EVOLUTION -- Biology 4250
Dr. Adams
Review Sheet 3 -- Test 3
Chapter 10: Adaptation: Evolutionary analysis of
form and function
What is an adaptation?
In this
chapter we discuss HOW evolutionary biologists study adaptation and how it
is
possible to demonstrate that certain traits of organisms ARE indeed
adaptations.
White Arrowheads on South African Iris petals.
So, when we study adaptation, we must
start by determining WHAT a trait is used for,
and then
show that individuals with the trait are more fit.
To study adaptations, we must consider the following (see page 373)
1. A nice
hypothesis about the adaptive significance of a trait that seems plausible
is
not
necessarily correct. (Giraffe necks, for instance.) It needs to be tested (duh!).
2.
Differences between populations/species are not necessarily adaptive (eg.,
spots)
3. Not
every trait or use of a trait by an organism is adaptive -- the use by giraffes
of their necks for feeding is a side benefit. However, this could be
considered a preadaptation for
feeding at height; this concept will become important in later
discussions.
4.
Adaptations are not perfect. Can only work with existing genes, and a
trait that
is adaptive in one respect may make doing something else more difficult
(giraffes drinking).
Methods for evaluating hypothesis about adaptation:
I. Experiments -- Important to minimize bias and maximize
precision
(standards, controls,
etc.;
see page 376)
Examples: 1. Large mammals and Oxpeckers
2. The tephritid fly Zonosemata and
Salticids (Jumping Spiders)
3. Choreutid (Brenthia) moths and Jumping Spiders
II. Observational Studies -- Some hypotheses are difficult/impossible to
test with experiments
Example: Behavioral Thermoregulation -- if want
to show that thermoregulatory behavior
is adaptive,
must show two things:
1. That organisms choose particular temps more frequently than they would
encounter those temps at random in the environment.
2. That the above choice is adaptive.
Examples: nematode worms, garter snakes
III. Comparative Methods -- using comparative data between
populations/species
Example: Bats testes size, Tiger moths and
defensive mechanisms
In this example, or any example, when
comparing correlation of two traits, you MUST
be aware of the evolutionary relationships between the species being compared.
You could
be looking at two traits that evolved in tandem, or are simply
correlated due
to evolutionary his-
tory. The evolutionary history is VITAL to making
claims of independence
of data points.
Complexities in organismal form and function: difficulties in
understanding adaptation
Phenotypic Plasticity: phenotypic influence by
the environment
Example: Water fleas (Daphnia)
phototactic behavior (pages 387 - 389)
This behavior is both plastic
(genotype-by-environment) and can be selected.
Trade-offs & Constraints:
Constraint: Since all
genes (traits) originate from something else, the new gene (trait)
can only
develop in certain ways because it is constrained by its own evolutionary
history.
It is also true that there can be physiological constraints on traits.
Trade-off: two (or more) traits
which collectively are required for fitness, but selection
on these two
traits is in opposing directions.
Examples: Flower
size/number in Begonias, Flower color change in Fuchsia,
host shifts in herbivorous insects (or other animals)
Evolutionary history of traits: All traits
were originally some other trait
Examples: mammalian ear,
feathers, metamorphosis hormones and prolactin
To understand
the development of a new trait, we need to be able to:
1. establish the ancestral connection
2. be able to illustrate the transformational sequence
A couple of
key aspects here are
1. intermediate states of the characters along the way MUST HAVE HAD a
selective advantage (useful function) to the species at the time.
2. these intermediate states can be said to be preadaptive.
The key to understanding evolution of
a trait is being able to establish homology with
an ancestral trait.
Chapter 8: The Adaptive Significance of Sex (pgs. 314 - 324)
As we have just finished talking about how to gather empirical evidence for the
adaptive value of traits, let us turn to sexual reproduction and its value.
Sexual reproduction – an advantage?
Sexual reproduction is complicated, costly and dangerous. Finding mates exposes the
searcher to predation, and takes time and energy which could be used for other tasks. Once
found, potential mates must be convinced – which may require gifts, dances, songs,
construction of nests, etc., in other words, more time and energy. Once convinced, intercourse
may expose both to disease, and compromise their ability to retreat should predators find them.
In some, males may BE the food gift. And, after all that, a number of "acts" result in no
offspring at all.
