Chapter 3- Evolution, Biodiversity, and Population Ecology
Chapter 3- Evolution, Biodiversity, and Population Ecology
This lecture will help you understand:
Natural selection
How evolution influences biodiversity
Reasons for species extinction
Ecological organization
Population characteristics
Population ecology
Conserving biodiversity
Central Case Study: Saving Hawaii’s Native Forest Birds
Evolution in the Hawaiian Islands has generated hundreds of species, many unique to the islands
The island chain was once home to 140 species of native birds
In recent times, half of the native bird species have gone extinct
a)Introduced species (like pigs, cattle, rats, and cats) destroyed habitat and killed eggs and young
b)Avian malaria killed native birds at lower elevations
Evolution: The Source of Earth’s Biodiversity
Species = a population or group of populations whose members share characteristics
a)They can breed with one another and produce fertile offspring
Population = a group of individuals of a species that live in the same area
Evolution = change over time
a)Biological evolution = change in populations of organisms over generations
b)Genetic changes lead to changes in appearance, functioning, or behavior
Evolution: The Source of Earth’s Biodiversity
Genetic changes in evolution may be random
a)But may be directed by natural selection
Natural selection = process in which traits that enhance survival and reproduction are passed on more frequently to future generations than those that do not
a)Genetic makeup of future populations is changed
Evolution: The Source of Earth’s Biodiversity
Evolution is one of the best-supported and most illuminating concepts in all science
a)It is the foundation of modern biology
We must understand it to appreciate environmental science
a)Understanding how species change over time and adapt to their surroundings is crucial for comprehending ecology and the history of life
b)Evolutionary processes influence pesticide resistance, agriculture, medicine, health, etc.
Natural selection shapes organisms and diversity
In 1858, both Darwin and Wallace proposed natural selection as the mechanism of evolution
Premises of natural selection:
a)Organisms struggle to survive and reproduce
b)Organisms produce more offspring than can survive
c)Individuals of a species vary in their characteristics due to genes and the environment
d)Some individuals are better suited to their environment and reproduce more effectively
Organisms with better adapted traits will produce more offspring
Natural selection shapes organisms and diversity
Adaptation = the process where, over time, characteristics (traits) that lead to better reproductive success become more prevalent in the population
Adaptive trait (adaptation) = a trait that promotes reproductive success
Mutations = accidental changes in DNA that may be passed on to the next generation
a)Non-lethal mutations provide the genetic variation on which natural selection acts
Sexual reproduction also leads to variation
Directional selection = drives a feature in one direction
Selective pressures from the environment influence adaptation
Related species in different environments experience different pressures and evolve different traits
Convergent evolution = unrelated species may acquire similar traits because they live in similar environments
Evidence of natural selection is all around us
It is evident in every adaptation of every organism
Artificial selection = the process of selection conducted under human direction
a)Produced the great variety of dog breeds and food crops
Evolution generates biodiversity
Biological diversity (biodiversity) = the variety of life across all levels of biological organization
a)Species
b)Genes
c)Populations
d)Communities
Scientists have described 1.8 million species
a)Estimates of the total number of species that exist range from 3 million to 100 million
b)Biodiversity exists nearly everywhere
Speciation produces new types of organisms
The process of generating new species from a single species
Allopatric speciation = species formation due to physical separation of populations
a)The main mode of speciation
b)Populations can be separated by glaciers, rivers, mountains
c)Each population gets its own set of mutations
d)Natural selection can speed the process
We can infer the history of life’s diversification by comparing organisms
How did the major groups of organisms come to be?
Phylogenetic trees = diagrams that show relationships among species, groups, genes, etc.
