Darwin, speciation and species diversity

• Geographic isolation creates conditions for speciation, but reproductive barriers must develop within the gene pool for speciation to occur.
• Reproductive barriers can arise due to environmental factors.
• Natural selection acting on variations, genetic drift, or mutations can drive changes in traits, leading to reproductive barriers.
• Diane Dodd's experiments demonstrated that reproductive isolation could evolve as a by-product of adaptation to different environments.
• Fruit flies raised on starch or maltose adapted to their respective food sources.
• Starch-fed flies digested starch more efficiently, and maltose-fed flies digested maltose more efficiently.
• This supports the idea that environmental adaptation can lead to reproductive barriers over time.
• Fruit flies from "starch populations" mixed with flies from "maltose populations" showed a mating preference for partners raised on the same food source.
• Flies from different populations which had both been adapted to starch mated indiscriminately.
• The barrier's presence was linked to environmental adaptation rather than population origin.
• The emergence of a prezygotic barrier reduces the likelihood of mating between groups exposed to different environments.
• The barrier wasn't absolute, but it demonstrated that reproductive isolation was beginning to develop.
• Pollinator preferences can create reproductive barriers in plants, where populations of an ancestral species may adapt to environments dominated by either hummingbirds or bees.
• Flowers evolve to attract the most common pollinator, separating species even if they share the same region later.
• In the Sierra Nevada, pink-flowered Mimulus lewisii attracts bumblebees, while red-flowered Mimulus cardinalis attracts hummingbirds.
• Flower color changes can influence pollinator preferences, reinforcing reproductive barriers.
• Females of the Galápagos finch Geospiza difficilis respond to the songs of males from their island but ignore songs of males from other islands.
• Behavioral barriers to reproduction can develop in allopatric finch populations.
• Sympatric speciation occurs when a new species emerges within the same geographic area as its parent species.
• Factors like polyploidy, habitat differentiation, and sexual selection can limit gene flow within sympatric populations.
• Sympatric speciation in plants often results from polyploidy, which involves the duplication of chromosome sets during cell division.
• A diploid (2n) plant can produce tetraploid (4n) cells with four sets of chromosomes.
• The flowers on the tetraploid branch will produce diploid (2n) gametes.
• Diploid gametes fuse to form tetraploid (4n) zygotes.
• The tetraploid plants are reproductively isolated from their diploid parent species.
• The fusion of a diploid gamete from the tetraploid plant with a haploid (n) gamete from the parent species results in triploid (3n) offspring, which are sterile.
• Odd number of chromosomes in triploid plants prevents proper pairing during meiosis, leading to reproductive failure.
• The tetraploid plant becomes a distinct species, separate from its diploid ancestors.
• Most polyploid species arise from the hybridization of two different species.
• The hybrid has an odd number of chromosomes and is sterile.
• It may reproduce asexually, and subsequent chromosome duplications can restore a diploid chromosome set.
• Polyploid speciation occurs rarely in animals but is thought to have happened in species like the gray tree frog.
• Sympatric speciation in animals is more commonly driven by habitat differentiation or sexual selection.
• In Lake Victoria where hundreds of cichlid fish species likely evolved through adaptations to different food sources and habitats, reducing interbreeding.
• Sexual selection, where females choose mates based on coloration, likely contributed to reproductive barriers that helped form new species.
• Reduced hybrid fertility isolates a viable, fertile polyploid plant from its parental species.
• Isolated island chains often host unique species due to their diverse habitats.
• Islands spaced to allow isolated evolution but enable occasional dispersals become hotspots for multiple speciation events.
• The process where many species evolve from a common ancestor is called adaptive radiation.
• The Galápagos Archipelago exemplifies adaptive radiation
• The islands were colonized by plants, animals, and microorganisms carried by ocean currents and winds from nearby regions.
• The Galápagos Islands are home to unique species, including 14 closely related finches known as Darwin's finches.
• Their feeding habits and beak shapes are specialized for different diets, showcasing a link between structure and function.
• The woodpecker finch uses tools to find insects, and the warbler finch's thin beak is ideal for catching small insects.
• Darwin's finches likely evolved from a small population of ancestral birds that colonized one island.
• Natural selection led to adaptations in isolation, forming a new species.
• Some individuals then migrated to other islands, where different environments drove further speciation.
• Birds that recolonized the original island might coexist with the ancestral species if reproductive barriers kept them distinct.
• Each Galápagos Island hosts multiple finch species, with up to 10 species on some islands.
• Adaptive radiation is seen in their diverse beak types and habitats, such as living in trees or on the ground.
• Species-specific songs maintain reproductive isolation.
• Chance colonization of an island often presents a species with new resources and an absence of predators.
• Through natural selection acting on existing variation, the colonizing population becomes adapted to its new habitat and may evolve into a new species.
• Subsequent colonizations of nearby islands provide additional opportunities for adaptation and genetic drift, which could lead to further speciations.

Defining Species

• "Species" comes from the Latin for "kind" or "appearance".
• Defining species precisely is challenging.
• Some species look alike but are distinct (e.g., eastern and western meadowlarks).
• Members of some species show limited physical variation, while others display remarkable diversity.
• All humans belong to the same species, Homo sapiens, despite physical diversity.

