1. The Origin of Species
Chapter 24

2. The Origin of Species
Speciation- the process by which one species splits into two or more species
Microevolution- changes over time in allele frequencies in a population
Macroevolution- the broad pattern of evolution over long time spans
Described by Darwin. Speciation is responsible for the tremendous diversity of life, repeatedly yielding new species that differ from existing ones. Speciation explains not only differences between species, but also similarities between them. When one species splits, the species that result share many characteristics because they descended from this common ancestral species. Speciation also forms a conceptual bridge between microevolution and macroevolution. The word species is Latin for “kind” or “appearance”
Examples include mutations, natural selection, genetic drift, and gene flow
To be discussed more in next chapter

3. Biological Species Concept
Species- a group of population whose members have the potential to interbreed in nature and produce viable, fertile offspring.
Reproductive Isolation- the existence of biological factors (barriers) that impede members of two species from producing viable, fertile offspring.
Prezygotic Barriers
Postzygotic Barriers
*See Figure 24.4
The primary definition of species used in this textbook is referred to as the biological species concept, described in 1942 by Ernst Mayr. According to this a species is a group of population whose members have the potential to interbreed in nature and produce viable, fertile offspring- but do not produce viable, fertile offspring with members of other such groups. Thus the members of a species are united by being reproductively compatible, at least potentially. (all humans belong to the same species, but humans and chimps remain in distinct specie because many things keep them from interbreeding and producing viable offspring). Remember that diversity within a species is a result of gene flow, genetic drift, and natural selection.
*The greater the distance between two populations, the more reproductive isolation that occurs!
Because biological species are defined in terms of reproductive compatibility, the formation of new species hinges on reproductive isolation. Such barriers block gene flow between species and limit the formation of hybrids, offspring that result from an interspecific mating.
Prezygotic barriers block fertilization from occuring. Such barriers typically act in one of three ways: by impeding members of different species from attempting to mate, by preventing an attempted mating from being completed successfully, or by hindering fertilization if mating is completed successfully. If a sperm from one species overcomes prezygotic barriers and fertilizes an ovum from another species, a variety of postzygotic barriers may contribute to reproductive isolation after the zygote is formed. (ie: development errors may reduce survival of embryo or problems after birth may cause hybrids to be infertile or may decrease their chance of surviving long enough to reproduce).
Discuss figure 24.4 (pg )

4. Other Definitions of Species
Morphological Species Concept- characterizes a species by body shape and other structural features.
Ecological Species Concept- views species in terms of their ecological niche.
Phylogenetic Species Concept- defines a species as the smallest group of individuals that share a common ancestor.
*more than 20 other species definitions have been proposed
The biological species concept emphasizes the separateness of species from one another due to reproductive barriers, but other definitions emphasize the unity within a species.
Advantages to morphological species concept: it can be applied to asexual and sexual organisms (whereas biological can’t) and you don’t have to rely on gene flow (such as the fertility and viability of an offspring). In practice, this is how scientists distinguish most species. The disadvantage is that it has subjective criteria and researchers may disagree on which structural features distinguish a species.
Ecological niche- the sum of how members of the species interact with the nonliving and living parts of their environment. (Ex: two species of amphibians might be similar in appearance but differ in the foods they eat or their ability to tolerate certain environmental conditions. This definition also accommodates sexual and asexual species and emphasizes the role of disruptive natural selection as organisms adapt to different environmental conditions).
Phylogenetic- form one branch on the phylogenetic tree. Biologists trace the phylogeny of a species by comparing its characteristics (morphology, molecular sequences, etc) with that of other organisms. They then distinguish groups of individuals different enough to be separate species. The difficulty with this concept is determining the degree of difference required to indicate separate species.
*Each definition is useful in different situations. For our study of how species originate, we will look mostly at the biological species concept.

5. Types of Speciation Allopatric Speciation Sympatric Speciation
A population forms a new species while geographically isolated from its parent population
Sympatric Speciation
A small population becomes
a new species without
geographic separation.
Adaptive Radiation
Creates a better match
between organism and
environment
*Which picture depicts which type of speciation?
Allos=other, patric=homeland (Greek)
Ex: the water level in a lake may subside, resulting in two or more smaller lakes that are now home to separated populations. Or a river may change course and divide a population of animals that cannot cross it. It can also occur without geographic remodeling, such as when individuals colonize a remote area and their descendants become geographically isolated from the parent population. (likely in many of the Galapagos finches). *The level of geographical separation may be different based on the organism. (Different species of squirrels on opposing sides of the grand canyon, but the same species of birds). Once allopatric speciation occurs, gene pools may diverge. Different mutations arise, natural selection occurs, and genetic drift alters allelic frequencies. Gene pools of highly isolated populations experience very little gene flow and are particularly likely to undergo allopatric speciation. (Ex: the few animals and plants from the S. and N. American mainlands that colonized the Galapagos Islands gave rise to all the new species now found there). Evidence: when Madagascar broke off of India, two frogs that shared a common ancestor lived in India before it broke apart. Following the break up, allopatric speciation occurred within the separated populations of this common ancestor, resulting in the formation of many new species in each location.
Sym= together. Sympatric speciation is less common than allopatric speciation because of gene flow, etc. Sympatric speciation can occur if gene flow is reduced by such factors as habitat differentiation, sexual selection, and polyploidy. (note that these factors can also promote allopatric speciation).
Habitat Differentiation- when genetic factors enable a subpopulation to exploit a habitat or resource not used by the parent population. (when individual organisms move to a new habitat-ie: maggot flies move to different trees- they then begin to show isolation from the original habitat – sometimes they will form a new subspecies)
Sexual Selection- mate choice is a reproductive barrier that normally keeps the gene pools of species separate (if you have two species of an organism that are different colors, they are not likely to mix because the females of each species prefer their respective color)
Polyploidy can cause a new species. *What is polyploidy (extra set of chromosomes). There are two distinct forms of polyploidy, autopolyploidy and allopolyploidy.
*Review recap on pg

