Evolution, Biodiversity and Population Ecology
Chapter 3
After this lecture, you will be able to:
Explain the process of natural selection and cite evidence for this process
Describe the ways in which evolution influences biodiversity
Discuss reasons for species extinction and mass extinction events
List the levels of ecological organization
Outline the characteristics of populations that help predict population
growth
Assess logistic growth, carrying capacity, limiting factors, and other
fundamental concepts in population ecology
Central Case Study: Striking Gold in a Costa Rican Cloud Forest
Golden toads were discovered in Monteverde, Costa Rica in 1964
200 males in a 16-ft. area
The mountainous cloud forest was ideal for amphibians
The toads vanished from Earth within 25 years of their discovery
Climate change, a fungus and deforestation are responsible
Evolution is the wellspring of biodiversity
Evolution simply means change over time
Biological evolution is genetic change in populations over time
These genetic changes lead to changes in appearance, functioning, or
behavior over generations
Natural selection
Evolution may be random or can be driven by natural selection
Natural selection is the process whereby inherited characteristics that
enhance survival and reproduction are passed on more frequently to
future generations than those that do not
Evolution by natural selection
Evidence of natural selection is everywhere
Artificial selection is the process of selection conducted under human
direction
Artificial selection
Natural selection shapes organisms
Natural selection explains nature’s patterns
Premises of natural selection:
Organisms struggle to survive and reproduce
They produce more offspring than can survive
Individuals of a species vary in their characteristics because of genes and
the environment
Some individuals are better suited to their environment and reproduce
more effectively
Natural selection acts on genetic variation
Populations (NOT individuals) adapt to the environment
A parent that produces more offspring passes on more genes to future
generations
Over time, characteristics (traits) that lead to better reproductive success
become more prevalent
Adaptive trait (adaptation): a trait that promotes reproductive success
Sexual reproduction also leads to genetic variation
Producing new combinations of genes
Environmental conditions determine the pressures of natural selection
Organisms need time to adapt to changing conditions
Resolving religion and evolution
Literal biblical account is not compatible with evolution
What we know is not compatible with biblical account
E.g., species are unchanging
Many religions accept evolution
Speciation
Given enough geographical isolation or selective pressure (from natural
selection), members of a population become so different from their
ancestors that they may be considered an entirely new species
Speciation produces new types of organisms
The process of generating new species from a single species
Allopatric speciation occurs when species form as a result of physical
separation of populations
Main mode of speciation
Populations can be separated by glaciers, rivers, mountains
Each population, with its own set of mutations, diverges
Speciation results in diverse life forms
How do major groups diverge over time?
Phylogenetic trees are branching diagrams show relationships among
species, groups, genes, etc.
Represent life’s history
Evolution generates biological diversity
Biological diversity (biodiversity) is 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
Convergent evolution
Similar conditions lead to similar adaptations
Species look alike even if they are not related
Divergent evolution
Species evolve into different types to take advantage of opportunities in
environment and reduce competition
Fossils show life’s history
A fossil is an imprint in stone of a dead organism
Phylogenetic trees and the fossil record show:
Life has existed on Earth for 3.5 billion years
Life evolved complex structures from simple ones
Life evolved large sizes from small ones
But natural selection can also favor simplicity and small size
Extinction
Most species that once lived are now gone
Extinction is 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
Some species are vulnerable to extinction
Extinction occurs when the environment changes rapidly
Natural selection can’t keep up, so species can’t adapt
Many factors cause extinction:
Climate change, changing sea levels, severe weather
Arrival of new species, small populations
Specialized species
Endemic species exist only in a certain, specialized area
Very susceptible to extinction
These species usually have small populations
Earth has had several mass extinctions
Background extinction rate: extinction usually occurs slowly, 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
A gigantic asteroid caused dinosaur extinction
End-Permian event: 250 million years ago
75–95% of species went extinct from unknown causes
The sixth mass extinction is upon us
Humans are causing the sixth mass extinction event
Population growth, development, resource depletion
Destruction of natural habitats
Hunting and harvesting of species
Introduction of non-native species
This loss affects humans
We need organisms for food, fiber, medicine, services
Amphibians are disappearing faster than any other group
170 species have already vanished
We study ecology at several levels
E
cology studies interactions among organisms and their environment
Ecology and evolution are tightly intertwined
Species to ecosystems
A species is a “type” of organism
A population is all members of a species living in a given area at the
same time
A community consists of all of the populations of organisms living and
interacting in a particular area
An ecosystem is the biological community and its physical environment
Species
Populations
Community
Ecosystem
Levels of ecological studies
Organismal ecology examines relationships between individuals and their
environment
Population ecology investigates population changes
The distribution and abundance of individuals
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
A habitat is the environment where an organism lives
It includes living and nonliving elements
Species use different criteria to select habitat
Soil, topography, vegetation, other species
Species have different habitat needs
Depending on body size, season, etc.
