Chapter 17-18: The Theory of Evolution

The formation of Earth

Earth is estimated to be 4.5 billion years old (700,000 x recorded history)

Fossils

Preserved remains of, or tracks of ancient organisms

• Can be perfectly preserved

• Reveal information about structures of ancient organisms, evolution, and common ancestors

Radiometric dating

A method of dating a geological or archaeological specimen by determining the relative properties of particular radioactive isotopes present in a sample

• Method to determine age based on radioisotopes

• How energy is released from atoms

• Length of time energy is released - half life

• Can test fossils, rocks, soil, etc.

Earth’s early stage

• Atmosphere had little to no oxygen

• Made of carbon dioxide, nitrogen, water vapor, and hydrogen sulfide

• Most living things today would not have survived

• Earth’s atmosphere changed over time

  • Concentration of oxygen increased

  • Many anaerobic organisms died

  • Some evolved to use oxygen in metabolic pathways

The first cells

• We do not have concrete evidence of the first cells

  • Data suggests molecules needed to build amino acids and other molecular compounds were present on Earth

  • Inferences based on today’s prokaryotic cells and Earth’s early forms

• Fossil evidence of prokaryotes found to be 3.5 billion years old

• The first cells:

  • Prokaryotic

  • Aerobic (lined on O₂)

  • Hetertophic

  • Had RNA

  • Membrane of lipids

• Fossil evidence of eukaryotes found to be 2.1 billion years old

  • Eukaryotes cells are complex

    • Many organelles

    • Complex metabolic pathways

  • Very little evidence on how they came to be

  • Endosymbiotic theory

Endosymbiotic theory

Theory suggesting complex organelles originated from prokaryotic cells that were engulfed to form a symbiotic relationship with ancestral eukaryotic cell

Charles Darwin

• Traveled the world

• Galapagos Islands

• Noticed three distinctive patterns of biological diversity

  • Species vary globally

  • Species vary locally

  • Species vary over time

Species vary globally

Different but ecologically similar species inhabited separate but similar habitats around the globe

• Ex: Ostrich in Africa (grasslands) vs. Emu in Australia (grasslands)

• Some species only found in retain parts of the world despite similar habitats in other places

  • Ex: Kangaroos in Australia’s grasslands

Species vary locally

Different yet related species occupy different habitats within a local area

• Ex: Tortoises from different islands of Galapagos

Species vary over time

Some fossils of extinct animals were similar to living species

• Ex: Armadillos and Glyptodont fossils

Evolution

Heritable change in the characteristics within a population from one generation to the next

• Expressions of genes passed from one generation to the next

  • Changes in amino acid sequences due to changes in DNA (mutations)

• Changes in characteristics explain changes in organisms

Beliefs during Darwin’s time

• Organisms were permanent and perfect

• Earth was thousands of years old

• Religion held answers about formation of Earth

• George Cuvier

• Charles Lyell

• Jean Lamarck

George Cuvier (1769-1832)

• Studied fossils and anatomical structures of living animals

• Hypothesized that habitats determined anatomical form

• Did not believe in change, all animals have anatomical forms for their needs

• Catastrophes cause new species

• Work was still important for Darwin

Charles Lyell (1797-1875)

• Studied geology - relatively new science

• Hypothesized that Earth was extremely old and processes that changed Earth in the past still operate in the present

Jean Lamarck (1744-1829)

• Believed populations change over time

• Explanation:

  • Organisms change in their lifetime by selectively using or not using various parts of their bodies

  • “Use and disuse”

  • Pass traits on

The Origin of Species

• Darwin gathered all of his ideas

• Published The Origin of Species

• Main idea:

  • Descent with modification - every organisms comes about by reproduction and can change over time

    • Traits from generation to generation

• Proposed a theory explaining the mechanism of evolution

  • Natural selection

Natural selection

Process by which individuals with certain inherited traits tend to survive and reproduce at higher rates because of those traits (works on individuals)

• How evolution happens

• 4 parts to natural selection

• Nature changes species by “selecting” traits

• “Survival of the fittest”

  • Fitness

• Darwin believed natural selection and adaptations can produce new species (over time)

• Implied that all organisms are related

• Organisms diversified

4 parts to natural selection

1. Overproduction - a species produces more offspring that can survive to maturity

   • Population limited by environment

   • Food, space, predators, etc.

2. Genetic variation - every individual has different traits leading to variation among populations (Natural selection only works with variation)

   • Variation is inherited

   • New traits occasionally appear, may provide advantage

3. Struggle to survive - individuals must compete to survive and reproduce

   • Food, water, habitat, reproduction

   • Variations in individuals increase or decrease chances

   • Adaptation - traits that make an individual successful in its environment

4. Differential reproduction - organisms with the best adaptations are more likely to survive and reproduct

   • Adaptations become more frequent

   • Populations slowly change

Fitness

Measure of how well an organism can survive and reproduce

• An individual’s heredity contribution to the next generation

• Higher fitness = greater chance of individual and its descendants

• Natural selection depends on individuals’ ability to reproduce and leave descendants (take trait into consideration)

Effects of Natural selection

• Evolution is an ongoing process

• Operates on variation of traits within a population

  • Body size, color, etc.

