CHAPTER 24CHAPTER 24

The Origin Of SpeciesThe Origin Of Species

What is Speciation?What is Speciation?

• Speciation is the origin of new species

• Speciation increases our planet’s biological diversity.

• Therefore, speciation is an important part of the theory of evolution.

Macroevolution

• Microevolution is small changes in the gene pool of a species over a smaller period of time.

• Macroevolution is cumulative effects of speciation over vast periods of time

What is a Species?What is a Species?

• A species is a population or group of populations whose members are able to mate and produce fertile offspring.

• Cannot produce fertile offspring with other distinct populations or species.

Biological Species Concept

• What prevents biologically similar species from interbreeding?

Answer:

Reproductive Barriers. There are 2 kinds of reproductive barriers:

1. Prezygotic Barriers (Before gametes meet)

2. Postzygotic Isolation (After fertilization)

Prezygotic BarriersPrezygotic Barriers

1. Habitat IsolationTwo species live in the same area but in different habitats may encounter each other but will rarely mate.

Example:

2 species of garter snakes live in the same geographic area, but one lives in water, the other on land. So they will never mate.

Prezygotic BarriersPrezygotic Barriers

2. Temporal IsolationSpecies may mate at different times of the year, day, season, etc.

Example:The Eastern and Western Spotted skunks have some overlap of their geographic territories, but one mates in the winter and the other in the summer.

Prezygotic BarriersPrezygotic Barriers

3. Behavioral IsolationIn many animals, courtship displays and rituals are vital to reproduction.

Such behaviors can be very specific, varying between closely related species.

Male behaviors such as courtship calls, songs, and dances will only be recognized by females of the same species.

Prezygotic BarriersPrezygotic Barriers

4. Mechanical Isolation- cannot mate because not anatomically compatible. Often this occurs because the genitalia of different species are incompatible.

Example:Bush babies, a group of small arboreal primates, are divided into several species based on mechanical isolation. Each species has distinctly shaped genitalia that, like locks and keys, only fit with the genitalia of its own species.

Prezygotic BarriersPrezygotic Barriers

5. Gametic Isolation The gametes may not be able to fuse and fertilization does not take place.

• The sperm (or pollen) of one species may not be able to survive in the environment of the female reproductive tract of the other species

• The female immune system may recognize the sperm (or pollen) as foreign and attack it.

• The sperm or pollen may not have the correct apparatus/ surface proteins to fertilize the egg or ovule.

Postzygotic BarriersPostzygotic Barriers

Even if the sperm of one species manages to Even if the sperm of one species manages to fertilize an ovum of another species, other fertilize an ovum of another species, other barriers prevent the development of a fertile barriers prevent the development of a fertile adult.adult.

Postzygotic BarriersPostzygotic Barriers

1.1. Reduced hybrid viabilityReduced hybrid viability genetic incompatibility between two species genetic incompatibility between two species

causes abortion of development or the birth of causes abortion of development or the birth of unhealthy, weak individualsunhealthy, weak individuals

Example:Example: 2 particular salamander species mate and produce 2 particular salamander species mate and produce

offspring. The offspring usually die soon after offspring. The offspring usually die soon after birth or do not survive to sexual maturity.birth or do not survive to sexual maturity.

Postzygotic BarriersPostzygotic Barriers

2.2. Reduced hybrid fertilityReduced hybrid fertilityeven if hybrid is born, lack of fertility keeps even if hybrid is born, lack of fertility keeps

reproductive isolation.reproductive isolation.

Example:Example:

Horses mate with donkeys to produce mules.Horses mate with donkeys to produce mules.

Mules are healthy, but sterile.Mules are healthy, but sterile.

Postzygotic BarriersPostzygotic Barriers

3.3. Hybrid BreakdownHybrid Breakdownsometimes you will have a fertile hybrid that cannot sometimes you will have a fertile hybrid that cannot

produce fertile offspring.produce fertile offspring.

Example:Example:

2 different species of rice can produce offspring 2 different species of rice can produce offspring that are fertile. But when those offspring mate, that are fertile. But when those offspring mate, their offspring (F2 generation) are steriletheir offspring (F2 generation) are sterile

Biological Species Concept doesn’t work???Biological Species Concept doesn’t work???

• Well, it doesn’t work in all situations.

• Organisms that reproduce asexually will not be limited by this trend of barriers.

• Does not apply to some sexual organisms

  Table 24.1 Six Concepts of Species Compared 

Biological species concept

Emphasizes reproductive isolation, the potential of members of a species to interbreed with each other but not with members of other species.

Morphological species concept

Emphasizes measurable anatomical differences between species. Most species recognized by taxonomists have been designated as separate species based on morphological criteria.

Recognition species concept 

Emphasizes mating adaptations that become fixed in a population as individuals "recognize" certain characteristics of suitable mates.

Cohesion species concept

Emphasizes cohesion of phenotype as the basis of species integrity, with each species defined by its integrated complex of genes and set of adaptations. 

Ecological species concept Emphasizes species' roles (niches), their

positions and functions in the environment.

Evolutionary species concept Emphasizes evolutionary lineages and

ecological roles. 

