BIOLOGY NOW: Adaptation and Speciation (Chapter 13)
Objectives
Describe the relationships among adaptive traits, biological fitness, and evolution by natural selection.
Explain the usefulness of the biological species concept, and discuss its shortcomings.
Define speciation and the role of genetic divergence among populations in creating new species.
Differentiate between allopatric and sympatric speciation.
Identify two kinds of reproductive isolating mechanisms, and explain their role in speciation.
Define and give an example of coevolution.
Penguins on Thin Ice: Case Study
Penguins on Thin Ice refers to a series of snapshots illustrating penguin biology and responses to environmental change.
Species covered include Emperor penguins and King penguins, with notes on Adelie penguins and broader penguin diversity.
Key themes: life history strategies, parental care, and how climate change affects survival, reproduction, and distribution.
Emperor Penguins: Breeding and Life Cycle
Emperor penguins breed on the Brunt Ice Shelf after months of foraging.
Reproductive sequence:
A female lays an egg and transfers it to her male partner.
The male incubates the egg in a brood pouch above his feet for 3 months while the female returns to the sea to forage.
After hatching, both parents share incubation and chick-rearing duties until the chick becomes an independent adult.
This parental system creates a robust, biparental care strategy essential for chick survival in extreme environments.
Climate Change Effects on Penguins
In years 2016, 2017, 2018, sea ice broke up months early, causing a "catastrophic breeding failure" because chicks could not be raised to fledging before conditions deteriorated.
In 2022, NASA reported sea ice around Antarctica at its lowest levels in 40 years of satellite monitoring.
Overall trend: climate change alters food availability and nesting habitat, affecting survival and reproduction.
Example: on a southern Indian Ocean island, a colony of roughly 5 imes 10^5 breeding pairs of King penguins lost about 90\% of its population since the 1980s.
Penguin Species and Adaptation to Diverse Habitats
In Antarctica, AdeĢlie penguin colonies decline where sea surface temperatures rise.
Penguins occupy a range of habitats from tropical-like to frigid ice shelves.
Understanding penguin responses to changing environments is critical for predicting species fates in a warming world.
Biological Fitness and Adaptive Traits
Biological fitness refers to an individualās probability of surviving and reproducing.
Survival and reproduction together constitute an organismās biological fitness.
Adaptive traits increase biological fitness by improving survival, reproduction, or both.
Types of adaptive traits:
Structural features (morphology)
Biochemical traits
Behaviors
These traits enable a species to survive and reproduce in its environment.
What is Adaptation?
Adaptation is the combination of adaptive traits and the evolutionary process (natural selection) that produces those traits.
Can be:
A trait advantageous to an individual or population
The evolutionary process that aligns a population with its environment
In penguins, adaptations enable life as aquatic birds and efficient foragers.
Expression: ext{Adaptation}
ightarrow ext{improved match between organism and environment}.
Penguin Adaptations to Life as Aquatic Birds (Examples)
Salt gland near the eye to remove excess salt from seawater.
Nictitating membrane: clear eyelid-like protection for underwater vision.
Feathers: fluffy down for insulation; stiff, overlapping tips for waterproofing.
Dense, overlapping feathers: ~70 per square inch, aiding insulation and waterproofing.
Muscles that store oxygen: enable prolonged dives of a few minutes or more.
Torpedo-shaped body: improved propulsion underwater.
Layer of blubber: additional insulation in cold water.
Webbed feet: steering while swimming.
Short, stiff, paddle-like wings: power for underwater propulsion.
Solid, heavy bones: aid diving and underwater stability.
Textually noted: adaptations summarized in the Penguin anatomy figure (reprinted courtesy of the New England Aquarium).
Study Questions: Penguin Adaptations
Q1: How might a salt gland serve as an adaptation for an aquatic bird (penguin)?
Answer: The salt gland removes excess salt from seawater, allowing penguins to drink seawater and maintain osmotic balance.
Q2: How does muscle-based oxygen storage increase penguin fitness?
Answer: Muscles that store large amounts of oxygen enable longer, deeper dives, increasing foraging efficiency and thus survival and reproductive success.
Q3: Why would heavy bones be an adaptation for penguins but not for most bird species?
Answer: Heavy bones aid diving by increasing buoyancy control and momentum during underwater locomotion, which is advantageous for penguins but not for most flying birds.
What Makes a Species?
The biological species concept (BSC): a species is a group of natural populations that can interbreed to produce fertile offspring and cannot breed with other such groups.
