Principles of Evolution

1
Principles of Evolution

• Chapter 11
• Life History Strategies
• James F. Thompson, Ph.D., MT(ASCP)

2
Life History Strategies

• A Life History Strategy is the adaptive pattern
  and lifelong time course of individual growth and
  development and the mode of reproduction for a
  given species.
• Maturity ? age of first reproduction (most
  important)
• Parity ? number of episodes of reproduction
• Fecundity ? number of offspring produced per
  reproductive episode
• A Life History Strategy includes aspects the
  anatomy, physiology, and behaviors of the members
  of the species.
• A Life History Strategy evolves as a part of
  increasing adaptation to a species ecological
  niche.

3
Life History Strategies

• Variations in these Life History characteristics
  (maturity, parity, fecundity) reflect different
  allocations of an individual's resources (i.e.,
  time, effort, and energy expenditure) to
  competing life functions, especially growth, body
  maintenance, and reproduction.
• For any given individual, available resources in
  any particular environment are finite.

4
Life History Strategies

• Time, effort, and energy used for one purpose
  reduce the time effort, and energy available for
  another.
• The allocation of resources involves trade-offs
  which can be studied ? increase in one activity
  (growth, body maintenance, and reproduction )
  requires a decrease in another.

5
Life History Strategies

• Natural selection shapes adaptive modifications
  in a Life History Strategy to serve two purposes
• To increase resources available to individuals
• To use those resources to the best advantage ? to
  maximize survival and reproduction, i.e., fitness

6
Reproductive Strategies (1)

• Obligate Asexual reproduction by
• cell division
• fragmentation
• parthenogensis
• Alternation between Asexual and Sexual
  reproduction within a species, often controlled
  by changes in the environment

7
Reproductive Strategies (2)

• Sexual reproduction
• Individuals belong to separate sexes (males and
  females) common in animals rarer in
  (dioecious) plants
• Individuals have both male and female
  reproductive organs (hermaphrodites) rare in
  animals common in plants (monoecious or perfect
  flowers)

8
Reproductive Strategies (2)

• Sexual reproduction
• Individuals may change sex over time (protoandry
  male first protogyny female first)
• A species may use internal or external
  fertilization

9
Biological Life Cycles

• Protistans, especially parasites, have complex
  cellular stages in their life cycles
• In sexually reproducing multicellular organisms,
  the haploid or the diploid phase may be dominant
  (e.g., gametophyte vs sporophyte).
• In metazoans, developmental complexity varies
• cell colonies, e.g. sponges
• zygote ? embryo ? larva or larval stages ? adult
• zygote ? embryo ? adult

10
Schistosomiasis, A Complex Parasitic Life Cycle

• In blood vessels of the human gut, adult worms
  (Schistosoma sp.) mate and release eggs that
  reach the interior of the gut and are shed with
  the feces.
• Larvae hatch in water and enter their second
  host, a snail, in a form known as the mother
  sporocyst.
• Eventually the larvae within leave the snail and
  enter the water.
• The larvae pierce the skin of humans walking
  bare-foot in the water, usually tending
  agricultural fields.
• These larvae mature into adults again becoming
  lodged in the blood vessels of the gut to
  complete the cycle.

11
Reproductive Frequency (1)

• Semelparity the adults produce all of their
  offspring in a single massive fatal reproductive
  event
• Semelparous organisms are less common but widely
  distributed among the higher taxa in nature
• The majority are among the annual flowering
  plants
• Some flowering plants live many seasons but then
  reproduce and die bamboos, lobelias, and yucca
  plants, etc.
• A minority of animals certain salmon, trout,
  octopi, spiders, etc.
• The semelparous lifestyle makes it difficult for
  a herbivore or predator species to specialize on
  the semelparous species whose offspring are found
  in huge numbers but for only a very short time

12
Reproductive Frequency (2)

• Iteroparity the individual produces offspring
  in several discrete episodes, often in a
  particular season of the year
• Iteroparous organisms may require several seasons
  or years before they are reproductively mature
• Iteroparous organisms are common and widely
  distributed in nature
• The majority are biannual and perennial flowering
  plants as well as most animals
• Iteroparous species vary in the number of
  clutches they produce in a lifetime and the
  number of offspring per clutch

