Title: Principles of Evolution
1Principles of Evolution
- Chapter 12
- Life in Groups
- James F. Thompson, Ph.D., MT(ASCP)
2Life in Groups
- Social organization allows organisms to share
labor, to specialize in tasks and to coordinate
efforts. - Groups may be better at finding resources,
including mates, protecting against dangers, and
caring for their young.
3Types of Social Interactions
4Alarm Calls
- (a) Vervet monkeys fall prey to leopards, as well
as to snakes and eagles. - Each-snakes, leopards, and eaglespractice
different predatory styles and arrive from
different directions.
5Alarm Calls
- Depending upon the predator, the monkeys emit
different alarm calls that elicit different
adaptive escape responses. - Shown are the abrupt chirp call (leopard) and
staccato grunts of the threat-alarm bark (eagle).
6Beldings Ground Squirrel
- Beldings Ground Squirrels also have different
alarm calls for predators approaching by land or
air. - With hawks, the individual giving the call is
rarely taken as the prey. - With coyotes and other terrestrial predators, the
individual giving the call is about twice as
likely to be take as a neighbor.
7In Beldings Ground Squirrels, Most Alarm Calling
is Done by Females
?
?
8Why Would an Animal RiskGiving an Alarm Call?
- The caller benefits because the predator learns
it has lost the element of surprise? - The caller signals the predator it is ready to
escape? - The caller creates a scramble of escaping
neighbors which confuses the predator? - The caller, by reducing the predators success,
teaches the predator not to feed on this social
group? - Is giving an alarm call an act of Altruism?
9Altruism
- An act of altruism is a behavior by an individual
that increases the reproductive fitness of
another individual while decreasing the fitness
of the altruist. - The recognition of altruistic behaviors in
animals created a seeming paradox for the theory
of evolution? - How could natural selection act to preserve
behavior which reduces individual fitness?
10Altruism
- One possibility was that benefits to the group
outweighed the individual advantage. - If so, then the group was subject to natural
selection the group became a unit of selection. - The concept of Group Selection has been a
controversy for more than half a century.
11Group Selection?
- V.C. Wynne-Edwards (1906-1997) proposed this
hypothesis in his book Animal Dispersion in
Relation to Social Behavior (1962) and continued
in Evolution Through Group Selection (1986).
12Wynne-Edwards Group Selection
- Wynne-Edwards proposed that social behaviors act
to keep social species from exceeding the
carrying capacity of their environments that
social behaviors evolve to limit reproduction or
fecundity. - For example, occupying territories or
establishing dominance hierarchies would reduce
the number of breeding males. - His logic was supported by the success of
K-selected species in having stable population
sizes. - This could be seen as a form of altruism.
13Wynne-Edwards Group Selection
- Wynne-Edwards hypothesis stimulated much good
scientific work, but it also received immediate
sharp criticism as a concept. - The populations of social animals are kept in
check by predation, disease, and famine. - There is no check against new mutations that
would increase an individuals reproduction
fecundity. - Cheater genotypes would spread due to
individual natural selection.
14Kin Selection
- A better hypothesis was already under discussion
Kin Selection. - Basic concepts had been published by architects
of the Modern Synthesis well before Wynne-Edwards
proposed Group Selection. - R.A. Fisher in 1930 and J.B.S. Haldane in 1955,
two of the strongest mathematical population
geneticists of their generation.
15W.D. Hamilton Kin Selection
- The term, kin selection was coined by another
mathematical biologist, John Maynard Smith, but
Smith acknowledged the contributions of his
predecessors. - Hamilton (1936-2000), was another of the world
class evolutionary biologists of the post-WW II
generation.
16W.D. Hamilton (1936-2000)
- Hamilton was British, though born in Egypt and
much of his professional life was spent in
America. - Hamilton was another mathematical population
geneticist. - He advocated for the importance of the gene as a
unit of selection, a perspective shared by E.O.
Wilson and Richard Dawkins.
