11. Speciation

Question 1

What is a Species?

Answer 1

group of organisms whose members can breed and produce viable and fertile offspring typically by sexual reproduction

Question 2

What is Anagenesis?

Answer 2

• Also called phyletic evolution (nonbranching evolution)
• Accumulation of changes that gradually transform a given ENTIRE species into a species with different characteristics (evolution within a lineage) – so NO net increase in species diversity

Question 3

What is Cladogenesis?

Answer 3

• Also called branching evolution (diversifying evolution)
• One or more species arising from one original species (an evolution that results in the splitting of a lineage) – so net increase in species diversity
• Cladogenesis, unlike anagenesis, is a process of evolution in which a small subset of an existing species evolves, but NOT the entire species (this leaves the original species more or less intact, allowing for both the older species and the new species to co-exist)

Question 4

What is the Gradual Speciation Model?

Answer 4

In the gradual speciation model, species diverge gradually over time in small steps

Examples:

1. Small variations occur over time in a population of wolves — larger ears, longer teeth and a heightened sense of smell. Wolves with these helpful traits tend to survive better than those without; as time progresses slowly, the traits gradually become the norm among the population.
2. A species of butterfly is yellow and black in colour. However, a butterfly is born that happens to be orange and yellow in color making it difficult to see. Over a long period of time, the yellow and black butterflies die out, because the orange and yellow color combination makes the butterflies less visible to predators.

Question 5

What is the Punctuated Equilibrium Model?

Answer 5

In the punctuated equilibrium model, a new species undergoes changes quickly from the parent species, and then remains largely unchanged for long periods of time afterward

Examples:

1. A species of birds exist in stasis for many thousands of years. Suddenly, bacteria cause their primary tree of sheltering choice to die. The birds must adapt within the environment to trees that are much higher, requiring more wing strength. Some birds die. The remaining birds’ bodies adapt longer, stronger wings to survive. The strong birds survive meaning more are born until the others die out. The species returns to a state of stasis.
2. A cheetah species has no spots. However, due to a gene mutation, a cheetah cub is born with spots. Because this adaptation helps the cheetah to hide and survive, more cheetahs are born with spots. This continues until the spotless cheetahs are all replaced by spotted cheetahs. The species stays that way for many years.

Question 6

What is the Biological Species Concept?

Answer 6

The biological species concept defines species as:

– A group of individuals that can potentially interbreed in nature and produce viable, fertile offspring

• Individuals of different species are REPRODUCTIVELY ISOLATED from each other

Question 7

Why can’t we apply the biological species concept to all situations? (2)

Answer 7

– Does not apply to organisms that reproduce asexually all or most of the time, such as prokaryotes

– No way to evaluate the reproductive isolation of fossils (obviously they cannot mate any longer, although we still have to classify them)

Question 8

What is the Morphological Species Concept?

Answer 8

For example, most species can be classified based on similarities with respect to measurable physical traits (morphological species concept)

Question 9

What is the Ecological Species Concept?

Answer 9

Another approach is defining species as groups or populations that share the exact same ecological niche – or the functional role it plays within an ecosystem (ecological species concept)

Question 10

What is the Genetic Species Concept?

Answer 10

Also, one can define species with respect to similarities solely on the basis of molecular data (genetic species concept)

Question 11

What are Prezygotic Barriers?

Answer 11

Impede mating between species or hinder the fertilization of ova if members of different species attempt to mate

• before zygote is formed

Question 12

What are Postzygotic barriers?

Answer 12

Prevent zygote from developing into a viable and fertile adult

Question 13

What is Temporal Isolation?

Answer 13

– Species breed at different times (this means different times of day or different seasons or even different years!)

– Example: The western spotted skunk and the eastern spotted skunk overlap in distribution. Even though these species are very similar, the western spotted skunk breeds in late summer and the eastern in late winter.

Question 14

What is Habitat Isolation?

Answer 14

– Species found in different habitats but in same area that rarely comes across each other even though there are no obvious physical barriers

– Example: Two species of garter snake live in the same area, but one species lives mainly in water, and the other is mainly terrestrial.

Question 15

What is Behavioral Isolation?

Answer 15

– Every species has its own mating/courtship rituals and behaviors and because of this, different species will not attract each other which prevents them from mating

– Example: Male fireflies have different light flashing patterns, and only females of same species recognizes the pattern as being courtship behavior.

Question 16

What is Mechanical Isolation?

