Phenotypic variation results from the interplay between environmental and genetic factors. The genotypic composition of a population can change due to selection on phenotypes that are genetically determined.
Coadaptation of larval traits to environmental conditions
There is strong selection for adults to spawn at times and in locations that are conducive to offspring survival.
Early life adaptations are commonly observed in migratory fish, including lake sturgeon. Adaptations can be seen as differences in population phenotypes as a function of distance due to limited dispersal and due to adaptations associated with environmental conditions that vary over time (e.g., during early and late spawning periods in the same population.
Points represent spawning time (time). The lines represent distributions of trait values (e.g., size and age at sexual maturity, which may evolve in reproductively isolated sub-populations.
Examples of traits that vary as a function of environmental variation:
- Egg size
- Incubation time
- Larval size
Environmental Effects on Incubation and Larval Phenotypes
Incubation time decreases linearly with increases in incubation temperature.
Body size at hatch varies as a function of incubation temperature. There is a size advantage afforded to offspring from early-spawning females is retained through the larval stage over the period larvae remain in the stream substrate prior to dispersal.
Offspring Body Size at Hatch Varies Significantly as a Function of the Spawning Time of Parents Due to Differences in Water Temperature
The heritability for offspring body size at hatch is 0.23 which means that 23% of the variance in body size is due to genetic factors (shared ancestry).
Environmental conditions (temperature) during incubation affects the timing of egg mortality but not the percentages of offspring that survive to hatch. However, indirect effects (differences in body size at hatch) may predispose individuals produced later in the year by late spawning females that spawn at warmer temperatures to higher levels of mortality (if ‘bigger is better’— if larger body size at hatch confers greater probability of surviving predation).
Anthropogenic Effects to Riverine Environments Constitute an Ecological Trap that can Affect Probabilities of Population Persistence
“Organisms often rely on environmental cues to make behavioral and life history decisions. However, in environments that have been altered suddenly by humans, formerly reliable cues might no longer be associated with adaptive outcomes. In such cases, organisms can become ‘trapped’ by their evolutionary responses to the cues and experience reduced survival or reproduction.”
Schlaepfer, Runge and Sherman (2002)
Factors affecting age-specific sturgeon survival:
- current/dissolved O2
- microbial communities
The effects of dams is seen based on water temperatures in free-flowing stretches of river above the reservoir and dam and temperature below the dam (downstream) where are both elevated and more variable within each day.
- Long-term disruption in natural
- Habitat loss and fragmentation
- Altered selection regimes
In natural systems, traits such as body size follow a normal distribution with a mean and range of values reflecting environmental conditions that in part affect the traits (such as flow or temperature and body size). Changes in the environment, such as elevated water temperature in impoundments above dams result in a new selection regime that favors different phenotypes (e.g., body sizes).
Evolutionary forces affect levels and apportionment of genetic diversity within and among natural populations (changes in levels of additive genetic variance as a function of time — dVg/dt)
- Natural populations are generally adapted to selective regimes associated with their local environments.
- At any location and time the intensity and direction of selection is likely to change such that adaptation is an ongoing and ever-changing process (Hendry et al. 2008).
- We wish to describe and quantify how humans are changing directions and accelerating rates of change that are likely to be faster than organisms can respond using naturally evolved options.