17W.D. Hamilton (1936-2000)
- In addition to his mathematical analysis of
Inclusive Fitness Theory (Kin Selection),
Hamilton also investigated sex ratios and the
cost-benefit analyses for the evolution of sexual
reproduction. - Hamilton was a proponent of the Red Queen
Hypothesis in that regard, supporting the idea
that sexual recombination originated as a defense
against parasitism, a form of the evolutionary
arms race.
18Hamilton Kin Selection
- Hamiltons Rule A costly action should be
performed if - R x B gt C
- Where C is the cost in fitness to the actor
- R is the genetic relatedness between the actor
and the recipient, and - B is the fitness benefit to the recipient.
- Fitness costs and benefits are measured in
fecundity.
19Kin Selection
- Hamilton developed the Theory of Kin Selection,
in part, to resolve the challenge of explaining
how sterile worker castes in social insects could
evolve, which followed on an insight from Darwin. - Hymenopterans exhibit haplodiploidy drones are
haploid with all their genes coming from their
mother, the queen, while workers are typically
sterile females with half their genes coming from
the queen and half from their father, a drone.
20Kin Selection
- In brief, workers are sisters and share 3/4s of
their genome on average, and half their genome
with their mother, the queen. - Therefore, if the workers contribute to the care
of the queens other young, more female workers,
they are preserving more of their genes in the
next generation, than they would if they had
offspring of their own, who would share only ½ of
their genome. - The workers increase their Inclusive Fitness.
21Kin Selection
- The Theory of Kin Selection predicts that
individuals should behave more altruistically and
less competitively toward their relatives,
because they share a relatively high proportion
of their genes (e.g., one-half for siblings and
one-eighth for cousins). - Consequently, by helping a relative reproduce, an
individual passes its genes to the next
generation, increasing their Darwinian fitness
22Inclusive Fitness
- The genetic contribution an individual makes to
future generations includes the genes of its own
surviving offspring and any like genes preserved
in the surviving offspring of its relatives. - Kin Selection is one mechanism (but not the only
means) by which an individual can increase its
inclusive fitness.
23Inclusive Fitness
- The genetic contribution an individual makes to
future generations includes the genes of its own
surviving offspring and any like genes preserved
in the surviving offspring of its relatives. - Kin Selection is one mechanism (but not the only
means) by which an individual can increase its
inclusive fitness
24Parental Genetic Investment
- When diploid organisms (P) produce offspring,
each offspring (F1) receives half of its genotype
from each parent. - In turn, these offspring (only the female is
shown) produce progeny (F2) carrying a quarter of
the genotype of the original parents. - The genetic contribution of the female is
followed, although the same outcome applies to
the male. - Each individual is diploid, represented by the
divided half the amount of the original females
genetic contribution (cross-hatch) is followed
here.
½ ¼
25Kin Selection
- The most obvious example of Kin Selection is
simply the general phenomenon of Parental Care. - Any activity or resource allocation by a parent
to the reproductive fitness of its young will be
favored by natural selection. - Thus, no sacrifice short of death on the part of
a parent for its offspring should be thought of
as altruistic! - And death as a sacrifice is justified for three
or more young - R x B gt C ? ½ x 3 live gt 1 dies
26Parental Care
- The female parent defends and saves her three
offspring, but she dies in the effort.
Nevertheless, because half of her genotype is
carried in the offspring (F1), a total of 1½ of
her genotype survives. - No parental defense and the female survives, but
all offspring (F1) perish, leaving a total of
only one (1) of her genotype surviving (the same
applies to males).
27Brood Parasites Exploit Parental Care
- ? The large young Cuckoo (right) has evicted the
smaller young of the host Meadow Pipit (left) and
begs for food. - Rock Pipit and Cuckoo below.
28Levels of Selection
- Darwin, and most evolutionary biologists since
Darwin have recognized the individual organism as
the unit of selection. - However, some additional levels of selection have
been proposed, both above and below the level of
the individual organism.
