Macroevolution: The Genesis of Reproductively Isolated Populations from an Ancestral Population

Over generations, a population can undergo a great deal of change from its original state. But all members of that population are still members of the same species unless some members become reproductively isolated from one another. Speciation is the separation of two previously interbreeding populations into two populations that can no longer mate to produce fertile, viable offpring.

Modes of Speciation

  • allopatric speciation - a single population is divided into two by a geographic barrier.
  • peripatric - a new species arises at the edge of the range of the orignal population.
  • parapatric speciation - a "gradient" of genetic (and possibly phenotypic) difference develops across a species' range.
  • sympatric speciation - speciation occurs without physical separation, within the range of the ancestral population.

    These can be graphically represented as:

    Let's have a LOOK. (Thanks, Berkeley!)

    The Pace of Evolution

    How quickly does evolution proceed? It depends.

    PHYLETIC GRADUALISM
    This is the classical, traditional view stating that large changes (reproductive isolation and morphological differentiation) occur due to the gradual accumulation of many genetic changes. The classic example put forth in many natural history museums in the form of a nice display is that of the evolution of the modern horse.

    (NOTE: The modern systematist would not suggest that each of these species evolved into the next, more recent species. Rather, all these ancestral "steps" to the moder horse share a common ancestor that may have looked a lot like the "Dawn Horse," Hyracotherium (formerly known as Eohippus.

    PUNCTUATED EQUILIBRIUM
    This hypothesis was published in 1972 by Niles Eldredge and Stephen J. Gould.
    They suggested that major changes can occur relatively suddenly, and that they "punctuate" long periods of relatively little change. Let's have a LOOK.

    Remember: "suddenly" is a relative term, geologically speaking, and can mean over thousands of generations (quick!) instead of over millions (not so quick!)

    Eldredge and Gould suggested that this could explain how "awkward" intermediate forms such as the reptile-->flying bird and the terrestrial tetrapod-->swimming cetacean might have been "skipped". A major genetic event could have produced a phenotype that was drastically different from the original, and that this trait could become modified and fixed in the population over relatively few generations.

    Examples:

    Speciation Speciation is a temporal process. Populations exist in various stages of this process at any given time, and present day populations are even now undergoing microevolutionary processes which may eventually give rise to macroevolution.

    Species that are on the verge of becoming separated are known as incipient species.

    How do species change into different species?

    There are two major competing hypotheses:

    Adaptive Radiation An ancestral species can give rise to a variety of diverse species through repeated cladogenesis if its descendant species "radiate" into new ecological niches, a process that helps drive their diversification. When this occurs, the group of related species is said to have undergone adaptive radiation. This diversification can be driven by any (or more than one) of the five factors that can alter allele frequencies: mutation, migration, assortative mating, genetic isolation, natural selection.

    Example:
    On the Hawaiian islands, a single, finchlike ancestor gave rise to about 40 different species of Honey Creepers. Each is specialized in bill shape and size, as selected by its particular microhabitat and diet. Colors may have evolved in response to sexual selection.

    What do phenotypic characters tell us about evolution?
    Phenotype (at many levels, including the level of the DNA) provides the the biologist with the most basic information s/he needs in trying to accurately reconstruct evolutionary relationships.

    The goal of the modern systematist (i.e., a biologist who studies the evolutionary relationships between organisms) is to construct taxa (classification groups) that are monophyletic - derived from a single common ancestor.

    In so doing, the systematist considers homologies, analogies, primitive and derived characters in the taxa under study at the level of morphology, ontogeny, and the biological macromolecules (DNA, RNA, proteins) themselves.

    Reproducive Isolating Mechanisms

    When one species gives rise to two new species (cladogenesis), what is it that determines whether or not the two can reproduce, if allowed to regain physical contact (i.e., if they become sympatric once again)? We can define separate species by considering the mechanisms that restrict gene flow between them.

    Two related species may be separated by one or more of these types of reproductive isolating mechanisms, which may evolve--once again--due to the five factors already discussed that can change allele and genotype frequencies in a population.

    Sociobiology

    In 1975, Edward O. Wilson published Sociobiology: The New Synthesis. The controversy generated by one idea in his book spread from biology to many other disciplines, including those in the humanities and the social sciences.

    Wilson's main tenet:

    Social behavior is at least partly under genetic control.

    The book consists of 26 chapters, but the only one that caused a ruckus is the one that applies the hypothesis to Our Favorite Ape: Homo sapiens.