Organismal Response to Human-Induced Change
- Plastic response: change in phenotype, behavior, or physiology by individuals to changing environment
- Microevolutionary response: change in genetic architecture where most ‘fit’ genotypes survive and reproduce in the next generation
Factors affecting organism’s ability to track changes due to disturbances:
Alternative perspectives pertaining to how people view human effects of fish and wildlife populations:
- a thinning process – changes in numbers not characteristics, response – density dependent, where populations will respond numerically (i.e., relax selection and the population will simply rebound to pre-selection levels
- a selective process – change in population characteristics by removing genotypes., response – change in genetic architecture which has long-term consequences to population viability
Two Dimensions of Human Influences
- Ecological – changes in abundance, density, structure, and distribution
- Evolutionary – changes in heritable features and adapted traits
Contrasts between natural and human-based selection:
- Human-induced evolution is not always beneficial
- Significant evolution can occur in only a few generations within most management horizons
- Changes can be difficult to reverse (“Darwinian Debt”)
Examples of traits at risk to change:
- Age and size at sexual maturity
- Growth rates
- Reproductive effort
Mechanisms of change:
- climate change
- selective harvest
- invasive species
Example of Potentially Negative Effects of Directed or Inadvertent Selection
Breeders equation R=Sh2
The response to selection (R) which is the magnitude of change in the population mean for a trait is predicted based on the intensity of selection (S) and the heritability of the trait. Intensity of selection could be the extreme trait values used in parents (e.g., large body size to produce progeny.
What Does this Mean for Long-Lived Species?
- Sturgeon have a unique life history
- Females mature ~14-18 years
- Males mature ~8-12 years
- Broadcast, riverine spawners
- Environmental cues (temperature, discharge, depth) initiate migration to spawning areas and determine reproductive success
The species’ long generation time and delayed sexual maturity means that they are less able to adapt to rapid change genetically.
How Does this Relate to the Black Lake, Michigan Sturgeon Population?
- Well studied closed population
- Adults reside in Black Lake until the spawning season
- Two or more spawning runs:
- Early run = Cold water temperature, high discharge
- Late run = Warm water temperatures, low discharge
- Stream-side rearing facility allows for experimental monitoring across ontogenetic stages
- Opportunities for selection to act during early life stages
- Egg mortality (~99%, Forsythe 2010)
- High larval mortality
Examination of Survival Rates Under Different Flow Conditions
Examples of environmental variation associated with dams on the upper Black River. Sections of the river that are only a few miles apart differ considerably in daily flow and temperature. Daily temperatures are generally lower in the section above the dam and daily discharge is generally lower and less variable.
Water temperature and flow are 2 important environmental cues used by fish to initiate spawning and to locate suitable spawning locations that will provide suitable environments for eggs and offspring following hatch. These important cues can be disrupted by hydroelectric operations. Dams pond water in reservoirs and this water warms faster than in rivers. Discharge is more variable in amplitude and duration following precipitation events.
Experimental Design – Mean Water Velocity (± 1SD) Between Three Flow Treatments Designed to Test Whether Velocities Simulating Stream Flows With and Without Dams Affects Egg Survival.
We established 3 water flow treatments to evaluate the effects of different flow regimes on egg survival. The ‘variable’ flow regime simulated effects of an unregulated hydro-electric operation. We fertilized eggs from multiple sturgeon females and placed the eggs in flumes where water was allowed to flow at high (H), low (L) and variable (V) velocities. The same families were used in all 3 flow treatments.
Mean daily survival (+/- 1SD) for lake sturgeon incubated in three different flow regimes
Results from the experiment indicated that eggs incubating in ‘variable’ flows experienced significantly higher rates of mortality than eggs exposed to high or low flows.
Mean daily proportion (+/- 1SD) of dead lake sturgeon eggs incubated in three different flow regimes with microbial infection.
Experimental results also showed that the proportion of dead eggs That appeared to be associated with microbial infection was also higher in the variable flow treatments.
As the rate of change in environmental conditions increases, as indicated by the slopes (time 1 vs time 2 below), populations will be less likely to be able to track these changes. With the change experienced, some individuals will be selected against (will fail to reproduce or will die), which will reduce the population size. If the population consists of genotypes that are adapted to the new environment, the population can potentially recover numerically based on reproduction by the remaining (selected) genotypes. As the rate of environmental change increases, there will likely be fewer individuals that can survive, and thus the population will spiral to extinction. If the changing environmental conditions are accompanied by loss and fragmentation of habitats, increasing isolation of populations and decreasing population size, the probability of population extinction will increase. These scenarios are depicted in cases a, b, and c below.
Conservation programs would be well advised to prioritize goals for current and future adaptation and for preservation of genetic diversity.