13
Reproductive Frequency (3)

• Polycyclic species reproduce intermittently
  throughout their lives.
• Species whose individual life spans are less than
  a season or a year (microorganisms, algae and
  protistans, many insects and marine
  invertebrates, etc.)
• Species whose environments are stable (oceans,
  tropical forests) so that resources to support
  the young are generally available (some tropical
  plants, many tropical and aquatic animals,
  humans, etc.)

14
Reproductive Frequency

• Semelparous, Iteroparous, and Polycyclic species
  may reproduce asexually or sexually, but the
  terms are rarely used in reference to asexually
  reproducing species.
• Semelparous and Polycyclic species are more
  likely to be found where the environment is
  stable.
• Iteroparous species have a higher survival
  potential in unstable environments, but may be
  found in the stable environments too.

15
Mating Systems

• Mating systemThe behaviors and the nature of the
  social organization of a species which determine
  how an organism acquires a mate
• the number of mates acquired
• the manner in which they are acquired
• the nature of the relationship between mates
• the kind and degree of parental care provided by
  each sex

16
Mating Systems

• Self-Fertilization Hermaphrodites which do not
  exchange gametes with other individuals in the
  species. Common in plants uncommon in animals.
• Monogamy, more usually called pair bonding One
  male and one female have an exclusive mating
  relationship.
• Polygamy One or more males have an exclusive
  relationship with one or more females. Three
  types are recognized
• Polygyny (the most common polygamous mating
  system in vertebrates so far studied) One male
  has an exclusive relationship with two or more
  females
• Polyandry One female has an exclusive
  relationship with two or more males
• Polygynandry Two or more males have an exclusive
  relationship with two or more females the
  numbers of males and females need not be equal,
  and in vertebrate species studied so far, the
  number of males is usually less.
• Promiscuity Any male within the social group
  mates with any female.
• These different mating systems form a continuum
  and some species make use of more than one mating
  system, often depending on environmental
  conditions.

17
Embryo Development/Care Systems

• Egg are discharged into the environment for
  external fertilization with or without parental
  care
• Ovipary the embryo develops in an egg
  discharged from the females body
• Ovoviviparity the embryo develops in an egg
  (membrane) and hatches before being discharged
  from the females body (vipers)
• Vivipary the embryo develops in the female
  reproductive tract before birth (the duration of
  gestation varies among species)
• Marsupial vivipary
• Placental vivipary

18
Parental Care

• Parental Care begins with provision of nutrients
  to the zygote, usually a contribution from the
  female
• yolk in the ovum, endosperm in seeds, etc.
• deposition of the fertilized egg in an
  environment where nutrient resources are likely
  to be present
• providing additional nutrition after birth in
  various animal species by lactation, feeding the
  young, etc.

19
Parental Care

• Parental Care may also include some provision of
  protection for eggs or young
• Establishments of territories, construction of
  nests, guarding of nests and young are examples
• Protective behaviors may not be directly related
  to the degree of nutrient provisioning

20
Parental Care in Plants

• Parental Care in plants is usually limited to
  resource provision to the zygote in the spore or
  seed
• Some plants evolve mechanisms, such as producing
  fruit attractive to seed distributing herbivores,
  to increase the likelihood that seeds will be
  deposited widely and with some fertilizer applied

21
Parental Care in Animals

• Parental Care in animals spans the entire
  spectrum from minimal to elaborate.
• In general, the greater the degree of parental
  care provided by a species, the lower its
  fecundity ( of offspring/reproductive event)
• A general trend to greater parental care on the
  part of the female, with much greater variability
  of male parental contributions from none to
  nearly all by the male

22
Parental Care in Animals

• A general trend to greater parental care in
  proportion to the degree of sociality in a
  species, but single mothers are still capable of
  enormous contributions of care
• A general trend to greater parental care on the
  continuum of invertebrates to ectothermic
  vertebrates to endothermic vertebrates, but many
  dramatic exceptions exist in each grade
• A general trend toward greater parental care in
  species with larger brains and whose young
  require more learning to survive

23
Maturity of Offspring at Birthin Animals

• The extent and kind of parental care provided is
  dependent on the maturity of young when born.
• This is a continuum not a dichotomy.
• Precocial offspring exhibit a high level of
  independent activity and self-maintenance from
  birth.
• Altricial offspring show a marked delay in
  attainment of independent activity and
  self-maintenance.