29The Gene as the Unit of Selection
- The view that the gene is a unit of selection, is
an addition to the Modern Synthesis of Evolution. - In this model, those alleles and allele
combinations whose phenotypic effects
successfully promote their own propagation will
be preferentially selected in contrast to their
competitors. - This process produces adaptations to the benefit
of alleles, which promotes the reproductive
success of the organism, or of other organisms
containing the same gene (inclusive fitness), or
even only its own propagation in detriment to the
other genes of the genome.
30The Gene as the Unit of Selection
- The concept was first proposed by Colin
Pittendrigh in 1958 and further explored by W.D.
Hamilton in classic papers in 1963 and 1964. - Soon after, it was more thoroughly developed by
George C. Williams in his book, Adaptation and
Natural Selection (1966). - One necessary condition is that the selected
entity must have a high degree of permanence and
a low rate of endogenous change. - This characterizes genes as well as individuals.
31George C. Williams (1926- )
- Williams is an American evolutionary biologist
and ichthyologist - His diverse interests include the evolution of
sex, the gene as a unit of selection, evolution
applied to medicine, philosophy of science and
human ethics. - Williams is also a major critic of Wynne-Edwards
Theory of Group Selection.
32Richard Dawkins (1941- )
- Dawkins is a British evolutionary biologist and
ethologist, perhaps the premier living
evolutionary biologist. - Dawkins advocates for the gene as the unit of
selection in The Selfish Gene (1976) and The
Extended Phenotype (1982). - Dawkins crusades against creationism and
Intelligent Design, e.g., The Blind Watchmaker
(1987) and against religion, The God Delusion
(2006). - He also writes on human overpopulation,
environmental concerns and the evolution of human
culture
33The Gene as the Unit of Selection
- The concept of the gene as the unit of selection
is well substantiated by data, but also receives
some sharp criticism. - It is a fascinating topic but beyond the scope of
this course. - To follow the debate you need a much stronger
foundation in genetics than this course provides. - But the classic books aimed at general readers on
the subject are very readable.
34Higher Levels as Units of Selection
- There is little evidence for the population, or
species, as a unit of evolution, as hypothesized
by V.C. Wynne-Edwards. - However, there are more modern hypotheses which
explore the question of the species as a unit of
selection. - To explore them, we need to take a detour into
paleontology.
35Micro- vs. Macroevolution
- Microevolution Natural Selection - differential
survival due to inherited traits (individual
selection) - Macroevolution differential survival of branches
within phylogenetic lineages
36Micro- vs. Macroevolution
Sauropod evolution
- The overall pattern of vertebrate tetrapod
evolution (macroevolution) can be examined in
closer detail. - The cross section in time reveals the multiple
species lineages of which it is composed.
37Micro- vs. Macroevolution
- At the bottom, one lineage, one species, is
enlarged, showing the populations comprising the
species (microevolution). - The key question is IF there is any difference in
the mechanisms for macroevolution versus
microevolution?
38Gaps in the Fossil Record
- Darwin and his contemporaries recognized that
there were major gaps in the fossil record which
they attributed primarily to the incompleteness
of the study of the strata and the difficulties
of fossil preservation. - A century later, after massive explorations, many
major gaps still remained. - Old species and higher taxa disappeared and new
species and higher taxa suddenly appeared in
their place in later strata.
39Gaps in the Fossil Record
- The real pattern of evolution depends on the
completeness of the fossil record.
40George Gaylord Simpson (1902-1984)
- American vertebrate paleontologist and major
contributor to the Modern Synthesis of evolution - Simpsons interests also included biogeography,
the theory of systematics, and horses and
penguins - Tempo and Mode in Evolution (1944) and The Major
Features of Evolution (1953) addressed mechanisms
and rates of evolution
I don't think that evolution is supremely
important because it is my specialty it is my
specialty because I think it is supremely
important.
