Chapter 24

Allopatric speciation

Speciation that begins with geographic isolation

Sympatric speciation

Speciation that occurs even though populations live within the same geographical area 2 events that can initiate SS - External events (disruptive selection for extreme phenotypes) - Internal events (chromosomal mutations)

The origin of a new plant species by hybridizing two existing species, coupled with accidents during cell division, is an example of ________.`

Symmpatric speciation and allopolyploidy

Among known plant species, which of these have been the two most commonly occurring phenomena that have led to the origin of new species?

Symmpatric speciation and polyploidy

3 criteria used to identify species

- Biological Species Concept - Morphospecies Concept - Phylogenetic Speices Concept

Why is speciation by polyploidy so common in plants?

- Diploid species have close relatives that are polyploid Genetic implications of polyploidy - Highter heterozygosity than their diploid relatives - Can tolerate high levels of self fertilization because they are not as affected by inbreeding depression as their diploid relatives - Genes on duplicated chromosomes can diverage independently increasing genetic variation *Driven by chromosome level mutations and occurs in sympatry. It is fast, sympatric, and common

Geographic isolation occurs in

- Dispersal = A population moves to a new habitat, colonizes it, and founds a new population - Vicariance = A physical barrier splits a population into subgroups that are physically isolated from each other

Disadvantage of the phylogeneic species concept

- Estimated phylogenies are available

Hybrid zones

- Geographic area where interbreeding occurs and hybrid offspring are common - Can be narrow or wide, long or short lived, and stable in one place or moving over time -, hybridization does not lead to extinction, but rather the opposite—the formation of new species.

Advantages of the phylogenetic concept

- It can be applied to any population (fossil, asexual, sexual) - It is logical because different species have different synamorphies only if they are isolated from gene flow and have evolved independently

Allopolyploidy

- It is functionally polyploid because it has two copies each of two sets of chromosomes (indicated by blue and red). In terms of genetic information, this offspring has four copies of genes that are common to both parental species, instead of just two.

Disadvantages of Morphospecies concept

- It may lead to naming two or more species when there is only one polymorphic species with differing phenotypes - It cannot identify cryptic species which differ in traits other than morphology - The morphological features used to distinguish species are subjective

Autoploidy

- Less common than allopolyploidy - Polyploid individuals are repeoductively isolated from the original diploid population

Reinforcement

- Natural selection for traits that prevent interbreeding among populations - Individuals that do not interbreed due to prezygotic isolation should be favored because they produce more viable or fertile offspring. - If closely related species are sympatric—meaning that they live in the same area—individuals from the two species will seldom mate with one another in the lab. - If the species are allopatric—meaning that they live in differ- ent areas—then individuals from the two species will often mate with one another in the lab

Sympatric speciation by disruptive selection

- Niche - range of ecological resources that a species can use and the range of conditions it can tolerate - Disruptive selection for mate choice has the potential to cause rapid divergence among sympatric species because it affects gene flow directly via prezygotic isolation

Sympatric speciation by poluploidization

- Polyploidy - condition of possessing more than two complete sets of chromosomes 2 Types of polyploidy - Audopolyploid - Individuals are produced when a mutation results in a doubling of chromosome # and the chromosomes all come from the same species - Allopolyploid - Individuals are created when parents of different speices mate and then an error in mitosis occurs resulting in viable offspring with 2 full sets of chromosomes

What happens when isolated populations come into contact?

- Populations fuse + gene flow erases any distinctions between them 3 Possibilities: - The distinctions between the populations may be reinforced if hybrids have low fitness. - Hybrid zones may be established if hybrids have intermediate fitness. - Speciation by hybridization may occur if hybrids have high fitnes

Mechanisms that stop gene flow between populations

- Prezgotic isolation - prevents individuals of different species from mating successfully - Postzygotic isolation - hybrid offspring of matings between members of different species do not survive/reproduce

Allopatric speciation by vicariance

1. Geographic isolation (chance event that physically separates population into subgroups) 2. Divergence... mutation, genetic drift, and selection 3. Genetic Isolation... the two populations are genetically isolated from one another

Allopatric speciation by dispersal

1. Geographic isolation (colonize a new habitat) 2. Divergence... new population begins to diverge due to mutation, genetic drift, and selection 3. Genetic isolation... the two populations are genetically isolated from one another

The largest unit within gene flow can readily occur is

A species

Speciation

A splitting event that forms two or more distinct species from a single ancestral species

How are two different species most likely to evolve from one ancestral species

Allopartically, after the ancestral species has split into two populations

Which process led to the formation of the species pairs of Pacific and Caribbean snapping shrimp?

Allopatric speciation and vicraince

Which of the following statements explains why animals are less likely than plants to speciate by polyploidy?

Animals self fertilize less often than plants so diploid gametes are less likely to fuse

Species

Evolutionary independent population or group of population

Which of the following does NOT tend to promote speciation?

Gene flow

Speciation results from

Genetic isolation followed by genetic divergence

What keeps the two populations separate?

Habitat isolation

Which of the following statements about species, as defined by the biological species concept is correct? I. Biological species are defined by reproductive isolation II. Biological species are the model used for grouping extinct forms of life III. All members of a species can potentially interbreed

I and III

The Phylogenetic species concept

Identifies species based on the evolutionary history of populations - All species form a monophyletic group - Species are defined as the smallest monophyletic groups on the tree of life - Species are made up of populations that share synamorphies

The Morphospecies concept

Identifying evolutionary independent lineages by differences in size or shape -Distinguishing features are most likely to arise if populations are independent and isolated from gene flow

Genetic isolation happens when

Populations become geographically separated

Allopatry

Populations that are geographically separated

Subspecies

Populations that live in discrete geopraphic areas and have distinguishing features such as coloration or calls but are not considered distinct enough to be called separate species

Biological species concept

The main criterion for identifying species is reproductive isolation. -If 2 different populations do not interbreed in nature they are considered distinct species

Sympatry

When populations/species live in the same geographic area or close to another to make interbreeding possible