Download Workshop on Macroevolution

Workshop on Macroevolution
by Julian Lee
(revised by Dana Krempels)
Evolutionary Processes
In this workshop, we will contrast macroevolutionary patterns and processes with
microevolutionary ones. This will help you (1) have a better understanding of the similarities
and differences between these two categories of evolutionary mechanisms, (2) help enhance your
vocabulary of evolutionary terms and concepts, and (3) help you understand some of the ways in
which small genetic changes can translate into large phenotypic differences, thereby contributing
to large-scale evolutionary change.
Macroevolution encompasses evolutionary processes and patterns at or above the level of
species. Examples include
•
Reproductive isolation (speciation)
•
Appearance of major evolutionary novelties (e.g., the evolution of vertebrate jaws
from precursor characteristics in jawless ancestors)
•
Patterns of evolutionary diversification (e.g., adaptive radiation)
These are in contrast to processes of microevolution, that operate below the level of species.
Specifically, microevolution is both
1.
change the genetic constitution of populations, and
2.
the processes that produce such changes (e.g., mutation, migration, genetic drift, nonrandom mating, natural selection).
3.
Given these distinctions, consider each of the following scenarios and determine whether they
are more appropriately associated with microevolution, macroevolution, or neither.
Scenario
1. changes in allele frequencies within a population from one
generation to the next
2. evolutionary diversification of flowering plants in the late
Cretaceous
3. changes in genotypic frequencies in a population within a
single generation
4. differential survivorship by varied phenotypes within a
population
5. evolutionary transition from theropod dinosaurs to birds
Macro, Micro, Neither?
micro
6. adaptive radiation of marsupials in Australia
macro
7. decrease in the allele that causes sickle cell anemia in
parts of the world where malaria has been eradicated
micro
macro
neither
neither
macro
Ideas about the Nature of Evolutionary Change
Although Darwin's theory of evolution by natural selection is by far the most powerful and
well-supported idea so far, there have been many new proposals and modifications to his original
ideas, some of which are more controversial than others. You decide which is which.
Phyletic Gradualism
Darwin proposed that evolutionary forces (especially natural selection, including sexual
selection), operating generation after generation on individual variation within populations
would, over the immensity of geological time, bring about ancestor-to-descendent evolutionary
change. To use his expression, it would result in "descent with modification." Darwin was
thus an advocate of phyletic gradualism, the idea that evolution is a consequence of the
culmination over many generations of small differences in reproductive success among
individuals within populations.
Is phyletic gradualism sufficient to explain macroevolutionary events? Can the evolution of
structures as complex as the photoreceptors of arthropods, cephalopod molluscs, and vertebrates,
for example, be properly attributed to the cumulative effects of microevolutionary processes
operating over the long term? Darwin thought so, but even his staunch supporter and advocate,
Thomas Huxley ("Darwin's Bulldog"), chided Darwin for his unwillingness to embrace other
evolutionary scenarios that involved more rapid and abrupt evolutionary changes.
Can you think of some complex characteristics (in any living organisms) that seem unlikely
to have resulted solely from very gradual genetic changes within populations? List them here.
This part is designed to get students to address their pre-conceived notion about characters
that they “believe” could not evolve gradually. You can name any number of characters,
but a LOT of things have the potential to have arisen either gradually or via sudden genetic
change. Examples of characters that could arise relatively quickly include things like:
1. flight in birds (i.e., would not feathers be maladaptive, if not for flight?)
2. color vision (trichromatic) – could arise via gene duplication and mutation in a
relatively short time
3. Snake venom – Again, could have arisen relatively quickly due to gene duplication (of
existing enzymes expressed in multiople tissues), mutation, and conscription for different
function if expressed in a new tissue (e.g., salivary gland or other mouth tissue).
4. insert your favorite trait here, and try Googling for more if you get stuck.
Punctuated Equilibrium
Only a small portion of the world has been geologically explored. Only organic
beings of certain classes can be preserved in a fossil condition, at least in any
great number. Widely ranging species vary most, and varieties are often at first
local--both causes rendering the discovery of intermediate links less likely. Local
varieties will not spread into other and distant regions until they are considerably
modified and improved; and when they do spread, if discovered in a geological
formation, they will appear as if suddenly created there, and will be simply
classed as new species.
