/ Patterns of Evolution / Descent with Modification Chapter / 19 Chapter 23 / 19

Impacts, Issues Video Measuring Time

Evolutionary Theories Widely used to interpret the past and present, and even to predict the future Reveal connections between the geological record, fossil record, and organism diversity

Geological Discoveries Similar rock layers throughout world Certain layers contain fossils Deeper layers contain simpler fossils than shallow layers Some fossils seem to be related to known species

Fossils Recognizable evidence of ancient life What do fossils tell us? –Each species is a mosaic of ancestral and novel traits –All species that ever evolved are related to one another by way of descent –The fossil record indicates that there have been great changes in the kinds of organisms on Earth at different points in time

Few individuals have fossilized, and even fewer have been discovered The fossil record is biased in favor of species that –Existed for a long time –Were abundant and widespread –Had hard parts

Radiometric dating Radiometric Dating

How Rocks and Fossils Are Dated Sedimentary strata reveal the relative ages of fossils The absolute ages of fossils can be determined by radiometric dating A “parent” isotope decays to a “daughter” isotope at a constant rate Each isotope has a known half-life, the time required for half the parent isotope to decay

Radiocarbon dating can be used to date fossils up to 75,000 years old For older fossils, some isotopes can be used to date volcanic rock layers above and below the fossil

Geologic time scale Geologic Time Scale

Drifting continents Changing Land Masses

Plate margins Plate Margins

Evidence of Movement Wegener cited evidence from glacial deposits and fossils Magnetic orientations in ancient rocks do not align with the magnetic poles Discovery of seafloor spreading provided a possible mechanism

Geologic forces Geologic Forces

Scientists attempt to reconcile evidence of change with traditional belief in a single creation event –Georger Cuvier Paleontologist speculated that each boundary between strata represents a catastrophe that destroyed many species 19th Century - New Theories

–James Hutton and Charles Lyell geologists perceived that changes in Earth’s surface can result from slow, continuous actions still operating today Lyell further proposed that the mechanisms of change are constant over time This view strongly influenced Darwin’s thinking 19th Century - New Theories

–Lamarck hypothesized that species evolve through use and disuse of body parts and the inheritance of acquired characteristics The mechanisms he proposed are unsupported by evidence 19th Century - New Theories

DARWIN Galapagos Finches Darwin observed finches with a variety of lifestyles and body forms On his return, he learned that there were 13 species He attempted to correlate variations in their traits with environmental challenges

Finches of the Galapagos Islands Galapagos Finches

Darwin’s Theory A population can change over time when individuals differ in one or more heritable traits that are responsible for differences in the ability to survive and reproduce.

Darwin’s Focus on Adaptation In reassessing his observations, Darwin perceived adaptation to the environment and the origin of new species as closely related processes From studies made years after Darwin’s voyage, biologists have concluded that this is what happened to the Galápagos finches

Ideas from The Origin of Species Darwin explained three broad observations about life –The unity of life –The diversity of life –The match between organisms and their environment

Descent with Modification Darwin never used the word evolution in the first edition of The Origin of Species The phrase descent with modification summarized Darwin’s perception of the unity of life The phrase refers to the view that all organisms are related through descent from an ancestor that lived in the remote past

Artificial Selection, Natural Selection, and Adaptation Darwin noted that humans have modified other species by selecting and breeding individuals with desired traits, a process called artificial selection Darwin argued that a similar process occurs in nature

Wild mustard Kohlrabi Cabbage Kale Broccoli Selection for leaves Selection for stems Selection for flowers and stems Selection for axillary (side) buds Selection for apical (tip) bud Brussels sprouts

Darwin drew two inferences from two observations –Observation #1: Members of a population often vary in their inherited traits –Inference #1: Individuals whose inherited traits give them a higher probability of surviving and reproducing in a given environment tend to leave more offspring than other individuals

–Observation #2: All species can produce more offspring than the environment can support, and many of these offspring fail to survive and reproduce –Inference #2: This unequal ability of individuals to survive and reproduce will lead to the accumulation of favorable traits in the population over generations

Darwin was influenced by Thomas Malthus, who noted the potential for human population to increase faster than food supplies and other resources If some heritable traits are advantageous, these will accumulate in a population over time, and this will increase the frequency of individuals with these traits This process explains the match between organisms and their environment

Natural Selection: A Summary Individuals with certain heritable traits survive and reproduce at a higher rate than other individuals Over time, natural selection increases the match between organisms and their environment If an environment changes over time, natural selection may result in adaptation to these new conditions and may give rise to new species

Note that individuals do not evolve; populations evolve over time Natural selection can only increase or decrease heritable traits that vary in a population Adaptations vary with different environments

Natural Selection A difference in the survival and reproductive success of different phenotypes Acts directly on phenotypes and indirectly on genotypes

Variation in Populations All individuals have the same genes that specify the same assortment of traits Most genes occur in different forms (alleles) that produce different phenotypes Some phenotypes compete better than others

Change over Time Over time, the alleles that produce the most successful phenotypes will increase in the population Less successful alleles will become less common Change leads to increased fitness –Increased adaptation to environment

Comparative Morphology Study of similarities and differences in body plans of major groups Puzzling patterns: –Animals as different as whales and bats have similar bones in forelimbs –Some parts seem to have no function Guiding principle: –When it comes to introducing change in morphology, evolution tends to follow the path of least resistance

Comparative pelvic anatomy Comparative Morphology

Morphological Divergence Change from body form of a common ancestor Produces homologous structures

Morphological Convergence Individuals of different lineages evolve in similar ways under similar environmental pressures Produces analogous structures that serve similar functions

Morphological Convergence

AUSTRALIA NORTH AMERICA Sugar glider Flying squirrel

Comparative Development Each animal or plant proceeds through a series of changes in form Similarities in these stages may be clues to evolutionary relationships Mutations that disrupt a key stage of development are selected against

Similar Vertebrate Embryos FISHREPTILEBIRDMAMMAL

Human embryo Chick embryo (LM) Pharyngeal arches Post-anal tail

Altering Developmental Programs Some mutations shift a step in a way that natural selection favors Small changes at key steps may bring about major differences Insertion of transposons or gene mutations

Mutation and proportional changes Proportional Changes in Skull

Comparative Biochemistry Kinds and numbers of biochemical traits that species share is a clue to how closely they are related Can compare DNA, RNA, or proteins More similarity means species are more closely related

Comparing Proteins Compare amino acid sequence of proteins produced by the same gene Human cytochrome c (a protein) –Cytochrome c functions in electron transport –Deficits in this vital protein would be lethal Identical amino acids in chimpanzee protein Chicken protein differs by 18 amino acids Yeast protein differs by 56

Sequence Conservation Cytochrome C comparison

Nucleic Acid Comparison Use single-stranded DNA or RNA Hybrid molecules are created, then heated The more heat required to break hybrid, the more closely related the species