Macroevolution

Macroevolution

evolution on a scale at or above the level of species (more than one species)

Microevolution

smaller evolutionary changes of allele frequencies within a species or population

What is Neo-Darwinism and how did it develop?

• Neo-Darwinism combines:

  • Charles Darwin's theory of natural selection

  • Gregor Mendel's theory of genetics

• Emerged in the early 20th century, particularly in 1942 with the Modern Synthesis

• The Modern Synthesis:

  • Fused Mendelian genetics with Darwinian evolution

  • Created a unified theory of evolution

What are the patterns of biodiversity and the processes involved?

• Processes generating biodiversity (Speciation):

  • Speciation is the formation of new and distinct species over time.

  • Caused by factors like geographic isolation, environmental changes, and genetic divergence.

  • Biodiversity pump refers to the mechanisms driving speciation and increasing biodiversity over time.

• Processes reducing biodiversity (Extinction):

  • Extinction is the complete disappearance of a species.

  • Caused by factors like habitat loss, climate change, or invasive species.

  • Cull biodiversity refers to natural or human-driven reduction in species diversity through extinction events.

What is speciation?

Speciation is the process by which new and distinct species are formed.

What are the key factors in speciation?

• Geological and environmental changes can lead to the separation of populations.

• Natural selection favors specific alleles, leading to adaptive traits in isolated groups.

• Populations can become separated due to physical barriers like:

  • Mountains

  • Lakes

  • Islands

• Over time, these isolated populations may become genetically different.

• When there is no interbreeding between these groups, they are considered different species.

What is extinction?

Extinction is the complete disappearance of a species from Earth.

What are the causes and features of extinction?

• Extinction occurs when a species completely dies out.

• Contributing factors include:

  • New competition, predators, or disease affecting the species.

  • Environmental changes that the species cannot adapt to.

• Catastrophic events like:

  • Volcanic eruptions

  • Earthquakes
    can cause rapid extinction.

• Extinction is part of the cyclical nature of speciation and taxonomic changes, where species emerge and disappear over time.

Allopatric Speciation

Speciation that occurs when populations are geographically isolated by a barrier (e.g., river, mountain) leading to genetic divergence over time.

Peripatric Speciation

A form of allopatric speciation where a small population at the edge of a larger population's range becomes isolated and undergoes genetic drift, leading to speciation.

Parapatric Speciation

Speciation that occurs when populations are adjacent and have a small overlapping area, with limited gene flow due to environmental differences, leading to divergence.

Sympatric Speciation

Speciation that occurs within the same geographical area, often due to ecological, behavioral, or genetic factors that create reproductive isolation.

Cladogenesis

an evolutionary splitting of a parent species into two distinct species, forming a clade.

Anogenesis

evolution (changes) within a lineage

Genetic drift

the change in the composition of a gene pool as a result of a random or chance event

Selection

the change in the gene pool as a result of differentially selective environmental pressures

Adaptive radiation

Event in which a lineage rapidly diversifies, with the newly formed lineages evolving different adaptations. Different factors may trigger adaptive radiations, but each is a response to an opportunity

Key example of adaptive radiation

Darwin’s Finches: On the Galápagos Islands, a common ancestor of finches diversified into different species with various beak shapes suited to different food sources.

George Gaylord Simpson (1902-1984)

• American paleontologist and professor of zoology.

• Key figure in the modern evolutionary synthesis.

• Dispelled the myth of linear horse evolution; showed it was more complex.

• Studied rates of evolution, emphasizing gradual and rapid changes.

• Influential work in "Tempo and Mode in Evolution" (1944).

The Evolution of Horses

• Where did it take place?

• What is the process called?

• How many lineages survive today?

• Took place in North America.

• Adaptive radiation: many species evolved from a common ancestor.

• Only one lineage survives today: the modern horse (Equus).

Hyracotherium (Earliest Horse-like Form)

• Features?

• Diet and tooth structure?

• Evolutionary significance?

• Dog-like appearance with pads for feet.

• 25-50 cm at the shoulders, 3 hind toes, 4 front toes.

• Small brain case.

• Teeth: flattened molars & pre-molars for herbivory.

• Short tooth crowns suggest browsing (not grazing).

• Grasses were rare in the Eocene, became common in the Miocene.

• Common in the Northern Hemisphere in the early Eocene.

• Laurasia split into separate continents, isolating descendants and leading to distinct evolutionary paths due to continental drift.