Other species concepts

Modes Of SpeciationModes Of Speciation

• Allopatric Speciation- a geographical barrier that physically isolates populations initially blocking gene flow. Populations that are segregated by a geographic barrier are known as allopatric populations

• Sympatric Speciation- New species arise amidst an existing parent population

Allopatric SpeciationAllopatric Speciation

• If new individuals colonize a new, geographically isolated area, the colonizing population may vary from the parent population

• When populations become allopatric, speciation can occur as isolated gene pools accumulate genetic differences by microevolution.

Allopatric Speciation

 First geographic isolation...•Unless populations are geographically isolated they will continue to interbreed ...then genetic isolation•Populations diverge to the point where they no longer interbreed •This may be due to adaptation to different environments, or genetic drift

Allopatric speciation of squirrels in the Grand Canyon (see Campbell fig. 24.7, p. 451).The canyon is a barrier to dispersal by small mammals, and as a consequence the isolated populations can diverge.

Sympatric SpeciationSympatric Speciation

• New species arise within the parent New species arise within the parent populationspopulations

• How can reproductive barriers evolve How can reproductive barriers evolve if the species are in contact with each if the species are in contact with each other?other?

How Sympatric Speciation How Sympatric Speciation OccursOccurs

1.1. Polyploidy- accident in cell division Polyploidy- accident in cell division resulting in extra sets of chromosomesresulting in extra sets of chromosomes– Autopolyploid- an individual with more than Autopolyploid- an individual with more than

two chromosome sets as a result of polyploidy two chromosome sets as a result of polyploidy - all chromosomes came from the same - all chromosomes came from the same speciesspecies

– Allopolyploid – results from two different Allopolyploid – results from two different species breeding and producing polyploid species breeding and producing polyploid hybrids. The chromosomes came from hybrids. The chromosomes came from different speciesdifferent species– The hybrids may be sterile and reproduce asexuallyThe hybrids may be sterile and reproduce asexually– The hybrids can, in subsequent generation become The hybrids can, in subsequent generation become

fertile and reproduce sexually.fertile and reproduce sexually.

Polyploidy is more common in plants than Polyploidy is more common in plants than in animalsin animals

Sympatric Speciation, Sympatric Speciation, cont’d.cont’d.2.2. Habitat differentiation and Sexual SelectionHabitat differentiation and Sexual Selection

Sometimes, members of a population of the same Sometimes, members of a population of the same species select a different niche. species select a different niche.

This can lead to the formation of certain This can lead to the formation of certain reproductive barriers such as temporal, behavioral, reproductive barriers such as temporal, behavioral, habitat, etc. habitat, etc.

Even though they are still part of the same species, Even though they are still part of the same species, this is sympatric speciation in progressthis is sympatric speciation in progress

Adaptive RadiationAdaptive Radiation

• The evolution of many diverse The evolution of many diverse species from one common ancestorspecies from one common ancestor

• Example: Galapagos finchesExample: Galapagos finches

Adaptive RadiationAdaptive Radiation

• Galapagos Island- finches( peripheral isolate formed new species from south America - mainland)

• Multiple invasions would eventually lead to coexistence of distinct species

• Islands- far enough to evolve in isolation, close enough to occasionally disperse.

Adaptive Radiation

Gradualism VS. Punctuated Gradualism VS. Punctuated EquilibriumEquilibrium

Gradualism- species descended from a common Gradualism- species descended from a common ancestor, gradually diverge more and more in ancestor, gradually diverge more and more in morphology as they acquire adaptationsmorphology as they acquire adaptations

Punctuated equilibrium- new species diverge Punctuated equilibrium- new species diverge from the parent species in “spurts of rapid from the parent species in “spurts of rapid change” instead of the gradual change change” instead of the gradual change mentioned in gradualism, then there is a long mentioned in gradualism, then there is a long period of equilibrium where no changes occur.period of equilibrium where no changes occur.

In other words – species change the most when they first In other words – species change the most when they first “bud” from the parent species and then don’t change “bud” from the parent species and then don’t change much for the rest of the existence of that species.much for the rest of the existence of that species.

Gradualism vs. Punctuated Equilibrium

But which theory does the fossil record support?

• Paleontologists rarely find gradual transitions in fossil forms.

• Instead, when they look in rocks, they find that new species appeared quite suddenly, then stayed for a while and then disappeared rather suddenly

• Paleontologists believe (as did Darwin) that the period of time during which a species underwent change were quite short compared to the period when they remained unchanged.– Which theory of speciation does that sound like?

Heterochrony

• Evolutionary changes in the rate of development of a species

Examples of heterochrony:

- Allometric growth

- Paedomorphosis

Heterochrony• Allometric Growth: When different parts of the body grow at

different rates in order to give that organism its native and proportionate shape. The head, limbs, and body grow at different rates, resulting in a human adult with proportions completely different from those of the newborn baby:

Paedomorphosis

• When an organism grows to its full size, reaches sexual maturity and reproduces, while maintaining certain juvenile characteristics.

This axolotl This axolotl salamander is full salamander is full grown and able to grown and able to reproduce, but reproduce, but maintains larval maintains larval characteristics characteristics such as its gillssuch as its gills

Homeosis• The

transformation of one body part into another, due to mutations in or misexpression of specific developmentally critical genes.

Which set of genes are we talking about? Which set of genes are we talking about?

Branched Evolution of

Horses

• Horses got bigger, developed teeth for grazing and reduced the number of toes

THE ENDTHE END