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Principles of Evolution

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Title: Principles of Evolution


1
Principles of Evolution
  • Chapter 12
  • Life in Groups
  • James F. Thompson, Ph.D., MT(ASCP)

2
Life 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.

3
Types of Social Interactions
4
Alarm 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.

5
Alarm 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).

6
Beldings 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.

7
In Beldings Ground Squirrels, Most Alarm Calling
is Done by Females
?
?
8
Why 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?

9
Altruism
  • 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?

10
Altruism
  • 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.

11
Group 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).

12
Wynne-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.

13
Wynne-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.

14
Kin 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.

15
W.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.

16
W.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.

17
W.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.

18
Hamilton 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.

19
Kin 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.

20
Kin 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.

21
Kin 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

22
Inclusive 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.

23
Inclusive 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

24
Parental 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
25
Kin 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

26
Parental 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).

27
Brood 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.

28
Levels 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.

29
The 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.

30
The 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.

31
George 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.

32
Richard 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

33
The 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.

34
Higher 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.

35
Micro- vs. Macroevolution
  • Microevolution Natural Selection - differential
    survival due to inherited traits (individual
    selection)
  • Macroevolution differential survival of branches
    within phylogenetic lineages

36
Micro- 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.

37
Micro- 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?

38
Gaps 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.

39
Gaps in the Fossil Record
  • The real pattern of evolution depends on the
    completeness of the fossil record.

40
George 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.
41
Quantum 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.

42
Quantum 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
43
Quantum 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
44
Micro- 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.

45
Niles 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.

46
Stephen 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.

47
Punctuated 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.

48
Punctuated 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.

49
Punctuated 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.

50
Patterns 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.

51
Patterns of Macroevolution
  • Punctuated equilibrium (cladogenesis) envisions
    long periods of more or less unchanging species
    persistence, suddenly interrupted by speciation,
    producing a new species.

52
Punctuated 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.

53
I. 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.

54
Punctuated 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.

55
Punctuationist 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.

56
Cladogenesis, 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.

57
Peripheral 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.

58
Patterns 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).

59
Patterns 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.

60
Patterns 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.

61
Horse Phylogeny
  • Environmental Transitions
  • forests to prairies (wetter to drier)
  • slower to faster predators

62
Lake 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.

63
A Comparison of Models
Raw data
64
Pace 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.

65
Stephen 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)

66
Species 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).

67
Species 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.

68
High 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.

69
Mode 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!

70
Peripheral 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.

71
Sewell 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.

72
Genetic Drift
73
Genetic 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.

74
Empirical 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.

75
The 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.

76
The Founder Effect in an Island-Hopping Bird
5
4
1
3
2
77
The 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.
78
Drift 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.

79
Genetic 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.

80
Genetic 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.

81
Inbreeding
  • 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.

82
Inbreeding 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.

83
Inbreeding 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.

84
Inbreeding
  • 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.

85
Lizards-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.

86
Hox 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).

87
Hox 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.

88
Preadaptations
  • 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.

89
Preadaptations
  • 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.

90
Preadaptations
  • 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

91
Social 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

92
Life 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.

93
Sociobiology
  • 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.

94
Sociobiology
  • 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.

95
Sociobiology
  • 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.

96
Reciprocal 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.

97
Reciprocal 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.

98
Reciprocal 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.

99
Reciprocal 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.

100
Reciprocal 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).

101
End Chapter 12
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