Asexual sounds pretty advantageous. There are many organisms that do reproduce
asexually, at least some of the time; many organisms can choose. Some aphids are
parthenogenetic for spring and summer broods, but produce males in the fall generation.
Volvox (a cool colonial organism) can reproduce asexually or sexually, and a population may
include males, females and asexual individuals (see fig. 8.22). The animal Hydra may produce
new individuals by budding, as well as producing sperm/eggs. Many plants reproduce
vegetatively, with running roots underground. Even aspen trees do this.
So, less energy required, fewer encounters with predators, less exposure to disease. It
sounds like, if given a choice, asexual should replace sexual reproduction in any organism that
can do both, yet there are virtually no organisms that are exclusively asexual, and most
complex organisms can ONLY reproduce sexually. Why aren’t more organisms asexual? (A
question to ponder: Were the original organisms on the face of the planet asexual or sexual?)
Sexual reproduction MUST be quite advantageous for some reason. What’s the answer?
The answer, of course, is adaptability. It has been shown in a convoluted experiment
using Tribolium flour beetles that, even given a distinct numerical advantage, in changing
conditions, the "asexual" population loses out in the long run, with numbers going to zero.
That is because asexual reproduction produces clones, genetically identical individuals with
virtually NO variation. No variation, no adaptability.
So, what are the advantages of sexual reproduction? Many theories (including the
generation of variable offspring), but just a couple will be covered for the moment.
Sex generates genetic recombination
Sex does generate variability, in three ways: 1. mixing of two individuals genetic
material, 2. mixing and matching chromosomes from pairs when making gametes, and 3.
crossing over (2 & 3 both happen during early meiosis I). Remember from earlier, that genetic
recombination, by shuffling multilocus
haplotypes, helps reduce linkage disequilibrium (D),
providing a way to shuffle out deleterious
alleles (at the cost of some offspring, however!; see
below *). Interestingly, in population
genetics, for traits that are already in equilibrium, sex has
no effect, in and of itself. So, any
model to explain why sex continues to be advantageous must
minimally have two components:
1. some factor eliminating certain multilocus haplotypes and
creating excess of others,
resulting in D; and 2. a reason why genes that reduce D – by
promoting sex – are favored.
Mutation, Drift, Inbreeding and Sex:
A deleterious mutation in an
asexual species is a tremendous hindrance – all of her offspring
will receive it, generation after
generation. The only long term survival possibility would be a
back-mutation, or another
mutation that allows compensation for the first. Neither likely to
happen quickly. But sexual
reproduction can eliminate the allele in one generation. So out-
crossing will be selected for, which means that having males will then be
selected for as well.
When examined further, the variation seen in asexual organisms will be due to
mutation, and if most mutation is bad, then . . . selection can be a b----! Eventually, the
genetic load of too many mildly deleterious mutations dooms the population to extinction. But
sex gives the opportunity to recombine your genetic material with another and eliminate a
particularly deleterious allele in the combination of gametes. It also allows the
possibility of
recreating missing multilocus combinations (genotypes) through crossover. So,
mutation with
drift creates D, and sex eliminates it. "So, genes responsible
for sex are maintained in populations
because they help to create
low-mutation genotypes. As these low-mutation genotypes increase
in
frequency, the genes for sex hitchhike to high frequency with them." (pg. 321)
A natural
example – Asexual/sexual Tinema
walkingstick species and nonsynonymous
mutations (Fig. 8.28).
Selection imposed by changing environment can make sex beneficial. In a stable
environ-
ment, asexual reproduction may actually be better – a combination of alleles that is well
adapted to a particular set of conditions could be passed precisely in that combination to
offspring; sexual reproduction cannot maintain precise combinations of well adapted alleles.
However, there are NO environments that are stable, certainly not over the long term, and
many environments are changing DAILY in terms of selective pressures for some organisms.
So, in a changing environment, the variable offspring of sexually reproducing organisms more
likely ensures that *SOME offspring will survive; for asexual organisms, even a small
environmental change could mean extinction. This explains why changing
environmental
conditions (evolutionary stress) promotes bouts of sexual reproduction, even in organisms that
seem largely asexual. Understand, too, that the changes experienced can be interaction driven
– predators and their prey, parasites and their hosts, herbivores and the plants they eat, etc.,
resulting in a coevolutionary arms race.
