Notes
Chapter 4 Evolution and Biodiversity
Core Case Study: Earth: The Just-Right, Adaptable Planet
· The planet appears to be uniquely suited for life as we know it.
o Distance from the sun
o Rotation allows even distribution of heating
o Gravitational mass enough to keep Fe and Ni core molten and atmospheric gases from escaping
o In 3.7 billion years, surface temperature has averaged in a narrow range of 10-20 oC.
o An ozone layer whose origin dates back to first photosynthesizing bacteria some 2 bya.
· Understanding how organisms adapt to changing environmental conditions is important for:
o Understanding how nature works
o Understanding how our activities affect life on earth
o Understanding how we can prevent unnecessary loss of the planet’s biodiversity.
I. Origins of Life
A. Development of Life on the Primitive Earth: The Big Picture
1. Today’s biodiversity can be explained by the scientific theory of evolution.
a. Mechanism for evolution, called natural selection, proposed independently by Charles Darwin (1809-1882) and Alfred Russell Wallace (1823-1913).
i. Individuals in population struggle to obtain food and other resources to survive.
ii. Some individuals in the population have some edge over others at successfully obtain resources, mates, and leaving behind offspring,
iii. Offspring inherit this edge/traits that make them more successful in the environment.
iv. The inherited trait becomes more prevalent in the population.
b. There are still many unanswered questions regarding biological evolution however the evidence shows that:
i. It is a fact that life has changed on this planet over time.
ii. Natural selection is the best-supported explanation for how life changes.
iii. Healthy scientific debate continues regarding details of certain mechanisms influencing change, and on the rate at which evolution occurs
2. Life on Earth developed in two phases
a. Chemical evolution – took first 1 billion years; the formation of organic molecules, biopolymers and systems of chemical reactions needed to form first cells (Fig 4.2).
i. Evidence comes from chemical and radionuclide analyses of rocks and fossils.
b. Biological evolution by natural selection – for the last 3.7 billion years; from single-celled prokaryote to multicellular protests, Fungi, plants and animals (Fig 4.3).
i. Fossil record among many other lines of evidence.
B. How Do We Know Which Organisms Lived in the Past (and Their Relationships to Extant Organisms)?
1. Fossil record
a. Fossils – mineralized or petrified remains of organisms (e.g., skeletons, bones, teeth, shells, leaves, and seeds) or impressions of such items found in rocks.
2. Radioisotope dating
3. Principle of superposition
4. Cores from ice
5. Genetic code and DNA comparisons
6. Homologous structures
II. Evolution, Natural Selection, and Adaptation
A. Genetic Mutations: Changes in a Population’s Gene Pool
1. Populations – not individuals – evolve by becoming genetically different.
2. The key is to develop genetic variability.
a. Occurs by mutations – random changes in nucleotide sequence or number of chromosomes.
i. Can occur because of mutanogens.
ii. Or true random mistakes during replication
b. Only mutations present in germ cells/gametes can be passed on to offspring.
B. Natural Section and Adaptation: Leaving More Offspring with Beneficial Genetic Traits
1. Occurs when some individuals have inherited traits that increase their changes of survival and to reproduce in the environment.
a. Favorable mutations are maintained or increased in the population as generations pass.
2. Three conditions for biological evolution by natural selection:
a. Genetic variability
b. Trait must be heritable
c. Trait leads to differential reproduction and survivorship
3. Natural selection acts on individuals, but evolution occurs in populations.
4. Adaptation (or adaptive trait) – a heritable trait that enables and organism to survive and reproduce under the prevailing environmental conditions.
5. When faced with significant changes in the environment, there are three possibilities:
a. Adapt
b. Migrate
c. Become extinct
C. Co-evolution: A Biological Arms Race
1. Coevolution – when two populations of two different species interact over a long time, changes in the gene pool of one species can lead to changes in the gene pool of another.
a. Bat-moth example
D. Hybridization and Gene Swapping: Other Ways to Exchange Genes
1. Hybridization
2. Horizontal gene transfer in microbes.
** The above two processes can lead to rapid changes in populations compared to thousands
to millions of years for sexually reproducing species through natural selection.
