Chapter 3 Topics:  Natural selection  How evolution influences biodiversity  Reasons for species extinction  Ecological organization  Population characteristics  Population ecology  Conserving biodiversity

Some definitions  Species = a group of organisms that share characteristics and can breed with one another producing fertile offspring  Population = a group of individuals of a species that live in the same area  Evolution = means change over time  Biological evolution: genetic changes lead to changes in appearance, functioning or behavior over generations

“Directed evolution”  Evolution may be random or directed by natural selection  Natural Selection = traits that enhance survival and reproduction are passed on more frequently to future generations than those that do not

The process of natural selection  Individuals vary in their characteristics due to variations in genes inherited from parents  Some individuals will be better suited to their environment due to their characteristics (traits)  Those better suited to their environment will be more likely to live and to successfully reproduce  Future generations will have more of the genes that provide reproductive success, changing the species over time  Species evolve, individual organisms live or die

Variation and adaptation  Genetic variation arises from  Mutations = accidental changes in DNA that may be passed on to the next generation (if non-lethal)  Sexual reproduction = the normal mixing of parental genes in offspring  Adaptation = the process through which a species becomes better suited to its environment  Adaptive trait = a gene-based trait that promotes reproductive success

Types of selection  Natural selection changes characteristics through:  Directional selection = drives a feature in one direction  Stabilizing selection = favors intermediate traits, preserving the status quo  Disruptive selection = traits diverge in two or more directions

Selective pressures drive adaptation  Divergent evolution = related species experience different pressures and evolve different traits  Convergent evolution = unrelated species evolve similar traits because they live in similar environments

Selection depends on place and time  Environmental conditions determine the pressures natural selection exerts  But traits that promote success at one time or place may not do so at another  Rapid environmental change makes adaptation difficult (e.g., current global climate change)  Rapid reproduction makes adaptation easier  Antibiotic resistance  Pesticide resistance

Speciation  Allopatric speciation: species form due to physical separation of populations  The main mode of speciation  Populations can be separated by glaciers, rivers, mountains  Each population gets its own set of mutations  Sympatric speciation: species form from reproductively isolated populations within the same area  Feed in different areas  Mate in different seasons  Hybridization between two species

The fossil record  Fossil: an imprint in stone of a dead organism  Fossil record: the cumulative body of fossils worldwide  The fossil record shows:  Life has existed on Earth for at least 3.5 billion years  Earlier types of organisms evolved into later ones  The number of species has increased over time  Most species have gone extinct  There have been several mass extinctions in the past

Speciation through time  Phylogenetic trees (cladograms) = show relationships among related organisms allowing scientists to trace how certain traits evolved

Extinction  Extinction = the disappearance of a species from Earth  Extinction is natural  When environmental change outpaces natural selection  Species last 1-10 m.y.  Specialized organisms  Small populations

Extinction in the fossil record  Earth has had five mass extinctions – periods of time when very large numbers of species became extinct  At the end of the Cretaceous Period (Mesozoic Era): dinosaurs became extinct (K-T boundary event)  At the end of the Permian Period (Paleozoic Era): percent of all species became extinct

Extinction in the modern world  Human actions appear to be causing a sixth mass extinction event  Modern extinction rate is 100-1,000 times higher than the background rate  Destruction of natural habitats  Hunting and harvesting of species  Introduction of non-native species  Loss of biodiversity reduces available resources  Impacts to ecosystems impair the delivery of services

Ecological organization  Biosphere = the total living things on Earth and the areas they inhabit  Ecosystem = communities and the nonliving material they interact with  Community = interacting species living in the same area

Levels of ecological studies  Population ecology = investigates the dynamics of population change  The factors affecting the distribution and abundance of members of a population  Why some populations increase and others decrease  Community ecology = focuses on patterns of species diversity and interactions  Ecosystem ecology = studies living and nonliving components of ecosystems to reveal patterns