A natural example – trematodes and asexual/sexual snails (in New Zealand). Lively
studied frequencies of males in populations and found that trematodes selected for sex in their
snail hosts. Why??? (We’ll discuss it.)
Chapter 11: Sexual Selection
We have established that sexual reproduction is adaptive. As such, that makes possible
another very powerful and intriguing part of evolution – sexual selection.
Sexual dimorphism
Many species have males and females that are strikingly different: in size, pattern,
certain structures (antlers, throat dewlaps, for instance) and behavior. In some species, very
easy to mistake the two sexes for different species. Just look at humans. WHY the difference?
First, we must assume that the differences have ADAPTIVE value, otherwise
differences
would not occur. And yet some obvious differences seem maladaptive (mostly for
the males)
– bright color, long tail feathers, loud calls, etc. which take energy and can attract
predators.
It is sexual reproduction itself that explains sexual dimorphism – it involves mate
choice. One sex (usually the female) chooses mates based on characteristics of the other sex.
If not chosen, even if they are reproductively mature, that individual’s fitness will be ZERO.
So, it is important to be able to CONVINCE potential mates to actually mate.
Differences in parental investment
Eggs vs. sperm/semen
Gestation
Parental care (none, one sex, both sexes)
All of this results in a profound difference in the lifetime reprod. success of males vs.
females. For females, typically a lot more is required to make offspring than for males (even if
there is no parental care) – this limits females overall number of potential offspring. Males
typically have much more POTENTIAL, and the limit of their reproductive success depends
on the number of females they can convince to mate with them. So, the limiting factor for
males is access to females, but the reverse is not true (sperm are cheap!).
Females are typically
limited by the number of eggs they can produce or
number of pregnancies they can carry.
Understand that
when there is significant male parental care, access to males may be a limiting
factor for
females. So, we understand that there are exceptions, and we’ll return to this later.
Bladder
snail example: researchers discovered that male success (at producing offspring)
increased as number of matings increased (big winners and big losers). Female success did not
substantially increase with more than one mate (few big winners or losers).
See Fig. 11.8.
Newts
The fact that females are a limiting resource for males of most species, we should
predict that:
1. Males should COMPETE for mates. In some cases, males will fight with each
other (intrasexual selection), with winners gaining access to mates
2. Females should be CHOOSY. This in turn means that females would have, in most
cases, a method of intuiting males fitness before mating (intersexual selection).
I. Intrasexual selection:
A. Male-male combat
True combat selects for large body size, weaponry, armor, etc.; understand here that
losing in combat can be catastrophic for the loser in many cases, with death as a result.
If we see in a species that males are larger than females, can we say this is for combat?
Examples here are many: marine iguanas (see book) have stabilizing selection on size – large
size enhances ability to compete and to get mates, but food is limiting, and very large iguanas
tend to lose weight in bad algal years. The only factors involved here is competition for food
and competition for mates, as they have no predators and don’t have to compete significantly
with OTHER species for food. Males make no investment in parental care; females
dig a nest,
lay eggs (20% of body mass), and guard the nest site for a few days. So our prediction is that
females would select large males, because large males represent "fit" individuals that can both
find food well and win in combat; we would also predict males would attempt multiple matings
in a season, and establishing territories may also occur.
Actual behavior: males establish territories (to guard "resources": basking sites in this
case – remember, temperature is a resource for the iguanas), attempt to mate many times with
different females, if they have successfully fought/defended territory; females mate only once
in a season, so mating is a DEFINITE limiting factor for males. No surprise, the biggest males
get to mate the most because they are best at defending the females favorite basking sites.
Alternative strategies -- "cheaters";
sneaky satellite males in fish (Fig. 11.19), frogs
B. Sperm competition – this can occur in several ways:
Examples: 1. more ejaculate with more sperm -- fruit flies raised with rivals ejaculate 2.5
times as many sperm than if raised in isolation in the presence of females; 2. long mating time;
3. a copulatory plug – a sphragis in Parnassius butterflies; 4. chemically "mark" (with a
pheromone) the female as mated; 5. remove previous males sperm with special structures –
"scoop" in damselflies; 6. (not in text) different KINDS of sperm (eg., kamikaze sperm).
C. Infanticide – example: Lions, killing of previous males non-weaned cubs (why?)
II. Intersexual Selection:
A. Female Choice – this is an obvious and driving force for why many males seemingly
have unusual, and in some cases, maladaptive traits.