Miohippus (Evolution from Hyracotherium)

• Key features and changes from Hyracotherium?

• What did it lead to?

• Evolved from Hyracotherium by the late Oligocene in North America.

• Larger size, bigger brain case, and different forefeet.

• Loss of the little digit and reduction of lateral digits.

• Several lineages existed in this part of the evolutionary tree.

• One branch eventually led to true horses in the Miocene.

Merychippus (True Horse Line)

• What was significant about Merychippus?

• Key features and changes?

• Arisen in the Miocene, a larger-bodied animal adapted to open habitats.

• Teeth were modern horse-like, with long/high-crowned premolars and molars.

• Enamel ridges/hollows greatly lengthened.

• Gaps between ridges filled with cement.

• Major change in tooth structure to adapt to grassland diet.

• Coincided with the extensive development of grasslands.

What are the characteristics of grasses that shaped horse evolution?

• Tough with high silica content.

• Apical meristem close to the ground, an adaptation to resist herbivory.

How did teeth evolve in response to grasses?

• High-crowned teeth to resist wear from the silica in grasses.

• Merychippus developed deeper skulls to accommodate longer teeth.

What is the "spring foot," and how does it aid horse evolution?

• Hoofed feet with strong elastic ligaments binding toes to main leg bones.

• More efficient than pad-footed animals for larger-bodied creatures.

• Foot bends upon impact with the ground, then straightens when weight is lifted.

• Suspensory ligament provides lift to the limb.

• Adaptation for running and utilizing open spaces.

Descendants of Merychippus

• What evolutionary path led to modern horses?

• What happened to horses after that?

• At least 6 separate lines evolved from Merychippus (adaptive radiation).

• One line led to Pliohippus, with single-toed feet.

• Pliohippus gave rise to Equus (modern horses).

• Equus rapidly spread to Europe, Asia, Africa, and South America.

• North America connected to Asia via the Bering land bridge, and South America connected to North America.

• Eventually, horses became extinct in North America.

Early Views on Horse Evolution

• What was the view of horse evolution in the late 1800s?

• Very few transitional fossils were known.

• Horse evolution was seen as a gradual, straight-line process.

• Equus was viewed as the "goal" of equine evolution.

• O.C. Marsh discovered new fossils in the 1870s showing clear intermediates from Hyracotherium to Equus.

• The discovery revealed that the old model of horse evolution was an oversimplification.

Horse Evolution: Not a Straight Line

• Why is the idea of a smooth, straight-line evolution misleading?

• Equus is just one twig on a once-flourishing bush of horse species.

• Many other branches existed but went extinct.

• The illusion of straight-line evolution comes from only Equus surviving.

• Evolution was not always gradual—some traits evolved at different rates or even reversed direction.

Evolutionary Processes in Horses

• How did horse species evolve over time?

• Evolution occurred through both anagenesis (gradual transformation) and cladogenesis (splitting into new species).

• Some species evolved gradually, while others appeared suddenly.

• The horse family shows diverse evolutionary mechanisms, making oversimplified models misleading.

Examples of Adaptive Radiation

• Darwin’s Finches – Beak specialization in Galápagos.

• Cichlid Fish – Many species in African lakes with different feeding habits.

• Hawaiian Honeycreepers – Diverse beak shapes for various food sources.

• Mammals After Dinosaur Extinction – Rapid diversification into primates, whales, bats, etc.

• Marsupials in Australia – Filled roles of placental mammals elsewhere.

• Anole Lizards – Specialized for different habitats in the Caribbean.

Convergent Evolution

• What is it, and why does it occur?

• Unrelated organisms evolve similar features independently.

• Traits arise as solutions to the same problems (e.g., burrowing, swimming).

• Leads to matching body shapes, colors, or abilities.

• Example: Marsupial vs. Placental mammals evolved separately but filled similar ecological roles.

• Driven by similar habitat selection pressures.

How many species have been described, and how many are predicted to exist on Earth? What percentage of organisms remain undescribed?

• Described species: 2 million

• Predicted total species: 15 million

• Percentage of described species: 13%

• Undescribed terrestrial species: 86%

• Undescribed marine species: 91%

How are extinction, change, and persistence linked in the evolution of species?

• Change (evolution) helps species adapt to their environment.

• If adaptation is successful, the species persists over time.

• If a species fails to adapt, it faces extinction.

