1. Macroevolution
• Macroevolution: major patterns and changes among living organisms over long periods of time.
• The evidence comes from 2 main sources: fossils and comparisons between living organisms.
How macroevolutionary Changes Occur
The classical Neo-Darwinists thought that the same forces that drive microeveolution also cause macroevolutionary
changes, given enough time. That is, selection pressure gradually changes the form of a species,and speciation events
cause two species to slowly diverge from each other. This theory can be called the “gradualist” model of
macroevolution.
A more recenttheory,“punctuated equilibrium”, saysthat the large scale changesoccur rapidly in small, isolated groups,
due to mutations that significantly alter the form of the organism. Gradual changes occur in between bouts of major
changes. This theory’s modern version is due to Stephen Jay Gould and Niles Eldredge.
Fossils Fossils are the remains of living organisms, often converted into rock. Bones, teeth, shells,
seeds,footprints, leaf prints, etc. If an organism is buried so that large and small decay organisms don’t destroy it, water
slowly dissolves away the organic material and replaces it with inorganic compounds: calcium carbonate is a common
form. As sediments accumulate above, pressure squeezes fossils, so they are often distorted and flattened. Fossils are
exposed when erosion removes the overlaying rock, or when people dig them up n rock quarries and road cuts.
What does the Fossil Record Show?
It is quite spotty. If you are looking for specific fossils, they are hard to find. Estimated 250,000 fossil species known,
mostly from the past 600 million years. Currently alive: estimated 4 million. So, lots are missing. Bias in the fossil
record: hard parts are easier to fossilize. Very few insect fossils, for instance, despite their prevalence in the world
today. General, obvious trend: living things get more complex over time. There were invertebrates before there were
fish, fish before reptiles, reptiles before mammals, for example. Clear intermediate forms are rare: the “missing link”
betweenapesand humans, for example. However,there is a list of 139 examples of gradual speciesto speciestransitions
that are very well documented in the fossil record. “Explosions” of new species—adaptive radiation– is a common
event in the fossil record. Mass extinctions are also common. Note that mass extinctions can occur over thousands of
years are still seem almost instantaneous in the fossil record—sediments are usually laid down slowly.
Plate Tectonics Plate tectonics = slow movement of continents over long periods of
time.
From the earliest maps of the Atlantic Ocean and surrounding land it was obvious that the bulge of South America
appears to fit into the side of Africa. The idea that continents can move, that this isn’t just a coincidence, was proposed
in the 1930’s by Albert Wegener. No good mechanism, and it didn’t fit current theories—his theory was ignored or
attacked. Mapping of the ocean floors in the 1960’s showed that new ocean floor was being created by volcanoes in the
mid-ocean ridge, and then spreading out from there. Rocks get older as you move away from the ridge. Conintental
rock is ligher than ocean rock—continents float on top. The plates ocean rocks are pushed underneath continents at
deep ocean trenches. Plates can also slide past each other (as in California) or crash into each other (as in India).
Volcanoes erupt near plate boundaries: the plates going underneath melt and buddle up to the surface again. The theory
is very widely accepted today, and it explains most of the world’s geology.
Continents Colliding The Great American Interchange. North and South America were not connected until
about 3 million years ago. Separate groups of animals developed on each. South America had many marsupials
(mammals with pouches,like kangaroos and opossums), armadillos, sloths. North America had rodents, canines,felines,
bears. When the continents were joined, it became possible for animals to pass between them. Many South American
animals became extinct: giant ground sloths, marsupial carnivores—as their habitat was taken over by North American
types. Some South American animals have flourished in North America: opossums, armadillos, anteaters.
Comparative Morphology
Many creatures share characteristics with each other. We have arms and hands, so do monkeys, and other mammals
have similar forelegs. The bones among these structuresare similar acrossspecies.Similarly, all vertebrateshave skulls,
and the bones among them are quite similar.
Morphological divergence: starting form a common ancestor,different species have modified their body parts to fit their
situations. The forelimbs of primitive reptiles have been modified to become human hands, rid and bat wings, penguin
flippers, horse hooves, dog paws, etc. The basic bones and muscles are all still present, but they have grown or shrunk
in the different species. Examining the similarities and differences between structures is one of the main ways species
are grouped together. Structures sharing a common origin are called “homologous” structures.