24
Milestones in Offspring Development

• Age at which offspring can feed itself (related
  to age of weaning in mammals)
• Age of attainment of adult body size
• Age of sexual maturity or puberty
• Age of first sexual activity or mating
• Age of first successful reproduction

25
Some Other Life History Parameters of Interest

• Age at which the offspring leaves the parents
  presence, if this occurs
• Age specific mortality schedules
• Age specific fertility
• Age specific fecundity (litter size)
• Interbirth interval
• Age specific adult survival and longevity
• Type of Social System, if any
• Population age structure

26
Life History Strategies and Population Growth

• Life history strategies incorporate
• Reproductive strategies and mating systems
• Survival strategies
• Habitat use
• Competition with other organisms
• Life history strategies occur along a continuum
  characterized as r- and K-selection
• r- and K-selection refer to parameters found in
  population growth models

27
r- and K- Selection

• Life history strategy is correlated with many
  aspects of an organism's reproductive strategy
  and life cycle, as well as with demographic
  variables such as generation time and life span,
  and population parameters such as population
  density and population dynamics.
• Where individual species fall on the r-K
  continuum is largely determined by the
  environment in which they live.

28
r- and K- Selection

• The concepts of r- and K- selection were
  established by Robert H. MacArthur and Edward O.
  Wilson in The Theory of Island Biogeography
  (1967).

29
Robert H. MacArthur (1930-1972)

• One of Americas major 20th century ecologists.
• A leader in moving ecology from a descriptive to
  an experimental discipline.
• Did research on niches and foraging behaviors.

• Other important books include The Biology of
  Populations (1966) and Geographical Ecology
  (1972).
• The Robert H. MacArthur Award is awarded for
  meritorious contributions to ecology.

30
Edward O. Wilson (1929- )

• One of Americas major 20th century evolutionary
  biologists.
• A specialist in social insects who became a major
  theorist and later a major advocate for
  understanding and protecting biodiversity.
• Founder of a subdiscipline Sociobiology The
  New Snythesis (1975).

• Other important books include The Diversity of
  Life (1992), Consilience the Unity of Knowledge
  (1998) and On Human Nature (2004).

31
r- and K- Selection

• McArthur Wilson developed an elegant system for
  describing the stability and age distribution of
  natural populations known as r/K selection.
• Now, 'r' and 'K' are symbols in numerous
  equations of theoretical ecology, representing
  components of an organism's life history strategy
  (reproductive capacity and environmental carrying
  capacity, respectively).

32
r- and K- Selection

• The variables r and k in r-and k-selection come
  from the logistic equation for population growth.
  
• The annual growth of a population is calculated
  with the equation I rN (K-N / K), where I
  the annual increase for the population, r the
  annual growth rate, N the population size, and
  K the carrying capacity.

33
r- and K- Selection
34
r- and K- Selection

• Mathematically, r is the birth rate plus the
  immigration rate, minus the death rate and the
  emigration rate.
• The K refers to the maximum density at which a
  population is able to exist in a given
  environment, and is called the carrying capacity
  of that environment.

35
r- and K- Selection

• The r-growth curve is the standard exponential
  growth which leads to population crashes.
• The functional difference between r and K
  selected growth is shown above left.
• In reality, K-selected growth fluctuates
  qualitatively.