    While it's no doubt true (as some critics have suggested) that one cannot apply the biology of social insects to the biology of humans, it is also quite likely that a great deal of human behavior is at least partly under the control of our genes, and hence, subject to natural selection and other evolutionary forces. (Thought it's a bit harder to study the connection in our species, due to ethical constraints!)

    In 1962, British zoologist Verno Copner Wynne-Edwards published Animal Dispersion in Relation to Social Behavior, in which he suggested that animals regulated their own population density via behavior called altruism. defined as risking the loss of one's own fitness via an act that could improve the fitness of another individual.

    For example, Wynne-Edwards noted that under crowded conditions, many animals' reproduction is severely reduced or ceased altogether. He interpreted this behavior as "altruism" that was for the "good for the group." He hypothesized that a group that restrained its reproductive output and did not "overeat" its food supply would be more likely to survive than a group that reproduced without restraint, to the point of destroying its food supply and then starving.

    Wynne-Edwards believed that behaviors that improved the survival of some of a group's members would give that entire group an adaptive advantage over groups that did not have altruistic members.

    The idea of "group selection" was subject to severe criticism for many reasons.

    So how can such phenomena as

    ...be explained without invoking altruism?

    W.D. Hamilton was the first (in 1964) to develop ideas that explained apparently altruistic acts without resorting to the illogical "group selection" idea.

    Perhaps his most profound concept was that natural selection would favor an allele that promoted altruistic behavior toward relatives, since relatives share the alleles of the altruistic organism. By being altruistic to a relative, you are actually increasing the likelihood that some of your alleles will be passed on to future generations.

    (Those interested in human medicine might take note that W.D. Hamilton was also well known for his support of the hypothesis that the AIDS pandemic in Africa might have involved a polio vaccine campaign. The idea has been dismissed by many as having been well refuted, but it persists in some circles.)

    Inclusive Fitness, Individual Fitness and Kin Selection, oh my.

    We already know that the Darwinian fitness of a particular phenotype/genotype is its reproductive contribution to subsequent generations relative to an alternative phenotype/genotype.

    An individual's inclusive fitness may have a greater contribution from individual fitness or from kin selection, depending on the species' natural history, depending to a great degree on whether a species is solitary or social. Why is kin selection not altruism?

    Consider The Marmoset.

    This is a tiny, New World monkey who lives in social groups consisting of

  • Group living is critical to the survival of these monkeys.
  • Queen supresses ovulation in her daughters by behavioral bullying/stress.
  • Aunts help rear their siblings.
  • How could this possibly be adaptive for the aunt monkeys? (Of course, the monkeys aren't aware of the math. Genes that foster kin selection promote their own passage to future generations simply by fostering the 50% likelihood that they'll be passed along in any given individual.)

    Why should an "aunt" not take the chance to contribute all of her genes to future generations (In the form of multiple offspring, as the queen does)?

    Now consider the Honeybee.

    These are social hymenopteran insects whose populations are haplodiploid.

    The kin selection advantage is even greater in this case.

    Granted, the above scenarios make some rather arguable assumptions:
  • The behaviors that foster these genetic events are heritable
  • The worker bees all have the same drone father

    But the probability of promoting one's own genes' survival is a much more logical explanation for apparently "altruistic" behaviors than Wynne-Edwards's idea of group selection.

    Like physical traits, heritable behavioral traits (and if you believe E.O. Wilson and many others, all animal--including human--behaviors have at least some genetic component at their root) may be either

    And recall that neutral traits may becoem adaptive or maladaptive if the organism's environment changes.

    What makes a species a species? It depends on whom you ask.

  • morphological species concept: based on phenotypic characters. "If they look the same, they are the same." (This is the most "old fashioned" concept, and is largely obsolete)

  • biological species concept: based largely on reproductive isolation. If two individuals can produce fertile, viable offspring together under natural conditions, they are the same species.

  • recognition species concept: Mating and courtship specializations should also be considered as part of the species if they contribute to reproductive isolation.

  • cohesion species concept: Even if--under certain circumstances--two populations share genes, they can be considered two separate species if one population has a phenotype distinct from that of a related population with a different, distinct phenotype.

    "A population does not lose its species status when an individual belonging to it makes a mistake." --Ernst Mayr

    Unanswered questions:

  • What makes some species cohesive (unlikely to undergo genetic divergence), and others highly likely to undergo speciation?

  • How long does speciation take?

    It depends on the population. Speciation rate is affected by such factors as generation time, and what appears to be the poorly understood relative stability of certain species' genetic makeup.

    But unanswered questions is what makes all this fun, right?