• Influences distribution of traits in a population

• Types of effects

  • Stabilizing selection

  • Disruptive selection

  • Directional selection

Stabilizing selection

A type of natural selection in which genetic diversity decreases as the population stabilizes on a particular trait value

• Individuals with the average form of a trait have the highest fitness

• Population includes more individuals with the average form of a trait

Disruptive selection

Individuals with extreme variations of a trait have greater fitness than individuals with the average form of the trait

• Population includes more individuals with extreme versions of the trait

Directional selection

individuals that display a more extreme version of a traits have greater fitness than individuals with an average form of the trait

• Population includes more individuals with the most extreme version of a trait

Rate of evolution

• Evolution does not occur at exactly the same rate for all species

  • Can occur at different rates and at different times

  • Evidence to show some organisms have experienced periods of little or no visible change

  • Ex: crocodiles and horse shoe crabs

• Two different patterns for the rate of evolution:

  • Gradualism

  • Punctuated equilibrium

• Time frames

  • Adaptations - change in traits in a population over time

  • Acclimation

• As environmental conditions change, natural selection enables some species to adapt

  • Slow change: organisms adapt

  • Rapid change: acclimate or perish

    • Natural disasters, disease, habitual destruction, people

• Species become extinct when they fail to adapt to competition and changing environments

  • Background extinction

  • Mass extinction

Gradualism

Variation is gradual in nature and happens steadily over time

• Idea proposed by Darwin

• Organisms that have not appeared to change are in a state of “equilibrium”

Punctuated equilibrium

Equilibrium is interrupted by brief periods of rapid change

• Existing species change rapidly

Acclimation

Short term process, physiological changes take place in a lifetime

• Adjusting to temperature changes

Background extinction

Normal, ongoing extinction

Mass extinction

Large number of species become extinct in a relatively short amount of time

• 6th mass extinction

Evidence of Evolution

• Fossils

• Biogeography

• Anatomy

• Embryology

• Modern studies of DNA

Biogeography

Study of where organisms live now and where they and their ancestors lived in the past

• Patterns in distribution of living and fossil species

• Helps explain how modern organisms evolved

• Two biogeographical patterns to Darwin’s theory

  • Closely related but different

    • Species on Galapages evolved from mainland species

    • Natural selection provided variation among populations

  • Distantly related but similar

    • Similar habitats around the world are home to plants and animals that are distantly related

    • Similar selections processes (pressures) caused distantly related species to develop similar adaptations

Anatomy

• Homologous structures - structures that are shared between different but related species

  • Usually look the same or very similar

  • Inherited from a common ancestor

  • Help determine common ancestor

  • Ex: forearm bone structure of mammals

• Analogous structures - structure that are similar in function but among species that do not share a common ancestor

  • Ex: wings of a butterfly, wings of a bird, wings of a bat

• Vestigial structures: structure that seem to serve no function but resemble functional structures in related organisms

  • Ex: appendix, tailbone

Embryology

The study of embryos and their development

• Early developmental stages of related individuals look very similar

Modern studies of DNA

• Traces similarities and differences of DNA codes

• Universal genetic code and homologous molecules provide evidence of common descent

• The greater the similarities - the more closely related

Convergent evolution

The process by which different species evolve similar traits

Divergent evolution

The process by which descendants of a single ancestor diversify into species that each fit into different parts of the environment

Artificial selection

Selectively breeding individuals with desired traits

• Human driven process

• Hope desired traits are passed to next generation

Coevolution

The process of two species evolving in response to long-term interactions with each other

• Hawaiian honeycreeper and lobelia flower

Species

Group of closely related organisms capable of producing viable and fertile offspring

• Physical features

• Internal structures

Speciation

The formation of new species as a result of evolution

• Explanation for how species can form

• Requires isolation

• 3 types of isolation:

  • Geographic

  • Reproductive

  • Temporal

Geographic isolation

Physical separation of members of a population

• Physical habitat divided

• Forcing original population to separate

• Generation of subpopulations

• Natural selection between members of subpopulation

• Difference in genetics create new species

• Ex:

  • Death Valley, CA - used to have a lake

  • Dried up - isolation between each side of the lake

  • Each contains different species of pupfish

Reproductive isolation

Members of a population can no longer interbreed eventually creating two populations

• Disruptive selection - two extremes of a trait emerge

• Average trait phases out

• A shift in both directions away from middle

• Ex:

  • Wood frog and Leopard frog

  • Differences in mating call

  • Differences in recognition

Temporal isolation

Members of a population can’t interbreed because mating occurs at different times

• Difference in seasons, time of day, etc.

• Ex:

  • Cicadas that emerge every 13 or 17 years