Species survival depends on having suitable habitats
The niche
A niche is an organism’s use of resources
Along with its functional role in a community
Habitat use, food selection, role in energy and matter flow, interactions
with other individuals
Specialists have narrow niches and specific needs
Extremely good at what they do
But vulnerable when conditions change
Generalists are species with broad niches
They use a wide array of habitats and resources
Survive in many different places
Population
Populations can be described by vital statistics or demographics
Size
Age structure
Sex ratio
Density
Distribution
Population size
Number of individuals that contribute to a population’s gene pool
Immigration & Births add individuals
Emigration & Deaths subtract individuals
Population density
Population density is 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
Also increase transmission of diseases
Low densities make it harder to find mates
But individuals enjoy more space and resources
Population distribution
Population distribution (dispersion) is the spatial arrangement of
organisms
Random - haphazardly located individuals, with no pattern
Resources are widespread
Uniform - evenly spaced individuals
Territoriality, competition
Clumped - most common in nature
Arranged according to resources
Sex ratios and age structure
Sex ratio is the proportion of males to females
In monogamous species, a 1:1 sex ratio maximizes population growth
Age structure (distribution) is the relative numbers of organisms of each
age in a population
Helps predict population growth or decline
In species that continue growing as they age
Older individuals reproduce more (e.g., a tree)
Experience makes older individuals better breeders
Population age structure
Divide population into age categories
Four factors of population growth or decline
Natality - the births within the population
Mortality - deaths within the population
Immigration - arrival of individuals from outside the population
Births and immigration add individuals to a population
Emigration - departure of individuals from the population
Deaths and emigration remove individuals
Population growth rate
Growth rate is the rate of change in a population’s size per unit time
Equals (birth rate + immigration rate) – (death rate + emigration rate)
Tells us the net changes in a population’s size per 1000 individuals per
year
Growth rate is expressed as a percent:
Population growth rate * 100%
Populations of different sizes can be compared
Exponential population growth
In exponential growth, a population increases by a fixed percent
Graphed as a J-shaped curve
It occurs in nature with:
Small populations
Low competition
Ideal conditions
Limiting factors restrain population growth
Exponential growth rarely lasts
Limiting factors: physical, chemical, and biological attributes of the
environment limiting population growth
Environmental resistance is all limiting factors together
Stabilizes the population size at its carrying capacity
Terrestrial animals -space, food, water, mates, shelter, breeding sites,
temperature, disease, predators
Plants- sunlight, moisture, soil chemistry
Aquatic systems - salinity, sunlight, temperature, etc.
Carrying capacity
Carrying capacity is the maximum population size the environment can
sustain
Determined by limiting factors
Limiting factors slow and stop exponential growth
Forms an S-shaped logistic growth curve
Population density affects limiting factors
Density-dependent factors are limiting factors whose influence is affected
by population density
Increased density increases the risk of predation, competition for mates,
and disease
Results in the logistic growth curve
Environmental resistance has a stronger effect on larger populations
Density-independent factors are limiting factors whose influence is not
affected by population density
Temperature extremes, floods, fires, and landslides
Perfect logistic curves aren’t often found
Carrying capacities can change
Environments are complex and ever-changing
The carrying capacity can change
Humans lower environmental resistance for ourselves
Increasing our carrying capacity
Technologies have overcome limiting factors
We have appropriated immense amounts of resources
But by increasing the 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 is an organism’s capacity to produce offspring
K-selected species are species with long gestation periods and few
offspring (i.e., a low biotic potential)
Offspring have a high likelihood of survival
The population stabilizes at or near carrying capacity
Good competitors
r-selected species are species that reproduce quickly
Have a high biotic potential
Little parental care, populations fluctuate greatly
Population changes affect communities
Scientists have noticed troubling changes in the environment
As Monteverde dried out, species have disappeared
Golden toads, harlequin frogs, and more had been pushed from their
cloud-forest habitat into extinction
Species from lower, drier habitats moved into the cloud forest
Population sizes of cloud-forest bird species declined
Changing climate and disease are causing population fluctuations and
changing the makeup of communities
Conserving biodiversity
Human development, resource use, and population pressure are changing
populations and communities
Factors threatening biodiversity have complex social, economic, and
political roots
We must understand these factors to solve problems
Millions of people are working to protect biodiversity and to safeguard
ecological and evolutionary processes
Costa Rica’s protection is paying off
Costa Rica was losing forests at the world’s fastest rate
Now, 25% of its area is under protection
Ecotourism is when tourists visit protected areas
Ecotourism provides thousands of jobs and billions of dollars to Costa
Rica’s economy