From Biol301

Contents 1 The Origin of Species 2 What is a Species 2.1 The Biological Species Concept Emphasizes Reproductive Isolation 2.1.1 Prezygotic Barriers 2.1.2 Postzygotic barriers 2.2 The Biological Species Concept has some Major Limitations 2.3 Biologists ahve Proposed Several Alternative Concepts of Species 3 Modes of Speciation 3.1 Allopatric Speciation: Geographic barriers can lead to the origin of species 3.1.1 Condistions for Allopatric Speciation 3.1.2 Ring Species: Allopatric Speciation in Progres? 3.1.3 Adaptive Radiation on Island Chains 3.1.4 Ho Do Reproductive Barriers Evolve? 3.1.5 Summary of Allopatric Speciation 3.2 Sympatric Speciation: A new species can originate in the geographic midst of the parent species 3.2.1 Polyploid Speciation in Plants 3.2.2 Sympatric Speciation in Animals 3.2.3 Summary of Sympatric Speciation 3.3 The punctuated Equilibrium Model has Stimulated Research on the Tempo of Speciation 4 From Speciation to Macroevolution 4.1 Most Evolutionary Novelties are Modified Versions of Older Structures 4.2 "Evo-devo": Genes that control development play a major role in evolution 4.3 An Evolutionary Trend does Not Mean that Evolution is Goal Oriented

[edit] The Origin of Species

• "it is in new species that biological diversity arises. It is not enought to explain how adaptations evolve.... Evolutionary theory must also explaine macroevolution, the origin of new taxonomic groups."
• The fossil record shows evidence of two types of speciation:

1. Anagenesisn (phyletic): the accumulation of changes associated with the transormation of one species into another.
2. Cladogenesis (brancing): the budding of one or more enw species from a parent species that continue to exist. Only cladogenesis can promote biological diversity by increasing the number of species.

What is a Species

• Species comes from the latin word kind or appearance.
• Categorizing isn't quite as easy as just comparative biology.

The Biological Species Concept Emphasizes Reproductive Isolation

• Earnst Mayr in 1942 suggested the classic definition of a species: a species is the collection of animals that can breed to form viable offspring but cannot mate outside the same collection. (biological species concept)
• A species is the largest set of populations in which genetic exchange is possible.
• Reproductive barriers (defining the boundary of species) are split into two categories: Presygotic and postzygotic (before and after the gamete formed).

Prezygotic Barriers

• Habitat isolation: when the habitat two animals live in are different they are less likely to mate. (example: two garter snakes; one lives in water one is terrestrial)
• Behavior Isolation: probably the most important barrier between two close species; slight differences in the way animals act that keep them from mating. (examples: blinking pattern of fireflies, mating calls, etc.)
• Temporal Isolation: mating at different time of the day, season or year keeps species from interbreeding. (example: two skunks in one area, but one mates in the winter and one mates in the summer)
• Mechanical Isolation: barrier between species because they are anatomically incapable of mating. (example: some insects' genetilia won't fit together.)
• Gametic Isolation: a mechanistic barrier between the fusion of gametes into a zygote. (example: flowers and their pollen.)

Postzygotic barriers

• Reduced Hybrid Viability: even if a zygote is formed, it usually doesn't finish through growth; even if it does finish through growth, it will probably be frail.
• Hybrid Breakdown: when the first generation of a crossbreed is healthy and viable but the children of the cross (no matter who it mates with) are feeble or sterile (i.e. mules).

The Biological Species Concept has some Major Limitations

• One problem with the BSC is that it doesn't help for extinct or asexual species.
• Another problem is that in most situations we do not have enough data to determine if interbreeding is possible.

Biologists ahve Proposed Several Alternative Concepts of Species

• Ecological species concept: defining a species by the resources it uses, the habitat or host it lives in, and the special adaptations it has. This solves the asexual problem of using the Biological Species Concept.
• Pluralistic Species Concept: argues that each species must be defined in a different way; for some the environment they live in is extremely important, for some it's their isolation.
• Ecological, Pluralistic and Biological species concepts rely on explaining the existence of each species (i.e. why did this species come about from that, why did they differentiate, etc.).
• The Morphological Species Concept focuses more on letting morphology define the species. (structural features)
• Genealogical Species Concept: defining each species as a leaf on the tree of life. Each species is seen as one with a specific genetic history (where protein sequencing is coming in quite handy). Again, this concept focuses more on defining the species not determining why they came about.
• The Biological Species concept is particularly useful when discussing the origin of new species.

Modes of Speciation

• Speciation occurs in one of two modes based on the initial state of the population and their gene-flow:

1. Allopatric speciation (other-homeland): when two populations are geographically separated and therefore gene-flow decreases.
2. Sympatric speciation (together-homeland): when two populations overlap in geographic area, but biological factors (chromosomal changes and non-random mating) cause a decrease in gene flow.

Allopatric Speciation: Geographic barriers can lead to the origin of species

Condistions for Allopatric Speciation

• The geographic separation can be the cause of a mountain range, lake splitting, etc. or simply a group breaks off to found a new colony too far away to interbreed.
• As the population size decreases, the likelihood of allopatric speciation increases.
• Most populations that are cut / broken off end with extinction --only a few survive to produce a separate species.
• After recognizing the split, the question is "can the two split populations now interbreed?". (Scientists can either artificially put the two subjects together or make observations in the wild.) If they can they are still one species, if not speciation has occured.

Ring Species: Allopatric Speciation in Progres?