Limitations:
It does not always apply (e.g., asexual organisms, fossil species, and some populations with no opportunities to interbreed).
It is often impractical to test interbreeding capacity in the wild.
Therefore, many species are not defined solely by interbreeding ability.
Alternative Species Delimitation Criteria
When interbreeding information is insufficient, scientists use a combination of data:
Biogeographic information
DNA sequence similarity
Morphology (physical characteristics)
The ability to interbreed (reproductive isolation) as part of the assessment
These criteria help identify and distinguish species in practical settings.
Speciation and Genetic Divergence
Speciation: the process by which one species splits into two or more species.
Genetic divergence: the accumulation of differences in DNA sequences between populations over time, leading to genetic dissimilarity.
Consequence: populations become sufficiently different to be considered separate species.
Geographic Isolation and Allopatric Speciation
Geographic isolation is the physical separation of populations (e.g., rivers, canyons, mountains).
It reduces or eliminates gene flow between populations, allowing genetic divergence to accumulate.
Allopatric speciation: formation of new species from geographically isolated populations.
Penguin speciation can be driven by island formation, which is a classic form of allopatric speciation.
Gene Flow and Barriers
Gene flow: movement of alleles between populations through interbreeding.
When gene flow is blocked by a geographic barrier, populations diverge and may form new species.
Figure-based prompts in the text (e.g., Grand Canyon as a barrier) illustrate how barriers impede gene flow.
Allopatric Speciation: Key Concepts
A physical barrier is a prerequisite for allopatric speciation to occur (to restrict gene flow).
If the barrier is removed and populations interbreed, compatibility of offspring (per the BSC) determines whether they are still the same species.
If offspring remain fertile and interbreeding continues, speciation has not occurred; otherwise, divergence may have progressed to speciation.
Costs and Benefits of Sex
Costs of sex:
1) Time and energy to find or attract a mate.
2) Offspring receive only 50% of the parentās genetics vs. 100% in asexual reproduction.
3) Beneficial gene combinations can be shuffled during meiosis and recombination.Benefits of sex:
1) Increases genetic diversity, aiding adaptation to new environments.
2) Helps eliminate detrimental alleles and generate beneficial ones.
3) Rapid genetic change via recombination can enable resistance to infections.
Reproductive Barriers and Coevolution
Coevolution: when two species rely on each other for survival and co-evolve in tandem, with adaptations in one species shaping complementary adaptations in the other.
Example framing in the text: coevolution can be contrasted with other evolutionary patterns (e.g., convergent evolution) discussed in earlier chapters.
Coevolution: Conceptual Questions
Q1: How does coevolution (e.g., hummingbird beak and flower) differ from earlier chaptersā evolutionary discussions?
Q2: Is coevolution the same as convergent evolution? Why or why not?
Q3: Can one speciesā extreme adaptation to feed on another be considered coevolution? Why or why not?
GalƔpagos Finches: Evolution in Action
Charles Darwin observed GalƔpagos finches on different islands with related but distinct beak morphologies.
In The Voyage of the Beagle (1839), Darwin noted gradation and diversity suggesting descent with modification from a common ancestor.
Grants ( Rosemary and Peter) documented rapid speciation: a new finch species arising in just two generations.
Key points:
Speciation can be slow or rapid.
Beak size and shape adaptations relate to island environments and resource availability.
Allopatric vs Sympatric Speciation in GalƔpagos Finches
Q1: Is a given finch divergence example allopatric or sympatric? (Consider geography and gene flow.)
Q2: Are large cactus finches and medium ground finches, though from a shared ancestor, an example of allopatric or sympatric speciation?
Q3: How might beak-size differences reflect environmental adaptations?
Ecological Isolation and Sympatric Speciation
Ecological isolation: closely related species in the same area may be reproductively isolated due to habitat preferences (e.g., different microhabitats or resources) despite geographic proximity.
Sympatric speciation: the formation of new species within the same geographic area, common in plants and large aquatic systems.
Questions on Speciation Modes
Q1: What is the main difference between allopatric and sympatric speciation?
Q2: Name two events that must happen for both allopatric and sympatric speciation to occur.
Q3: Do you think all 500 species in Lake Victoria arose through sympatric speciation? Why or why not?
Reproductive Barriers: Prezygotic and Postzygotic
Reproductive barrier: an obstacle that prevents species from interbreeding.