Examples: NUMEROUS! 1. Color/tail feather length in many birds º
Red-collared
widowbird example in text – longer tailed mails attract mates
QUICKER (why important?), &
able to
solicit extra-pair copulations more easily. 2. Calls (in birds, frogs, insects, etc.) º
gray
tree
frog example in text – males differ in pace, components, length of calls. Males may increase
pace and length when they hear other males calling, suggesting both aspects mean something
to females. Females indeed preferred fast to slow, and long to short, calls, even when slow
short calls were closer. Bull frog females may select males base on the
deepness of the call
(indicates size=age); satellite males may steal mates. 3. Gifts provided by male (hangingflies,
see below). 4. Flash patterns
in bioluminescent
organisms. 5. Pheromonal quality and quantity . . .
and many other
examples. Remember, such male traits may increase males exposure to
predators
(though for
pheromones, the female is as frequently if not more frequently involved in
production --
why?
Pheromones are usually species specific, and are very unlikely to be detected
by predators.)
Why should females choose such males? Three possible explanations:
1. Females insure better genes for her offspring. A simple, elegant experiment with tree
frogs indeed showed that longer callers fathered more fit offspring than shorter
callers (pg.
437).
2. Females may acquire more resources from male. (food, parental care, territorial shelter)
Hangingflies offer nuptial food gifts º
more food, longer mating, more sperm passed.
3. Females may have previous sensory biases. (We’ll not discuss this at length)
4. Other explanations. Genetic "fashionability" – the sexy son hypothesis. Once preferred
(and selected) by females, a certain fancy trait is genetically engrained in the choice.
Understand that all are mutually compatible, and can be functioning collectively. Also, as
stated above, remember that such male traits may increase males exposure to predators.
Diversity in Sex Roles
Understand that the "typical" sex roles (females provide "more" than males) is not
always how it works in nature. There are many organisms where males provide most of the
parental care – seahorses/pipefish, ptarmigans, back-brooding frogs, etc. In this case, females
may be bigger, with mate attracting patterns, and in turn may compete for males.
Does bigger female size mean reversed sex roles in every species? No, in insects,
females are typically larger for the simple fact that they must carry numerous eggs.
Sexual selection in Plants
Many plants are markedly sexually dimorphic – different male and female flowers on
different plants. The female, however, will be required to make the fruit/seed, while the male
supplies only the packets of sperm – the pollen.
Examples:
1. Wild radish, with white/yellow flowers (see Fig.
11.46) – turns out, being a yellow
MALE increased a plants success
2. In general, it should be animal pollinated (AP) dioecious plants where sexual
dimorphism would be the most pronounced; wind-pollinated (WP) plants should not
"care". Delph, et al, investigated perianth (sepal/petal) size in WP and AP plants. In
WP, function of perianth is protective, meaning
the sex with largest reproductive parts
should have biggest perianths – true in all 11 species they investigated. But if attraction
important, as in AP plants, then sex that "needs" to attract more should have biggest
perianth
(and odor). True for some (though not all) AP plants. Also indicates
sexual
selection going on through pollinators, and results suggest often stronger for
male
(stronger odor, larger perianth).
3. Euglossine bees and orchids – male orchids forcibly attach pollinaria to bees (this
unpleasant experience makes the bees avoid other male orchids); males then visit
female flowers which remove a pollinarium and immediately seal the stigmatic cleft (so
female fertilized by only one male). So, in essence, male
flowers get sole access to at
least one female’s offspring, and prevents other males from mating by the same bee.
How far can sexual selection push the expression of (outlandish) traits?
To the point where the
negative effects of predatory pressures outweigh the positive effects of mate
attraction.
Chapter 12: Kin Selection and Social Behavior
Interaction Definitions: Mutualistic – both parties benefit
Altruistic – giver potentially has reduced fitness, recipient benefits
Selfish – practicer benefits, recipient has reduced fitness
Spite – both parties have reduced fitness; for obvious reasons, truly spiteful behavior
is probably
quite rare (see bacterial example in book [pg. 459] and don't forget us)
Evolution of truly mutualistic behavior should be easy to understand --
traits that are beneficial
to both parties involved would be selected for and
emphasized. Example: communal breeding.
Selfishness also easy to explain, and can explain behaviors such as cannibalism.
Altruism is the difficult one to explain, at least on the surface. But the explanation is
. . .
