Species and Their Formation - Laboratory of Visual Systems

(a)SPECIES AND THEIR FORMATION 483(b)Agelaius phoeniceus (male, NY) Agelaius phoeniceus (male, CA) Agelaius phoeniceus (female)24.2 Members of the Same Species Look Alike—or Not (a) Bothof these male red-winged blackbirds are obviously members of thesame species, even though one is from the eastern United States andthe other is from California. (b) Because red-winged blackbirds aresexually dimorphic (see Chapter 23), the female of the speciesappears quite different from the male.productively incompatible. Alternatively, they may becomereproductively incompatible before they evolve any noticeablemorphological differences. But how do these differencesbetween populations come about?How Do New Species Arise?Speciation is the process by which one species splitsinto two or more daughter species, which thereafter evolveas distinct lineages. Although Charles Darwin titled his bookThe Origin of Species, he did not extensively discuss speciation,a process he called “the mystery of mysteries.” He devotedmost of his attention to demonstrating that species areIncreasingGenetic distanceIncreasing01 A barrier isestablished.Interbreedingpopulation(parent species)2 The two populationsdiverge genetically butare still reproductivelycompatible.TimeDaughterspecies AReproductiveincompatibilityis established.Daughterspecies B24.3 Speciation May Be a Gradual Process In this hypotheticalexample, genetic divergence between two separated populationsbegins before reproductive incompatibility evolves.3altered by natural selection over time. But not all evolutionarychanges result in new species: A single lineage maychange over time without giving rise to a new species.The critical event in speciation is the separation of the genepool of the ancestral species into two or more separate andisolated gene pools. Subsequently, within each isolated genepool, allele and genotype frequencies may change as a resultof the action of evolutionary agents. If two populations areisolated from each other, and sufficient differences in theirgenetic structure accumulate during the period of isolation,then the two populations may not be able to exchange geneswhen they come together again. As we will see, the amountof genetic difference that is needed to prevent gene exchangeis highly variable. Gene flow among populations may be interruptedin two major ways, each of which characterizes amode of speciation.Allopatric speciation requires total genetic isolationSpeciation that results when a population is divided by aphysical barrier is known as allopatric (allo-, “different”; patris,“country”) or geographic speciation (Figure 24.4). Allopatricspeciation is thought to be the dominant mode ofspeciation among most groups of organisms. The physicalbarrier that divides the range of a species may be a waterbody for terrestrial organisms, dry land for aquatic organisms,or a mountain range. Barriers can form when continentsdrift, sea levels rise, glaciers advance and retreat, andclimates change. These processes continue to generate physicalbarriers today. The populations separated by such barriersare often large initially. They evolve differences for a varietyof reasons, especially because the environments inwhich they live are, or become, different.Allopatric speciation may also result when some membersof a population cross an existing barrier and found a new, isolatedpopulation. Populations established in this way usuallydiffer genetically from their parent populations because ofthe founder effect: A small group of founding individuals has