New specise with the help of a suitable example
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Evolutionary Biology, research related to the formation of new species - 'speciation ' - is rich in historical and current debate. Here, we review both early and modern views on speciation, starting with Darwin and finishing with current genomics-era insights.
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Darwin's "Mystery of Mysteries"
"... these forms may still be only ... varieties; but we have only to suppose the steps of modification to be more numerous or greater in amount, to convert these forms into species ... thus species are multiplied" (Darwin 1859, p. 120).
Discussion of most topics within Evolutionary Biology begins with Darwin. Indeed, On The Origin of Species (1859) continues to influence much of modern Evolutionary Biology. Darwin viewed evolution by natural selection as a very gradual mechanism of change within populations, and postulated that new species could be the product of this very same process, but over even longer periods of time. This eventual process of speciation by natural selection is illustrated by a sketch drawn by Darwin in his personal notebook nearly 20 years before the Origin of Species was published (Figure 1). Here, he proposed a model whereby lineages form from their ancestors by evolving different characters over relatively long periods of time. Darwin indicated that species could form by the evolution of one species splitting into two, or via a population diverging from its extant ancestor to the point it was a new species. Darwin's insights into evolution were brilliant, especially in light of their being made in the absence of genetics. Indeed, ideas about heredity and the introduction of new genetic material via mutation were to come long after Darwin's founding theories of evolution.
Darwin’s famous sketch indicating that evolution within species may eventually give rise to entirely new ones.
Figure 1: Darwin’s famous sketch indicating that evolution within species may eventually give rise to entirely new ones.
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The Modern Synthesis
A major turning point for evolutionary research occurred in the 1930s when Fisher, Haldane, Wright, Dobzhansky, and others, developed mathematical population genetic models to illuminate the genetic mechanisms of evolutionary change (Mayr & Provine 1998). The integration of genetics with models of natural selection shed tremendous light on, and strengthened Darwin's views on, evolution — here was the missing mechanism that introduced new variation into populations via mutation and recombination. Indeed, thanks to the Modern Synthesis, much of current research in Evolutionary Biology is strongly tied to genetics, and current methods for studying speciation are no exception. As discussed below, the Modern Synthesis led to advances not only in the study of evolution within populations, but also changes in the way species were defined, and in how new species were considered to form.
Barriers to reproduction.
Under the commonly used ‘Biological Species Concept' (Mayr 1942), the formation of new species involves the evolution of reproductive barriers to the production of viable offspring either before (pre-zygotic barriers) or soon after (post-zygotic barriers) mating. Thus, new species form when individuals from diverging populations no longer recognize one another as potential mates, or opportunities for mating become limited by differences in habitat use or reproductive schedules. In some cases, these pre-zygotic isolating mechanisms fail to prevent inter-breeding among individuals from separate populations. In these cases, viable hybrids may form, or the consequences of a successful mating attempt may end in failure, either due to the production of inviable zygotes or sterile, non-reproductive offspring. These diverse pre- and post-zygotic barriers are of great importance to speciation biologists because they determine how reproductively-isolated populations are from one another, which indicates how far along the often continuous process of speciation that populations are. For example, reproductive isolation is weak in the early stages of speciation, but changes to strong or complete in later stages of speciation (Figure 2). One or more of the many types of isolating mechanisms may play a role in the evolution of species along a continuum (Figure 2). But how and why might reproductive barriers to genetic exchange evolve?