• Extinction can create opportunities for new species to evolve (change) and fill ecological gaps.

Why is the coelacanth considered a "living fossil"?

• Example of species persistence (unchanged for millions of years).

• Carnivorous fish, can live up to 60 years.

• Grows up to 2 meters in length.

• Arose in Devonian (> 400 MYA), prevalent in Triassic period.

• Lobed fins link it to early tetrapod lineage.

• Thought to be extinct by late Cretaceous (> 65 MYA) but rediscovered in 1938.

Where was the coelacanth rediscovered, and how does its morphology compare to ancient fossils?

• Rediscovered off the coast of South Africa in 1938.

• Its morphology closely resembles Devonian fossils, showing minimal evolutionary change.

What are some examples of Pleistocene megafauna that went extinct, and how were they impacted by human influence?

• Smilodon (sabre-toothed cat)

• Woolly mammoth

• Giant ground sloth

• Island species like the moa were heavily impacted by human hunting and climate change, leading to their extinction.

What is a global mass extinction?

A global mass extinction is a rapid loss of species across the planet, caused by events like climate change, asteroid impacts, or human activity.

How long has life existed on Earth, and which group dominated for the first 3.1 billion years?

• Earth is 4.55 billion years old.

• Life has existed for the last 3.8 billion years.

• Bacteria dominated Earth for the first 3.1 billion years.

What did the primitive atmosphere consist of, and how did it contribute to the development of prokaryotes?

• The primitive atmosphere comprised nitrogen, CO2, hydrogen, and water vapor.

• This environment created a primordial ‘soup’ where prokaryotes developed a range of metabolic adaptations.

What is the difference between bacteria and cyanobacteria?

What was the Atmospheric Revolution, and how did cyanobacteria play a role?

• Cyanobacteria appeared (~2.6 BYA) and began producing oxygen through photosynthesis.

• Initially, oxygen was absorbed by iron in the oceans, forming iron oxides.

• Around 2.4 BYA, oxygen started accumulating in the atmosphere.

• This led to the Great Oxygenation Event, causing a mass extinction of anaerobic organisms.

What happened after the Great Oxygenation Event (GOE)?

• Mitochondria evolved after the GOE.

• Diversity gradually increased over time and exploded during the Cambrian period.

How does the rate of biodiversity accumulation change through time?

The rate of biodiversity accumulation is not constant, with periods of diversification followed by stable periods and mass extinctions.

Key extinction events

• Ordovician (~500 MYA): 50% of animal families, including trilobites.

• Devonian (~350 MYA): 30% of animal families, including fish and trilobites.

• Permian (~250 MYA): 50% of animal families, most marine life.

• Triassic (~180 MYA): 35% of animal families, including reptiles and molluscs.

• Cretaceous (~65 MYA): Extinction of dinosaurs and much of marine life.

What was the K-T Event, and when did it occur?

• The K-T Event occurred at the end of the Cretaceous and the start of the Tertiary (~65 MYA).

• It marked the extinction of many species, including dinosaurs, pterosaurs, plesiosaurs, mosasaurs, ammonites, and belemnites.

What are the two likely causes of the K-T Event?

• Giant meteorite impacts, with evidence of impact sites near southeast Mexico and the Ukraine.

• A mantle plume eruption on the Indian subcontinent (Deccan Traps) around 66.25 MYA.

What are the two main factors thought to have caused the K-T mass extinctions?

• Short-term acid rain caused by the impact of giant meteorites.

• Long-term atmospheric barrier that blocked solar radiation following the event.

How were early mammals likely able to survive the K-T event's harsh conditions?

Early mammals likely survived due to their burrowing habits, feeding on roots and invertebrates, which protected them from extreme conditions.

What occurred by the Eocene period after the K-T event?

A major radiation of mammals took place, driven by competitive release as ecosystems shifted and dinosaurs went extinct.

What mass extinction event are we currently living through?

We are living during the sixth mass extinction event.

How do large animal extinctions relate to the Pleistocene era and modern humans?

• Oscillations in global temperature (ice ages) started in the Pleistocene era (~2 MYA).

• However, large animal extinctions align more closely with the arrival of modern humans.

Where have species gone extinct in recent times?

• Australia: Extinctions like the Thylacine (Tasmanian tiger) due to habitat destruction and invasive species.

• Pacific Islands: Extinctions like the Moa (New Zealand) due to hunting and invasive species.