36
r - Strategist
37
K - Strategist
38
r- and K- Selection
a few large offspring
many small offspring
trade-off between size and number of offspring
39
Ecological Succession
r-selected species
K-selected
species
40
Early Succession Communities
emphasis on rapid reproduction
41
Climax Community
emphasis on survival
42
Life History Strategies

• The various parameters of Life History Strategies
  (reproductive mode, life cycle, mating system,
  parental care activities, growth and development)
  are useful ways to distinguish species and
  niches.
• Natural Selection drives the trade-offs in time,
  effort, and energy allocations between survival
  and reproductive function among species.
• The allocation of resources involves trade-offs
  which can be studied ? increase in one activity
  (growth, body maintenance, and reproduction )
  requires a decrease in another.

43
Life History Strategies

• The individuals within a species are able to make
  limited shifts in reproductive strategies in
  response to the prevailing environments.
• Depending on the abundance of resources, probable
  longevity, and co-evolutionary relationships,
  selection pressures on populations can shift
  their reproductive strategy in one direction or
  another to take advantage of available resources
  or to compensate for resource shortages.

44
Life History Strategies

• It is difficult to find fossil evidence to
  demonstrate the evolution life history strategies
  over geologic time.
• Instead, scientists document the smaller
  microevolutionary changes in living populations
  which suggest mechanisms which could lead to the
  development of the larger differences in life
  history strategies observed between species and
  higher taxa.

45
Side-Blotched Lizards

• The Side-blotched Lizard (Uta stansburiana)
  inhabitats rocky, sandy, dry areas with scrub
  vegetation in Western North America
• It is an r-selected predator of arthropods.
• Its predators are other lizards, snakes and birds.

46
Side-Blotched Lizards
Predation More
Less

• There is clinal variation north to south (r- to
  K-).
• Greater nutrient resources in the south allow
  larger body size and larger clutches, but larger
  lizards may attract more predators.

47
Guppies (Poecilia reticulata)

• Guppies (Poecilia reticulata) occupy pools
  separated by waterfalls in Venezuela and
  Trinidad.

48
Effect of Predation on Male Guppies

• In pools where predators, such as the pike
  cichlid (Crenicichla alta), are present, males
  are drab.
• Where predators are absent, male guppies are
  brightly colored which attracts females more
  successfully.
• Thus, predation pressures modify mating systems.

49
Male Guppies, Evolution of Color Varieties

• After several generations, guppies raised in low-
  and high-predation environments evolve different
  features.
• As measured by the number of bright, conspicuous
  spots, males become more brightly colored (low
  predation) or drab (high predation).

? female male ?
50
Guppies and Predators

• Guppies in pools with only Pike-Cichlid predators
  were more r-selected (smaller, early maturing,
  larger broods)
• But when those guppies were transferred to
  natural pools with the smaller Killifish
  predators (Rivulus hartii), natural selection
  drove the guppies to a more K-selected life
  history and anatomy (larger, later maturing,
  smaller broods) in 11 years
• These modified guppies resembled the K-selected
  guppies found in natural pools with the smaller
  Killifish predators

51
Rock Pipit (Anthus petrosus)

• The Rock Pipit (Anthus petrosus) inhabits rocky
  coasts in Northern Europe.
• They are omnivores eating arthropods, molluscs,
  small fish, seeds, etc.
• They are territorial breeders.
• Many populations overwinter at their breeding
  grounds, though some migrate to warmer habitats

52
Rock Pipit (Anthus petrosus)

• A classic time allocation study demonstrated the
  trade-off between territorial defense and
  foraging in a comparison of mild and harsh
  winters
• Such changes in territorial defense due to
  environment might impact reproduction in the
  following season

53
Common Swifts (Apus apus)

• The Common Swift (Apus apus) is a migratory
  insectivore
• Females are determinate egg layers a genetic
  polymorphism
• Some females always lay two eggs the other
  phenotype lays three eggs
• Why does the polymorphism persist?

breeding
wintering
54
Common Swifts (Apus apus)

• Relative fitness varies in different environments
• In mild years, mothers with clutch size 3 have
  more offspring survive to fledge, but
• In harsh years, mothers with clutch size 2 have
  more offspring survive to fledge
• So natural selection preserves both genotypes in
  the population over time

55
Seed Predatore Selection Pressure on Bean Plants
The Leguminosae are a family of bean plants the
screwbean mesquite is illustrated on the left.
The Bruchinae are a subfamily of weevils
(beetles) which lay their eggs in seeds where
their larva mature and pupate, destroying the
seed in the process.

