Two fundamental aspects of reproductive biology are largely responsible for the planet's remarkable diversity

Sexual Reproduction

Anisogamy

These in turn influence the timing of reproduction (Life History) and lead to many conflicts of interest between individuals, often in the form of Sexual Selection

Sexual Reproduction

Question: If the ability to reproduce one's genes is a mark of evolutionary success, why have sex?

Consider the benefits and costs

Benefits of Sex

The main benefit of sex appears to be the mixing of genes into new combinations

Meiosis separates and mixes genes on different chromosomes

Crossing-over separates and mixes genes on the same chromosome.

Assuming that specific groups of genes are adapted to specific environmental conditions, the new combinations of genes produced by sexual reproduction may create phenotypes that are better able to invade new habitats or survive environmental changes

Sex also improves the removal of deleterious genes.

Costs of Sex

There are several reasons why sex is costly relative to simple asexual reproduction

1) Meiotic cost of sex: the number of offspring a female produces during her lifetime is limited by time, access to resources, and the cost/offspring.

Asexually produced offspring will each carry an entire copy of the mother's genome

Sexually derived offspring will carry only half the mother's genome

Thus, as a gene spreading mechanism, sex is half as effective as asexual reproduction (sometimes referred to as "the two-fold" cost of sex).

2) Recombination cost of sex: by mixing genes, sex may produce offspring that are, on average, less well adapted to specific environments than the parents (and, thus, for a given level of reproductive output, more asexually produced offspring are fitter, though some may not be as fit as some sexually produced offspring).

3) Additional costs of sex: finding mates, avoiding unnecessary or unwanted matings, injuries from fighting for mates, sexually transmitted diseases, etc....

At first glance, these costs would seem to outweigh the benefits.... yet sex is a common form of reproduction in prokaryotes and the dominant form of reproduction in eukaryotes.... so the question remains: why have sex?

First ask: do the benefits exceed the costs enough to permit the evolution and maintenance of sex?

3 types of answer:

1) Yes, and the benefits are ecological

Group selection hypothesis (lowered rates of extinction in changing environments)

"Red Queen" hypothesis (sex allows variation to keep up with sexually derived parasites, predators, prey, etc.).

Broader niche widths in saturated environments hypothesis (sexual offspring can use a larger fraction of available resources with lower within-family competition when compared with asexuals.... this assumes that different kinds of asexual clones are rare).

2) Yes, and the benefits are cytological (DNA repair hypothesis)

Recombination functions to repair DNA lesions and complementation better insures against deleterious mutations.

Note: secondarily evolved asexuals tend to appear in sites where DNA damage from irradiation is highest: Bacteria and viruses (which lack meiosis and recombination) repair DNA at same relative rates to sexual organisms.

3) No, sex is the result of selfish parasitic genes forcing the organism to do things which allow these genes to replicate more easily (even at 50% cost to the "host"). This might, for example explain the maintenance of high recombination rates (2-3 recombination events/gamete) observed in Drosophila

A book by Burt and Trivers, Genes in Conflict: The Biology of Selfish Elements, gives an excellent overview of this idea (click here for a review). It is available in Waztek (Call no.: QH447.8.S45 B87 2006)

Irrespective of the ultimate reason that sex has evolved, recombination between generations has important implications for the evolution of behavior

With asexual individuals the interests of genes and individuals are identical (because they are always together).

With sexual organisms, genes are effectively "emancipated" from a given chromosome and from a given set of fellow genes at other loci.

With sex, each gene is on its own, and it will be replicated most often if selfish

While the hypotheses listed above are not exclusive, there is evidence for selfish genes

1) C Value Paradox: 80-90% of most eukaryotic genomes produce no gene products.

This could be due to accumulations of "old" genes that are no longer needed...

... or it could be due to parasitic DNA

... or it may influence the shape of coiled DNA, which promotes particular function or efficiency

(note, these are not mutually exclusive)

2) Y chromosome wars: A heterogametic chromosome (such as Y in mammals) shares no genes with the X chromosome.

An increased production of Y gametes over X gametes (of benefit to genes on Y chromosome) is sometimes found, along with X genes that suppress distortion of "sex ratio" at the gamete level.

The inactivity of Y genes in many species may be the result of past gene wars.

3) Different genomes in the same organism... mitochondria, chloroplasts, endosperm in seeds, etc.

This can produce competition for control of the organism

Sexual reproduction generates conflicts of interest between genes, between parts of cells, between tissues, and between individuals (male vs. female; parent vs. offspring; brother vs. sister, etc.)...

These conflicts of interest are important source of diversity in animal behavior (mating systems, parental care, etc.)

Anisogamy

Anisogamy (literally, different sized gametes) reflects differential investment in gametes.

This differential allocation in resources contributes directly and indirectly to an enormous variety of animal behaviors.

Anisogamy presumably evolved via an arms race as smaller-than-average gametes parasitized larger ones

By definition, females produce larger, generally immobile and energetically costly gametes while males produce small, motile, energetically "cheap" gametes.

Given the costs of producing eggs, female reproductive success is often limited by access to resources, while male RS is limited by access to females

If females align themselves to patterns of resource availability, and males align themselves to patterns of female availability, then we expect correlation between resource distribution and mating system.

Patterns of parental investment and mating system are often linked...

Life History Theory

Now, Consider the scheduling of reproduction

This includes considerations of both the timing (when) and the intensity (how much) of reproductive schedules

Such investigations are generally referred to as the study of life history

Understanding life history strategies (the timing of reproductive events over an organism's lifetime) are central to discussions of mating system, parental care, and the dynamics of sexual reproduction.

Some basic assumptions in life history modeling:

1) There is a tradeoff between growth, reproduction, and survival

Investing energy in one of these takes away from the others.

