11.4 Speciation and Macroevolution
4 min read•august 7, 2024
Evolution doesn't just happen slowly over time. Speciation and macroevolution show how new form and big changes occur. From geographic barriers to reproductive isolation, these processes shape life's diversity.
, , and coevolution highlight nature's creativity. Whether it's finches adapting to different food sources or flowers and pollinators evolving together, these mechanisms drive the incredible variety of life on Earth.
Speciation Processes
Types of Speciation
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- Speciation occurs when one species splits into two or more separate species over time
- happens when populations become physically separated by a geographic barrier (mountains, rivers, or oceans) and evolve independently
- and act on each separately causing them to diverge genetically over time
- If populations remain separated long enough, they may no longer be able to interbreed if reunited becoming separate species
- takes place within the same geographic area without physical separation
- Can occur through polyploidy in plants where offspring have multiple sets of chromosomes and are reproductively isolated from parent species
- May also happen if subpopulations exploit different resources or niches within the same habitat reducing between them
Reproductive Isolation Mechanisms
- Reproductive isolation is the inability of two populations to interbreed and produce viable, fertile offspring
- Prezygotic barriers prevent fertilization from occurring and include:
- Habitat isolation: Populations occupy different habitats within the same area
- Temporal isolation: Populations breed at different times of day, season, or year
- Behavioral isolation: Unique courtship rituals, mating calls, or other behaviors prevent mating
- Mechanical isolation: Incompatible reproductive structures prevent successful mating (flower shapes that attract different pollinators)
- Postzygotic barriers occur after fertilization and include:
- Reduced hybrid viability: Hybrid offspring have reduced survival rates
- Reduced hybrid fertility: Hybrid offspring are sterile and cannot produce viable gametes (mules, offspring of horse and donkey)
Evolutionary Patterns
Rates of Evolutionary Change
- Gradualism suggests that species evolve slowly and continuously over long periods of time
- Small genetic changes accumulate gradually in populations eventually leading to speciation
- Supported by transitional fossils showing gradual changes in traits over time (evolution of horse from small, multi-toed ancestor to larger, single-toed modern species)
- proposes that species remain relatively stable for long periods punctuated by brief periods of rapid evolutionary change
- Speciation occurs quickly in small, isolated populations that diverge rapidly from the parent species
- Supported by fossil record showing long periods of stasis followed by sudden appearance of new species
Adaptive Radiation
- Adaptive radiation is the rapid diversification of a single ancestral species into many descendant species adapted to different ecological niches
- Often occurs when a species encounters a new environment with many available niches and little competition
- Classic example is Darwin's finches in the Galapagos Islands which evolved into 15 species with specialized beak shapes adapted to different food sources (seeds, insects, cacti)
- Hawaiian honeycreepers are another example having radiated into over 50 species from a single finch-like ancestor
Evolutionary Relationships
Convergent Evolution
- Convergent evolution occurs when unrelated species independently evolve similar traits or adaptations in response to similar environmental pressures
- Results in analogous structures that have similar form or function but different evolutionary origins
- Examples include flight in birds, bats, and insects; streamlined body shapes of sharks and dolphins; and succulent stems of cacti and euphorbs
- Divergent evolution, in contrast, is when closely related species evolve different traits over time as they adapt to different environments or niches (Darwin's finches)
Coevolution
- Coevolution happens when two or more species reciprocally affect each other's evolution
- Changes in one species exert selective pressures on the other species which then evolves in response, causing further changes in the first species in an ongoing cycle
- Mutualistic coevolution occurs when both species benefit from the interaction
- Flowers and their pollinators (hummingbirds, bees) evolve matching adaptations such as long nectar tubes and tongues
- Acacia trees and ants: trees provide hollow thorns for ants to live in and nutrient-rich nectar; ants defend the tree from herbivores and competing plants
- Antagonistic coevolution involves predator-prey or host-parasite relationships where each species evolves adaptations and counter-adaptations against the other
- Cheetahs and gazelles: cheetahs evolve to run faster to catch prey; gazelles evolve to run faster to escape predators
- Rough-skinned newts produce potent tetrodotoxin in skin; common garter snakes evolve resistance to the toxin to prey on newts
Key Terms to Review (18)
Adaptive radiation: Adaptive radiation is an evolutionary process in which organisms rapidly diversify and adapt to a variety of ecological niches. This phenomenon often occurs when a single ancestral species spreads into different environments, leading to the emergence of new species that are uniquely adapted to their specific habitats and lifestyles. This process highlights the relationship between environmental factors, natural selection, and the diversification of life forms.
Allopatric speciation: Allopatric speciation is the process through which new species arise when populations are geographically isolated from one another, leading to reproductive barriers and genetic divergence. This isolation can be caused by various factors such as mountains, rivers, or human activities, and it plays a crucial role in how biodiversity develops over time. The mechanism highlights the importance of geographical separation in the evolution of species and illustrates the concept of natural selection acting on isolated populations.