Kin
Selection, and the idea of
Inclusive Fitness.
Altruistic behavior is seen in organisms when the behavior benefits close relatives; the closer
in relation the individuals are to the "giver", the more likely the giver will practice the altruistic
behavior. We can examine this by figuring the coefficient of relatedness (r;
we will do a bit of
practice to make sure you can determine this), and predict that
altruistic behavior can happen
(but not necessarily) when:
Br – C > 0 (B = benefit to recipient; C = cost to giver);
this is Hamilton's Rule
This is in terms of inclusive fitness, namely number of offspring and the relatedness of them to
the giver (meaning when the giver’s fitness is > 0, then the altruistic behavior makes some
sense).
Understand that an individual’s total fitness includes both direct and indirect fitness. The
indirect component is also called kin selection.
Examples of altruistic behavior:
Alarm calls – numerous examples from social mammals/birds. Book examples:
1. Belding’s
Ground
Squirrel (Fig. 12.5) -- calls mostly by females (why?), and almost
exclusively
when "next" of kin nearby (daughter/granddaughter/sister/mother).
Two types of calls:
trills for predatory mammals, whistles for predatory birds. Whistler's
less likely to die
than nearby individuals (selfish), but trillers actually more likely to die (8
vs. 4%). Why?
Therefore, trilling is, by definition, altruistic. These same related
individuals also much
more likely to cooperate when chasing off territorial "invaders".
2. Black-Tailed Prairie
Dogs (pg. 462) -- calls by both sexes, more frequent when kin are near than
without kin.
Also equally likely to call in presence of parents, siblings and offspring (is r
= for these?)
Cooperative breeding – again numerous examples from social birds/mammals.
Book example of White-winged
Choughs: incapable of fledging young without help.
Most helpers are kin, but occasional recent fledgings are kidnapped and used as
helpers
later.
Lower promiscuity has apparently led to at least potential cooperative breeding
in
multiple lineages of birds. Why?
Even so, this type of helping behavior usually seen when breeding opportunities
extremely
restricted, either because of available mates or nesting sites.
Understand the examples given above require a very important ability – kin recognition;
this is
because altruistic behavior clearly is only likely to occur when it is nepotistic, helping
close kin.
Parent – Offspring Conflict:
Parents and offspring have DIFFERENT fitness interests – offspring SHOULD try to
monopolize parental care for as long as possible, while parents SHOULD try to wean offspring
once costs outweigh benefits (so they can make more). (examples in text)
Siblicide:
Siblicide, on the surface seems counterintuitive – parents and siblings are equally
closely related (r = ½). Siblicide occurs in a number of bird species, but also in insects (and
probably other groups of organisms as well). For birds, eggs laid a few days apart will hatch
at different times, with the first to hatch having access to all the food to
begin with. When the
second hatches, if enough food is available, the first will not demand all the food, or may even
refuse a small portion. But if food shortages occur, the younger may be killed by the older –
this increases its own chance of survival and therefore the fitness of the parents (who could
lose both if they continued to try to feed both). The youngest (weakest) may be actively killed,
or simply pushed out of the way when food is brought to the nest. However, examples like the
Masked Booby, where virtually ALL younger siblings are killed, leaves an unanswered
question, which is . . . ??
Reciprocal Altruism:
What about cooperation among non-kin? There are many examples out there
(including humans, of course). A possible answer would be if reciprocal altruism can be
expected. Difficult to imagine, however, how this would evolve – two conditions would have
to be met: 1. small cost to the provider (< or at most = to benefit to the receiver), and 2.
recipient’s failing to reciprocate would ultimately have fitness reduction. The end result should
be that altruists would ultimately punish cheaters in some fashion. An
expectation: quick
punishment for cheating should bring cheaters back into cooperation.
Under what circumstances should such evolve? Reasonably stable groups (individuals
long-lived, interacting with each other), multiple opportunities for altruism, and individuals
have good memory (!). (What are other characteristics of organisms who exhibit reciprocal
altruism?) Certain emotions in humans (moral aggression, gratitude, guilt, trust) may have
evolved under just such social circumstances.
Any good examples?
1. Grooming and help in agressive encounters in
unrelated baboons.
2. Blood-sharing in Vampire Bats.
3. Nest protection in Great Tits, with neighbors
from the previous year.
