in the hind limb; the median toe bore most of the body weight.

  • Pliohippus: pliocene; one of the later intermediates.

  • Equus: Pleistocene, ancestral to the modern horse; by this stage the anatomy resembled the modern horse with a single prominent toe (hoof) bearing weight.

MORPHOLOGY AND COMPARATIVE ANATOMY

  • Morphology: study of external form and structure of organisms.

  • Anatomy: study of internal structure.

  • Homologous structures: anatomical features that share a common evolutionary origin even if they perform different functions (e.g., forelimbs of mammals).

  • Analogous structures: similar in appearance and function but have different evolutionary origins (convergent evolution).

  • Vestigial organs: structures that have lost most or all of their ancestral function.

HOMOLOGOUS STRUCTURES

  • Forelimbs of vertebrates (frog, birds, bat, seal, human) share the same basic arrangement: humerus, radius, ulna, carpals, metacarpals, and phalanges.

  • Modifications (shortening, lengthening, fusion, or reduction) reflect different functions not different ancestry.

ANALOGOUS STRUCTURES

  • Examples include organs that look similar and perform similar functions but arise from different embryonic plans.

  • They illustrate convergent evolution.

  • Example group: tendrils in various plants can be modified parts of leaves, stems, or leaf tips, illustrating multiple evolutionary solutions to climbing and support.

VESTIGIAL ORGANS

  • Vermiform (appendix) in humans is a remnant of a larger caecum used for digestion of cellulose; reduction over time indicates a shift away from high-cellulose diet.

  • Nictitating membrane (plica semilunaris) near the inner angle of the eye is vestigial in humans; in some animals it can be stretched across the eye when underwater.

  • In snakes, bones of hind limbs and pelvic girdle persist as vestiges within the abdominal flesh.

EMBRYOLOGY

  • Embryology: study of embryo development.

  • Vertebrate development begins with a zygote, which divides to form a morula (solid ball of cells), which becomes a blastula (hollow ball), then a gastrula (two/three layers) that organizes into germ layers.

  • Germ layers give rise to various tissues and organs.

  • A key observation is the unity of developmental plan across vertebrates.

  • In vertebrates:

    • Notochord is present in embryos.

    • Paired pharyngeal pouches appear during development.

    • In fish and amphibians, pouches develop into gills.

    • In humans, the first pair of pouches becomes the middle ear cavity and auditory tube; the second pair becomes the tonsils; the third and fourth pairs become thymus and parathyroid glands.

RECAPITULATION THEORY (HAECKEL)

  • Also known as the biogenetic law.

  • States: Ontogeny recapitulates phylogeny.

  • Ontogeny: development of an organism from the fertilized egg onward.

  • Phylogeny: evolutionary history of the species.

  • According to the theory, embryonic development briefly traverses the organism's entire evolutionary history.

RECAPITULATION DETAILS

1) The fertilized egg (zygote) is comparable to a single-celled ancestor.
2) The blastula resembles a colonial protozoan.
3) The gastrula resembles a coelenterate (radiate animals).
4) The three-layered embryo resembles a flatworm.
5) Development progresses to resemble fish, then reptilian, and finally mammalian characteristics.

  • By around the seventh month, the human embryo may resemble a baby ape in certain features (hair, limb proportions).

COMPARATIVE BIOCHEMISTRY

  • Almost all living organisms use the same basic biochemical molecules: DNA, ATP, enzymes.

  • Similarities in amino acid sequences and DNA codes can be explained by descent from a common ancestor.

  • Hemoglobin and myoglobin molecules show similarity across diverse taxa.

  • Vertebrates share similar hormones with conserved functions.

GEOGRAPHICAL DISTRIBUTION (BIOGEOGRAPHY)

  • Biogeography studies the geographical distributions of plants and animals.

  • Discontinuous distribution: species may be widely distributed but become extinct in some areas, leaving isolated populations.

  • Populations derived from extinct widespread populations can diverge due to isolation and local adaptation.

AUSTRALIA, MONOTREMES, AND MARSUPIALS

  • Marsupials and monotremes are found primarily in Australia and New Zealand.

  • The continents were historically connected with Asia, but separated before true mammals evolved.