Two categories:
Prezygotic barriers: prevent fusion of gametes before zygote formation; act before fertilization.
Postzygotic barriers: restrict development or viability of offspring after zygote formation; affect fitness of hybrids.
Prezygotic Barriers
Examples include:
Ecological isolation: different habitats or niches prevent mating.
Behavioral isolation: courtship or mating signals are not recognized by the other species.
Mechanical isolation: physical incompatibilities prevent mating.
Gametic isolation: gametes do not fuse or fail to survive in the other's tract.
Figure-based prompts discuss prezygotic barriers (e.g., a prezygotic courtship example in the booby).
Postzygotic Barriers
Examples include:
Zygote death: zygotes fail to develop.
Hybrid sterility: hybrids survive but are infertile (e.g., mules).
Hybrid performance: hybrids have reduced viability or reproductive success.
Case study prompts discuss a mule as an example of postzygotic isolation.
Summary Table: Reproductive Barriers (Table 13.1)
Prezygotic barriers: ecological isolation, behavioral isolation, mechanical isolation, gametic isolation.
Postzygotic barriers: zygote death, hybrid sterility, hybrid performance.
These barriers help maintain species integrity in the same geographic region.
Emperor Penguins and Climate Change: Population Trends
Despite catastrophic loss of Emperor penguin chicks in some colonies, others have grown (e.g., Dawson-Lambton has grown more than tenfold in the past 3 years).
Trathan and Fretwell note relocation of some emperor penguin populations to safer colonies.
Future climate change is predicted to cause species-wide extinction for some penguin species; among all living penguin species:
Five are in danger of extinction.
Five are vulnerable.
Three are threatened.
Action: protecting habitats now is crucial to preserve evolutionary potential for future speciation.
Diversity of Species on Earth
There are an estimated 8.74 imes 10^6 eukaryotic species on Earth, but scientists are familiar with only a fraction.
Estimates suggest that 86\% of land species and 91\% of aquatic species have not been discovered.
Among known species, insects are the most numerous (~1{,}000{,}000 species).
Known counts by major groups (approximate from the slide):
Insects: 1 imes 10^6
Mammals: 5{,}500
Fish: 31{,}200
Sponges: 6{,}000
Crustaceans: 47{,}000
Amphibians: 6{,}500
Mollusks: 85{,}000
Fungi: 99{,}000
Reptiles: 8{,}700
Jellyfish and polyps: 9{,}800
Arachnids: 102{,}200
Plants: 310{,}100
Birds: 10{,}000
Millipedes and centipedes: 16{,}100
Segmented worms: 16{,}800
Flatworms: 20{,}000
Roundworms: 25{,}000
Practice Questions and Answers (selected)
Question: In order to declare that two populations are two distinct species, which must be demonstrated?
Answer: a) Reproductive isolation (though genetic divergence, morphology, etc., contribute evidence under various criteria).
Question: Which of the following can create reproductive isolation?
Options discussed include: Prezygotic mating rituals, rapid geographic changes, gamete incompatibility, and all of the above.
Answer: d) All of the above.
Question: A question about ant- Acacia tree coevolution (ant and hollow thorns) illustrates coevolution.
Question: Horses and donkeys produce a mule; mules are infertile. What barrier is this?
Answer: c) Hybrid sterility.
Question: Which scenario is least likely to result in genetic divergence?
Answer: c) Gene flow.
Key Figures and Concepts Note
Penguin species tree based on mitochondrial genomes and morphology (Theresa Cole et al.)
Speciation concepts illustrated via Grand Canyon barrier (geographic isolation) and Lake Victoria cichlid diversification (sympatric context)
Coevolution examples (e.g., hummingbird flowers) highlighted to contrast with other evolutionary processes
The GalƔpagos finches as a classic demonstration of adaptive divergence and rapid speciation under different island conditions
Quick Recap: Core Concepts
Adaptation and natural selection drive changes in populations that increase biological fitness.
The Biological Species Concept focuses on reproductive isolation but is not universally applicable.
Speciation requires genetic divergence and often a barrier to gene flow (geographic or ecological).
Reproductive barriers (prezygotic and postzygotic) help maintain species boundaries.
Coevolution describes reciprocal evolutionary changes between interacting species.
Global biodiversity is vast but incompletely described; most undiscovered diversity lies in insects and tropical/remote habitats.
Climate change threatens many species by altering habitats, food webs, and reproductive success, potentially reducing future speciation opportunities.