• The ring species example used salamanders in california. It is best explained by figure 24.9 on page 470 of the book.

Adaptive Radiation on Island Chains

• Adaptive radiation: when one species diversifies into several adapted forms.
• The Hawaiian Islands are very new and show diversity in their geographic and ecological states thus providing an ideal location for speciation.

Ho Do Reproductive Barriers Evolve?

• Note that geographic barriers are not biological barriers --just because they are separate doesn't mean they can't interbreed.
• Note that there is no drive to keep one species from interbreeding with another. The inability of two species to interbreed is usually a genetic coincidence (genetic drift / natural selection).
• An example of Evolution of a Prezygotic Barrier: Dodd and Drosophila
  • Dodd separated one strand of Drosophila into two families and grew them through several generations on different supplements (maltose and sucrose). When she performed mating experiments the families mated within the family (even different crops of the family) when presented with both maltose and sucrose mate choices.
  • There was some mating between sucrose and maltose strains.
  • This should be seen as a behavior barrier (as there was some interbreeding of the maltose and sucrose strands so it isn't mechanical isolation).
  • Pleiotropy is a suggested hypothesis for why divergence of supplement preference would affect mating. Pleiotropy is the idea that one allele affects more than one phenotypic trait. In this case, it is suggested that one of the alleles selected for for a given supplement also affects the courting process phenotype or the pherenomes phenotype.
• Example of Evolution of a Postzygotic Barrier: Vickery, monkey flower (Mimulus glabratus)
  • Vickery imported monkey flowers from all over the US. He then crossed them and observed the offspring for sterility and growth.
  • Although most grew, only nearby populations (i.e. Wisconsin / Michigan vs. Wisconsin / Mexico) produced offspring with significant portions being fertile.
  • Nearby populations probably retained the ability to crossbreed because they were naturally exposed to each other enough for it to be advantageous to be capable of interbreeding. There was no reason for a Wisonsin breed to retain the ability to mate with a Mexican version.

Summary of Allopatric Speciation

• "A new species forms while geographically isolated from its ancestor. As the isolated population's gene pool evolves by genetic drift and natrual selection, reproductive isolation from the ancestral species may evolve as a by-product of the genetic change. Such reproductive barriers prevent interbreeding with the ancestor, even if the populations come back into contact."

Sympatric Speciation: A new species can originate in the geographic midst of the parent species

• "new species arise within the range of parent populations rather than in geographically separate populations

Polyploid Speciation in Plants

• Autopolyploids can come-about if meiosis separation fails to occur leaving a tetraploid offspring. This offspring can then self-fertilize to become many offspring. This often leads to a new species because the tetraploid will produce triploid offspring when mating with an haploid mate; triploids are generally sterile.
  • Hugo de Vries discovered this in the early 1900s.
• Alloploids occur when parents from two different species mate. Although this would usually cause the death of offspring, it can sometimes cause a vigorous, autoreplication-only offspring. This being can then propagate itself long enought for some semi-random mechanisms to change it back to a sexually reproducing plant.
  • Scientists are able to document cases of alloploidy because it occurs so quickly.
  • We use allopolyploidy techniques to fuse the qualities of mutliple species (i.e. wheat and rye).

Sympatric Speciation in Animals

• Allopolyploidy is less likely in animals but it does occur.
• Genetic changes can have a huge effect on sympatric speciation in animals:
  • A change in what type of food source an organims likes, or a change in the mating habit of an organim can be the cause of a behavioral isolation within the domain of the parents.

Summary of Sympatric Speciation

• "Sympatric speciation requires the emergence fo some type of reproductive barrier that isolates the gene pool of a subset of a population without geographic separation from the parent population."

The punctuated Equilibrium Model has Stimulated Research on the Tempo of Speciation

• One theory points out that we would not see the new species in the form of fossils until their parent form had died out.
• Another theory suggests that diverging species undergo the most change at the beginning of the divergent process.
• Some argue that stasis is an illusion: that a species is constantly evolving even if its environment is not.

From Speciation to Macroevolution

• "Speciation is the boundary between microevolution and macroevolution."
• Speciation occurs when microevolution of a subgroup of a species causes reproductive isolation.

Most Evolutionary Novelties are Modified Versions of Older Structures

• Explanation of the logic for evolution of the eye: it started as a single photoreceptor cell that helped the organism stay alive simply because it could see a huge change in light. It developed into a focusable eye --helpful at every stage.
• "...structures that evolve in one context but become co-opted for another function are called exaptations."
  • Example: bird bones had a light-weight design before in the birds' ancestors before they took flight.

"Evo-devo": Genes that control development play a major role in evolution

• Evo-devo: "the interface between evolutionary biology and the study of how organisms develop".
• Allometric growth: the proportioning of growth of different parts of the body that give an organism its final look.
• Heterochrony is "evolutionary change in the rate or timing of developmental events."
• Paedomorphosis (child formation): when sexual maturity is sped up such that adults retain child-like traits.
• Macroevolution can occur because of changes in the layout of the body via homeotic genes.
  • These genes determine where body parts develop on the body (i.e. wing position, flower parts, etc.)
  • Hox genes in particular determine positioning of body parts on the developing embryo.
  • A hox gene is probably responsible for the movement of animals from fish-like creatures with a fin to terestrial animals with legs.

An Evolutionary Trend does Not Mean that Evolution is Goal Oriented

• Note that just because we see a trend doesn't mean there is some internal drive.
• The idea of evolution via natural selection can be applied to species in terms of determining how many species (offspring) are rooted from the subject species.