E. Limits on Adaptation through Natural Selection
1. Humans: Can we adapt to increased UV radiation, air pollution, and other chemical pollutants?
a. No, because of two limits.
i. Variable traits must be present in the population
ii. Rate of reproduction/generation time too slow
2. Even if a favorable trait were present, most of the population would have to die or become sterile.
a. Many individuals (w/ unfavorable traits) prematurely over thousands of generations for a new trait to predominate.
3. The limits are less restrictive in genetically diverse and quickly reproducing species:
a. Insects, weeds, grasses, bacteria, etc.
4. The rules above do not apply to the development of new species through hybridization and horizontal gene flow.
F. Common Myths about Evolution through Natural Selection
1. Three common misconceptions:
a. (1.) Survival of the fittest means survival of the strongest.
i. Fitness is a measure of reproductive success, i.e., ability to survive and pass genes on to the next generation.
b. (2.) Organisms develop traits because they want them or need them.
i. Novel traits in the population are a result of random mutations that happened to be selected for by the prevailing environmental conditions.
c. (3.) Evolution by natural selection involves a grand plan for nature in which species become more perfectly adapted.
III. Geologic Processes, Climate Change, Catastrophes, and Evolution
A. Geologic Processes and Evolution by Natural Selection
1. Flows/currents of molten rock in earth’s interior breaks crust into gigantic solid plates – tectonic plates.
a. Drift very slowly over thousands to millions of years (Fig. 4.5).
i. Super continent Pangaea 225 mya
ii. Laurasia and Gondwanaland 135 mya
iii. Present continents began split about 65 mya
b. Continental drift has two important effects evolution and location of life on earth – a study called biogeography.
i. Location of continents and ocean basins influence earth’s climate.
ii. Allows species to move, adapt forming new species by natural selection.
c. Volcanic eruptions
d. Earthquakes
** Geographic isolation caused by plate tectonics phenomena – vicariance event – leads to population/ genetic isolation. (What influence does this have on the population?)
B. Climate Change and Natural Selection
1. What is climate?
2. Geologic data suggests that earth’s climate has changed drastically. Sometimes cooling. Sometimes warming.
a. Cycles lead to advancement and retreat of ice sheets (Fig. 4.6)
3. Effects biological evolution by changing the locations of different types of ecosystems.
a. Some species became extinct because climate change is too rapid.
C. Catastrophes and Natural Selection
1. Asteroid collisions have caused widespread destruction of ecosystems and wiped out long numbers of species.
a. Catastrophe results in long period of continental stress.
b. Some caused mass extinctions
IV. Ecological Niches and Adaptation
A. Ecological Niches: How Species Live and Coexist
1. Ecological niche, or simply niche – a species’ role in a community or ecosystem and includes everything that affects its survival and reproduction.
2. Fundamental niche – full potential range of physical, chemical, and biological conditions and resources it could potentially use if it could avoid interspecific competition.
a. Niches may overlap.
3. Realized niche – to avoid competition, a species occupies only part of its fundamental niche.
B. Generalist and Specialist Species: Broad and Narrow Niches
1. Generalist species (Fig. 4.7, p. 91)
a. Have a broad niche
i. Live in a variety of places
ii. Varied diet
iii. Broad tolerance to environmental conditions
b. Examples: flies, cockroaches (Spotlight, p. 90), many rodents, white-tailed deer, coyotes, humans, and weeds.
2. Specialist species
a. Have a narrow niche
i. Live in one type of habitat
ii. Restricted diet
iii. or, tolerate only a narrow range of climate and other environmental conditions.
b. These species are more prone to extinction.
c. Examples: tiger salamander, giant panda
d. Waterbirds show specialization in feeding habits (Fig. 4.8, p. 91)
i. This reduces competition
3. What’s better, generalist or specialist strategy?
a. When environmental conditions are constant, it is better to be specialized.
i. Example: Rainforest communities
b. In rapidly changing environmental conditions, it is better to have a generalized niche.
4. Natural selection can lead to an increase in specialization when several species must compete intensely for scarce resources.
a. Evolutionary divergence in honeycreepers (Fig. 1.9, p. 91)
V. Speciation, Extinction, and Biodiversity
A. How Do New Species Evolve?
1. Speciation – when two new species arise from one.
a. Occurs when some members of a species of a population can no longer breed with other members.
