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Transcript of © 2011 Pearson Education, Inc. This lecture will help you understand: Natural selection How...
© 2011 Pearson Education, Inc.
This lecture will help you understand:
• Natural selection
• How evolution influences biodiversity
• Reasons for species extinction
• Ecological organization
• Population characteristics
• Population ecology
• Conserving biodiversity
© 2011 Pearson Education, Inc.
Striking gold in Costa Rica
• In 1964, Golden toads were discovered in Monteverde, Costa Rica
• The mountainous cloud forest has a perfect climate for amphibians
• They became extinct within 25 years
- Climate change’s drying effect on the forest
- A lethal fungus
© 2011 Pearson Education, Inc.
Evolution generates biodiversity
• Species = a population or group of populations
- Whose members share characteristics
- 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 = means change over time
- Biological evolution: genetic change in populations over time
- Genetic changes lead to changes in appearance, functioning or behavior over generations
© 2011 Pearson Education, Inc.
Natural selection
• 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
© 2011 Pearson Education, Inc.
Evolution by natural selection
• It is one of the best-supported and most illuminating concepts in all science
– It is the standpoint of modern biology
• We must understand it to appreciate environmental science
– Knowing ecology and learning the history of life
• Evolutionary processes influence pesticide resistance, agriculture, medicine, health, etc.
• In 1858, both Darwin and Wallace proposed natural selection as the mechanism of evolution
© 2011 Pearson Education, Inc.
Natural selection shapes organisms
• Premises of natural selection:
- Organisms struggle to survive and reproduce
- Organisms produce more offspring than can survive
- Individuals of a species vary in their characteristics due to genes and the environment
- Some individuals are better suited to their environment and reproduce more effectively
• Natural selection acts on genetic variation
© 2011 Pearson Education, Inc.
Genetic variation
• 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
- Non-lethal mutations provide the genetic variation on which natural selection acts
• Sexual reproduction also leads to variation
© 2011 Pearson Education, Inc.
Natural selection acts on genetic variation
• 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
© 2011 Pearson Education, Inc.
Environmental conditions affect selection
• Environmental conditions determine the pressures natural selection exerts
- These pressures affect who survives and reproduces
- Traits evolve that allow success in that environment
• But traits that promote success at one time or place may not do so at another
• Natural selection weeds out unfit individuals
- It also elaborates and diversifies traits that may produce new species
© 2011 Pearson Education, Inc.
Selective pressures influence adaptation
• Related species in different environments
- Experience different pressures
- Evolve different traits
• Convergent evolution = unrelated species may evolve similar traits
- Because they live in similar environments
© 2011 Pearson Education, Inc.
Evidence of natural selection is everywhere• It is evident in every adaptation of every organism
• Artificial Selection = the process of selection conducted under human direction
- Producing the great variety of dog breeds and food crops
© 2011 Pearson Education, Inc.
Evolution generates biological diversity
• Biological diversity (biodiversity) = the variety of life across all levels of biological organization
- Species
- Genes
- Populations
- Communities
• Scientists have described 1.8 million species
- Up to 100 million species may exist
- Tropical rainforests are rich in biodiversity
© 2011 Pearson Education, Inc.
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
- The main mode of speciation
- Populations can be separated by glaciers, rivers, mountains
- Each population gets its own set of mutations
© 2011 Pearson Education, Inc.
Another type of speciation
• Sympatric speciation = species form from populations that become reproductively isolated within the same area
- Feed in different areas
- Mate in different seasons
- Hybridization between two species
- Mutations
© 2011 Pearson Education, Inc.
Speciation results in diverse life forms
• How do major groups diverge over time?
• Phylogenetic trees (cladograms) = show relationships among species, groups, genes, etc.
- Scientists can trace how certain traits evolved
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
Extinction
• Extinction = the disappearance of a species from Earth
- Species last 1-10 million years
• Biological diversity is now being lost at an astounding rate
- This loss of species is irreversible
Number of species = speciation - extinction
© 2011 Pearson Education, Inc.
Extinction is a natural process, but …
• Humans profoundly affect rates of extinction
• Biodiversity loss affects people directly
- Food, fiber, medicine, ecosystem services
© 2011 Pearson Education, Inc.