- Charles Darwin, 1859 in On the Origin of Species
In 1972, Niles Eldredge and Stephen Jay Gould revisited this idea, noting that while the
fossil record did contain records of what appeared to be "smooth transitions" from one ancestral
form to new species, there were also--as Darwin had suggested--many examples of apparently
rapid changes from one form to another, in which transitional fossil forms were lacking.
In their paper, they suggested that long periods of evolutionary stasis--during which a
population underwent little or no evolutionary change--were "punctuated" by relatively short
spans during which a smaller subset of the population underwent relatively rapid evolutionary
change. Such punctuated equilibrium would be more likely to occur in a small, genetically
isolated deme of a parent population. In some ways be considered a case of allopatric speciation.
What is meant by "rapid" change is relative. Eldredge and Gould were not proposing that
species undergo a sudden, one-generation split from a parent species. Both authors have
suggested that speciation via punctuated equilibrium could take place over periods of as little as
five- to fifty-thousand years, or as long as hundreds of thousands to millions of years. Although
this may sound like a long time, such periods are negligible in terms of the geologic time scale.
The idea of punctuated equilibrium has generated a great deal of controversy, partly because
many people have confused it with the (largely discounted) idea of saltational evolution, whose
proponents suggest that a new species may arise in single generation through a macromutation.
While there are documented cases of reproductive isolation taking place in one generation in
flowering plants via polyploidy, such events are rare in other types of organisms.
Unlike saltation, punctuated equilibrium is more an expansion of Darwin's original gradualist
model, adding the dimension of differential rates of change to the evolutionary equation.
Can you think of some complex characteristics (in any living organisms) that might be good
"candidates" for evolution via punctuated equilibrium? List them below.
The same traits mentioned above as NOT good candidates for gradual evolution would fit
here. Try to think of as many as you can, and MAKE SURE THE STUDENTS GIVE
LOGICAL REASONS WHY they think these traits could arise relatively rapidly.
Consider genetic drift as a contributing factor. Polyploidy in plants, too.
Forces of Evolutionary Change: Heterochrony
Humans (Homo sapiens), chimpanzees (Pan troglodytes) and Bonobos (Pan paniscus) are
each other's closest living relatives, sharing about 98% similarity of their genomes.
Notwithstanding the fact that chimps and humans share many phenotypic traits, they differ in
many obvious and important respects. For example, chimps are not fully bipedal, have a smaller
cranial capacity than humans, have forward-projecting jaws, heavy brow ridges, and a sagittal
crest on the skull that serves as a surface for the attachment of powerful jaw muscles. Chimps
and humans are placed in different genera, and--until recently--were considered members of
different families (Pongidae, and Hominidae, respectively; currently accepted classification
places all Great Apes, including humans, in Homindae).
Despite this close genetic relationship, chimp-human hybrids are unknown, and members of
the two species do not normally attempt to breed with one another. Clearly, in the evolutionary
divergence between the lineage that led to chimps and the lineage that led to humans, the
accumulation of small genetic differences has had profound phenotypic consequences.
Heterochrony--an evolutionary change in the timing of developmental events--provides a
plausible explanation for how this might have happened. Even small genetic changes in the
developmental program, when expressed early in development, can have profound and farreaching effects on the resultant adult phenotype.
Imagine an evolutionary change in the timing
of developmental events such that, relative to
the development of an ancestral species,
1. development of the reproductive system is
accelerated with respect to the same process
in an ancestral lineage (PAEDOGENESIS)
2.
development
of
non-reproductive
(somatic) tissues is retarded with respect to
the same process in an ancestral lineage
(NEOTENY)
The result could be an organism that is a
"mosaic" of adult characteristics (e.g.,
reproductive competence) and juvenile
characteristics
(body
form).
An
reproductively competent species that retains
the juvenile form of an ancestral species is
said to be paedomorphic (from the Greek
paed meaning "child") and morph meaning
"form"). Its relatively juvenile form can be
generated via one of the two types of
heterochrony described above, or possibly a
combination of both.
Figure 1. Comparison of developmental
change in human and chimpanzee skulls.
