Transcript Macroevolution - NIU Department of Biological Sciences

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.
Examples
How Large Scale Changes Occur
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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 recent theory, “punctuated equilibrium”, says that the large scale changes
occur 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
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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.
Some fossils are not turned to rock: insects in amber (fossilized tree sap) and
sometimes ancient bones. Occasionally possible to extract DNA from them.
Fossils are exposed when erosion removes the overlaying rock, or when people dig
them up n rock quarries and road cuts.
Becoming a fossil is very unusual: most organisms decay to nothing.
Fossils in Sedimentary layers
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Two main sources of rock layers:
sediments piling up at the bottom
of lakes and oceans, and volcanic
ash.
Disruption of layers is caused by
erosion, which can remove whole
layers, and mountain-building,
which folds the layers and
sometimes even turns them over.
But, in most cases, older rocks are
underneath newer ones.
Maps of rock strata show where
different layers lie relative to each
other, and where they are
exposed on the surface. The
maps can be used to detect loss
of layers by erosion.
What does the Fossil Record
Show?
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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” between apes and humans, for
example. However, there is a list of 139 examples of gradual species to species
transitions that are very well documented in the fossil record.
The fossil record is like taking single frames from a movie—we miss much of the
action and need to fill in the gaps ourselves. Lack of intermediate forms has
stimulated the punctuated equilibrium idea.
“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
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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.
More Plate Tectonics
Still More
Biogeography
• How plate tectonics affects life. Biogeography is the
study of how the spatial patterns of living things
developed.
• Continents have split apart, moved around, then
rejoined, all very slowly. There are coal beds in
Antarctica, for example, laid down when that part of the
world was near the equator.
• Identical rock layers with particular fossils in them are
found in different continents—they were laid down as
one bed, then the continents broke up and now they are
widely separated.
• Distribution of current species can also be explained by
continental movements.
Continents Colliding
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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.
A More Recent Collision
• Still ongoing is a collision between the Australian and Asian plates.
They are colliding in Indonesia, as a consequence a very volcanic
region, home of Krakatoa.
• Alfred Wallace mapped a line between Borneo and Sulawesi, where
entirely different sets of animals, birds, and plants lived, as a result
of having evolved on separate plates. Some islands are less than 20
miles apart. On the Australian side, the mammals are marsupials,
and on the Asian side, the mammals are placental.
Comparative Morphology
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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
structures are similar across species.
Similarly, all vertebrates have 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.
More Divergence
• Divergent evolution: starting
with a common ancestor, then
diverging into altered forms.
Vertebrate forelimb is an
example.
• Another example: insect
wings. Started with 4 wings,
equal in size, like a dragonfly.
Rear wings have shrunk to tiny
balancers in flies. Front wings
converted to cases in beetles.
All wings expanded in
butterflies.
Morphological Convergence
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Convergent evolution: 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 ocean and 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.
Sequence Analysis Example
• Using a bit of text from the
sixth century Irish monk, the
Venerable Bede. Manuscripts
were copied by hand, leading
to errors, just like DNA
sequences. Errors are
propagated with each
successive copying, and new
errors appear. The errors can
be analyzed to show the order
and derivation of the
manuscripts.
• “Before the inevitable journey”
Example
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MS Century
Text
1 9 FORE TH’E
NEIDFAERAE
2 10 FORE THAE
NEIDFAERAE
3 12 FORE TH_E
NEIDFAERAE
4 12 FORE TH_E
NEIDFAER_E
5 15 FORE TH_E
NEYDFAER_E
6 13 FORE TH_E
NEYDFAOR_E
7 12 FORE TH_E
NEIDFAOR_E
Cytochrome C
• Cytochrome C is part of the electron transport system in the
mitochondria. It is found in all eukaryotes, and some aerobic
prokaryotes as well. The number of amino acid differences between
the cytochrome c found in different species is proportional to the
time since they diverged.
Protein and DNA Comparisons
• Genes have functions that are important for life, and so they 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 each amino 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
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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 species by
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
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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).
Higher Taxa
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There is clearly order among living things above the level of genus.
Taxonomists have developed a hierarchy to describe any organism’s
classification: kingdom, phylum, class, order, family, genus, species. “Kings
play chess on fine ground sand” is a good mnemonic device for this.
For humans: we are in the animal kingdom, the chordate phylum, the class of
mammals, the order of primates, family of hominids, genus Homo, species
sapiens.
The classification scheme roughly indicates evolutionary relationships. But in
reality, all evolutionary changes come from one species splitting into 2. A true
representation of evolutionary history would be a tree diagram showing when
each species split.
Large Scale Classification
Schemes
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Linnaeus originally had two kingdoms: plant and animal. This works OK, but there is
no real place for bacteria. They got lumped in with plants, but that isn’t reasonable.
A later scheme had 5 kingdoms: the bacteria were in the kingdom Monera, single
celled eukaryotes were the kingdom Protista, Fungi had their own kingdom, and the
Plants and Animals had separate kingdoms.
Recent work with DNA sequencing of the ribosomal RNA genes has shown that the
bacteria are deeply divided into 2 groups: the Eubacteria (most of the common
bacteria) and the Archaebacteria (dwellers in extreme temperature, pH, salinity, etc.).
The Archaebacteria and the Eubacteria are as different from each other as they are
from all eukaryotes, as far as the time since they diverged goes. This has led to the
“3 domain” model”: life can be classified as Archaebacteria, Eubacteria, or Eukaryote.
This scheme is favored by microbiologists.
Problem: although the 3 domain model fits evolutionary history well, the differences
between Eubacteria and Archaebacteria are not easy to state for non-scientists.
I like to think of prokaryote vs. eukaryote as the largest distinction between groups.
The book uses a 6 kingdom scheme: eubacteria, archaebacteria, protists, fungi,
plants, animals.
There is no universal agreement about any scheme, and all schemes are artificial: in
all cases, one species split into 2 species are different point in time.
Three Domain Scheme
Five Kingdom Scheme