2. Most common mechanism – Geographic isolation (or Allopatric speciation) – a physical isolation between populations of the same species for long periods (Fig. 4.10, p. 92).
a. A cohort of the population may migrate to new area
b. Streams, lakes, mountain ranges, roads
c. Volcanic eruption, earthquakes
d. A few individuals being carried away by streams, wind, or ocean currents
3. Reproductive isolation – mutation and change by natural selection operates in gene pools of geographically separated populations, leading to differences that make individuals the two populations incapable of reproducing offspring.
a. Prezygotic: spatial, temporal, behavioral, structural
b. Postzygotic: gamete mortality
4. Make take hundred to millions of years depending on the generation time of a species.
B. Extinction: Lights Out
1. Extinction – when species ceases to exist; dies out.
a. Process affects the number and types of species.
2. Endemic species are found in only one area.
a. Especially vulnerable to extinction (Fig. 4.11, p. 92).
C. Background Extinction, Mass Extinction, and Mass Depletion
1. Extinction is the ultimate fate of all species.
2. An estimated 99.9% of all species that ever existed have gone extinct.
3. As environmental conditions change a certain number of species disappear – called background extinction.
a. Based on geological record, this rate is 1-5 species for each million species on earth.
4. Mass extinction – significant rise in extinction rates above the background rate.
a. Extinctions are global and comprise 25-70% of species disappearing over a period lasting 5 m.y.
b. During past 500 m.y. there have been five mass extinctions (20-60 million years apart; Fig. 4.12, p. 93).
c. Mass extinctions are opportunities for other species to fill unoccupied niches, speciation.
i. Biodiversity must be there however
5. Speciation rates on average had kept ahead of extinction rates during the last 250 m.y.; but in last 1.8 m.y. has leveled off (Fig. 4.13)
D. Effects of Human Activities on the Earth’s Biodiversity: Are We a Wise Species?
1. Speciation minus extinction equals biodiversity, defined here as the planet’s genetic raw material for future evolution in response to changing environmental conditions.
2. Evidence indicates humans have become a major selective force leading to premature extinction of a growing number of species.
a. Extinction rates are estimated to be 100-1000 times greater than background extinction rate.
b. At current rate:
i. By 2030, one-fifth of all species will be lost
ii. One-half by the end of the century
3. On such a short time scale, such major losses cannot be recouped by the formation of new species.
4. We are also destroying or depleting hotspots (rainforests, coral reefs, and wetlands) for future speciation.
VI. Genetic Engineering and the Future of Evolution
A. Artificial Selection and Genetic Engineering
1. Artificial selection – when humans select one or more desirable genetic traits in the population of a plant or animal species and then use selective breeding to produce populations containing many individuals with the desired traits.
a. What artificial selection given us?
i. Food crops with higher yields
ii. Cows that give more milk
iii. Trees that grow faster
iv. Varieties of cats and dogs
2. Genetic engineering, or gene splicing – is the alteration of an organism’s genetic material through adding, deleting or changing segments of its DNA to produce desirable traits or eliminate negative ones.
a. Can allow genes from unrelated species to be transferred between them.
i. Example: gene from a fish can be inserted to a tomato or strawberry.
b. Genetically modified organisms (GMOs), or transgenic organisms (Fig. 4.14, p. 95), have recombinant DNA – DNA that has been altered to contain genes from organisms of different species.
c. Gene splicing takes about half as much time to develop a new crop or animal variety than traditional selective breeding.
d. Has been used to develop:
i. Modified crop plants
ii. Drugs
iii. Pest-resistant plants
iv. Animals that grow rapidly (Fig. 4.15)
v. Bacteria to extract minerals such as copper from ore
vi. Bacteria to clean up oil spills and other toxic pollutants
vii. Chickens that lay low-cholesterol eggs
viii. Tomatoes with genes that help prevent some types of cancer
ix. Bananas and potatoes that contain oral vaccines to treat certain viral diseases
B. Synthetic Biology and Cloning
1. Goal of synthetic biology is to go beyond conventional genetic engineering, separate cells into their fundamental components, and use them to rebuild new organisms.
a. As of now synthetic biologist have created:
i. A polio virus by stitching together various individual genes.
ii. E. coli to produce the anti-malaria drug artenisin
iii. World’s smallest computer using DNA to carry out mathematical functions.
2. Clone – a genetically identical version of an individual in a population.
a. Have cloned sheep, horses, cows, and dogs
b. Researchers hope to use cloning from cells found in bones, hair, teeth, or frozen specimens of extinct animals to bring vanished species back from extinction.