Evolution, Biodiversity, and Population Ecology Chapter 3

Evolution: The Source of Earth’s Biodiversity Natural Selection Shapes Organisms and Diversity: Darwin and Wallace proposed natural selection as mechanism of evolution Premises of natural selection: Organisms struggle to survive/reproduce and produce more offspring than survive Individuals of species vary in characteristics due to genes and environment Some individuals are better suited to their environment, reproduce more effectively Organisms with better adapted traits produce more offspring

Evolution Generates Biodiversity: Biological diversity (biodiversity) - variety of life across all levels of biological organization Species Genes Populations Communities

Speciation Produces New Organisms: Allopatric speciation - species formation due to physical separation of populations Main method of speciation Caused when population are separated by glaciers, rivers, mountains, etc. Each population gets own set of mutations Natural selection can speed process

Speciation and Extinction Determine Earth’s Biodiversity: Extinction - disappearance of a species from Earth Species last 1–10 million years Human activity greatly affects rates of extinction Biodiversity loss affects people directly Food, fiber, medicine, ecosystem services

Some species Particularly Vulnerable to Extinction: Extinction can occur when environment changes faster than natural selection keeps up Many factors cause extinction: Severe weather, climate change, changing sea levels Arrival of new species Being a small population or specialized species

Endemic species - species only existing in a certain, specialized area Very susceptible to extinction Usually have small populations Island species are often endemic and thus at risk

Earth’s Episodes of Mass Extinctions: Mass extinction events - episodes that kill massive numbers of species at one time Five in Earth’s history Cretaceous–Tertiary (K–T) event: 65 million years ago Dinosaurs extinct End-Permian event: 250 million years ago 75–95% of species extinct

A Sixth Mass Extinction is Beginning: Humans are causing sixth mass extinction event Resource depletion, population growth, development Destruction of natural habitats Hunting and harvesting of species Introduction of non-native species It will take millions of years for life to recover

Population Ecology Population Characteristics Predict Dynamics: Population size - number of individuals present at a given time Size will fluctuate depending on a number of factors Population density - number of individuals in a population per unit area Population distribution (dispersion) - spatial arrangement of organisms Random, uniform, or clumped Age distribution (structure) = the relative numbers of organisms of each age in a population

Birth and Death Rates: Survivorship curves – graphs showing the likelihood of death varying with age Type I: higher death rate at older ages Larger animals (e.g., humans) Type II: same death rate at all ages Medium-sized animals (e.g., birds) Type III: higher death rate at young ages Small animals, plants

Populations Grow, Shrink, or Remain Stable: Natural rate of population increase is (Crude birth rate)  (crude death rate) Population change due to internal factors Population growth rate is (Crude birth rate  immigration rate)  (Crude death rate  emigration rate) Net changes in a population’s size/1000/year Growth rate as a percent = Population growth rate  100% Populations of different sizes can be compared

Limiting Factors: Limiting factors - physical, chemical, and biological attributes of environment restrain population growth Space, food, water, mates, shelter, suitable breeding sites, temperature, disease, predators

Factors Influencing Density: Density-dependent factors - limiting factors whose influence is affected by population density Increased density increases the risk of predation, disease, and competition Results in the logistic growth curve Density-independent factors - limiting factors whose influence is not affected by population density Events such as floods, fires, and landslides

Reproductive Strategies: Biotic potential - organism’s capacity to produce offspring K-selected species - species with long gestation periods, few offspring, and strong parental care low biotic potential Stabilize at or near carrying capacity; good competitors r-selected species - species that reproduce quickly and offer little or no care for offspring high biotic potential Populations fluctuate greatly

Climate Change: Climate change alters how we protect species and habitats Land is typically protected to conserve the species that live there As the climate changes, the protected land may no longer support the same species