Cambrian Explosion: The Cambrian Explosion refers to a significant event in Earth's history that occurred around 541 million years ago, marked by a rapid diversification of life forms and the emergence of most major animal phyla. This period is crucial for understanding the patterns of speciation and macroevolution, as it represents a time when complex organisms evolved from simpler ancestors, leading to the rich biodiversity we see today.
Charles Darwin: Charles Darwin was an English naturalist and biologist best known for his theory of evolution through natural selection. His ideas laid the foundation for understanding how species adapt and evolve over time, influencing various scientific fields including ecology, genetics, and behavior.
Convergent Evolution: Convergent evolution is the process where different species independently evolve similar traits or adaptations in response to similar environmental pressures or challenges. This phenomenon illustrates how unrelated organisms can develop comparable features, demonstrating the influence of natural selection in shaping life forms in similar ecological niches.
Gene flow: Gene flow is the transfer of genetic material between populations through processes like migration and interbreeding. This movement of alleles can significantly affect the genetic diversity of populations, introducing new genetic variations and influencing evolutionary changes over time.
Genetic drift: Genetic drift is a mechanism of evolution that refers to random changes in allele frequencies within a population, particularly affecting small populations. This process can lead to significant evolutionary changes over time, especially when certain alleles become more or less common purely by chance, rather than through natural selection. It highlights how chance events can influence genetic variation and the evolutionary trajectory of a species.
Great American Interchange: The Great American Interchange refers to the significant exchange of flora and fauna between North and South America that occurred around 3 million years ago, driven by the formation of the Isthmus of Panama. This geological event allowed for the migration of species across what had previously been a barrier, resulting in the mixing of previously isolated ecosystems. This interchange had profound effects on the biodiversity and evolutionary trajectories of both continents, leading to increased competition and speciation events.
Mass extinction: Mass extinction refers to a significant and rapid loss of biodiversity on Earth, where a large number of species become extinct in a relatively short period of geological time. This phenomenon can reshape ecosystems and has been linked to various factors, including catastrophic events, climate change, and human activities. The understanding of mass extinctions is crucial for studying speciation and macroevolution, as these events can reset ecological communities and lead to the emergence of new species.
Mutation: A mutation is a permanent change in the nucleotide sequence of an organism's DNA, which can result in alterations to genes and the proteins they encode. These changes can arise from various factors such as errors during DNA replication, exposure to radiation, or chemical mutagens. Mutations play a crucial role in evolution and genetic diversity, impacting everything from individual traits to population dynamics.
Natural selection: Natural selection is the process by which organisms that are better adapted to their environment tend to survive and produce more offspring. This mechanism drives evolution by favoring traits that enhance survival and reproduction, leading to gradual changes in species over generations. It connects to understanding variations in populations, the evidence supporting evolutionary theory, genetic principles, and the emergence of new species.
Population: A population refers to a group of individuals of the same species that live in a specific area and interact with each other. This concept is fundamental to understanding how organisms thrive and evolve, as it encompasses the dynamics of reproduction, survival, and interactions within their environment. Analyzing populations helps in comprehending larger biological principles like ecosystem balance, speciation processes, and overall biodiversity.
Postzygotic barrier: A postzygotic barrier is a reproductive mechanism that occurs after fertilization, preventing the hybrid offspring from developing into viable, fertile adults. These barriers act to reduce gene flow between different species, thus promoting speciation by ensuring that even if fertilization occurs, the resulting hybrids are less likely to survive or reproduce successfully.
Prezygotic barrier: A prezygotic barrier is a reproductive mechanism that prevents mating or fertilization between species before a zygote can form. These barriers play a crucial role in speciation by maintaining the genetic separation between populations, thus contributing to the process of macroevolution. They can arise from various factors including temporal, behavioral, mechanical, and ecological differences that hinder successful reproduction.
Punctuated equilibrium: Punctuated equilibrium is a theory in evolutionary biology that proposes that species experience long periods of stability, or equilibrium, interrupted by brief episodes of rapid change during which new species emerge. This concept challenges the traditional view of gradual evolution, suggesting that significant evolutionary changes occur relatively quickly, often in response to environmental shifts or other factors. It emphasizes the role of speciation and the dynamics of macroevolution.
Species: A species is a group of organisms that can interbreed and produce fertile offspring in natural conditions, sharing common characteristics and genetic similarities. Understanding species is crucial for classifying living organisms, studying biodiversity, and examining evolutionary relationships among various groups of life.
Sympatric speciation: Sympatric speciation is the process through which new species arise from a single ancestral species while inhabiting the same geographic region. This form of speciation occurs without physical barriers, often driven by factors such as behavioral changes, polyploidy in plants, or resource partitioning. It highlights the complexities of evolution and how organisms can diverge while still sharing their environment.
Theodosius Dobzhansky: Theodosius Dobzhansky was a prominent geneticist and evolutionary biologist known for his work in the field of population genetics and the modern synthesis of evolutionary theory. He emphasized the importance of genetic variation and its role in speciation and macroevolution, arguing that evolution is driven by changes in gene frequencies within populations over time.