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Starling Shakespeare’s Henry the Fourth 1890 ~ 120 ind. 1990 ~ 100 million ind. Chapter 4, 5, 7 Human population? Natural Selection Conditions Natural Selection • • Natural Selection - Process that determines which individuals within a species will reproduce and pass their genes to the next generation. Conditions: – Individuals within a species show genetically determined variation. – Organisms within a species typically produce huge numbers of offspring, most of which die. – – – Excess number of individuals results in a shortage of specific resources. Due to individual variation, some individuals have a greater chance of obtaining needed resources and thus have a greater likelihood of survival and reproduction. As time passes, percentage of individuals showing favorable variations will increase while percentage showing unfavorable variations will decrease. 1 Evolutionary Patterns • • Evolution - A change in the kinds of organisms that exist and in their characteristics. Speciation - Production of new species from previously existing species. – Thought to occur as a result of a species dividing into two reproductively isolated subpopulations. Population Density Evolutionary Patterns • • Extinction - Loss of entire species. – Of estimated 500 million species believed to have ever existed on earth, 98-99% have gone extinct. Co-Evolution - Two or more species can reciprocally influence the evolutionary direction of the other. – Grazing animals and grass species. Dispersion pattern • Clumped • Uniform • Random • Actually count • Sampling • Mark-recapture method 2 Population Growth Carrying capacity Carrying capacity Density-dependent Factor Density-independent Factor 3 Life History Opportunistic life history Equilibrial life history Human Population Human Population 4 Population Principles Chapter 7 Age-structure Diagram Outline • • • • • • Population Characteristics Age Structure Diagrams Exponential Growth Carrying Capacity – Environmental Resistance Reproductive Strategies – (r and K) Human Population Growth – Doubling Time Population Characteristics • Population - Group of individuals of the same species inhabiting the same area simultaneously. – Natality and Mortality Natality - Number of individuals added through reproduction. ¾ Birth Rate (Humans Born / 1,000) – Mortality - Number of individuals removed via death. Death Rate (Humans Died / 1,000) 5 Population Characteristics • • Sex Ratio and Age Distribution • Population Growth Rate - Birthrate minus the death rate. Often expressed as a percentage of the total population. Survivorship Curve - Shows proportion of individuals likely to survive to each age. – High mortality in young. – Mortality equitable among age classes. – Mortality high only in old age. Age Distribution in Human Populations Sex Ratio - Age Distribution – Sex Ratio - Relative number of males and females in a population. Females determine the number of offspring produced in sexually reproducing populations. – Age Distribution - Number of individuals of each age in the population. Greatly influences population growth rate. 6 Population Density and Spatial Distribution • Population Density - Number of individuals per unit area. – High population may lead to increased competition for resources. Dispersal - Movement of individuals from densely populated locations to new areas. ¾ Emigration - Movement from an area. ¾ Immigration - Movement into an area. Exponential Growth Curve • • • Population Growth Curve • Biotic Potential - Inherent reproductive capacity. (Biological ability to produce offspring) – Generally, biotic potential is much above replacement level. Natural tendency for increase. ¾ All living populations follow an exponential growth curve. Typical Population Growth Curve Lag Phase - First portion of the curve; slow population growth. Exponential Growth Phase (Log Phase)More organisms reproducing causing accelerated growth; continues as long as birth rate exceeds death rate. Stable Equilibrium Phase - Death rate and birth rate equilibrate; population stops growing. 7 Carrying Capacity • Carrying Capacity - Number of individuals of a species that can be indefinitely sustained in a given area without harming the habitat. Environmental Resistance • Reproductive Strategies and Population Fluctuations • Not all species reach a stable carrying capacity. – Species can be broadly lumped into two categories: K-strategists r-strategists Environmental Resistance - Any factor (limiting factor) in the environment limiting carrying capacity. – Four main factors: Raw Material Availability Energy Availability Waste Accumulation and Disposal Organism Interactions K - Strategists • • • • • • • Large organisms Long-lived Produce few offspring Provide substantial parental care Populations typically stabilize at a carrying capacity. Usually occupy relatively stable environments. Reproductive strategy is to invest in a few, quality offspring. 8 K - Strategists • • Controlled by density-dependent limiting factors. – Factors that become more severe as the size of the population increases. Diseases Deer - Lions - Swans r - Strategists • • • • • • • r - Strategists • • Controlled by density-independent limiting factors. – Population size is irrelevant to the limiting factor. Weather Conditions Grasshoppers - Gypsy Moths - Mice Small organisms Short-lived Produce many offspring Little if any parental care Usually do not reach carrying capacity (boom-bust cycles). Exploit unstable environments. Reproductive strategy is to produce large numbers of offspring to overcome high mortality. Human Population Growth • Major reason for increasing human population growth rate is an increase in medical care, and a consequential decrease in death rates. 9 Historical Human Population Growth Doubling Time • Doubling Time of a Population (years) : • 70 / Population Growth Rate (%) ( 70 / 2.0% = 35 years ) Doubling Time for the Human Population Human Population Growth • Interactions Affecting Carrying Capacity – Available Raw Materials – Available Energy – Waste Disposal – Interaction With Other Organisms 10 Social Factors Influence Human Population • Humans are social animals who have freedom of choice. – People make decisions based on history, social situations, ethical and religious beliefs, and personal desires. Biggest obstacles to controlling human population are not biological, but are the province of philosophers, theologians, politicians, and sociologists. Ultimate Size Limitation • • If the world continues to grow at current rate, population will surpass 12 billion by 2060. Human population subject to same biological constraints as other species. – Human population will ultimately reach a carrying capacity and stabilize. Disagreement about exact size and primary limiting factors. Review • • • • • • Population Characteristics Age Structure Diagrams Exponential Growth Carrying Capacity – Environmental Resistance Reproductive Strategies – (r and K) Human Population Growth – Doubling Time 11