Answer 16

– When male and female sex organs are not compatible which means fertilization cannot occur

– Example: An example of this is the direction a snail’s shell spirals. If it spirals in the direction opposite from another snail, they will not be able to mate since their reproductive organs will not align.

• Example: Bush Babies, a group of small nocturnal 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.

Question 17

What is Gametic Isolation?

Answer 17

– Gametes do not unite to form a zygote (so egg and sperm cells are incompatible)

– Example: Many marine species broadcast their eggs and sperm in the water. The gametes of more than one species can encounter each other, but they generally will not fuse.

– Example: In sea urchins, a protein called bindin allows sperm cells to penetrate eggs. Differences in the amino acid sequence of binding can cause the failure of fertilization to occur between closely related populations.

Question 18

What is Hybrid Inviability?

Answer 18

– Zygote forms, but dies after a few series of cell divisions (genetic information from male and female parents were insufficient to carry the organism through morphogenesis)

– Most often, the hybrid embryo dies before birth…however, sometimes, the offspring develops fully with mixed traits from each parent, forming a frail, often infertile adult

– Example: Species of frogs (genus Rana) overlap in habitat use and sometimes hybridize. Their offspring usually die during development. A few frail offspring sometimes survive development but die shortly after they hatch into tadpoles.

Question 19

What is Hybrid Sterility?

Answer 19

– When the offspring of two parents cannot reproduce (offspring are sterile)

– Example: Male horse mates with a female donkey to produce a hinny that is sterile…or male donkey mates with female horse to produce a mule that is sterile

Question 20

What is Hybrid Breakdown?

Answer 20

– Some first-generation hybrids are viable and fertile, but when they mate with one another or with either parent species, offspring of the next generation are feeble or sterile

– Example: Different rice species can produce fertile hybrid offspring, but in the next generation, the hybrid’s offspring die as seeds or grow into weak defective plants

Question 21

What is Incipient Species?

Answer 21

The key to speciation is the evolution of genetic differences between the incipient species (a group of organisms that is about to become a separate species from other, related individuals)

For a lineage to split once and for all, the two incipient species must have genetic differences that are expressed in some way
that causes matings between them to either not happen or to be unsuccessful

Question 22

What are the Five general modes of speciation?

Answer 22

1. Allopatric speciation
2. Peripatric speciation
3. Parapatric speciation
4. Sympatric speciation
5. Artificial speciation

Question 23

What is Allopatric Speciation?

Answer 23

1. One large population is split into two populations that are separated by a physical barrier
   * A population can colonize a new habitat (dispersal and colonization)
   * A new physical barrier can split a widespread population into two or more isolated groups (a vicariance event)
2. The two populations diverge from each other due to genetic drift, natural selection, and mutation
3. END RESULT: The two populations become so different that they can no longer reproduce with each other due to pre- or post-zygotic barriers à they are separate species

Question 24

What is Peripatric speciation?

Answer 24

When small groups of individuals break off from the larger group and form a new species, this is called peripatric speciation

– As in allopatric speciation, physical barriers make it impossible for members of the groups to interbreed with one another

– The main difference between allopatric speciation and peripatric speciation is that in peripatric speciation, one group is much smaller than the other

Question 25

What is the Founder Population?

Answer 25

When small groups of individuals break off from the larger group and form a new species, this is called peripatric speciation

– The small population is referred to as the founder population

– Unique characteristics of the smaller groups are passed on to future generations of the group, making those traits more common among that group and distinguishing it from the others

Question 26

What is a well-documented example of peripatric speciation?

Answer 26

The evolution of the polar bear from the brown bear is a well-documented example of peripatric speciation

As brown bears dispersed northward from their original range, glaciation may have made movement southward difficult, isolating them from the more southern populations

Question 27

What is Parapatric speciation?

Answer 27

Parapatric speciation occurs when subpopulations of the same species are mostly isolated from each other, but have a narrow area where their ranges overlap (i.e., narrow contact zone)

– In parapatric speciation, a species is spread out over a large geographic area

– Although it is possible for any member of the species to mate with another member, individuals only mate with those in their own geographic region

– Like allopatric and peripatric speciation, different habitats influence the development of different species in parapatric speciation

– Instead of being separated by a physical barrier, the species are separated by differences in the same environment

Question 28

What are ring species?

Answer 28

This type of speciation is applicable to ring species (we will look at a couple of examples in upcoming slides – Ensantina salamanders and greenish warblers)

– Typical ring species are supposed to form a chain of intergrading populations (or a connected series of neighbouring populations that can interbreed with each other) encircling a geographic barrier, and the terminal forms in the secondary contact coexist without interbreeding

Question 29

What is Artificial speciation?

