© 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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


Extinction is a natural process, but …

• Humans profoundly affect rates of extinction

• Biodiversity loss affects people directly

- Food, fiber, medicine, ecosystem services


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


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


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


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


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


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


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


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


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


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


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



• 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



• 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


• 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



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


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


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


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


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.


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


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


Perfect logistic curves aren’t often found


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


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


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


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


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


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


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


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


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

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