41Quantum Evolution
- At the beginning of the Modern Synthesis,
Darwinian gradualism was disputed by various
Saltational hypotheses, mutation theory,
Lamarckian views, Goldschmidts hopeful monsters,
etc. - Simpson sought to explain the discontinuities in
the fossil record by advocating that Darwinian
natural selection, though gradual in
microevolutionary terms, could still show
different rates of change in larger, geological
time frames. - He termed this Quantum Evolution.
42Quantum Evolution
- Within the taeniodonts, a group of extinct
placental mammals, two lineages evolved. - One was the original group of taeniodonts, the
beaver-sized conoryctines that survived into the
late Paleocene
conoryctines
43Quantum Evolution
- The other taeniodont lineage was the
stylinodonts, which evolved rapidly (quantum
evolution) across a transition to a new adaptive
zone (lifestyle). - Compared to the conoryctines, the bear-sized
stylinodonts evolved specialized dentition
especially suited to rough and highly abrasive
foods, well-developed claws, and strong muscles
suggesting a digging foraging style. (After
Simpson 1953.)
quantum evolution
stylinodonts
44Micro- vs. Macroevolution
- The 1950s and the 1960s were a quiet time for
organismal biologists and paleontologists. - They continued to work, but the attention and
acclaim went to the rapidly developing fields of
genetics and molecular biology. - A new hypothesis to explain differences in the
mechanisms for macroevolution versus
microevolution arose in the 1970s.
45Niles Eldredge (1943 - )
- An American invertebrate paleontologist and
expert on trilobites. - Eldredges interests also include the history of
biology, criticism of creationism and Intelligent
Design, and protection of biodiversity. - He is a critic of the concepts of the gene as a
unit of selection and of sociobiology. - He is co-author of the model of Punctuated
Equilibria.
46Stephen Jay Gould (1941 2002)
- An American evolutionary biologist, invertebrate
paleontologist and expert on land snails. - Gouldss interests also included the history of
biology, developmental biology, and resolving
conflict between science and religion while
opposing creationism. - He was a critic of the concepts of the gene as a
unit of selection and of sociobiology. - He was co-author of the model of Punctuated
Equilibria.
47Punctuated Equilibria
- Prior to the 1970s, most paleontologists, unlike
G.G. Simpson, rarely interacted with organismal
and field biologists. - Willi Hennig, father of cladistics, had been an
entomologist and Dipteran (fly) systematist. - Eldredge and Gould championed interaction and
cooperation among paleontologists and
neontologists. - They helped integrate cladistic methodology into
paleontology.
48Punctuated Equilibria
- Eldredge and Gould championed interaction and
cooperation among paleontologists and
neontologists. - They helped integrate cladistic methodology into
paleontology. - Eldredge and Gould coined the term punctuated
equilibrium in 1972 to describe a pattern to
evolution which they contrasted to phyletic
gradualism.
49Punctuated Equilibria
- Punctuated equilibrium is a model of
macroevolution which states that most species
experience little change for most of their
geological history, and that when phenotypic
evolution does occur, it is localized in rare,
rapid events of branching speciation (called
cladogenesis). - Punctuated equilibrium is commonly contrasted
against the model of phyletic gradualism, which
states that evolution generally occurs uniformly
and by the steady and gradual transformation of
whole lineages (called anagenesis). In this view,
evolution is seen as generally smooth and
continuous.
50Patterns of Macroevolution
- Phyletic evolution (anagenesis) envisions gradual
divergence of a lineage as the bell-shaped mean
of successive populations changes, until a new
species is formed.
51Patterns of Macroevolution
- Punctuated equilibrium (cladogenesis) envisions
long periods of more or less unchanging species
persistence, suddenly interrupted by speciation,
producing a new species.
52Punctuated Equilibria
- Eldredge and Goulds Punctuated equilibrium model
of macroevolution was grounded in Ernst Mayr's
theory of geographic speciation (allopatric and
especially parapatric models), I. Michael
Lerner's theories of developmental and genetic
homeostasis, as well as their own empirical
research on the fossil record.