2. Morphological Divergence: starting with a common ancestor, then diverging into altered forms. Vertebrate
forelimb is an example.Another example: insect wings. Startedwith 4 wings, equal in size, like a dragonfly. Rearwings
have shrunk to tiny balancers in flies. Front wings converted to cases in beetles. All wings expanded in butterflies.
Morphological Convergence different organisms developing similar structures independently from each other.
Similar solutions to common problems, similar responses to common environmental conditions. Example: the shape of
dolphins (mammal), sharks (fish) and icthyosaurs (extinct reptile). All live in the oceanand need to swim fast. Common
streamlined body shape. Another example: birds, bats, and insects all have wings with very different structures.
Structures with similar functions that have different evolutionary origins are called ‘analogous” structures.Convergence
can confound the study of evolutionary relationships. Often attention is paid to small details that seem unimportant to
natural selection, to avoid being confused by convergence.
Developmental Patterns To quote Haekel: “Ontology recapitulates phylogeny”. That is, the embryo
goes through stages that look a lot like the evolutionary development of species. This idea is better stated as: the early
embryos of related species often resemble each other more than the adults do. Why—the basic body plan gets modified
from an ancestral pattern as species evolve. Modifications accumulate on top of the original pattern.
Comparative Biochemistry Many genes are found in all living things, because we all use similar metabolism.
Ribosomal RNA genes are a common example—all living things make proteins by essentially the same mechanism.
Also genes for basic metabolic functions like glycolysis and electron transport. Genes can also be described as
homologous and analogous. Homologous genes evolved from a common ancestor; analogous genes perform similar
functions, but have different evolutionary origins. Considering homologous genes, the genes of closely related species
are or similar than genes from more distantly related species. Increasing time since the divergence of two species gives
increasing numbers of random mutations.
Protein and DNA Comparisons Genes have functions that are important for life, and sothey are subject
to natural selection. Mutations that affect critical amino acids will be lethal. This causes some proteins to be almost
unvaried between all species. For example, histones are proteins that make up the basic structure of chromosomes.
There are only 2 amino acid differences between yeast histone H4 and the same protein in humans. Histone structure
is highly conserved, due to a high level of natural selection. However,recall that eachamino acid is coded for by a group
of 3 DNA bases, a codon. There are more codons (64) than amino acids (20), and several codons code for the same
amino acid. Synonymous mutations alter the codon but give the same amino acid. Although the amino acid sequence
of histone H4 is virtually identical in all eukaryotes, the number of synonymous changes is very high. Synonymous
mutations are selectively neutral. For this reason, most evolutionary studies today try to use DNA and not protein, and
they concentrate on synonymous codon changes.
Molecular Clock Mutations happen at random, and synonymous mutations are not subject to selection
pressure. So,the accumulation of synonymous mutations should occur at a relatively regular rate. This is the molecular
clock concept:the idea that you can date the time since divergence of two speciesby counting the number of synonymous
changes between homologous genes. It tends to work reasonably well as long as you stay within a single type of gene,
and if you have some outside evidence to verify it. But, the rate of change varies between genes and between different
groups of organism.
Taxonomy How to impose order on the chaos of 4 million species. Attempt to classify them into groups.
Some of it is pretty obvious: cats, lions and tigers are all felines; cats, dogs, monkeys, and rats are all mammals;
mammals, reptiles, fish are all vertebrates; vertebrates, insects, mollusks are all animals.
Carl Linne (Linnaeus) developed the classification scheme we use today, called the binomial system. In it, the first
word is the genus (general type), and the second word is the species. Both are in Latin, and the genus is capitalized
while the species is not. Thus humans are Homo sapiens. “Homo” is the genus, which we share with some extinct
species such as Homo erectus. “sapiens” is the species. Another example: the common black bear is Ursus americanus.
Other bears are also part of the genus Ursus: Ursus maritimus (polar bear), Ursus arctos (grizzly bear and Alaska brown
bear). Several species in different genera can have the same species name: americanus is a species of Ursus (bear),
Homarus (lobster), and Bufo (toad).
