BIOL 108 Topic 8

Speciation

Darwin

explained the role of natural selection in the process of speciation (as a branching event)

Speciation

the process by which new species arise; evolutionary process by which one ancestral species diverges into two or more genetically distinct descendant species that are reproductively isolated from each other, meaning they can no longer interbreed to produce fertile offspring; is a gradual process from 4000 yrs to 40 million years (average for plants and animals is 6.5 million; begins when gene flow between populations is interrupted

Microevolution

evolution at the population level. Changes in allele frequency in a population over time. It occurs mainly through natural selection (results in adaptation) or genetic drift. Can occur quickly (over generations).

Macroevolution

evolutionary changes occurring on geological time scales. Origin of novel traits, e.g. evolution of wings in ancestors of birds. Origin of new groups, e.g. the origin of mammals or flowering plants via a series of speciation events. Mass extinctions.; gradual compounding of small changes, i.e. cumulative microevolution + cumulative new genetic variation (mutations and sexual reproduction). Includes many speciation and extinction events

Species

a group of individuals that can interbreed, producing viable (healthy and fertile) offspring, and are reproductively isolated from other species (exchanging genes which keeps the group unified as a species); BSC (Biological Species Concept) is based on potential to interbreed rather than physical similarity

BSC limits

not applicable to fossils or asexual organisms (like bacteria and extinct species); cannot explain gene flow between species (hybridization -> Grolar bear)

Morphological species Concept

identifies a species based on its physical appearance or structural traits that make them distinguishable from other species - applies to sexual and asexual organisms but relies on subjective criteria

Ecological species concept

defines a species by its role in the environment, known as its ecological niche, how the species interacts with other organisms and its habitat. Members of the same species will have similar preferences and tolerances for environmental conditions, like temperature or food sources. This concept also applies to both sexual and asexual species. For example, Canada lynx (Lynx canadensis) and bobcat (Lynx rufus) hybridize in the wild, especially in regions where their ranges overlap, but occupy distinct ecological niches that define them as separate species

Phylogenetic species concept

defines a species as the smallest group of individuals that shares a common ancestor, based on evolutionary history. A species is a branch (or “tip”) on the phylogeny, i.e. the smallest set of organisms that come from a common ancestor is considered a species. This concept applies to sexual and asexual organisms and is particularly useful for analyzing evolutionary relationships. However, determining how much phylogenetic difference is required to separate species can be difficult. For example, giraffes were previously considered a single species, but recent phylogenetic analyses have revealed that giraffes consist of four distinct species

Prezygotic barriers

occur before fertilization and prevent mating or the formation of a zygote (habitat, temporal, behavioural, mechanical, or genetic isolation)

Postzygotic barriers

occur after fertilization, where hybrids may be sterile or not survive long enough to reproduce

Habitat isolation

Two species encounter each other rarely, or not at all, because they occupy different habitats, even though not isolated by physical barriers. e.g. insect species feeding and mating on different host plant species in the same habitat.

Temporal isolation

Species that breed at different times of the day, different seasons, or different years cannot mix their gametes. e.g. nocturnal vs. daytime-active species. e.g. plant species that flower and pollinate at different times of the year

Behavioural isolation

Courtship rituals and other behaviours unique to a species are effective barriers. Behavioural isolation is restricted to animals, because only animals undertake courtship

Mechanical isolation

Morphological differences can prevent successful mating (anatomically incompatible). Example: the shells of two species of snails in the genus Bradybaena spiral in different directions. Moving inward to the center, one spirals in a counterclockwise direction, the other in a clockwise direction. As a result, the snails’ genital openings (indicated by arrows) are not aligned, and mating cannot be completed.

Gametic isolation

Gametes (e.g. sperm) of one species may not be able to fertilize the eggs of another species. Gametes are often genetically or chemically incompatible. Important in open-pollinating plant species, i.e. flowers reject foreign pollen or aquatic species which are broadcast spawners (eg. sea urchins red and purple)

Postzygotic reproductive barriers

prevent hybrid zygote from developing into a viable, fertile adult through reduced hybrid viability/fertility or a hybrid breakdown

Reduced hybrid viability

Genes of the different parent species may interact to impair the hybrid’s development or survival. Reduced hybrid viability often results in zygote or embryo death; eg. salamander subspecies live in the same regions and habitats where they occasionally hybridize but they do not complete embryonic development

Reduced hybrid fertility

Even if hybrids are vigorous, they may

be sterile. If the chromosomes of the two parent species differ in number or structure, meiosis in the hybrids may fail to produce normal gametes.Example: male donkey (Equus asinus) × female horse (Equus caballus) = mule (sterile hybrid)

Hybrid breakdown

Some 1st-generation hybrids are vigorous and fertile, but when these hybrids mate with one another or with either parent species, offspring of subsequent generations are feeble or sterile. Hybrids are eventually eliminated from the population by natural selection

Allopatric speciation

occurs when a population is geographically separated into isolated groups, interrupting gene flow, and leading to genetic divergence and the formation of new species. Most speciation is thought to be through this; as subpopulations diverge, they may evolve traits that act as prezygotic or postzygotic barriers to reproduction