• Life history strategies evolve under different
  environmental demands.
• This can be diagrammatically represented with
  alternative energy budget allocations.
• Some plants can alter their reproductive
  strategies depending on the impact of herbivores
  on their energy budgets

56
Energy Budgets, Beans Free of and Under Beetle
Attack
defend against herbivory

• The size of the arrow represents the size of the
  energy investment.
• Free of beetle attack, beans allocate more to
  Toxins and Growth than to Reproduction.
• Under beetle attack, beans evolved a strategy of
  increased Reproduction, overwhelming beetles with
  a large output of seeds, but at the expense of
  Toxin production and vegetative Growth.

57
Phenotypic Plasticity

• The term phenotypic plasticity refers to the
  variation in phenotype (for a given genotype)
  which occurs due to the influence of
  environmental factors.
• Phenotypic plasticity can represent the small and
  sometimes trivial differences which we observe
  in identical twins.

58
Phenotypic Plasticity

• Phenotypic plasticity more often represents the
  sorts of regular changes observed when
  individuals of the same genotype develop or
  behave differently when they live in different
  environments.

59
Phenotypic Plasticity

• Phenotypic plasticity may be a negative
  consequence of an inadequate environment.
• organisms do not grow to their full size
  potential if denied adequate nutrients or water
• young song birds of some species may be unable to
  sing the correct songs if denied exposure to
  adults singing the songs at a critical period in
  development.

60
Phenotypic Plasticity

• Phenotypic plasticity can evolve!
• Like any other potentially adaptive trait,
  phenotypic plasticity must be examined
  scientifically and its adaptive value, conferring
  increased reproductive fitness on the organisms,
  must be demonstrated.

61
Phenotypic Plasticity in Daphnia

• Daphnia magna, the water flea, is a fresh water
  crustacean which is a popular invertebrate for
  laboratory studies in evolutionary biology and
  ecology.
• Because Daphnia normally reproduce asexually,
  they are a good experimental animal for studies
  of phenotypic plasticity
• Genetically identical clones can quickly and
  easily be produced and exposed to different
  environments in a controlled laboratory setting.

62
A Water Flea Daphnia sp.
63
Phenotypic Plasticity in Daphnia

• Phenotypic plasticity is observed in Daphnia sp.
  in response to predators.
• Individual water fleas have the potential to grow
  a helmet and longer spine when exposed to
  chemicals from various of their natural predators
  (fish).

? helmet
64
Phenotypic Plasticity in Daphnia

• The genetic potential was already present in the
  Daphnia genome, but it requires the action of the
  predator chemical dues to activate the genes for
  the altered morphology.
• Interestingly, the asexual daughters of water
  fleas who have grown the helmet during their
  lifetime may be able to start life by growing a
  helmet, and therefore, the helmet may be
  larger than the helmets offspring of nonhelmeted
  females living in the same environment.

65
Conclusions

• It is sometimes assumed that in most species,
  life history strategies are largely uniform and
  predetermined, produced by the usual genetic and
  developmental mechanisms.
• Those mechanisms have been developed by natural
  selection.
• However, there is evidence for variation in the
  components of a life history strategy even within
  a given species and that variation can be
  modified by natural selection.

66
Conclusions

• Observed variation in life history strategy
  within a given species suggests that natural
  selection acted in the past to produce the great
  diversity of life history strategies we observe
  within the living world.
• To put it another way We can assume all
  ecological relationships co-evolved, often in an
  incremental fashion, with the species in a
  community impacting each other in various ways.

67
Conclusions

• Most evolutionary changes are compromises and
  trade-offs and subject to various constraints.
• To increase the success of one structure or
  function, there is often a compensatory loss in
  performance of some other structure or function.
  

68
End Chapter 11