Answer 29

Creation of new species by people (which is achieved through lab experiments or animal husbandry)

• An experiment demonstrating artificial speciation in the fruit fly conducted by Diane Dodd of Yale University Artificial speciation

‒ A single population of flies was divided into two, with one of the populations fed with starch-based food and the other with maltose-based food

‒ After the populations had diverged over ~40 generations, the groups were again mixed; it was observed that the flies, for the most part, continued to prefer mating with others from the same original population

Flies raised on starch-based food mated with other flies raised on starch-based food

Flies raised on maltose-based food mated with
other flies raised on maltose-based food

Question 30

What is Sympatric speciation?

Answer 30

1. Occurs in groups that occupy the same geographic area
2. Within one population, two groups start to diverge from each other (with no physical/geographical barrier)
3. END RESULT: The two groups become separate species

Question 31

Apple maggot flies

Answer 31

• Around 200 years ago, the ancestors of apple maggot flies laid their eggs only on hawthorns
  — but today, these flies lay eggs on hawthorns (which are native to America) and domestic apples (which were introduced to America by immigrants and bred)
• Females generally choose to lay their eggs on the type of fruit they grew up in, and males tend to look for mates on the type of fruit they grew up in
• Therefore, hawthorn flies generally end up mating with other hawthorn flies and apple flies generally end up mating with other apple flies
• Some interbreeding still occurs between the apple-specialized flies and the hawthorn specialized flies, so they are not yet separate species (however, many scientists think this is a case of sympatric speciation in progress)

Question 32

Cichlid fish (Sympatric Speciation)

Answer 32

• Lake Victoria, in Eastern Africa, contains approximately 500 species of cichlid fish - all members of the same distinctive family, but differing in numerous details of morphology, behaviour and genetics
  – These closely related species probably all arose from one common ancestor with the driving force for speciation being competition for a limited resource (food) within the lake, and adaptation to new food sources

– This gave rise to different species that are kept from breeding with each other by distinctive coloration pattern

• Many studies point to sexual selection as a way of maintaining reproductive isolation
  – Female choice with regard to male coloration is one of the more studied modes of sexual selection in African cichlids

– Female choice is present in cichlids because the female does much of the work in raising the offspring, while the male has little energy input in the offspring

– She exerts sensory bias when picking males by choosing those that have colours similar to her or those that are the most colourful

Question 33

What is Polyploidy?

Answer 33

Polyploidy is a mechanism often attributed to causing some speciation events in sympatry
– Polyploidy is the presence of extra sets of chromosomes due to accidents during cell division

– When a meiotic error occurs, gametes (sperm and eggs) with the wrong number of chromosome copies can be produced

– Results in more than two full sets of chromosomes (i.e., triploid or 3n, tetraploid or 4n, hexaploid or 6n, etc.)

– If these gametes fuse, the new zygote, or fertilized egg, will have a different number of chromosomes than the parent species

Question 34

Where is polyploidy common?

Answer 34

Common in plants – important crops (oats, cotton, potatoes, tobacco and wheat) are polyploid

o Sympatric speciation by polyploidy is more common in plant species that can self fertilize

o This is because individuals with abnormal gametes are unlikely to find another individual with gametes of the same number of chromosomes to breed with

o Also, individuals grown from abnormal zygotes are unlikely to find an individual with matching gametes when they eventually mature and form gamete

Question 35

What is an autopolyploid?

Answer 35

An autopolyploid is an individual with more than two sets of chromosomes derived from one species

– EXAMPLE: Tetraploid plant might arise if chromosome separation errors in meiosis (non-disjunction) produced a diploid egg and a diploid sperm that then fertilized to make a tetraploid zygote

– When a tetraploid plant matures, it will make diploid (2n) eggs and sperm

– These eggs and sperm can readily combine with other diploid eggs and sperm via self-fertilization, which is common in plants, to make more tetraploids

– On the other hand, the diploid eggs and sperm may or may not combine effectively with the haploid (1n) eggs and sperm from the parental species

– Even if diploid and haploid gametes do get together to produce a triploid plant with three chromosome sets, this plant would likely be sterile because its three chromosome sets could not pair up properly (via synapsis) to form bivalents during prophase I of meiosis

– Because tetraploid plants and diploid species from which they came cannot produce fertile offspring together, we consider them two separate species