Chapter 13: The Origin of Species 13.1 What is a Species? ⢠Within a ...
Chapter 13: The Origin of Species 13.1 What is a Species?
Within a species, alleles flow among populations, evolutionary forces affecting one population also affect other populations Parts of the same whole, evolving as one linage; no cohesion between species Phylogenetic species concept – species are the smallest possible groups whose members are descended from a common ancestor and who all possess defining or derived characteristics that distinguish them from other such groups Biological species concept – Species are groups of actually (or potentially) interbreeding natural populations that are reproductively isolated from other such groups Metapopulation – a group of spatially separated populations of the same species that interact at some level (e.g., exchange alleles) General lineage species concept – species are metapopulations of organisms that exchange alleles frequently enough that they comprise the same gene pool, and therefore, the same evolutionary lineage
13.2 Barriers to Gene Flow: Keeping Species Apart
Isolating barrier – an aspect of the environment, genetics, behaviour, physiology, or ecology of a species that reduces or impedes gene flow from individuals of other species. Isolating barriers can be geographic or reproductive Geographic barriers to gene flow are features of the environment that physically separate populations from each other Most common barrier to gene flow Populations do not interbreed simply because they are geographically isolated from each other (i.e., they exist in allopatry) Allopatry – occurs when populations are in separate, non-overlapping geographic areas Sympatry – occurs when populations are in the same geographic area Separation of populations in space or in time reduces the likelihood they will exchange gametes Divergent behaviour (e.g., habitat preference, spawning time) acts as a reproductive barrier to gene flow Reproductive barriers to gene flow are intrinsic properties of organisms that reduce the likelihood of interbreeding between individuals of different populations May also play important roles in keeping some species apart Reproductive isolation – occurs when reproductive barriers prevent or strongly limit reproduction between populations. The result is that few or no genes are exchanged between the populations Floral traits can act as reproductive barriers to gene flow when their divergence causes them to attract different pollinators
Molecular incompatibilities between eggs and sperm can be important barriers to gene flow between species, causing conspecific sperm to be more likely to fertilize eggs than heterospecific sperm Gametic incompatibility – occurs when sperm or pollen from one species fails to penetrate and fertilize the egg of another species Genetic incompatibilities can be important barriers to gene flow between populations if they cause hybrid offspring to be sterile or to perform poorly Prezygotic reproductive barriers – aspects of the genetics, behaviour, physiology, or ecology of a species that prevent sperm from one species from fertilizing eggs from another species; reduce likelihood that a zygote will form Postzygotic reproductive barriers – aspects of the genetics, behaviour, physiology, or ecology of a species that prevent fertilized zygotes from successfully developing and reproducing themselves Bateson-Dobzhansky-Miller incompatibilities – molecular incompatibilities that arise when the genomes of two species are combined in hybrid offspring
13.3 The Origin of Isolating Barriers: How New Species Form
Populations can become allopatric if new geographic barriers appear, as occurs when continents drift apart, glacial ice sheets intrude, or mountains rise (vicariance), or if a subset of individuals disperses to a new and isolated location, like an island, watershed, or distant mountaintop (dispersal) Allopatric populations will begin to diverge genetically because they independently experience mutation, selection, and drift. Eventually, they may become sufficiently divergent that they no longer would or could interbreed, even if the physical barrier disappeared Reinforcement – the increase of reproductive isolation between populations through the selection against hybrid offspring Isolation by distance occurs because individuals tend to mate with individuals from the same or nearby populations, resulting in imperfect mixing of alleles across the geographic range of species Isolation by distance – populations that live nearby are genetically more similar to each other than populations that live farther apart Although ring species are relatively rare, they provide powerful illustrations of how new species can form in the absence of allopatry – that is, in populations that still experience some gene flow Ring species – a connected series of populations (each of which can interbreed with its neighbouring populations) that have diverged sufficiently across their ranges so that the populations at the end of the series are too different to interbreed Reproductive isolation can arise even when populations live in sympatry. New alleles accumulate independently in these populations anyway because individuals rarely interbreed Ecological speciation is a mechanism by which new species can evolve in sympatry (though it can apply to allopatric populations as well)
Ecological speciation – the evolution of reproductive barriers between populations by adaptation to different environments or ecological niches Sometimes when organisms adapt to different habitats or ecological situations, barriers to gene flow arise as a by-product
13.4 The Speed of Speciation
Because of the rapid and local coevolution that it triggers between males and females, sexual conflict can lead to evolution of reproductive barriers between populations The speed of speciation is highly dependent on the genetic architecture of organisms. In plants, mutations such as duplication events are common. In birds and mammals, however, reproductive isolation can take millions of years Allopolyploidy – polyploidy (more than two paired chromosomes) resulting from interspecific hybridization
