4b. Organismal Ecology

parthenogenesis: development of an individual from an egg that did not undergo fertilization

monoecious: separate male/female flowers on same plant

Dioecious: separate male/female plants

hermaphroditic: possessing both male and female organs

simultaneous hermaphrodites: male organ of one individual is mated with female organ of other individual and vice versa

sequential hermaphrodites: male in one part of life cycle and female in another part

life history: patter of growth, reproduction, and mortality for an individual

cohort: members of a population that are the same age

semelparous: produce offspring in a single reproductive event

iteroparous: produce offspring in a series of multiple reproductive events

net productive rate: number of female offspring left during a lifetime by a newborn female

gerontology: study of aging and older adults

life expectancy: average number of years to be lived in the future by members of a given age in a population

pleiotropic: more than one effect

antagonistically pleiotropic: one beneficial effect and one deleterious effect

Evolution of Life History Patterns

lifetime reproductive success = measure of fitness
- but all life histories involve trade-offs
how individuals allocate scarce resources is shaped by natural selection
natural selection cannot maximize all life history variable simultaneously

Life History Trade-Offs

modes of reproduction
age at reproduction
allocation to reproduction
timing of reproduction
number and size of young of seeds
parental care

Modes of Reproduction: Sexual or Asexual?

in asexual reproduction, offspring genetically identical to parent
asexual reproduction = common
- ex. strawberry “runners”
- ex. budding in hydras
- ex. aphid parthenogenesis
most asexual lineages revert to occasional sexual reproduction
- ex. hydras/aphids produce overwintering zygotes
- asexual plants produce seeds to survive times of environmental hardship
- obligate asexuals = rare
asexual reproduction has advantages
- parents adapted to environment? offspring = similarly adapted
- all individuals = reproductive, = potential for high population growth
females produce larger, non-motile, energetically costly gametes
males produce smaller, motile, less energetically costly gametes

Why Sex?

sex = recombination
recombination occurs in two ways

independent assortment
molecular recombination of DNA via crossing over

meiosis = halves chromosome compliment
main advantage = ability to outrun enemies via genetically variable offspring
some offspring may be less susceptible to predation + disease
sexual populations capable of more rapid evolutionary change

Costs of Sex

Genetic costs:

meiosis: each gamete only has half of your chromosome complement

sexual offspring = 50 percent related to you

Demographic costs:
- Asexual females will produce twice as many daughters as sexual females
- sexual reproduction is far inferior measured by reproductive output
Energetic costs:
- mates have to find one another
- courtship
- direct conflict over mating
increased predation risk
disease cost (STIs)
fertilization is often inefficient

Red Queen Hypothesis
coevolution between a species and its enemies leads to an arms race

in plants, sexual reproduction takes several forms
- dioecious
- hermaphroditic
- bisexual “perfect” flowers
- monoecious
in animals
- simultaneous hermaphrodites
- sequential hermaphrodites
- sex change may take place as individual grows
- change in sex ration may also stimulate sex change

Life History

gather data on these
- population size/density
- age of members
- sex ratio
- birth rates
- death rates
when collecting data, experimental error can accumulate at each of these steps
capturing, evaluating + tracking wildlife = imprecise af
demographic data will be determined by tracking cohort from birth to death
- good cohort data = hard to collect
- can take years + money
- rarely exists for non-game species and/or non-threatened populations

The Life Table

developed by human demographers
widely used by life insurance companies
to construct, follow a cohort or determine the age of organisms in question
cohort/static
- both = inaccurate because mortality and reproduction vary from year to year

Cohort Life Tables

constructed by following a cohort of individuals from birth to death of the last individual

Static Life Tables

constructed by sampling the population in some manner and aging organism to obtain an age distribution during a single time period
snapshots of a population at a specific time
assumes each age class is sampled in proportion to its abundance in population

Fecundity Schedules

age-specific schedule of births, the number of offspring per unit time by females in different age classes
- determining the mean number of females born to each group of females
combining survivorship and fecundity, we can determine the number of births produced per unit time per age class
net productive rates greater than 1? indicative of populations that are increasing, at least for that cohort

Age Structure

age distribution of a population determines in part reproductive rates and death rates
ratio of young to adults = informative
- increasing populations tend to have a high proportion of young
- decreasing populations tend to have a relatively few young

Survivorship Curves

life tables allow ecologists to make comparisons between sexes, cohorts, populations, and species
despite the great variety in life histories, there are patterns that are apparent in survivorship curves
survivorship curves depict age specific patterns of survival by plotting survivors against age (log scale)
- translates absolute numerical change into a per capita rate of change
useful in studying the influence of environmental conditions on survival

Type 1

individuals exhibit a high degree of survivorship throughout life and then experience heavy mortality in old age (many mammals)
kinda flat and then curves down

Type 2

linear
constant mortality rate

Type 3

concave
extremely high mortality rates in early life
curves down, flattens out

Mortality Curves

mortality rate against age
most common = J-shaped curve/fish hook

Sex Ratios

often weighted towards males
in later age classes, populations shifts towards females
- ex. Elk in the Jasper
- 113 males : 100 females
- after two years, 85 males : 100 females
- ex. Humans in Canada
- 0-4 years: 104 males :100 females
- 40 - 44 years: 100 males: 100 females
- 80 - 84 years: 54 males: 100 females
- indicates higher mortality rate in males
populations of nearly all females with just enough males to allow fertilization have the highest possible intrinsic growth rate
- why ratios usually near 1:1?
- when one sex is rare, advantageous to produce rarer sex’
- consider mechanistic constraints in meiosis that make a 1:1 ratio difficult to significantly alter

Ageing or Senescence?

organismal aging or senescence is characterized by the declining ability to respond to stress, increasing homeostatic imbalance, and increased risk of death
death = ultimate consequence of aging
some gerontologists regard aging itself as a disease that may be curable?
we live longer, just not as much as you’d think
- we just don’t die as young
most evolutionary biologists don’t think aging is curable
evolutionary explanations for aging = grounded in the idea that the strength of natural selection decreases with age
- if an allele contributes to heart attacks at 15, theres strong selection against that allele
- if allele contributes to heart attacks at 50, theres weaker selection against that allele
- individual with that allele may have already passed that allele on to offspring already
- little selection against late acting nasty genes
- weak selection against deleterious alleles is even weaker for alleles that are antagonistically pleiotropic
- ex. if beneficial effect is expressed in early age classes but deleterious effect is expressed in older age classes, there is strong selection for that allele
why do humans live for so long?
- “grandparenting theory”
- evolutionary advantage to retaining the elderly
- elderly can teach and babysit