53I. Michael Lerner (1910-1977)
- Lerner, a Russian immigrant to America, became an
important population geneticist, working with
domesticated animals and later the flour beetle. - His two most influential books were Genetic
Homeostasis, (1954) and Genetic Basis of
Selection (1958) - Lerner defined Genetic Homeostasis as the
tendency of a Mendelian population as a whole to
retain its genetic composition arrived at by
previous evolutionary history
- The major mechanism for maintaining this genetic
composition, through sexual reproduction, is the
selection of heterozygotes. This is especially
important in the genetic control of behaviors. - Increased heterozygosity in a population
preserves allelic diversity and permits large
numbers of individuals to display optimum
characteristics.
54Punctuated Equilibria
- Eldredge and Gould proposed that the degree of
gradualism they attribute to Charles Darwins
view was virtually nonexistent in the fossil
record, and that stasis dominates the history of
most fossil species. - To be fair to Darwin, knowledge of paleontology
has grown enormously between Darwins death and
Eldredge and Goulds proposal some ninety years
later.
55Punctuationist versus Gradualist Theories of the
Origin of Species
- Biologically "instantaneous" origin of a new
species through founder effect speciation may
occur in only a few generations. - Geologically "instantaneous" speciation by
allopatric divergence may occur over many
thousands of generations. - An incomplete fossil record will obscure the
difference between the two.
56Cladogenesis, Details
- Where sudden changes occur, they can be
represented with an angular, branching
phylogenetic tree. - Each independent lineage produced is a clade,
shown here as Clade 1 and Clade 2. - Vertical sections represent more or less
unchanging persistence of a species branch
points represent the time of speciation where
populations diverge and become two distinct
species.
- Time runs upward species divergence is
indicated along the horizontal scale. - The balloons show details of the phylogeny in a
species before speciation (light shading), at a
branching point of speciation wherein two species
form (light and dark shading), and the subsequent
fate of each species thereafter.
57Peripheral Isolates
- The geographic range of a species may include
major populations fragmented into several
smaller, isolated populations. - These isolated populations can be peripheral to
the main populations, or they may occur within a
main population that has contracted around it.
58Patterns of Fossils and Phylogeny
- The balloons indicate temporal and geographic
distribution of species A-H. - Gaps between species, as they might appear in the
fossil record, are indicated by short vertical
brackets at the left. - Location 1 indicates a fossil excavation site
that preserves only part of the actual history of
this evolving group.
gaps in the fossil record
Note also that our picture of the record would be
different if we had sampled different fossil
locations.
- The pattern of evolution expressed as a
phylogenetic tree. - Note that most new species originate in a
peripheral, isolated part of the distribution
(e.g., species B) or from a central, isolated
population (e.g., species E).
59Patterns of Macroevolution
One problem for interpreting the fossil record is
that scientists sometimes do not discover enough
fossil individuals to make a judgment about the
variations in phenotypes, much less genotypes, in
the populations.
unknown when I was in college ?
- As more transitional fossils are added to a
sequence, the morphological discontinuities are
likely to become smaller and smaller.
60Patterns of Macroevolution
- Recall the patterns of selection stabilizing,
directional, and disruptive. - If the environment is stable, stabilizing
selection should produce phenotypic stasis over
time. - Only when the environment changes, should
directional or disruptive selection take over to
transform phenotypes to adapt to the changes. - So the punctuated pattern is really not such as
new concept as Eldredge and Gould suggested it
was it was a new term for the pattern seen in
the fossil record.
61Horse Phylogeny
- Environmental Transitions
- forests to prairies (wetter to drier)
- slower to faster predators
62Lake Turkana Snail Phylogeny
- We can infer that the changes in lake levels not
only effect which times that fossilization are
likely to occur, but also that such changes in
the environment produce the selective pressures
that led to speciation or extinction of different
snail lineages.