• North America: Passenger Pigeon and Carolina Parakeet due to overhunting and habitat loss.

• Madagascar: Extinctions like the Elephant Bird due to hunting and habitat destruction.

What is the 6th mass extinction, and what is its cause?

• The 6th mass extinction, also known as the Holocene extinction, describes the ongoing extinction of species during the Holocene epoch.

• This extinction event is primarily driven by human activity, including overharvesting, deforestation, and habitat fragmentation.

What are the main reasons for species extinctions?

• Climate change:

  • Contributes to the decline of amphibians and other species.

  • Rising temperatures and ocean acidification threaten ecosystems.

  • Coral reefs predicted to be extinct if global temperatures rise by 2°C or more.

  • Coral reefs are also threatened by commercial collectors of tropical fish.

• Global movement of animals and plants:

  • Alien species become pests when they arrive without their natural predators.

  • Pathogens and alien diseases, like chytridiomycosis, spread due to the trade in amphibians.

What was Alfred Wegener's hypothesis about continental drift, and what inspired it?

• Alfred Wegener's (1912) hypothesis proposed that continents were once connected and have drifted apart over time.

• He was partly inspired by the unexpected distributions of plants and animals across the globe.

What examples of plant and animal distributions support Wegener's hypothesis of continental drift?

• Southern beech (Nothofagus) occurs in patches around Antarctica, Chile, New Zealand, Australia, and New Guinea.

• Glossopteris fern fossils are found in Antarctica, Australasia, and India.

• The extinct reptile Mesosaurus is found only in small regions on the east coast of Brazil and the west coast of Africa.

What is Wallace's Line, and how is it used in biogeography?

• Wallace's Line is an imaginary line used by biogeographers to delineate different faunas among islands in Southeast Asia and Melanesia.

• To the west of the line, the bird fauna shares more characteristics with mainland Asia.

• To the east of the line, the bird fauna shares more characteristics with Australia.

Are there similar lines used for other animals besides birds?

Yes, similar lines have been applied to the mammals, indicating a divide in the distribution of mammalian species between the eastern and western parts of the region.

Why do we see distinctive distributions of species across regions like Wallace's Line?

• Australia and New Guinea (Australasia) were once part of Gondwanaland.

• Asia, Indonesia, and Malaysia were part of Laurasia.

• During ice ages, lower sea levels allowed Australia and New Guinea to become connected, leading to species exchanges and distinctive distributions between these regions.

What are Ice Ages?

Ice Ages are prolonged periods of global cooling where large parts of Earth experience extensive glaciation (growth of ice sheets).

What are the genetic consequences of the Pleistocene Ice Age?

• The Pleistocene Ice Age, which began 3 million years ago and is still ongoing in a warm interglacial phase, led to significant genetic consequences for species.

• Fragmented species ranges meant that many populations survived only in isolated refugia (safe, stable areas), leading to the development of distinct genetic lineages.

How did the Pleistocene glaciation affect species and ecosystems?

• The Pleistocene glaciation (18,000-20,000 years ago) marked the last glacial maximum, altering the landscape and climate globally.

• Species were often forced into isolated refugia, where they persisted in smaller, fragmented populations, leading to changes in genetic diversity.

Effects of ice ages on faunas

Extinction of species (mass extinctions in some cases)

Formation of barriers to movement (glaciers in high latitudes, arid areas in low latitudes)

Splitting of populations & subsequent speciation

What were the refugial sites in Europe during the Pleistocene Ice Age?

Refugial sites in Europe during the Pleistocene Ice Age included:

• Iberian Peninsula

• Apennine (Italian) Peninsula

• Balkan Peninsula, Greece

How did species recolonize and experience genetic consequences after the Ice Age?

• Species that had survived in southern refugia recolonized northern areas.

• As populations moved away from refugial sites, they were bottlenecked, progressively losing alleles and genetic diversity.

• Northern European populations were expected to have less genetic diversity than those in the southern refugia.

What does the concept of southern richness and northern purity refer to?

• Southern richness refers to the genetic diversity and species richness in southern refugial areas.

• Northern purity refers to the genetic simplicity or less diversity in populations that recolonized from the south into northern Europe after the Ice Age.

How do molecular methods help in understanding the dispersal of European fauna after glaciation?

With molecular methods, it is now possible using DNA similarity to show from which refugia

particular genomes emerged to cover their present distribution