Population Biology Biotic Potential: highest rate of reproduction under ideal conditions. -populations very rarely reach their biotic potential, because conditions are not perfect all of the time. Population size: the number of organisms in a population (N)

Under perfect conditions, a population will continue to increase indefinitely. The human population shows this pattern (so far). Why? -unlimited resources, such as food and water, unlimited space, all waste is removed so that it does not build up, etc.

This is an example of an EXPONENTIAL growth curve.

Carrying Capacity: the number of individuals that a given environment can support. Ex: Number of lions in the African plains, or the number of zebras at carrying capacity, birth rate is equal to the death rate.

Carrying Capacity This is an example of a LOGISTIC growth curve.

Environmental Resistance: the sum of limiting factors facing a population. They all add up to the Environmental Resistance, which holds numbers down Ex: water requirements, space, need for food, competition, lack of mates.

Limiting Factors: -control population size and growth -can change depending on environmental conditions. Population Density: the number of organisms per unit area, or volume. Ex: rain after a long drought

Limiting Factors can be Density Dependent (related to the density of the population), or Density Independent (does not matter what the density of the population is) Ex: flood/storm (DI) vs levels of disease (DD).

Density Dependent Factors 1)Predation: size of prey population is held down by predators, size of predator population is dependent on size of prey population.

3) Food/water resources: the higher the density of organisms, the faster the food and water supply will be used up. 4) Space for nesting: High density populations will have a high level of competition for the best areas to raise young. 2) Disease: Disease spreads more rapidly through a dense population, populations can be reduced by disease

Interspecific and Intraspecific Competition: Interspecific: competition between different populations. Intraspecific: within a population. Ex: lions vs. hyenas Ex: lions vs. lions Both types are Density Dependent

Dispersion and Competition Dispersion is the pattern of distribution the individuals within a population take. Random: Individuals are placed by chance, or by self determination. Ex: trees in a forest, a coral reef.

Even Distribution: Individuals are evenly spaced, located at regular intervals. Usually occurs because of intraspecific competition. Ex: Competition between pine trees for sunlight

Clumped Distribution: Individuals are bunched together in clusters, for protection, reproduction, or because of narrow habitat tolerances. Ex: schools of fish, flocks of birds, clumps of one type of plant within a forest.

Population Growth Patterns 1)Rapidly growing populations: Many species, such as insects, plants, and fungi are found in rapidly changing environments. Such species are called r-strategists r-strategists populations grow exponentially when environmental conditions allow them to reproduce. When conditions worsen, the population size drops quickly.

R-strategists use the strategy of creating many offspring, but spending little energy and time on each individual. Many offspring, little input EX: Dandelions: Lots of seeds, blown by the wind, no care at all. Blue Crabs: Eggs are released by the thousands, but no energy is spent on the young.

Years Population R-Strategist Population Growth

2) Slowly Growing Populations Organisms that grow slowly often have small population sizes. These organisms are called k-strategists, because their population is usually below the carrying capacity (K). k-strategists have fewer young, but spend more time and energy on each individual.

K-strategists grow slowly, have a longer life span, and are less susceptible to environmental change. Many endangered k-strategists are in trouble because they are being hunted in numbers that cannot be supported. Examples: Sharks Gorillas Tigers Lions

Years Population K-strategist Population Growth

Summary of R vs. K strategists R: Mature rapidly Short-lived Have many offspring Invest little energy in young Boom or Bust population K: Mature Slowly Long-lived Few offspring at a time Care for their young More stable population

Calculating Population Growth Birth Rate: (b), is the number of births per unit time (B), divided by the total Population (N). b = B/N

Death Rate: (d), is the number of deaths per unit time (D), divided by the total Population (N). d = D/N Growth Rate: The # of births minus the # of deaths per unit time divided by the total population. g = (B – D) /N

Ex: A total population of 18,700, ,800 births in 1 year -130,900 deaths in 1 year Birth rate:261,800/18, 700,000 = 1.4% Death rate:130,900/18,700,000 = 0.7% Growth Rate: ( ,900) / 18,700,000 = 0.7%

Doubling Time: (T) the amount of time it takes for a population to double its size. Doubling time is estimated by the formula: T = 70/annual growth rate Where T is the doubling time, and the annual growth rate is expressed as a percentage. Ex: a population with a 1.7% annual growth will double in about 42 years. (70/1.7)

The Change in Population Allele Frequencies Allele frequency refers to the proportion of a population that has a particular allele. Natural selection alters the proportions of alleles, making those that give an advantage to a population higher. Natural selection puts genetic pressure on the population, and the frequency of alleles changes over large amounts of time.

The Hardy-Weinberg Principle In 1908, G.H. Hardy and Wilhelm Weinberg both showed that mathematically, dominant alleles do not replace recessive alleles in a population. Their theory states that allele frequencies do not change “unless acted upon by an evolutionary force”. The Hardy-Weinberg Principle only holds true if certain conditions are met.

Hardy-Weinberg Conditions 1)Mating must be random. There is no pressure to “select” a mate. All organisms also produce an equal number of offspring. 2) No genes may enter or leave the population, therefore, no immigration nor emigration is possible. 3) There can be no occurrence of mutations within the population. 4) The population must be large, so that changes in gene frequency are not the result of chance.

The circumstances under which the Hardy- Weinberg principle operate are never found in nature. Mating is almost never random, immigration and emigration do occur, mutations do occur, and some populations can be small and isolated. Hardy-Weinberg does indicate that evolution does occur, and is useful because of the conditions that is needs. These are the conditions under which evolution happens.

Gene Flow The movement of alleles into or out of a population. Created by movement of individuals into (immigration) or out of (emigration) the population. Nonrandom Mating Sometimes individuals prefer to mate with others that live nearby or are their own phenotype. Mating with relatives (inbreeding) causes a lower frequency of heterozygotes than predicted by the Hardy-Weinberg principle.

Inbreeding increases the proportion of homo- zygotes in the population. Genetic Drift In small populations, a chance event can wipe out a set of genes. Small populations may have genes that are very similar, or even identical, because they all came from a few individuals. This is called the “founder effect”, and can have large implications for the population.