63A Comparison of Models
Raw data
64Pace of Macroevolution
- For some biologists, each pattern implies a
different rate of new species appearance. - Punctuated equilibrium produces new species
relatively rapidly.
- Phyletic evolution produces new species more
gradually and is sometimes termed Gradualism.
65Stephen M. Stanley (1941- )
- An American paleontologist and evolutionary
biologist - Stanley was an early supporter of punctuated
equilibria and advocated for a mechanism of
selection associated with it species selection. - Macroevolution Pattern and Process (1998)
66Species Selection
- Stanley, Gould, and others, often other
paleontologists, have advocated for the species
as a distinct unit of selection. - Like the individual, the species is seen as a
real biological entity with a birth (a speciation
event) and a death (extinction or transformation
into a new species or clade over time).
67Species Selection
- Our text takes the majority position, which is
that there are few documented cases of species
selection and no mechanism for selection that is
operating at the species level as distinct from
the level of the individual. - The minority position is that some lineages are
more likely to speciate, and, therefore, less
likely to have all members of their clade go
extinct. - Some of these species may be more likely to
produce adaptive radiations. - It becomes a matter of semantics if this is a
separate form of selection, or just an aspect of
ordinary natural selection acting on individuals. - It is a part of the part of evolution.
68High Speciation Potential Taxa?
Hawaiian Honeycreepers
- Do small omnivorous birds have more potential to
speciate than small omnivorous species? - We do observe more adaptive radiations among such
birds, as opposed to lizards, on islands around
the world.
69Mode and Tempo of Evolution
- There are examples of both gradual and rapid
appearance of new species occurs in the fossil
record. - Evolution is not limited to gradualism nor to
punctuationism!
70Peripheral Isolates
- Recall the four modes of geographic speciation.
- The rate of evolutionary change in each mode will
be influenced by the size of the population
experiencing the new selective pressures. - We expect small populations to respond more
rapidly to natural selection. - There are two main reasons inbreeding and
genetic drift.
71Sewell Wright (1889-1988)
- Sewell Wright, American geneticist, one of the
founders of modern theoretical population
genetics and the Modern Synthesis. - He researched the effects of inbreeding and
crossbreeding with guinea pigs, and later on the
effects of gene action on inherited
characteristics. - He adopted statistical techniques to develop
evolutionary theory. - Wright is best known for his concept of genetic
drift, called the Sewell Wright effect - that when small populations of a species are
isolated, out of pure chance the few individuals
who carry certain relatively rare genes may fail
to transmit them. - The genes may therefore disappear and their loss
may lead to the emergence of new species,
although natural selection has played no part in
the process.
72Genetic Drift
73Genetic Drift
- In a small habitat, an island or habitat island,
resources may be very limited. - If so, chance may be more important than fitness
in determining who survives to leave offspring to
the next generation.
74Empirical Researchon the Founder Effect
- The Founder Effect is the effect on the resulting
gene pool that occurs when a new isolated
population is founded by a small number of
individuals possessing limited genetic variation
relative to the larger population from which they
have migrated. - It is the direct result of sampling error.
75The Founder Effect in an Island-Hopping Bird
- Silvereyes, Zosterops lateralis, are small
insectivorous songbirds. - They are native to Australia and Tasmania.
- Naturalists have documented 5 new colonizations
of offshore islands.
76The Founder Effect in an Island-Hopping Bird
5
4
1
3
2
77The Founder Effect in an Island-Hopping Bird
- Allele diversity declines as you follow the
successive colonizations through time just as
predicted if each colonization is due to a small
flock of founders. - The most recently colonized island population
shows half the diversity of the mainland source
population.
Similar founder effects have been documented
for Large Ground Finches on Isla Daphne in the
Galápagos Islands, and many other island bird
species.
78Drift After a Bottleneck
- Even a large population may suffer a population
crash after some catastrophe, such as a flood,
fire, drought, volcanic eruption. - The survivors will experience the same random
events which may dramatically alter the gene pool
of the survivors.