More specifically, paedomorphosis describes the appearance of juvenile (or larval) features
of an ancestor in the adult of a descendant. The phenomenon of paedomorphosis appears to have
been important in the evolution of our own species from the ape-like ancestor we shared with
chimpanzees. As an example, depicted are fetal, juvenile and adult skulls of human and chimp
(Figure 1). Superimposed on the skulls are transformation grids, which allow visualization of the
changes in shape that occur during development. Notice that the fetal skulls of the chimp and
human are quite similar in shape, much more so than in the adult. The juvenile skull of the
chimp, though already showing more of a protruding jaw than the human skull, is almost more
similar to that of the human than it is to that of the adult ape.
Questions
1. In what ways do the fetal skulls of the two species differ?
Chimp has larger jaw, smaller relative cranium size, more distinct sagittal crest (visible
from the front). Human face is more flattened. But overall, they are quite similar.
2. In what ways do the adult skulls differ?
Adult chimp is more prognathous, smaller cranium, larger jaw, teeth, and especially
canines; heavier brow ridges. Essentially, the human skull is arrested at the equivalent of
the juvenile chimp stage.
3. Which developmental sequence (chimp or human) involves the greatest amount of change in
shape (i.e., which shows the greatest deformation of the transformation grid?)?
The adult chimp shows the most distortion of the original grid.
4. Which species, then, is more "child shaped" as an adult?
The human.
Apparently, in the evolution of our species certain terminal developmental stages--those that
would further transform our skulls into those resembling a chimp's--have been suppressed,
producing a decidedly juvenile-shaped skull in adult humans.
5. How do we know that human evolution has involved the suppression of terminal
developmental stages rather than the addition of developmental stages in the chimp lineage?
Outgroup analysis would reveal that most adult primates most closely related to apes (i.e.,
Old World Monkeys) share more similarity with the apes than with the paedomorphic
human. It is more parsimonious to hypothesize that only the humans remained relatively
paedomorphic, rather than that all the other primates independently gained the more
distorted features many times.
Allometric and Isometric Growth
Allometry is the study of the relationship between the size of an organism and the size of any
of its parts. One can consider allometry at different levels, including
• during the growth and development of a single organism
• between different members of the same species (ontogenetic allometry)
• between different species (evolutionary allometry)
In graphic an allometric relationship at any of these levels, one would typically use body size
as the independent variable (x axis), and some body part’s measure as the dependent variable (y
axis). The points on the graph could reflect measurements from
• a single individual measured at different ages
• different individuals of a single species (scatter may be due to age differences)
(ontogenetic allometry)
• different species within a taxon of higher rank (genus; family, etc.)
(evolutionary allometry)
• Note: phylogenetic allometry is a type of evolutionary allometry in
which analysis are constrained to taxa within a monophyletic (i.e., having
a single common ancestor) lineage
•
Differential timing of developmental events may occur within a species, as well as across species
over evolutionary time.
Figure 2. Isometric growth in a
salamander
Figure 3. Allometric growth in a human.
Ontogenetic Allometry
As an organism grows larger during ontogeny, or as an evolving lineage of organisms
increases in size over evolutionary time, the various parts of the organisms also increase in size.
Bigger organisms have bigger parts. But not all of the parts necessarily grow at the same rate.
Such differential growth of different parts is termed allometry, and it results in shape changes as
the animal grows. When all parts of an organism grow at the same rate, the organism is said to
exhibit isometric growth, and this results in a change in size without a change in shape.
Although most organisms exhibit allometric growth, some, such as certain salamanders
(Figure 2) are essentially isometric in their growth. Note that the various body proportions
remain more or less constant as the animal increases in size.
Growth in our own species (Figure 3) is decidedly allometric, most obviously with respect to
the growth of the head relative to the body. At birth, the head is relatively enormous, and
comprises nearly a third of the length of the infant. As we grow larger, our body grows more
rapidly than the head such that the body "catches up" eventually to produce the proportions that
we recognize as normal in the adult. Maturation in humans thus involves both increase in size
and a change in shape.
Questions
1. In humans (and in vertebrates in general), the head is large at birth relative to the rest of the
body. Why is this so?
Vertebrates have succeeded so well in part because of their larger, more complex brains
that allow more complex behaviors. Note that the more altricial (helpless) the species is at
birth, the larger the head tends to be—relative to the body—at birth. It may be that
extended parental care has allowed these species (usually predators) the “luxury” of a large
head at birth at the expense of a functional, running body early in life. Compare a
newborn wolf (altricial) to a newborn caribou (precocial) to see what I mean.