Some species are more vulnerable to extinction
• Extinction occurs when the environment changes rapidly
- Natural selection can not keep up
• Many factors cause extinction:
- Severe weather, climate change, changing sea levels
- New species, small populations
- Specialized species
• Endemic species = a species only exists in a certain, specialized area
- Very susceptible to extinction
- These species usually have small populations
© 2011 Pearson Education, Inc.
Many U.S. amphibian species are vulnerable
• Many U.S. amphibians have very small ranges- They are vulnerable to extinction- The Yosemite toad, Houston toad, Florida bog frog
• 40 salamander species are restricted to areas the size of a typical county
Some U.S. salamander species live on top of single mountains
© 2011 Pearson Education, Inc.
Earth has had several mass extinctions
• Background extinction rate = extinction usually occurs one species at a time
• Mass extinction events = killed off massive numbers of species at once
- Occurred five times in Earth’s history - 50-95% of all species went extinct at one time• Cretaceous-Tertiary (K-T) event: 65 million years ago- Dinosaurs went extinct• End-Permian event: 250 million years ago- 75-95% of all species went extinct
© 2011 Pearson Education, Inc.
The sixth mass extinction is upon us
• Humans are causing the sixth mass extinction event- Resource depletion, population growth, development- Destruction of natural habitats- Hunting and harvesting of species- Introduction of non-native species
• It is 100-1,000 times higher than the background rate and rising
• Amphibians are disappearing the fastest- 170 species have already vanished
• It will take millions of years for life to recover
© 2011 Pearson Education, Inc.
Ecology is studied at several levels
• Ecology and evolution are tightly intertwined
• Biosphere = the total living things on Earth
- And the areas they inhabit
• Community = interacting species living in the same area
• Ecosystem = communities and the nonliving material and forces they interact with
© 2011 Pearson Education, Inc.
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 systems to reveal patterns
- Nutrient and energy flows
© 2011 Pearson Education, Inc.
Each organism has habitat needs
• Habitat = the environment where an organism lives
- It includes living and nonliving elements
• Habitat use = each organism thrives in certain habitats, but not in others
- Results in nonrandom 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
© 2011 Pearson Education, Inc.
A specialized frog
• Epiphytes grow on trees for support
- Obtaining water from the air
- They collect pools of rainwater and pockets of leaf litter
- Frogs lay their eggs in these rainwater pools
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
Organismal ecology: niche
• Niche = an organism’s use of resources - Along with its functional role in a community- Habitat use, food selection, role in energy and
nutrient flow, interactions with other individuals• 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
© 2011 Pearson Education, Inc.
Population characteristics
• All populations show characteristics that help scientists predict their future dynamics
• Population size = the number of individual organisms present at a given time
- Numbers can increase, decrease, cycle or remain the same
© 2011 Pearson Education, Inc.
Population characteristics
• Population density = the number of individuals in a population per unit area
• Large organisms usually have low densities
- They need many resources and a large area to survive
• High densities make it easier to find mates
- But increase competition and vulnerability to predation
- Increased transmission of diseases
• Low densities make it harder to find mates
- But individuals enjoy plentiful resources and space
© 2011 Pearson Education, Inc.
Population characteristics
• Population distribution (dispersion) = spatial arrangement of organisms
• Random = haphazardly located individuals, with no pattern
• Uniform = individuals are evenly spaced
- Territoriality, competition
• Clumped = arranged according to availability of resources
- Most common in nature
© 2011 Pearson Education, Inc.
• Sex ratio = proportion of males to females
- In monogamous species, a 1:1 sex ratio maximizes population growth
• Age distribution (structure) = the relative numbers of organisms of each age in a population
- Age structure diagrams (pyramids) = show the age structure of populations
• In species that continue growing as they age
- Older individuals reproduce more (i.e. a tree)
- Experience makes older individuals better breeders
Population characteristics
© 2011 Pearson Education, Inc.
Birth and death rates
• Type I: more deaths at older ages
• Type II: equal number of deaths at all ages
• Type III: more deaths at young ages
Survivorship curves = the likelihood of death varies with age
© 2011 Pearson Education, Inc.