a)Scientists can trace how certain traits evolved
b)Some traits evolved and were passed on
c)Other traits evolved more than once (e.g., the ability to fly)
We can infer the history of life’s diversification by comparing organisms
Knowing how organisms are related to one another helps scientists organize and name them
Categories reflect evolutionary relationships
a)Scientists use physical and genetic characteristics to organize
Each species gets a two-part Latinized scientific name
The fossil record teaches us about life’s long history
Fossil = an imprint in stone of a dead organism
Fossil record = the cumulative body of fossils worldwide
The fossil record shows:
a)Life has existed on Earth for at least 3.5 billion years
b)Earlier types of organisms evolved into later ones
c)The number of species has increased over time
d)Most species have gone extinct
e)There have been several mass extinctions in the past
Speciation and extinction together determine Earth’s biodiversity
Extinction = the disappearance of a species from Earth
a)Species last 1–10 million years
Extinction has historically been a natural occurrence
a)The loss of a species is irreversible
Speciation and extinction together determine Earth’s biodiversity
Human activity profoundly affects rates of extinction
Biodiversity loss affects people directly
a)Food, fiber, medicine, ecosystem services
Some species are especially vulnerable to extinction
Extinction can occur when the environment changes rapidly and natural selection can not keep up
Many factors cause extinction:
a)Severe weather, climate change, changing sea levels
b)Arrival of new species
c)Being a small population or specialized species
Some species are especially vulnerable to extinction
Endemic species = a species that only exists in a certain, specialized area
a)Very susceptible to extinction
b)Usually have small populations
c)Island species are often endemic and thus at risk
Some species are especially vulnerable to extinction
Many U.S. amphibians have very small ranges
a)They are vulnerable to extinction
b)For example, the Yosemite toad, Houston toad, Florida bog frog
Forty salamander species are restricted to areas the size of a typical county
Earth has seen several episodes of mass extinctions
Background extinction rate = a constant, slow rate of extinction that occurs as a part of evolution
Mass extinction events = episodes that killed off massive numbers of species at once
a)Occurred five times in Earth’s history
b)50–95% of all species go extinct at one time
Cretaceous–Tertiary (K–T) event: 65 million years ago
a)Dinosaurs went extinct
End-Permian event: 250 million years ago
a)75–95% of all species went extinct
The sixth mass extinction is upon us
Humans are causing the sixth mass extinction event
a)Resource depletion, population growth, development
b)Destruction of natural habitats
c)Hunting and harvesting of species
d)Introduction of non-native species
Today’s extinction rate is 100–1000 times higher than the background rate and rising
It will take millions of years for life to recover
We study ecology at several levels
Ecology and evolution are tightly intertwined
Biosphere = the total of living things on Earth and the areas they inhabit
Community = interacting species that live in the same area
Ecosystem = communities and the nonliving material and forces they interact with
We study ecology at several levels
Population ecology = investigates the dynamics of population change
a)The factors affecting the distribution and abundance of members of a population
b)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 systems to reveal patterns
a)Nutrient and energy flows
Each organism has habitat needs
Habitat = the environment where an organism lives
a)It includes living and nonliving elements
Habitat use = each organism thrives in certain habitats, but not in others
a)Results in nonrandom patterns of use
Habitat selection = the process by which organisms actively select habitats in which to live
a)Availability and quality of habitat are crucial to an organism’s well-being
b)Human developments conflict with this process
Each organism has habitat needs
Habitats vary with the body size and needs of species
a)A soil mite vs. an elephant
Species have different habitat needs at different times
a)Migratory birds use different habitats during migration, summer, and winter
Species use different criteria to select habitat
a)Soil, topography, vegetation, other species
b)Water temperature, salinity, prey
Species survival depends on having suitable habitat
Niche and specialization are key concepts in ecology
Niche = summary of everything an organism does
a)Use of resources
b)Functional role in a community: habitat use, food selection, role in energy and nutrient flow, interactions with other individuals
Niche and specialization are key concepts in ecology
Specialists = species that have narrow niches and specific needs
a)Extremely good at what they do
b)But vulnerable when conditions change
Generalists = species with broad niches
a)Use a wide array of habitats and resources
b)Can live in many different places
Population Ecology
Population = individuals of a particular species that inhabit an area
Species may have different arrangements of their populations
a)Some populations (like the nēnē) exist as isolated populations
b)Others (like humans) exist as
large continuous populations
Populations show characteristics that help predict their dynamics
Certain characteristics of a population help scientists predict what will happen to them in the future
a)Helps in managing threatened species
Population size = number of individuals present at a given time
a)Populations generally grow when resources are abundant and predators are few
b)Decline in response to loss of resources, other species, disasters
Population density
Population density = the number of individuals in a population per unit area
High densities have advantages and disadvantages
a)Easier to find mates
b)Increased competition and vulnerability to predation
c)Increased transmission of diseases
d)Sometimes causes organisms to leave an area if too dense