Newborn wolf pups (a few hours old)
About 8 inches long
Newborn caribou calf (a few hours old)
Stands about 24” at the shoulder
2. An increase in cranial capacity to accommodate the expansion of the cerebral hemispheres in
our species is considered one of the evolutionary innovations that has contributed to our success
as a species. (But note that the average cranial capacity of Neanderthal humans (Homo
neanderthalensis) slightly exceeded that of modern humans (Homo sapiens). What do you
suppose sets the upper limit on cranial capacity (and thus brain size) in our species?
The capacity of the mother’s pelvic opening. Death during childbirth is a major problem
in our species, especially where medical/surgical assistance is not available.
Evolutionary Allometry
A lineage of organisms can also exhibit
allometric growth over evolutionary time, as
exemplified by the brontotheres (pictured at the
right), an extinct group of large mammals. The
brontothere lineage shows a clear trend of increasing
size, as seen in the comparison of these four species.
From a small, hornless ancestral species, the trend
has been for an increase in overall body size, but an
even more rapid increase in horn size. Over
evolutionary time, the horns have increased in size
more rapidly than the head or body, producing a
change in shape as well as a change in size.
Questions
1. What might be the adaptive significance of
horns in brontotheres?
Feeding/digging; Defense against predators;
Fighting among members of same species, etc.
Let the students come up with as many
competing hypotheses as possible.
2. If horns in brontotheres are restricted to
males only, what would this suggest
concerning the selective agents involved in their
evolution?
Sexual selection: (1) males with larger horns
are better able to fight with other males or (2)
females prefer males with larger horns, or both.
Figure 4. Evolutionary allometry in
brontothere species.
The Results of Organic Evolution
Every person alive today is the descendant of ancestors who successfully mated and left
offspring. Those ancestors of ours faced various pressures of natural selection, and we are left
with much of that "evolutionary baggage" in our own morphology and even in our behaviors,
though we may not recognize them as such.
With a little bit of training, however, you can teach yourself to recognize traits in yourself
and other humans that have been handed down from a time (and environment) both different and
the same as the one in which we now live.
Cashing in on Ancestral Hardwiring: The Evolution of Mickey Mouse
Stephen Jay Gould noted that during the fifty years since his inception in the animated
cartoon Steamboat Willie in 1928 (Figure 5, Stage 1), Mickey Mouse has been gradually
transformed by Disney artists, from a small-headed, long-snouted, beady-eyed rodent (Stage 1)
to the large-eyed, more loveable version (Stage 3).
Figure 5. The evolution of Mickey Mouse from 1926 to the present.
For each of the evolutionary stages in Figure 5, calculate the following ratios:
1. EYE SIZE/HEAD HEIGHT X 100
stage 1:
stage 2:
stage3:
2. HEAD HEIGHT/BODY LENGTH
X 100
stage 1:
stage 2:
stage3:
3. HEAD HEIGHT/HEAD LENGTH
X 100
stage 1:
Figure 6. Mouse-o-gram indicating
proper measurement points.
stage 2:
stage3:
Now quantify this "evolutionary" transition: record the following measurements on the mouse at
each stage.
Have your students do these measurements and ratios with a small ruler.
characteristic
stage 1
stage 2
stage 3
EYE SIZE
(maximum height)
HEAD HEIGHT
(base of snout to top of rear ear)
(1 in the diagram)
HEAD LENGTH
(base of snout to posterior margin of the
anterior ear) (2 in the diagram)
BODY LENGTH
(bottom of foot to margin of top ear)
(3 in the diagram)
Questions
1. Over the fifty years of mouse "evolution" represented here, what has happened to the relative
size of the head? It has increased.
2. What has happened to the size of the head relative to the overall body size? Head is larger in
comparison to the body in the later Mouse.
3. What has happened with respect to the extent of the cranial bulging, as reflected by the
rearward displacement of the rear ear? In each subsequent mouse, the ear is farther back,
increasing the adorable, bulging cranium. Ew.
4. Assuming that Stage 1 is the ancestral configuration and that stage 2 is transitional to the
modern mouse of Stage 3, is the modern adult mouse more similar or less similar to a
juvenile mouse? More similar to a juvenile HUMAN than to a mouse:
Which one looks more like late Mickey
I think this one!
5. What motive(s) might Disney Studios have had in transforming Mickey this way?