Four factors of population change
• 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
© 2011 Pearson Education, Inc.
Population growth rate
• 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 changes in a population’s size/1000/year
• Growth rate as a percent =
- Population growth rate * 100%
- Populations of different sizes can be compared
© 2011 Pearson Education, Inc.
Exponential population growth
• Exponential growth- A population increases by a
fixed percent- Graphed as a J-shaped
curve• Exponential growth cannot
be sustained indefinitely• It occurs in nature with a:
- Small population- Low competition- Ideal conditions
© 2011 Pearson Education, Inc.
Limiting factors restrain population growth
• Limiting factors = physical, chemical and biological attributes of the environment
- They restrain population growth
• Environmental resistance = all limiting factors taken together
- Stabilizes the population size
- Space, food, water, mates, shelter, suitable breeding sites, temperature, disease, predators
- Aquatic systems: salinity, sunlight, temperature, etc.
© 2011 Pearson Education, Inc.
Carrying capacity
• Carrying capacity = the maximum population size of a species that its environment can sustain
• Limiting factors slow and stop exponential growth
- An S-shaped logistic growth curve
Many factors contribute to environmental resistance and influence a population’s growth rate and carrying capacity
© 2011 Pearson Education, Inc.
Population density impacts limiting factors
• Density-dependent factors = limiting factors whose influence is affected by population density
- Increased density increases the risk of predation and competition for mates
- Results in the logistic growth curve
- Larger populations have stronger environmental resistance
• Density-independent factors = limiting factors whose influence is not affected by population density
- Events such as floods, fires, and landslides
© 2011 Pearson Education, Inc.
Carrying capacities can change
• Environments are complex and ever-changing
- Changing carrying capacities
• Humans lower environmental resistance for themselves
- Increasing our carrying capacity
- Technologies overcome limiting factors
• By increasing carrying capacity for humans
- We have reduced the carrying capacity for countless other organisms
- Calling into question our own long-term survival
© 2011 Pearson Education, Inc.
Reproductive strategies vary among species
• 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
© 2011 Pearson Education, Inc.
Population changes affect communities
• As the population of one species declines, other species may appear
• Human development displaces other species and threatens biodiversity
• As Monteverde dried out, species disappeared
- Golden toads, harlequin frogs and more had been pushed from their cloud-forest habitat into extinction
- 20 of 50 frog species, 2 lizard species went extinct
• Species from lower, drier habitats appeared
- 15 species tolerant of drier conditions had moved in
© 2011 Pearson Education, Inc.
Conserving biodiversity
• Human development, resource extraction and population pressure are speeding changes
- In populations and communities
• Impacts threatening biodiversity have complex social, economic and political roots
- We must understand these factors to solve problems
© 2011 Pearson Education, Inc.
Costa Rica and its environment
• Past economic and social forces still threaten Costa Rica’s species and ecological communities
- Lush forests are seen as obstacles to agriculture and for timber
- The country’s population grew to 3.34 million
- Pasture land increased from 12% to 33%
- Forests decreased from 80% to 25%
- Species were declining and becoming endangered
© 2011 Pearson Education, Inc.
Costa Rica’s species were in danger
• Few people saw the need to conserve biological resources
- Until it became clear they were being rapidly lost
© 2011 Pearson Education, Inc.
Costa Rica is protecting its environment
• The privately managed 10,500 ha (26,000 acre) Monteverde Cloud Forest Reserve was founded in 1972
- To protect the forest, its plants and animals
• Costa Rica created the country’s first national parks and protected areas in 1970
- Centered on areas of spectacular scenery
- Protecting valuable tropical dry forests, turtle nesting beaches, coral reef systems
• Government support remains strong
- Over 25% of its land is under some type of protection
© 2011 Pearson Education, Inc.
Costa Rico’s protection is paying off• Ecotourism: tourists visit protected areas
- Providing thousands of jobs and billions of dollars to local economies
• But parks are still under-protected and underfunded
- Restoration is a step beyond preservation
© 2011 Pearson Education, Inc.
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