Clearly, social interactions can be complex, and it can be very difficult to ascertain the
fitness value of many specific behaviors; in the case of reciprocal altruism, there is the further
difficulty of analyzing the follow-up behavior (at a later time).
Evolution of Eusociality – the epitome of altruistic behavior
Characteristics of Eusociality: 1. overlapping generations, 2. cooperative brood care, 3.
a caste system, where at least some of the castes are non-reproductive
Which animals are eusocial? Many insects (particularly Hymenoptera and Isoptera),
one group of crustaceans (snapping shrimp), and one family of mammals (naked mole-rats).
Examples:
Hymenoptera – an unusual case because of the mechanism of sex determination.
Females diploid (from fertilized egg); males haploid (from unfertilized egg), a system called
haplodiploidy. This means females are most closely related to sisters (r = ¾) than they are to
their own daughters or mothers (r = ½) or brothers (r = ¼) (why these fractions? You
SHOULD be able to figure it out). So sterile female workers in the hive makes some sense IF
the workers in the hive are sisters to the queen, and, in turn helping to make more sister
workers. When reproductives are produced, the queen should not care about the sex ratio
(males to females), but the WORKERS should care (why?). So if workers have a say in what
reproductives are produced, there should be a strong female bias in the reproductives, and we
do see this in *some* species (authors mention the ant species Formica exsecta; not all have
been studied for sex ratios of reproductives produced). So the workers win the "tug-of-war"
over the fitness of future colonies by destroying a percentage of male eggs/larvae.
Is haplodiploidy the
entire explanation? No. Queens may mate with several males (so
. . . ?). More than one queen can found/be present in colonies of some (so . . . ?). Haplo-
diploidy is the sex determination mechanism for Hymenoptera, but NOT other species. And
lastly, in the evolution of Hymenoptera, eusociality appears to have developed several times
(not just once; see Fig. 12.37) – in ants, in paper wasps, and bees – and always together with
complex nest-building and care of young. This suggests the driving force is ecological (and is
not constrained by the genetics of these organisms,
the Ecology/Life-history Hypothesis); in
other words, it would be impossible for
an individual reproductive to care for and protect off-
spring. Indeed, the fact
we see this also in
the complex nest building, care giving, all diploid
termites reinforces this idea.
Other possibilities: The Monogamy hypothesis
-- if parents are monogamous, sisters and
brothers are equally related to
you as your own offspring.
The Paper Wasp story – multiple females on the nest; none sterile (but not all
contribute to eggs). Different strategies – 1. Join another nest, 2. found a new nest, 3. wait for
breeding opportunities. What circumstances should lead to each of these behaviors? Do
numbers/size of individuals involved make a difference in adopting a particular strategy?
(We’ll answer these questions in class). The take home message is that reproductive altruism
can be facultative, as an adaptive response to the current environmental conditions, largely
depending on size of individuals, r, and availability of nest building sites.
Chapter 13: Aging (senescence) and other Life History Characteristics
There is a diversity of reproductive strategies, intimately tied to lifespan of organisms.
Life history analysis – the study and interpretation of different reproductive strategies.
Technically, the "perfect" organism would have the following characteristics: maturity at
birth, continuous production of high-quality offspring, and live forever. Hasn’t happened in
3.8 billion years (or so) in the history of life. Why?
Some organisms come close on one characteristic or another, but not all:
Thrips egg mites – mature and inseminated (!) at birth, but produce only one set of eggs
and die at four days of age as young eat
their way out of mom.
Kiwi Birds – produce very large eggs (nearly self reliant at hatching), but production is
slow and one at a time (see Fig. 13.1 [holy cow!, or perhaps, holy kiwi!])
It’s all about resource/energy trade-offs. Organisms must "decide" where to allocate time,
resources and energy. Resources/energy are finite; once used, they can’t be used for
something else. Evolution should select for an optimal balance of traits which lower costs
and raise benefits as much as possible to maximize lifetime fitness (reproductive output).
This analysis of trade-offs leads to some very important questions:
1. Why do organisms age and die?
2 a. How many offspring should individuals produce?
b. How frequently should individuals produce?
c. What size offspring should individuals produce?
Additionally, tied in with the above are pace of maturation, growth rates, maximum size, etc.
Why do organisms age and die?