79Genetic Drift Bottleneck Effect, in Theory
- In a large collection of individuals, here the
blue and yellow marbles, approximately equal
numbers of both are present. - However, when just a few persist to start the
next generation, chance alone may yield mostly
blue. - Because most are blue, the next generation, even
if large numbers are produced, are now mostly
blue.
80Genetic Drift Bottleneck Effect, in Practice
- The original population of cheetahs contained
many alleles of a particular gene. - Habitat loss and excessive predatory control
brought their numbers down, by chance leaving
only a few to breed with much less genetic
diversity (inbreeding). - Even as their numbers were partially restored,
the limited genetic diversity in the reduced
cheetah population did not also bring recovery of
the lost genetic diversity. - Without such diversity, cheetahs have been
susceptible to breeding deficiencies such as
decreased resistance to disease and reduced
offspring survival.
81Inbreeding
- Inbreeding is sexual reproduction between close
relatives, whether plant or animal. - If practiced repeatedly, it leads to increased
homozygosity in the population. - A higher frequency of recessive, deleterious
traits in homozygous form in a population can,
over time, result in inbreeding depression. - Inbreeding depression may occur when inbred
individuals exhibit reduced health and fitness
and lower levels of fertility.
82Inbreeding Depression
- In Sweden, a change in allocation of farm lands
isolated a population of 40 Swedish adders,
Vipera berus. - Subsequent inbreeding depression produced more
stillborn and deformed offspring than normal. - Introduction of adders from another habitat
restored diversity in the gene pool and improved
offspring survivability.
83Inbreeding in Humans
- There are many examples of the effects of
inbreeding in humans. - A classic case relates to the discovery of the
specific gene, an autosomal dominant, which
causes the neurological disorder, Huntingdons
chorea. - Because the people of the isolated Venezuelan
fishing villages of Barranquitas and Lagunetas
were inbred, many individuals were available for
DNA analysis. - Having isolated the gene, medical scientists now
have the first molecular diagnostic test for a
human inborn error of metabolism.
84Inbreeding
- Inbreeding does not have to lead to negative
consequences. - A small population which carries an adaptive
allele better suiting it to a marginal habitat
may increase the speed at which the favored
genotype spreads n the population as a result of
inbreeding between the successful phenotypes.
85Lizards-From Limbed to Limbless
- Cross sections through the posterior end of the
lizard embryo are depicted. - Legged lizard. The somite, an embryonic
population of formative cells, grows downward to
meet mesenchymal cells, which together stimulate
the sprouting of the limb bud capped by an apical
epidermal ridge. - Legless lizard. The somites fail to grow
downward, thereby failing to initiate the limb
bud, which regresses, producing an embryo that is
limbless.
86Hox Genes and Rapid Evolution-Lizards to Snakes
- Hox genes associated with the chest region in
lizards (a), expand their influence, leading to
loss of forelimbs (b). By other changes in
embryology, more vertebrae are added to the
vertebral column, producing an elongated body
(c).
87Hox Genes and Rapid Evolution-Lizards to Snakes
- Either by a shift in influence of other Hox genes
and/or by changes in limb bud growth (for
example, see figure 12.12), hindlimbs are lost
and an essentially modern snake body is produced
(d). - These steps may have occurred in a different
order. - Certainly, other changes accompanied these three
basic steps to consolidate and integrate them. - But apparently the major steps from lizard to
snake are built upon only a few gene or embryonic
modifications. - Different Hox genes (Hox) are indicated at
locations wherein mutations in them are
hypothesized to produce a change in body design.
88Preadaptations
- The sudden loss of legs might be of great
disadvantage to most tetrapods, but in the case
of legless lizards it may actually have
stream-lined their bodies for moving through a
confining substrate. - If so, that is an example of a preadaptation.
