Chapter 4: Evolution

  • Genetic change in population of organisms across generations
  • Genetic change often leads to modifications in appearances, functioning, or behaviour of organism through time
  • Mechanism of evolution
  • Mutation
  • Migration
  • Random drift
  • Natural selection: traits that enhance survival and reproduction are passed on more frequently to future generations than those that do not

Charles Darwin

  • Proposed the concept of natural selection

Descent with Modification - Traits are passed on from parents to children

Genetic Variation

Mutations - Accidental change in DNA, can range from the addition, deletion, or substitution of single nucleotide

  • Mutation can be passed onto the next generation
  • Sexual reproduction can also generate variation

Directional selection

  • Selection that drives a feature in one direction rather than another

Stabilizing selection

  • Produces intermediate traits

Disruptive selection

  • Traits diverge in two or more directions

Divergent evolution

  • Closely related species that live in different environments and thus experience different selective pressures tend to diverge in their traits as the differing pressures drive the evolution of different adaptions

Convergent evolution

  • Unrelated species may have similar traits as a result of adapting to selective pressures from similar environments

Evolution generates biological diversity

Biodiversity - Total variety of all organisms in an area

Species - Particular type of organism

Population - Group of individuals of a particular species that live in the same area

Speciation - Process by which new species are generated

Allopatric Speciation - Emergence of new species as a result of physical separation of population

Population can be separated in many ways

Sympatric Speciation - Occur when species form from populations that become reproductively isolated, occupying a new ecological role or niche within the same geographic area


  • Disappearance of a species from earth

Mass extinction

  • Massive number of species being killed all at once
  • Have been five mass extinctions

Levels of ecological organization

Atoms and molecules represent the lowest levels in the hierarchy

Cells - Basic functional units of life

Population - Group of individuals of a particular species that live in the same area

Population Ecology - Quantitative dynamics of how individuals within a species interact with one another

Communities - Multiple interacting species that live in the same area

Community Ecology - Interactions among species

Ecosystems - Encompass communities and the abiotic materials with which their members interact

Ecosystem Ecology - Focus on patterns and nutrient flow by studying living and nonliving components of systems in conjunction


  • Specific environment in which an organism lives in
  • A specific habitat consists of both living and nonliving elements
  • Habitats vary with the body size and needs for species
  • Motile organisms: Organisms that are able to move our freely
  • Habitat selection: Motile organisms actively select habitats from among the range of options they encounter
  • Sessile animals: Those that are not freely mobile

Niche and specialization are key concepts in ecology

  • Niche: Reflects its use of resources and its functional role in a community
  • Specialists: Species with narrow breadth, and thus very specific requirements
  • Generalists: Those with broad tolerances, able to use a wide array of habitats or resources

Population show characteristics that help predict their dynamics

  • Population size: Expressed as the number of individual organisms present at a given time, may increase, decrease, undergo cyclical change or remain the same over time
  • Population density: Describes the number of individuals in a population per unit of area
  • Population distribution: Describes the spatial arrangement of organisms within an area. They are 3 types: random, uniform or clumped
  • Random distribution: Individuals are located haphazardly in space in no particular pattern
  • Uniform distribution: Individuals are evenly spaced
  • Clumped distribution: Pattern most common in nature, organisms arrange themselves according to the availability of the resources they need to survive
  • Sex Ratio - Proportion of males to females
  • Age structure: Describes the relative numbers of individuals of each age within a population
  • Birth and death rates: Measure the number of births and deaths per 1000 individuals for a given period of time

Survivorship curves

  • Type 1: Higher death rates at older ages
  • Type 2: Equal rates of deaths at all ages
  • Type 3: Higher death rates at younger ages

Population may grow, shrink, or remain stable

  • Natural rate of population growth: Crude birth rate-crude death rate
  • Population growth rate: (crude birth rate-crude death rate) + ( Immigration rate-emigration rate)

Limiting factors restrain population growth

Limiting Factors - Physical, chemical and biological characteristics of the environment that restrain population growth

Carrying Capacity - The maximum population size of a species that a given environment can sustain

Density-Dependent Factors - Density of a population can increase or decrease the impact of certain factors on that population.

  • Example \rightarrow Disease, predation

Density-Independent Factor - Influence is not affected by population density ex. Temperature extremes and catastrophic events

Reproductive strategies vary from species to species

K-Selected Species -Populations tend to stabilize over time at or near their carrying capacity, because their population stay close to carrying capacity , natural selection in these species favours individuals that invest in producing offspring of high quality that can be good competitors

R-Selected Species - Have high biotic potential and devote their energy and resources to producing as many offspring as possible in a relatively short time

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