Two theories:
I. Rate of Living Theory –
Live faster (metabolism), die younger. Errors during rep./
transcription/translation (mutations) and
toxins (metabolic by-products) accumulate to do
irreparable damage to cells. Predictions: (1) since aging tied to build up of metabolic by-
products, rate of aging and metabolism should be correlated; (2) since organisms already
selected for repair to maximum extent possible, a longer lifespan should not be "evolvable".
1. Testing the first prediction, namely -- species should expend similar amounts of
energy per unit weight per lifespan. Austad and Fischer compared different mammal species
and found tremendous variability (39 kcal/g/lifetime in elephant shrew, to 1102 kcal/g/lifetime
in one bat species). Variability even in closely related species: from 325 to 1102
kcal/g/lifetime in different bats. Additionally, bats metabolic rates similar to other small
mammals, but lifespans average three times longer. Marsupials have lower met. rates than
similar sized placentals (eutherians), but shorter lifespans as well. Oh well . . .
2. As for the second prediction: Luckinbill, et al, tried selecting for long life in lab
pops of Drosophila. Choosing slow maturing, late egg-laying "old" individuals, dramatically
increased lifespan from 35 days average at beginning to 60 days after 13 generations. The
older flies did have lower metabolic rates in the first 15 days of life, but not after that point –
that would explain the later reproduction but not necessarily the entire extended lifespan.
So, is it true that organisms that "live fast" do indeed "die young"? The above would
suggest that fast metabolism and short life do not necessarily occur together. However, if we
look at cell line division, it would appear that there is a certain limit to the # of times cells can
divide – telomeres with repetitive (and apparently protective) sequences of DNA
lose a bit
of
these sequences with each division, to be replaced by the enzyme telomerase with each
division. However, telomerase is not strongly expressed in any but stem and cancer cell lines.
As such, losing telomere portions could be correlated with eventual
senescence and death.
If the telomerase gene could be more
strongly expressed, increased number of divisions could
be possible (so why ISN’T it more
strongly expressed in these cell lines . . . .? Anyone?). More
detailed research, however, has shown that telomere length is NOT necessarily
correlated with
longer life in several different species of mammals (pgs.
498-499). Indeed, Gomes, et al, found
that longer-lived mammals tended to have SHORTER
telomeres (see fig. 13.8). Long telo-
meres appear to have evolved from
short several times, apparently to enhance protection from
oxidation.
However, the long telomeres apparently increase the risk of cancer. Mice,
with
extremely short life spans, have nearly the same incidence of cancer as
us. A trade-off --
cancer risk versus aging (see pg. 499).
Clearly, though there is a maximum cell division number for some cell lines, and faster
metabolism means faster cell divisions, the Rate of Living Theory has BIG holes. As indicated
above, some mammals with similar metabolic rates live much longer than others; longer life
can be selected in fruit flies; and telomere length may NOT be directly
correlated with more
cell divisions and longer life. So why hasn’t natural selection produced
a "better" life for species?
II.
Evolutionary Theory of Aging – The ETA suggests that it is problems with repair,
not with the damage itself. Why can’t organisms continue repair – should be easier than the
original genetic developmental program to simply maintain already formed tissues/organs.
The suggestion is that damage can be repaired only to a certain physiological limit (an
evo-
lutionary constraint). Why? Two ideas: (1) the mutation accumulation hypothesis; and
(2) trade-offs between repair & reproduction – the antagonistic pleiotropy hypothesis.
1. Deleterious mutations that affect older individuals will not be that strongly
selected
against, since reproduction will have already been possible, and have little effect
on lifetime reproductive success. This is one possible evolutionary
explanation for aging.
One direct example of such a mutation (that is typically expressed
later in life) is that for
nonpolyposis colon cancer in humans (see page 502).
2. Pleiotropy – genes influencing more than one trait. Suppose an allele affects two
traits antagonistically – for instance, both earlier reproduction but also earlier death. It’s a
trade-off to the individual. This allele could be selected for, since earlier reproduction could
be much more advantageous from a fitness standpoint (more generations, with your genes in
them) than deleterious from an early death standpoint. As long as reproductive success was
higher with such an allele, it should persist. See examples, pgs. 505-507
-- methuselah gene
in Drosophila is one, providing longer life but lower
reproductive output.
Other examples of trade off between early reproduction versus survival/reproduction
later in life: 1. Collared Flycatchers in Sweden – first year vs. second year breeders
2. Annuals vs. perennials (duh!)
A Natural Experiment – from right here in GA (and SC)!
