- Slides: 30
Chapter 15 - Populations Section 15 -1 - How Populations Grow
What is a population? • All individuals of a species that live together in one place at one time.
What Is a Population? • Ex: humans, bacteria, animals • Individuals usually have more than one offspring = population growth
3 Key Features to Populations • 1. Population Size - # of individuals in a pop. – Important effect on ability of pop. to survive – Small pop. @ greater risk of extinction – Interbreeding = risk of genetic disorders
3 Key Features to Populations • 2. Population Density - # of individuals in a given area – Large area may mean less reproduction
3 Key Features to Populations • 3. Dispersion - how individuals of the population are arranged – Random – Even – Clumped • Most common • Microhabitats
Modeling Population Growth • Population Model – hypothetical population that attempts to exhibit the key characteristics of a real population.
• What causes populations to grow? – More birth than death • What determines how fast they will grow? – Amount of resources – Rate of immigration – Success of birth rate • Is there anything that can slow their growth? – Death – Emigration – Lack of resources
Exponential Growth Curve • Pop. size plotted against time. – (r) remains constant – (∆N) pop. growth – rises quickly as pop. increases – Ex: single bacteria produces over 1 million in 10 hrs.
Carrying Capacity • Pop. size the environment can support – (K)
Resources and Population Size • Density-Dependent Factors – amount of resources available depends on population size.
Logistic Model • Accounts for declining resources as pop. grows. – Assumes birth and death rates are not constant – As pop. grows, births decline and death increases – As (N) approaches (K) growth decreases
Density-Independent Factors • Environmental conditions
Two Strategies of Population Growth • r - strategists – mature rapidly • Ex: insects, bacteria, some plants • Exponential growth • Temporarily large pop. followed by crash in size • Produce lots of offspring with little investment or care
Two Strategies of Population Growth • k - strategist – mature slowly • Ex: Redwood trees, whales, rhinos • Characterized by high degree of specialization – Environments stable and predictable • Competition • Produce few offspring • Danger of extinction
Rapidly Growing Human Population • What strategy? – r/K - strategist • Large investment in offspring • What led to human pop. growth? – Technology and agriculture increased carrying capacity, which led to rapid growth (∆N ) • Population growing explosively • Will it continue to grow explosively? – Something has to give (human population will reach carrying capacity)
Current population = 6. 8 billion people
Section 15 -2 How Populations Evolve
Allele Frequencies • Just because an allele is dominant doesn’t mean it will become more common – Dominant allele could be deadly
Hardy-Weinberg Principle • States that the frequency of alleles in a population do not change unless evolutionary forces act on the population. – Pop. must be large – No inbreeding – Evolutionary forces change alter genetic makeup of a population
5 Evolutionary Forces • 1. Mutation – Rate in nature is very slow – Not all mutations result in phenotypic changes. – Source of variation, makes evolution possible
• 2. Gene Flow – Result of migration – Immigrants = add alleles – Emigrants = take alleles away
• 3. Nonrandom Mating – Individuals mating with others nearby or of the same phenotype. • inbreeding
• 4. Genetic Drift – Random change in allele frequency – Small populations effected by catastrophic events • Less genetic drift = similar individuals = BAD
• 5. Natural Selection – Directly changes allele frequency – Acts on the population, not individuals
How Selection Acts • Only on characteristics that are expressed. • Hemophilia – homozygotes may die, but heterozygotes may still produce offspring. – Only homozygous individuals will be eliminated from the population.
Why Genes Persist • Individuals carry genes that are not expressed. • When two individuals that carry the gene reproduce, it may become expressed.
• Polygenic trait – a trait that is influenced by several genes. – Produce a normal distribution – Height
Directional Selection • Frequency of a particular trait moves in one direction – Eliminates one extreme
Stabilizing Selection • Reduces extremes from both ends – Individuals become more “average”