2) Because of different levels of investment into gamete production, life history models focus on females.

Male reproductive strategies evolve according to female reproductive schedules

Thus, female reproductive strategies evolve depending upon the contrast between the current prospects for successful reproduction vs. the expected level of reproduction at some future time.

 

Models of life history evolution consider three variables

A) The benefits of reproduction (copies of genes in next generation)

B) The survival of offspring (parental investment)

1) Prenatal investment

a) Oviparous vs. viviparous

b) Size of young/eggs

c) Length of development

d) Altricial vs. precocial offspring

2) Postnatal investment (parental care)

a) Brooding/incubating

b) Feeding of young

c) Guarding of eggs or babies

d) Post weaning retention

C) The costs of reproduction (female's longevity)

1) Risk of mortality

2) Increased energy expenditure

3) Time lost

Incorporate these to consider:

The fitness W of a female is a function of three components

M = The number of offspring produced (fecundity)

P = The quality of the offspring (offspring survivorship)

L = The length of the adult female reproductive life (adult survivorship)

Since these three variables cannot all be maximized, we should strive to optimize the product:

W = M x P x L

I) Holding offspring survivorship constant, consider the relationship between fecundity and life expectancy

R₀ = net reproductive rate

where l_x is the probability of surviving to age x and m_x is fecundity (number of female offspring at age x)

An example of an age/survivorship table for grasshoppers

Some facts to contemplate:

R₀ = 1 for a stable population (each female exactly replaces herself)

Clearly a tradeoff (negative relationship) between survival and fecundity

Note: for growing or shrinking populations, solving for r in the Euler-Lotka equation is more appropriate:

What determines whether a female should maximize fecundity or survival?

1) Consider the survival rate of adults, extrinsic of reproduction

If adult survival is already high, maximize l_x; animals can afford to reduce reproduction costs per reproductive episode, and fitness is maximized by breeding for as many years as possible

This leads to iteroparity: repeated reproduction of a limited number of offspring over many reproductive events

If adult survival is low, maximize m_x; it is better not to wait for future reproductive opportunities. Produce as many young as possible as early in life as possible

This leads to semelparity: a single, "big bang" bout of reproduction

Ignoring reproduction, survival rates are influenced by: body size, trophic level, environmental stability/disturbance

II) Holding total energy invested in reproduction constant (i.e. life expectancy is fixed)... what is the relationship between number of offspring and quality of offspring?

Any increase in parental effort expended per offspring will increase the fitness of the young

The tradeoff between quality and quantity: it's an optimization problem

Wy = fitness of offspring Wp = fitness of parent Iy = investment/offspring Ip = total investment by parent (constant) Ny = number of offspring

Factors leading to increased parental investment: "K" selected environments, harsh physical environments, specialized or scarce food sources, predation pressure

Relationships between all three components: l_x maximizers tent to invest more heavily in offspring survivorship and have low fecundity and vice versa. Thus, these life history traits tend to be associated:

The ability to put off the expression of deleterious genes until selection pressures are reduced leads to the condition of "aging" (falling apart).

The infirmities of old age are generally not an issue for wild animals.

The concept of "senescence" (an age-specific decline in survival) is associated with many characteristics we associate with "aging"

Sexual Selection, sexual tradeoffs, and alternative sexual strategies

Sexual conflicts of interest and competition for mating opportunities and resultant differences in mating success among individuals of the same sex can create variance in fitness.

This variance is called Sexual Selection.... a unique component of fitness that stands apart from other aspects of fitness that are influenced by Natural Selection.

Traits that confer fitness via Sexual Selection may run counter to the pressures of Natural Selection

Darwin recognized the special nature of sexual selection and identified two types of sexual selection

Both can lead to "runaway" selective pressures that favor the development of elaborate or exaggerated traits so often associated with mating behaviors and displays

 

Intrasexual selection: interactions between members of the same sex that cause different access to mates (usually male/male competition)

Traits that confer competitive superiority should be strongly selected for, leading to "arms races" for the elaboration of such traits.... this creates strong sexual dimorphism.

Obvious examples include the antlers of deer, horns in beetles, large body size in elephant seals, etc.

 

Intersexual selection: interactions between members of the opposite sex that cause differences in mating success for at least one sex (usually female choice of certain males).

Here, two traits are needed... the male trait that is chosen by females and the female trait of preference for a certain type of the male trait.

Once the link between preference and trait is established, the trait may quickly "run away" to a value that either causes extinction or is balanced by increased mortality (natural selection).

Choice by females for a specific trait presumably carries some cost, but there can still be selection for choice:

a) no benefit: traits selected for will be those that minimize female costs (little or no trait elaboration).

b) direct benefit: selected traits should correlate with benefit (e.g. indicate level of parental care, territory quality, low parasite or disease load, etc... leading to elaboration of trait).

c) indirect benefit: selected traits indicate to females which males have the best genetic make-up for offspring fitness (e.g., good foraging efficiency, disease resistance, high predator avoidance, etc.).

Handicap models of female choice center on "good genes" arguments and the idea that males with better genes will be able to either a) accept a greater handicap or b) survive better for a given handicap.

 

The potential strength of sexual selection can be assessed from the variance in mating success among all members of the same sex...

i.e., the greater the difference between "winners" and "losers" the stronger the potential selection

Careful: differences in winners and losers may have no genetic component (e.g. differences due to environment)... thus, no selection in this case.

Extreme variance in male reproductive success due to sexual selection can lead to alternative mating tactics.

Often linked to life history (make the best of a bad situation when young and small)

 

"Sneaky mating strategies", "jacks", etc...

And then... there is always sex-change...

 

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