Hypothesis: lower mortality should lead to later senescence.
Opossum populations on mainland vs. Sapelo Island (no big predators for 4-5000 yrs)
Sapelo island opossums: live longer, show no decline in ability nourish second-litter
offspring, and connective tissue strength declines more slowly. However, mainland opossums
have larger litter size. So, this is support for which theory above (under the ETA)?
Offspring – how many, how big, how often? Obviously, trade-offs are involved here.
How many? Example: Clutch size in birds (birds easy to study/manipulate) – # should
always be where fitness is maximized; should be a trade-off between # and probability of
survival of indivs. (Lack’s hypothesis).
Testing Lack’s Hypothesis: 1. Great Tits in England – # of surviving young highest in
clutch sizes of 12; avg. clutch
size was 8.53. Hmm . . .
Virtually all studies show the same results – smaller clutch size than predicted. Why?
Lack’s hypothesis does NOT take into account future reproduction. Indeed, the only factor in
Lack’s hypothesis is affect of clutch size on survival, not other potential costs of large clutch.
Beyond that, studies show clutch size is plastic – if there are indications of a "good" year,
clutch size can be adjusted.
2. Parasitoid wasps – number of eggs laid in host for Trichogramma;
# was varied
depending on host quality, but # laid was lower than predicted (as above). Why? A larger #
may stress host (while still alive) to point where individual fitness is lowered, and "too many
eggs in one basket" (host may die for other reasons). These wasps quickly lay several clutches.
So, total fitness (as with optimal foraging theory) would involve how many eggs she can lay in
however many hosts she can find, and how long it takes to find hosts (using energy in the
search/exposing herself to preds.)
How big? Should be a trade-off between numbers and size (see Fig. 13.30). However,
the
size/number ratio (as with many, many other traits we have now looked at) should be
optimized to maximize parental fitness. Increasing size of offspring does improve likelihood
of survival and maturation, but at a certain point, the larger size gives smaller benefits, and
certainly is more taxing on mom. As such, selection on mom may favor somewhat smaller
eggs than would selection on offspring.
Example: Beetle (Stator limbatus) on two host seeds: Acacia (excellent survival of
larvae) and Palo Verde (< half of larvae survive typically). Prediction: fewer larger eggs
should be laid on Palo Verde. Requires phenotypic plasticity in the egg-layer (which Stator
has). The prediction is exactly matched by the beetles: larger egg size on Palo Verde shortens
developmental time and increases competitive ability with other larvae.
Conflicts of Interest
Between male and female parents
Females may carry
offspring from more than one father. Each offspring is equally related
to her, but alleles from fathers that can coerce female to invest more in his
offspring should be
selected for. Can alleles be marked (imprinted) as
to paternal or maternal origin? The answer
is yes. This can lead
to an evolutionary tug of war in the uterus between these alleles.
Drosophila seminal fluid biochemicals influencing egg laying/attraction to other males;
turns out, such seminal fluid is mildly toxic to females (!). Chase-away sexual selection –
similar to a coevolutionary arms race between plant host and herbivore. If kept monogamous,
males reduced toxicity of seminal fluid over several generations.
Male genitalic structure in Papaipema moths.
Between parents and individual offspring – see "How Big?", above.
Also remember lions
and their offspring from Chapter 11.
Maintenance of Variation/Evolution of Novel Traits
We return to these important concepts and how Life History traits are involved.
Reproductive traits (which we’ve been discussing here) should be more tightly correlated with
fitness than numerous other traits. As such, selection should act on these traits very strongly,
and there should be less genetic variation, and review of heritabilities of many traits do suggest
less variation (high h2) in life history traits, though not insignificant.
What’s maintaining variation in life history traits? Same phenomena as before.
Example: Sea squirts: iteroparous and semelparous morphs. Different seasonal conditions
favored the different reproductive strategies, maintaining them both. Competition with another
species later in the year hinders the semelparous, but not iteroparous, morph.
A last take home message:
Life-history
traits and vulnerability of extinction: sauropods for instance.
Life histories are still evolving, which means that life histories could very
easily be
suboptimal. Stages along the way would be open to selection for better optimization,
but may
be limited at the moment by evolutionary constraint (either time or genetic).
Pink Lady’s Slipper orchid example