The Biological Species Concept

• A species is a group that can interbreed and produce fertile offspring.
• Reproductive compatibility unites members of a species.
• Reproductive isolation maintains species distinction through barriers.
• Barriers can be physical, behavioral, or genetic.
• Some species can interbreed and produce hybrids, for example grizzly bears (Ursus arctos) and polar bears (Ursus maritimus) producing "grolar bears".
• The biological species concept cannot be applied to fossils or asexual organisms.

Other Definitions of Species

• The morphological species concept classifies organisms based on physical traits.
• The ecological species concept defines species by their ecological niches.
• Two fish species can be visually similar, but distinguished by what they eat etc
• The phylogenetic species concept defines a species as the smallest group sharing a common ancestor.
• The phylogenetic species concept uses traits like morphology, DNA, and biochemical pathways to trace species' evolutionary histories.
• The biological species concept emphasizes how groups arise through reproductive isolation
• Morphological, ecological, and phylogenetic definitions can be applied to both asexual and sexual species.
• The biological species concept relies on reproductive compatibility, not applicable to asexual organisms.

Terminology

• Taxonomy: Identifying, naming, and classifying life.
• Reproductive barrier: Prevents interbreeding, maintaining genetic separation.
• Postzygotic barriers: Prevent hybrid zygotes from developing into fertile adults (e.g., reduced viability, fertility, breakdown).
• Species: Interbreeding populations capable of producing fertile offspring.
• Prezygotic barriers: Impede mating or fertilization (e.g., temporal, habitat, behavioral, mechanical, gametic isolation).
• Speciation: Evolution of a new species.
• Reproductive isolation: Barriers impeding viable, fertile offspring.
• Biological species concept: Interbreeding with fertile offspring but not with other groups.
• Morphological species concept: Defining species by anatomical criteria.
• Ecological species concept: Defining species by ecological niche.
• Phylogenetic species concept: Smallest group sharing a common ancestor.

Visualizing the Concept

• Closely related species are prevented from interbreeding by reproductive isolation, which relies on reproductive barriers.
• Reproductive barriers are inherent biological features that stop successful interbreeding between species.
• Prezygotic barriers prevent mating or fertilization.
• Postzygotic barriers act after hybrid zygotes are formed.

Prezygotic Barriers

• Prezygotic barriers are mechanisms preventing different species from mating and producing offspring.
• Habitat isolation: species live in different environments and don’t encounter each other to mate, like aquatic and terrestrial garter snakes.
• Temporal isolation: species breed at different times, such as eastern and western spotted skunks with distinct mating seasons.
• Behavioral isolation: species have unique courtship behaviors that prevent interbreeding, like the dances of blue-footed and masked boobies.
• Mechanical isolation: physical incompatibility in reproductive structures keeps species separate, like hummingbird pollinators for Heliconia plants.
• Gametic Isolation: Prevents eggs and sperm or pollen and stigma from different species from fusing, demonstrated by the surface protein incompatibility in sea urchins.

Postzygotic Barriers

• Postzygotic barriers prevent hybrid offspring from developing into viable or fertile adults.
• Reduced Hybrid Viability: hybrid offspring from species with incompatible parental genes fail to develop fully or survive long-term, as seen in certain salamanders.
• Reduced Hybrid Fertility: Hybrids, like mules (offspring of horses and donkeys), are healthy but cannot reproduce.
• Hybrid Breakdown: Initial hybrids are viable and fertile, but their descendants become weak or sterile, exemplified by specific rice plant hybrids.

Allopatric Speciation

• Allopatric speciation occurs when a population is geographically divided into isolated subpopulations, interrupting gene flow.
• The isolated splinter population evolves independently, with changes in allele frequencies driven by natural selection, genetic drift, and mutation.
• Geologic processes, such as lake formation, stream course changes, or canyon carving, can isolate populations.
• Continent splitting and remote area colonization can also contribute to allopatric speciation.
• The size of a geographic barrier depends on the movement ability of the organisms or their gametes.
• Birds and coyotes can easily cross rivers and canyons, while rodents may find such barriers insurmountable.
• The Grand Canyon and Colorado River have separated two species of antelope squirrels.
• Harris’s antelope squirrel resides on the south rim and the white-tailed antelope squirrel on the north rim.
• Studies on snapping shrimp (genus Alpheus) near the Isthmus of Panama provide evidence of allopatric speciation.
• The snapping shrimp are grouped into 15 species pairs, each consisting of closely related members.
• One member of each snapping shrimp species pair lives on the Atlantic side, while the other inhabits the Pacific side.
• Geographic separation caused by the formation of the Isthmus of Panama has led to the evolutionary divergence of snapping shrimp species.
• Snapping shrimp use a claw to produce shockwaves for hunting.
• The Isthmus of Panama gradually closed about 3 million years ago.
• Snapping shrimp species originated from 9 to 3 million years ago, with the species pairs in deepest water diverging first.
• Deep-water snapping shrimp populations were geographically isolated first as the Isthmus of Panama started to close millions of years ago.
• The deeper oceanic parts were cut off earlier than shallower regions, generating isolated populations.
• As the land bridge continued to close, populations in shallower waters also separated, following a similar evolutionary path.
• Deeper species diverged earlier, while those in shallower regions separated later.