Mammalian Evolution: Extinction and Adaptation

• Mass Extinction and Adaptive Radiation of Mammals
• The Cretaceous extinction occurred 65 million years ago, marking the boundary between the Mesozoic and Cenozoic eras
• Triggered by a large asteroid impact, there was massive forest fires, and huge amounts of particles were thrown into the air, blocking the Sun for months

• 50% of existing marine species and terrestrial plants/animals, including dinosaurs, were exterminated

• Climate cooled and sea levels changed
• Ended the age of the dinosaurs, but spurred adaptive radiation of mammals previously smaller than mice
• Species diversity recovers slowly after mass extinctions, taking ~10 million years to reach previous levels

Speciation

• Galapagos tortoise speciation illustrates how new species can form
• Tortoises on different Galapagos islands are all different
• Charles Darwin noted differences in 1835, with the vice-governor able to identify a tortoise's origin island by sight

New Species Formation in Galapagos Tortoises:

• Tortoises from South America reached the Galapagos Islands
• The ability to survive environmental conditions led to some individuals surviving and reproducing
• Natural selection caused descendants to change, leading to new species over time
• Tortoises moved to different islands, and adaptive radiation further changed populations

Human Activities and Population Decline

• Hunting and habitat removal can cause populations to decline
• A bottleneck effect can occur, followed by genetic drift
• Large-scale loss of genetic diversity can lead to inbreeding, which may cause further population decline
• Populations with low genetic diversity are more susceptible to environmental changes
• E.g., The American chestnut tree has little genetic variation after the chestnut blight fungus destroyed populations
• Conservation and wildlife management programs consider gene pools to maintain large, genetically diverse wild populations

Speciation, Extinction, and Human Activities

• Environmental factors heavily influence both speciation and extinction
• Environmental influences create selective pressure
• Influences can be both positive and negative
• In some cases, new species arise but in other cases species go extinct

Biological Diversity Since the Cambrian Period

• Overall biological diversity has generally increased
• Several sharp declines in species numbers occurred because of mass extinction events

Mass Extinction Events

• Five major mass extinctions identified
• The most severe occurred at the end of the Permian period ~250 million years ago

• 50% of all families or ~96% of all species went extinct

Consequences of Human Activities on Speciation

• Human activities can impact genetic diversity by
• converting wilderness to croplands
• creating recreation areas
• building roads, urban subdivisions, and hydroelectric dams
• Man-made barriers can prevent gene flow, leading to adaptive radiation if environments differ
• Severely fragmented populations may die with insufficient genetic diversity to adapt

Giant Pandas

• Live in China's bamboo forests at 1500-3100m elevation
• Habitat destruction from agriculture, harvesting herbs/bamboo, road construction, and illegal hunting threaten their survival
• Breeding panda pairs need ~30 km²
• Many panda populations are isolated in bamboo belts as narrow as 1.2 km
• Conservation efforts are helping pandas avoid extinction
• Surveys in 2006 showed >2000 pandas in the wild (75% higher than previous estimates)
• Conservation areas are increasing for panda habitats
• With low reproductive rates plus total dependence on bamboo, giant pandas have hovered near extinction
• Populations are recovering where habitat is being preserved/protected

Speed of Evolutionary Change: Gradualism and Punctuated Equilibrium

• Darwin's Gradualism Model: Views evolutionary change as slow and steady before and after divergence
• Big changes are the result of the accumulation of changes
• The fossil record rarely shows gradual transitions; instead it shows species appearing & disappearing suddenly
• Niles Eldredge & Stephen Jay Gould's Punctuated Equilibrium Model (1972): Evolutionary history consists of long periods of stasis punctuated by periods of divergence
• The fossil record dominated by periods of time where little to no change occurred, with only a few fossils from periods of rapid change
• Both evolutionary change models are now accepted
• Many species evolved rapidly during Earth's history, but gradual change is also observed for some species

Divergent and Convergent Evolution

• Divergent evolution
• Species diverge from an ancestral species & become different
• Populations change as they adapt to different environments
• The populations become less alike, resulting in distinct species
• Convergent evolution
• Unrelated species share similar traits
• Each independently adapts to similar environmental conditions
• E.g., Birds & bats evolved independently yet natural selection favored wings that are suitable for air, even though they do not share a common ancestor

Adaptive Radiation

• Adaptive radiation, a form of allopatric speciation, diversifies species into those adapted to different environments
• Islands are living laboratories for speciation because dispersed organisms change in response to new environmental conditions
• E.g., Hawaiian Islands:
• Being far from continents and varying in age make them ideal for studying evolution
• Colonization occurred gradually
• Unique physical characteristics create many exploitable niches
• Great biodiversity resulted
• Many endemic species: the Hawaiian honeycreepers found nowhere else, and believed to evolved from ancestors crossing the ocean ~5 million years ago

Adaptive Radiation Beyond Islands: The Red Crossbill:

• Two biologists demonstrated speciation by studying a specific finch
• The twisted beak of the crossbill allows it to pry the cones open
• Birds with different sized beaks open cones of different sizes
• Scientists trimmed and uncrossed "the beaks of birds that ate tightly closed cones of western hemlock
• These birds eat seeds from open cones but could no longer open closed cones
• When their bills grew back began to cross gradually and they gained the ability to open tightly closed cones again
• The novelty of the crossed bill allowed it to eat food no other bird ate
• Birds with adaptations radiate into other habitats and niches because they have an advantage

Major adaptive radiation occurs

• Following the evolution of a novel characteristic
• E.g., limbs in vertebrates and wings in insects allowed them to evolve into new habitats, giving them access to food supplies
• Insect wings are highly successful

Darwin's Finches

• Demonstrating allopatric speciation “in action”
• Ancestral finches reached one of the Galapagos islands (blown off course?)
• No other land birds were present
• Finches moved into/adopted unoccupied niches (ecological niche),
• Individual finches subjected to selective pressures
• Some flew to nearby islands with unoccupied niches
• Eventually divided into different Galapagos populations
• Different groups became geographically and reproductively isolated from one another due to the formation of new traits and niches
• By observing present-day finches scientists identified their phylogenetic tree showing common ancestry of 14 species based on beak length/DNA

Allopatric Speciation (Geographic Speciation)

• Occurs when a population splits into 2+ isolated groups by a geographical barrier preventing interbreeding from occurring

Examples of geographical barriers

• Includes glaciers/lava flow, fluctuations in ocean levels that turn a peninsula into an island, plus colonization of geographically separate areas. Even though the geographical barrier doesn't have to last forever, it must be maintained

How does divergence occur

• Allele begins to diverge in various ways, including those of natural selection, mutation, genetic drift and/or gene flow
• Isolated groups within population will not automatically survive & thrive once separated into a new population.
• Some may never be able to adapt

Generally, small populations are more likely to change

• Usually geographically isolated at the edge of their range

Sympatric Speciation

• Sympatric factors such as genetic or chromosomal changes in population allow divergence when in the same geographical area

Sympatric conditions

• When in the right conditions, a new species can be generated in a single generation
• Errors during meiosis for chromosome separation can result in the formation of polyploidy
• Can occur in organisms with 3+ chromosomes
• Animals: Typically diploid with chromosomes inherited from each parent + is rare
• Plants: Common due to self pollination (especially in flowering plants)

Meiosis

• A reproductive barrier is made when geneflow is interrupted between tetraploids & the parent population due to infertile offspring.

If offspring are sterile, they can asexually reproduce to create separate population. The hybrid can be transformed into a fertile hybrid, such is the case for multiple agriculture species

• These include wheat, rice, corn, cotton, & potatoes

Post-zygotic Isolating Mechanisms

• In nature, the sperm of one species fertilizes the egg of another successfully + zygote is produced prevent these hybrid zygotes from developing into viable, fertile individuals

Types of post-zygotic mechanism (post utilization barriers) Includes:

• Hybrid inviability- Genetic or Development of incompatability
• Hybrid sterility : Species mate but produce hybrid offspring that are sterile
• Hybrid breakdown – 1st Generation hybrids are viable, fertile and second generation are sterile or weak.

Types of Speciation – Speciation can be defined with morphological + phylogenetic concepts.

• Includes the morphological + phylogenetic species concepts. Organisms must become and remain genetically isolated.

Speciation types

• Sympatric – populations in same habitat diverge genetically.
• Allopatric – populations are geometrically separated but diverge genetically.

Isolating mechanisms

1. Pre-zygotic – impede mating between species of prevent fertilization
2. Post-zygotic - prevent hybrid
3. Behavioural - specific behaviours that prevent interbreeding (Eg. Meadowlark mating is based on song/mating ritual differences!)
4. Habitat – species remain separated to mate and reproduce (Eg. Garter Snakes living on land and in water) 5 *Temporal: Species in habitat do not reproduce (timing) (Eg. 3 orchid species open on different days in short window

• Mechanical: species may physically fail to reproduce due to anatomical structure
• Gambetic: Gametes fail fuse & fertilize

Non-random Mating: In-breeding

• In- breeding is when relatives reproduce together.
• This is a type for self fertilization and increases certain species to become overexpressed.

Non-random Mating: In-breeding

• There’s matie selection to increase the rate of reproduction
• Mate selection for animals are based on physical or behavioural traits( Eg. Caribou will only use their strong antlers

Natural Selection

Alleles

• Can change allele frequencies due to genetic advantage of competition and pressure
• This is because there is higher survival + trait over genes
• Genetic pressure & selection

Mutations

• Genetic changes to one set of DNA that have high probability to occur
• This affects the entire gene pool the rate of evolution for the species. This mutation provides selective advantage to survive
• warfarin is a selective advantage

Explore the Cretaceous extinction event 65 million years ago and its impact on mammalian evolution. Learn how the extinction of dinosaurs led to adaptive radiation of mammals. Also, explore how new species form using the example of Galapagos tortoises observed by Charles Darwin.