Chapter 6
What is the definition of natural selection?
The process by which individuals with certain heritable characteristics tend to survive and reproduce more successfully than other individuals because of those characteristics
Compare the benefits and costs associated with sexual versus asexual reproduction
The high levels of genetic variation resulting from sexual reproduction may be beneficial in challenging environments and there is less genetic variation with asexual reproduction
Define the terms population, population size, and population density
- A population is a group of individuals of the same species that live in the same area and at the same time and interact with one another
- Population size is the number of individuals in the population
- Population density is the number of individuals per unit of area
Justify why fluctuations in population growth rate can increase a population’s risk of extinction
- For a given average population size, a population that fluctuates in size shows a slower growth rate than one that does not vary.
- A slower growth rate results in a smaller population size, which has a greater risk of extinction
What is the equation for geometric population growth?
Nt+1 = λNt
Give an example of a mutation
Example: A plant growing in nutrient-rich soil growing larger than another individual of the same species growing in nutrient-poor soil, even if both have the same alleles of genes that influence size
Illustrate how the number of offspring may affect the size of those offspring
There is a trade-off between offspring size and number, such that organisms tend to produce large numbers of relatively small offspring or small numbers of relatively large offspring
Compare the different dispersion patterns of populations
The dispersion of individuals within a population may be regular, random, or clumped. In the field, clumped dispersions are more common
Compare the patterns of exponential growth with that of logistic growth
- Exponential growth occurs when the rate of growth increases (or decreases) in proportion to the current number of individuals and lasts for a limited time when conditions are favorable (or unfavorable)
- Logistic growth occurs when the rate of growth increases rapidly at first and then stabilizes as the population reaches carrying capacity (the maximum population size that can be supported indefinitely by the environment)
Define exponential population growth
Exponential growth occurs when a population with continuous reproduction changes in size by a constant proportion at each instant in time
Explain how natural selection can lead to adaptions in populations
By favoring individuals that have advantageous alleles over individuals that don't, leading to adaptive evolution. Over time, this advantageous trait will increase over time
Compare the benefits of semelparity and iteroparity in the context of total lifetime reproduction of an organism
Semelparous species reproduce only once in a lifetime, while iteroparous species reproduce multiple times.
Describe the factors important to the suitability of habitat for populations and species
The suitability of habitat depends on both abiotic and biotic features of the environment, including factors that affect physiological tolerances, resources, and species interactions.
Some species are thwarted or dependent on regular forms of disturbance, or abiotic events that kill or damage some individuals
Describe population size fluctuations and the special case of population cycling
All populations show fluctuations in size, some of which are erratic and others of which are deviations from exponential or logistic growth. Some populations fluctuate greatly over time; others fluctuate relatively little.
Some populations show regular population cycles, a special type of fluctuation in which alternating periods of high and low abundance occur after nearly constant intervals of time.
Justify the use of life tables to determine population growth and size
Life tables provide a summary of how survival and reproductive rates vary with the age, size, or life stage of the individuals, allowing more accurate determinations of population growth rates and sizes
Evaluate how random events can affect populations through time via genetic drift
Genetic drift, which occurs when random events determine which alleles are passed from one generation to the next, can have negative effects on small populations
Evaluate the environmental conditions that would favor the persistence of r-selected and K-selected species
Environments subject to frequent disturbances and low population size favor species exhibiting r-selected traits. Where the environment is stable with population sizes near the carrying capacity, K-selected traits are favored
What is a metapopulation?
A collection of discrete local populations of the same species, linked by a network of habitat patches that allow for some level of dispersal and migration between them, enabling recolonization after local extinction and ensuring the persistence of the species at a larger regional scale
List one way that chance events can drive small populations to extinction
Genetic drift and inbreeding reduce the genetic variation of small populations, limiting the ability of populations to respond to environmental change and harmful alleles
Define density-independent factors and describe how they affect population size and growth rate
Density-independent factors affect population size and growth rate independent of population density and include abiotic factors such as weather and climate and biotic factors such as hunting.
Evaluate the roles of speciation and extinction in determining the diversity of species
The number of species in a group of organisms increases when more species are produced by speciation than are lost to extinction, and decreases when the reverse is true.
Speciation: The process by which one species splits into two or more species.
What is r-selected and k-selected species? Give an example of each
r-selected; produce many offspring with little parental care and have short life spans. Example: insects
k-selected; produced few well cared for offspring and have long lifespans. Example: elephant
Describe how the amount of suitable habitat and population isolation can affect metapopulation persistence or extinction
A metapopulation can be doomed to extinction, even when some suitable habitat remains, if the remaining habitat is not large enough to sustain individual populations or if the habitat is isolated by distance and is thus unable to receive immigrants from other populations (rescue effect)
List another way and describe the ways that chance events can drive small populations to extinction
Demographic stochasticity and environmental stochasticity produce unpredictable changes in population sizes, thus increasing the risk of extinction, especially for small populations
Compare the three types of survivorship curves
In a population with a type I survivorship curve, most individuals survive to old age; death rates do not begin to increase until old age.
In a population with a type II survivorship curve, individuals experience a constant chance of surviving from one age to the next throughout their lives.
In a population with a type III survivorship curve—the most common type in nature—death rates are very high for young individuals, but adults survive well later in life