6. 2n = 6 4n = 12 2n 4n Failure of cell division after chromosome
Fig
2n = 6
4n = 12 2n 4n
Failure of cell
division after
chromosome
duplication gives
rise to tetraploid
tissue.
Gametes
produced
are diploid..
Offspring with
tetraploid
karyotypes may
be viable and
fertile.
2n = 4
Unreduced
gamete
with 4
chromosomes
Unreduced
gamete
with 7
chromosomes
Hybrid
with 7
chromosomes
Top is autopolyploidy- an individual that has more than two chromosomes sets and all are derived from a single species, ie: a failure of cell division could double a cell’s chromosome number from the diploid number, 2n, to a tetraploid number, 4n. This mutation causes the tetraploid to be reproductively isolated from diploid plants of the original population, because the triploid (3n) offspring of such unions have reduced fertility. However the tetraploid plants can produce fertile tetraploid offspring by self-pollinating or mating with other tetraploids. Thus in just one generation, autoploidy can generate reproductive isolation without any geographic separation.
Bottom is allopolyploidy- when two different species interbreed and produce hybrid offspring. Most hybrids are sterile because the set of chromosomes from one species cannot pair during meiosis with the set of chromosomes from the other species, because they are non-homologous. However, an infertile hybrid may be able to propagate itself asexually (as many plants can do). In subsequent generations, various mechanisms can change a sterile hybrid into a fertile polyploid called an allopolyploid. The allopolyploids are fertile when mating with each other but cannot interbreed with either parent species; thus they represent a new biological species. Wheat is allohexaploid, meaning it comes from two set of three different species.
Botanists estimate that more than 80% of the plant species alive today are descended from ancestors that formed by polyploid speciation.
Meiotic
error
Normal
gamete
n = 3
Viable fertile
hybrid
(allopolyploid)
2n = 10
Normal
gamete
n = 3
Species A
2n = 6

7. Reproductive Isolation
Hybrid Zone- a region in which members of different species meet and mate.
Reinforcement- strengthening reproductive barriers
Fusion- weakening reproductive barriers
Stability- continued formation of hybrid individuals
Produces some offspring of mixed ancestry. Results when separated populations come back into contact with one another. Discuss book example of yellow-bellied vs. fire-bellied toad- draw map on the board. Discuss how this affects allele frequencies. In this case the hybrid toads do not have as much relative fitness with higher rates of mortality and physiological abnormalities which lead to lower survival and reproductive rates. Therefore, these hybrid alleles are rarely passed on to other offspring.
There are three possible outcomes for the hybrid zone over time. Reproductive barriers between species may be strengthened over time (limiting the formation of hybrids), they may be weakened over time (causing the two species to form into a single species), or they can continue to be produced, creating a long-term stable hybrid zone.
Would each of these produce more or less hybrids?
Reinforcement occurs when hybrids are less fit than members of their parents species. With the strengthening or REINFORCEMENT of reproductive barriers, less hybrids are produced.
If the reproductive barriers are weak, the two hybrid species would likely fuse into a single species.
Stability- this can be seen with the frogs- biologists predict that if the hybrid zone were wider; this would be less likely to occur, since the center of the zone would receive little gene flow from distant populations outside the hybrid zone.

8. Speciation
Punctuated Equilibria- periods of apparent stasis punctuated by sudden change.
(a) Punctuated pattern
Other species do not show a punctuated pattern; instead, they change more gradually over long periods of time.
What do punctuated and gradual patterns tell us about how long it take a new species to form? Suppose that a species survived for 5 million years, but most of the morphological changes that caused it to be designated a new species occurred during the first 50,000 years of its existence- just 1% of its total lifetime. Time periods of this sort often cannot be distinguished in fossil strata, in part because the rate of sediment accumulation is too slow to separate layers this close in time. Thus, based on its fossils, the species would seem to have appeared suddenly and then lingered with little or not change before becoming extinct. Even though such a species may have originated more slowly than its fossils suggest (in this case take 50,000 years), a punctuated pattern indicates that speciation occurred relatively rapidly. For species whose fossils change much more gradually, we also cannot tell exactly when a new biological species forms, but it is likely that it occurred over a long period of time.
What is the total length of time between speciation events? This interval consists of the time that elapses before populations of a newly formed species start to diverge from one another plus the time it takes for speciation to be complete once divergence begins. It turns out that the total time between speciation events varies considerably (4,000 to 40 million years) with an average of 6.5 million but rarely fewer than 500,000 years.
Therefore, millions of years may pass before a newly formed species will give rise to another new species. Speciation begins only after gene flow between populations is interrupted, perhaps by an unpredictable event such as a storm that transports a few individuals to an isolated area.
How many genes change when a new species forms?
-in a few cases, it may be just a single gene such as in Japanese snails which determines the direction of the spiral, which effects the way that genitals are oriented making mating impossible. – a prezygotic barrier
In other organisms the speciation process is influenced by larger number of genes and gene interactions (20 or more)
Speciation may begin with very small differences but as it occurs over and over, differences accumulate and become more and more pronounced eventually leading to the formation of new organisms that differ greatly from their ancestors.
As one species increases in size by producing many new species, another may shrink, losing species to extinction for one reason or another. The cumulative effects effect the overall evolutionary changes that are observed.
Time
(b) Gradual pattern

9. Exit Slip
Is allopatric speciation more likely to occur on an island close to a mainland or on a more isolated island of the same size? Explain your prediction.