89Preadaptations
- A preadaptation exists whenever an organism has
and uses a preexisting molecule, biochemical
pathway, anatomical structure, physiological
process, or behavior inherited from an ancestor
for a new and unrelated purpose. - Thus, a preadaptation is a characteristic already
present which can be modified and improved by
natural selection once it begins to be used for
the new purpose.
90Preadaptations
- There are many excellent examples of
preadaptations in the living world - sweat glands ? mammary glands
- feathers for insulation ? for flight
- lobe fins for bottom-walking ? for terrestrial
walking - ancestral penguin wings for flight ? penguin
wings for swimming - marine vertebrate gill arches ? fish jaws
- quadrate/angular jaw joint ? malleus/anvil ear
ossicles - hemoglobin ? myoglobin
- Preadaptations should be considered the raw
materials for natural selection to use in the
same way that new mutations are the raw materials
for natural selection
91Social SystemsThe Evolution of Cooperation
- Advantages of sociality
- Reduced predation by improved detection or
repulsion - Improved foraging efficiency
- Improved territoriality against other groups of
conspecifics - Improved care of offspring
- Disadvantages of sociality
- Increased competition within group for food,
mates, nesting, etc. - Increased risk of infection
- Increased exploitation of parental care by
conspecifics - Increased risk that conspecifics will kill ones
progeny
92Life in Groups Sociobiology
- The term sociobiology was introduced in E. O.
Wilson's Sociobiology The New Synthesis (1975)
as the application of evolutionary theory to
social behavior. - Sociobiologists claim that many social behaviors
have been shaped by natural selection for
reproductive success, and they attempt to
reconstruct the evolutionary histories of
particular behaviors or behavioral strategies.
93Sociobiology
- Wilson's describes four pinnacles of social
evolution - Marine invertebrate colonies
- Insect societies
- Vertebrate social systems
- Human sociality
- Like group selection, punctuated equilibria, and
species selection, sociobiology has generated
much excellent science and many fierce
controversies.
94Sociobiology
- Wilson addresses altruism in discussing
sociobiology. - He recognizes the mathematical underpinning of
inclusive fitness, but he also hypothesizes that
certain environments may also be conducive to the
development of altruistic behaviors among
animals.
95Sociobiology
- There is some evidence that a variety of animals,
both invertebrates and vertebrates can recognize
other individuals in their groups and behave as
if they were changing their behavior in ways
which parallel the degree of relatedness between
the two. - There are also a few example of altruistic
behaviors that cannot be explained by kin
selection.
96Reciprocal Altruism Vampire Bats
- Vampire bats live in communal roosts.
- Within the roosts, the bats live in matrilineal
groups. - Vampire bats are able to distinguish other
vampire bats as individuals.
97Reciprocal Altruism Vampire Bats
- Vampire bats must have a blood meal at least
every 60 hours or they will starve. - Female bats carry some blood back to the roost in
their stomachs. - 70 of the time, that extra blood is distributed
to her offspring ? ordinary parental care. - The remaining 30 of the time, she may be coaxed
to regurgitate concentrated blood to another
roostmate.
98Reciprocal Altruism Vampire Bats
- Vampire bats are about as likely to receive as to
give a blood meal, especially in interactions
between individuals who share the same roost. - While some of the receiving bats are close
relatives, based on genetic studies, all are not. - It seems that vampire bats have learned that a
good deed will be repaid on average.
99Reciprocal Altruism Vampire Bats
- Vampire bats use a special grooming behavior to
elicit the donation of the blood meal. - Behaviors are subject to natural selection, but
finding the underlying genetic controls beneath
the environmental influences is challenging.
100Reciprocal Altruism
- In addition to Vampire bats, reciprocal altruism
has been demonstrated in - Humans
- Chimps
- Hyena
- Wild dogs
- Impalas
- Dwarf mongeese
- Naked mole rats (have a queen and temporarily
sterile workers due to suppressive behaviors and
pheromones from the queen).
101End Chapter 12