Low densities provide access to plentiful resources and space but make it harder to find mates
Population distribution
Population distribution (dispersion) = spatial arrangement of organisms
Random = haphazardly located individuals, with no pattern
Uniform = individuals are evenly spaced
a)Territoriality, competition
Clumped = organisms found close to other members of population
a)Most common in nature
b)Clustering around resources
c)Mutual defense
Sex ratio and age structure
Sex ratio = proportion of males to females
a)In monogamous species, a 1:1 sex ratio maximizes population growth
b)Most species are not monogamous, so ratios vary
Age distribution (structure) = the relative numbers of organisms of each age in a population
a)Age structure diagrams (pyramids) show the age structure of populations
In species that continue growing as they age, older individuals reproduce more (e.g., a tree)
a)Experience can help older individuals breed more
Birth and death rates
Survivorship curves = graphs that show that the likelihood of death varies with age
a)Type I: higher death rate at older ages
a)Larger animals (e.g., humans)
b)Type II: same death rate at all ages
a)Medium-sized animals (e.g., birds)
c)Type III: higher death rate at young ages
a)Small animals, plants
Populations may grow, shrink, or remain stable
Natality= births within the population
Mortality= deaths within the population
Immigration= arrival of individuals from outside the population
Emigration= departure of individuals from the population
Births and immigration add individuals; deaths and emigration remove individuals
Crude birth (death) rates = number of births (deaths) per 1000 individuals per year
Populations may grow, shrink, or remain stable
Natural rate of population increase=
(Crude birth rate) (crude death rate)
a)Population change due to internal factors
Population growth rate=
(Crude birth rate immigration rate) (Crude death rate emigration rate)
a)Net changes in a population’s size/1000/year
Growth rate as a percent = Population growth
rate 100%
a)Populations of different sizes can be compared
Unregulated populations increase by exponential growth
Exponential growth = whena population increases by a fixed percent
a)Graphed as a J-shaped curve
Exponential growth cannot be sustained indefinitely
It occurs in nature with:
a)Small population
b)Low competition
c)Ideal conditions
Occurs often with introduced species
Limiting factors restrain population growth
Limiting factors= physical, chemical, and biological attributes of the environment that restrain population growth
a)Space, food, water, mates, shelter, suitable breeding sites, temperature, disease, predators
Carrying capacity= the maximum population size of a species that its environment can sustain
Limiting factors slow and stop exponential growth
a)An S-shaped logisticgrowth curve
The influence of some factors depends on population density
Density-dependent factors= limiting factors whose influence is affected by population density
a)Increased density increases the risk of predation, disease, and competition
b)Results in the logistic growth curve
c)Larger populations have stronger effects of limiting factors
Density-independent factors= limiting factors whose influence is not affected by population density
a)Events such as floods, fires, and landslides
The logistic growth curve is a simplified model
Few populations in nature match the curve exactly
Carrying capacities can change
Environments are complex and ever-changing
a)Limiting factors can change, altering the carrying capacity
Humans lower environmental resistance for themselves
a)Increases our carrying capacity
b)Technologies overcome limiting factors
In increasing carrying capacity for humans, we now use immense portions of the planet’s resources
a)We have reduced the carrying capacity for countless other organisms
Reproductive strategies vary among species
Biotic potential= an organism’s capacity to produce offspring
K-selected species= species with long gestation periods, few offspring, and strong parental care
a)Have a low biotic potential
b)Stabilize at or near carrying capacity; good competitors
r-selected species= species that reproduce quickly and offer little or no care for offspring
a)Have a high biotic potential
b)Populations fluctuate greatly
Conserving Biodiversity
Humans are developing land, extracting resources, and growing as a population
a)This increases the rate of environmental change for other species
Science can help us understand how we are changing the environment
Impacts threatening biodiversity have complex social, economic, and political roots
a)We must understand these factors to solve problems
Introduced species pose challenges for native populations and communities
Some introduced species thrive in their new environments, eliminating native species
Native island species are particularly vulnerable
a)Evolved in isolation with limited need for defenses
Biologists and land managers often must eradicate introduced species to protect native habitats
a)In Hawaii, pigs are being hunted and pig-free areas are being fenced off
Innovative solutions are working
Scientists, land managers, and private citizens are protecting the native species and habitats of Hawaii
a)Invasive species are being removed
b)Native species (like the nēnē) are being protected, and new populations are being started
c)Ranch land is being restored to forest
d)Coral reef communities are part of the largest federally protected marine reserve in the world
This restored and protected land has resulted in ecotourism = the phenomenon of people visiting the islands to experience the natural areas
Climate change now poses an extra challenge
Climate change is altering how we protect species and habitats
Land is typically protected to conserve the species that live there
a)As the climate changes, the protected land may no longer support the same species
Conclusion
The fundamentals of evolution and population ecology are integral to environmental science
Natural selection, speciation, and extinction help determine Earth’s biodiversity
Understanding how ecological processes function
at the population level is crucial to protecting biodiversity
1