Sympatric speciation

occurs within the same geographic area and requires reproductive isolation despite potential for gene flow (through Chromosomal errors during meiosis or hybridization of closely related species. Polyploidy and hybrid speciation. Natural selection for reproductive isolation. Habitat differentiation (often by disruptive selection). Sexual selection (non-random mating))

Regions with many geographic barriers

(e.g. mountains, rivers, etc) typically have more species than regions with fewer barriers

Dispersal

(gives rise to geographic separation) where a small population becomes isolated at the edge of a larger population, leading to rapid genetic divergence. Also known as peripatric speciation. e.g. speciation of finches on the Galápagos Islands, where geographic isolation led to the evolution of different species from a common ancestor

Vicariance

the range of a species is split by a change in the environment, creating two subpopulations

Separated populations

… independently diverge via natural selection, mutation, and genetic drift. Reproductive isolation between subpopulations generally increases as the distance between them increases. Reproductive isolation may arise as a result of genetic divergence. Barriers to reproduction must be intrinsic; separation itself is not a biological barrier

Snapping shrimp

~3 million years ago, North and South America merged, creating a land bridge (Isthmus of Panama). Isolated the marine communities of the Caribbean and the Pacific (vicariance). Phylogeny of snapping shrimp (Alpheus) shows that 15 pairs of sister species are separated by the Isthmus of Panama. Mating experiments demonstrated that sister species from the Caribbean and the Pacific were unable to produce viable offspring

Hybridization

occurs when individuals from different species with incomplete reproductive barriers interbreed and produce offspring, known as hybrids. Depending on the extent of genetic divergence, these hybrids may: Have reduced survival (reduced hybrid viability) or fertility (reduced hybrid fertility, e.g. mule) or be fully viable and fertile, e.g. coywolf: a fertile hybrid of coyote × wolf; can result in speciation.

Polyploidy

… speciation occurs when changes in the number of chromosome sets (polyploidy) create genetically distinct descendants that are reproductively isolated from parental forms; can produce a new sympatric species within a single generation. Polyploidy occurs infrequently in animals. Most chromosomal abnormalities in animals are lethal. Polyploidy is common among plants. More the 50% of plant species have at least one ancestral polyploidy event in their evolutionary history. Many important crops are polyploids, e.g. cotton, potatoes, canola, and wheat

Hybrid speciation

occurs when interbreeding between two related species creates genetically distinct descendants that are reproductively isolated from the parent species

Allopolyploid

a species with multiple sets of chromosomes derived from the hybridization of different species; have double the number of chromosome sets than the parent species; at least five plant species with this have arisen naturally by hybrid speciation since 1850

Autopolyploids

an individual with more than two chromosome sets, derived from one species. Can arise spontaneously by genome doubling or by fusion of 2n gametes (failure of cell division during meiosis creates gametes with double the number of chromosomes)

Homoploid hybrid speciation

when hybrid speciation occurs with no change in chromosome number; when hybridization produces novel combinations of genes that can adapt hybrids to new habitats that cause them to be reproductively isolated from parental populations

Habitat differentiation

Natural selection for reproductive isolation between subpopulations can occur after the appearance of new ecological niches. Eg. Apple maggot flies (Rhagoletis pomonella) can live on native hawthorn trees as well as more recently introduced apple trees; this increases reproductive isolation between subpopulations

Sexual selection

occurs when individuals with certain inherited traits are more successful at obtaining mates. This often leads to sexual dimorphism: distinct differences in secondary sexual traits between males and females; natural selection for mating success (but is not Natural Selection because these traits may reduce chances of survival); a mechanism for sympatric speciation as they reduce interbreeding and create reproductive barriers (assortative mating - when females choose males with shared specific traits)

Cichlid fish

In Lake Victoria, female __ __ choose mates based on male colouration. In clear water, colour signals are more visible, leading to strong assortative mating. This has contributed to the rapid diversification of cichlid species: many closely related species coexist in the same habitat, separated by mate choice (sexual selection) rather than geography. When sexual selection breaks down, so do the reproductive barriers between species

Hybrid zones

regions where members of different species (or diverging populations) meet and mate, producing hybrids. Hybrid zones are informative: Show if and how much gene flow occurs between diverging species. Measure fitness of hybrids, revealing the strength of reproductive barriers and the factors causing reproductive isolation. Predict evolutionary outcomes: reinforcement, stability, fusion, and hybrid speciation (rare, e.g. allopolyploids and homoploids). Speciation is a process: Hybrid zones act as “natural experiments” in this process, demonstrating the tension between gene flow and reproductive isolation.

European toads

Bombina bombina (which inhabits lowland ponds) and B. variegata (which occupies more mountainous areas) form a long and narrow hybrid zone across central Europe. Hybrids survive but have reduced fitness compared to the parent species; may shift is habitats shift (climate change) - when species meet, the hybrids reinforce barrier (selected against with prezygotic barriers = less fit; stronger in sympatric vs. allopatric), fuses the two species (hybrids are selected for), are maintains equilibrium of hybrids and original (equal fitness and overpowers Nat. selection with gene flow)