Each organism has habitat needs  Habitat = the environment where an organism lives  Includes living and nonliving elements  Habitat use = each organism thrives in certain habitats, but not in others  Results in non-random patterns of use  Habitat selection = the process by which organisms actively select habitats in which to live  Availability and quality of habitat are crucial to an organism’s well-being  Human developments conflict with this process

Habitats vary  Habitats vary with the body size and needs of species  A soil mite vs. an elephant  Species have different habitat needs at different times  Migratory birds use different habitats during migration, summer and winter  Species use different criteria to select habitat  Soil, topography, vegetation, other species  Water temperature, salinity, prey  Species survival depends on having suitable habitat

How organisms fit in their habitat  Niche = an organism’s use of resources along with its functional role in a community  Habitat use, interactions with other individuals  Food selection, role in energy and nutrient flow  Specialists = have narrow niches and specific needs  Extremely good at what they do  But vulnerable when conditions change  Generalists = species with broad niches  They use a wide array of habitats and resources  They can live in many different places

Population Ecology  All populations show seven characteristics that help scientists predict their future dynamics  Size  Density  Distribution  Sex ratio  Age structure  Birth and death rates (survivorship)  Reproductive strategy

Population size  Population size = the number of individual organisms present at a given time  Numbers can increase, decrease, cycle or remain the same

Population density  Population density = individuals per unit area  Large organisms = low densities (need more resources, larger area)  High densities  Easier to find mates  More competition  More predation  Increased transmission of diseases

Population distribution  Population distribution = spatial arrangement of organisms  Random = individuals are haphazardly located (no pattern)  Uniform = individuals are evenly spaced (territoriality)  Clumped = individuals arranged according to availability of resources

Population sex and age structure  Sex ratio = proportion of males to females  Age distribution (structure) = the relative numbers of organisms of each age in a population  In species that continue growing as they age  Older individuals reproduce more (i.e. a tree)  Experience makes older individuals better breeders

Population births/deaths  Survivorship curves = the likelihood of death varies with age  Type I: more deaths at older ages  Type II: equal number of deaths at all ages  Type III: more deaths at young ages

Reproductive strategies  Biotic potential = an organism’s capacity to produce offspring  K-selected species = species with long gestation periods and few offspring  Have a low biotic potential  Stabilize at or near carrying capacity  Good competitors  r-selected species = species which reproduce quickly  Have a high biotic potential  Little parental care, populations fluctuate greatly

Changes in population size  Four factors determine the size of a population  Natality = births within the population  Mortality = deaths within the population  Immigration = individuals coming into the area  Emigration = individuals moving away from the area  Crude birth (death) rates: number of births (deaths) per 1000 individuals per year

Population growth rates  Natural rate of population growth =  (crude birth rate) – (crude death rate)  Population change due to internal factors  Population growth rate =  (crude birth rate + immigration rate) – (crude death rate + emigration rate)  Net change in a population’s size/1000/year  Percent growth rate =  Population growth rate * 100%  Populations of different sizes can be compared

Exponential growth pattern  Rate of population growth is proportional to population size  Graph: J-curve  Cannot be sustained indefinitely  Happens when/for  Small population  Low competition  Ideal conditions

Limiting factors – carrying capacity  Limiting factors = physical, chemical and biological attributes of the environment that restrain population growth  Space, food, water, mates, shelter, suitable breeding sites, temperature, disease, predators  Aquatic systems: salinity, sunlight, temperature, etc.  Carrying capacity = the maximum population size of a species that its environment can sustain

Density affects limiting factors  Density-dependent factors = limiting factors whose influence is affected by population density  Increased density increases the risk of predation, the rate of disease transmission, and competition for mates  Density-independent factors = limiting factors whose influence is not affected by population density  Events such as floods, fires, and landslides

Logistic growth pattern  Population size stabilizes at its carrying capacity due to the collective effect of limiting factors (environmental resistance)  Graph: S-curve

Population growth in nature

Carrying capacities can change  Environments are complex and ever-changing  And carrying capacities change as a result  Humans have used technology to reduce the environmental resistance we face  Increasing our carrying capacity  Looking back, as we have increased our carrying capacity  We have reduced the carrying capacity for countless other organisms