  • On the mainland (Asia), true mammals replaced monotremes and marsupials; in Australia, they persisted and evolved into a diverse array of marsupial and monotreme lineages.

BIOGEOGRAPHICAL REALMS

  • Eight biogeographical realms are identified, separated by geographical barriers (seas, mountains, deserts):

    • Antarctic Realm

    • Nearctic Realm

    • Neotropical Realm

    • Afrotropical Realm

    • Palearctic Realm

    • Indomalayan Realm

    • Australasian Realm

    • Oceanian Realm

DARWIN AND ENDEMIC SPECIES; GALAPAGOS FINCHES

  • Darwin observed many endemic species with small ranges.

  • The Galápagos finches vary in beak shape and size but are closely related, illustrating adaptive radiation.

ADAPTIVE RADIATION

  • Organisms radiate into new geographic areas with novel environmental conditions.

  • They undergo adaptive changes to utilize new habitats and resources, leading to the formation of new species.

VARYING RATES OF SPECIATION

  • Gradualism (gradual speciation): species diverge gradually over many small steps.

  • Punctuated equilibrium: new species change rapidly from the parent species, followed by long periods of little change.

  • The two models are not mutually exclusive; punctuated equilibrium does not necessarily exclude gradualism.

MECHANISM OF MICROEVOLUTION (DEFINITION OF TERMS)

  • Gene: a section of a chromosome that encodes information to build a protein; locus is the gene’s location.

  • Allele: variants of the information at a locus; an individual has two alleles per locus (could be the same or different).

  • Zygosity: Homozygous (two identical alleles) vs Heterozygous (two different alleles).

  • Dominant alleles: expressed when present; recessive alleles: expressed only when no dominant allele is present.

  • Gene Pool: the collection of all alleles in a population.

  • Allele Frequency: how common a particular allele is in a population.

    • Example: If there are 50 individuals, there are 100 alleles. If 25 alleles are R and 75 are W, then frequencies are:

    • p(R)=25100=0.25,p(R)=\frac{25}{100}=0.25, p(W)=75100=0.75.p(W)=\frac{75}{100}=0.75.

  • Genotype: the genetic information at a locus; possible genotypes depend on the alleles present (e.g., RR, RW, WW).

  • Phenotype: the observable appearance, influenced by genotype (e.g., red vs white if alleles R and w exhibit simple dominance).

HARDY-WEINBERG EQUILIBRIUM

  • The Hardy-Weinberg principle predicts that allele and genotype frequencies will remain constant across generations in the absence of evolutionary forces.

  • Conditions for HW equilibrium (population model):

    1. No mutations

    2. No immigration or emigration

    3. No natural selection

    4. No sexual selection

    5. A very large (ideally infinite) population size

  • In real populations these conditions are not perfectly met, but HW equilibrium provides a useful baseline model for population genetics.

  • Key formulas:

    • Allele frequencies: p+q=1p+q=1

    • Genotype frequencies: p2+2pq+q2=1p^2 + 2pq + q^2 = 1

    • Where p is the frequency of one allele (e.g., A) and q is the frequency of the other allele (e.g., a).

NOTES ON FORMULAS AND KEY IDEAS TO REMEMBER

  • Hardy-Weinberg baseline: in the absence of evolution, allele and genotype frequencies remain constant across generations.

  • Common evolutionary mechanisms:

    • Mutation: source of new genetic variation; most changes arise in gametes to affect offspring; mutations alone have small effects on allele frequencies but provide raw material for evolution.

    • Gene Flow: movement of alleles between populations via individuals or gametes; can introduce new alleles or alter allele proportions; strong evolutionary force.

    • Genetic Drift: random fluctuation in allele frequencies, especially in small populations; can lead to fixation or loss of alleles over time.

    • Bottleneck Effect: drastic reduction in population size; survivors’ allele frequencies may differ from the original population.

    • Founder Effect: a new population established by a small number of individuals; founder allele frequencies may differ from the source population.

    • Natural Selection: differential survival and reproduction based on phenotype; changes in allele frequencies as advantageous traits become more common; leads to higher Darwinian fitness.

  • Biogeography and adaptive radiation explain how geography and environmental niches drive diversification.

  • Embryology and recapitulation theory historically linked development to evolutionary history, but modern interpretation recognizes both shared developmental patterns and heterochrony/other processes.