Within a species, alleles flow among populations, evolutionary forces affecting one population also affect other populations Parts of the same whole, evolving as one linage; no cohesion between species Phylogenetic species concept – species are the smallest possible groups whose members are descended from a common ancestor and who all possess defining or derived characteristics that distinguish them from other such groups Biological species concept – Species are groups of actually (or potentially) interbreeding natural populations that are reproductively isolated from other such groups Metapopulation – a group of spatially separated populations of the same species that interact at some level (e.g., exchange alleles) General lineage species concept – species are metapopulations of organisms that exchange alleles frequently enough that they comprise the same gene pool, and therefore, the same evolutionary lineage
13.2 Barriers to Gene Flow: Keeping Species Apart
Isolating barrier – an aspect of the environment, genetics, behaviour, physiology, or ecology of a species that reduces or impedes gene flow from individuals of other species. Isolating barriers can be geographic or reproductive Geographic barriers to gene flow are features of the environment that physically separate populations from each other Most common barrier to gene flow Populations do not interbreed simply because they are geographically isolated from each other (i.e., they exist in allopatry) Allopatry – occurs when populations are in separate, non-overlapping geographic areas Sympatry – occurs when populations are in the same geographic area Separation of populations in space or in time reduces the likelihood they will exchange gametes Divergent behaviour (e.g., habitat preference, spawning time) acts as a reproductive barrier to gene flow Reproductive barriers to gene flow are intrinsic properties of organisms that reduce the likelihood of interbreeding between individuals of different populations May also play important roles in keeping some species apart Reproductive isolation – occurs when reproductive barriers prevent or strongly limit reproduction between populations. The result is that few or no genes are exchanged between the populations Floral traits can act as reproductive barriers to gene flow when their divergence causes them to attract different pollinators
Molecular incompatibilities between eggs and sperm can be important barriers to gene flow between species, causing conspecific sperm to be more likely to fertilize eggs than heterospecific sperm Gametic incompatibility – occurs when sperm or pollen from one species fails to penetrate and fertilize the egg of another species Genetic incompatibilities can be important barriers to gene flow between populations if they cause hybrid offspring to be sterile or to perform poorly Prezygotic reproductive barriers – aspects of the genetics, behaviour, physiology, or ecology of a species that prevent sperm from one species from fertilizing eggs from another species; reduce likelihood that a zygote will form Postzygotic reproductive barriers – aspects of the genetics, behaviour, physiology, or ecology of a species that prevent fertilized zygotes from successfully developing and reproducing themselves Bateson-Dobzhansky-Miller incompatibilities – molecular incompatibilities that arise when the genomes of two species are combined in hybrid offspring
13.3 The Origin of Isolating Barriers: How New Species Form
Populations can become allopatric if new geographic barriers appear, as occurs when continents drift apart, glacial ice sheets intrude, or mountains rise (vicariance), or if a subset of individuals disperses to a new and isolated location, like an island, watershed, or distant mountaintop (dispersal) Allopatric populations will begin to diverge genetically because they independently experience mutation, selection, and drift. Eventually, they may become sufficiently divergent that they no longer would or could interbreed, even if the physical barrier disappeared Reinforcement – the increase of reproductive isolation between populations through the selection against hybrid offspring Isolation by distance occurs because individuals tend to mate with individuals from the same or nearby populations, resulting in imperfect mixing of alleles across the geographic range of species Isolation by distance – populations that live nearby are genetically more similar to each other than populations that live farther apart Although ring species are relatively rare, they provide powerful illustrations of how new species can form in the absence of allopatry – that is, in populations that still experience some gene flow Ring species – a connected series of populations (each of which can interbreed with its neighbouring populations) that have diverged sufficiently across their ranges so that the populations at the end of the series are too different to interbreed Reproductive isolation can arise even when populations live in sympatry. New alleles accumulate independently in these populations anyway because individuals rarely interbreed Ecological speciation is a mechanism by which new species can evolve in sympatry (though it can apply to allopatric populations as well)
Ecological speciation – the evolution of reproductive barriers between populations by adaptation to different environments or ecological niches Sometimes when organisms adapt to different habitats or ecological situations, barriers to gene flow arise as a by-product
13.4 The Speed of Speciation
Because of the rapid and local coevolution that it triggers between males and females, sexual conflict can lead to evolution of reproductive barriers between populations The speed of speciation is highly dependent on the genetic architecture of organisms. In plants, mutations such as duplication events are common. In birds and mammals, however, reproductive isolation can take millions of years Allopolyploidy – polyploidy (more than two paired chromosomes) resulting from interspecific hybridization
