Biology Unit 3 Macroevolution

Introduction to Speciation

• If gene flow stops, allele frequencies in isolated populations can diverge

• They began to evolve independently

Why does divergence occur?

Occurs as a result of:

• Mutation

• Natural Selection

• Genetic Drift

• may eventually result in speciation

Speciation

a splitting event that creates two or more distinct species from an ancestral species

• can occur gradually or abruptly

Species

an evolutionary independent population or group of populations

Approaches for identifying species:

• The biological concept

• The morphological species (morphospecies) concept

• The phylogenetic species concept

According to the Biology species concept, what is the main criterion for identifying species?

reproductive isolation

What does reproductive isolation result in?

a lack of gene flow between populations

What is the mating pattern in the Biological Species Concept?

members of the population do not interbreed, or fail to produce viable, fertile offspring after mating

The Biological Species Concept

• mechanisms that stop gene flow between populations (reproductive isolation) are categorized as either:

  • Prezygotic isolation

    OR

  • Postzygotic isolation

Prezygotic Isolation- Biological Species Concept

individuals of different species are prevented from mating successfully

Postzygotic Isolation- Biological Species Concept

the hybrid offspring do not survive or reproduce

Disadvantages of Biological Species Concept

• reproductive isolation cannot be evaluated in fossils and asexually reproducing species

• Ex. Trilobites, look similar

Temporal Isolation- prezygotic isolation mechanism

• populations are isolated because they breed at different times

• Ex. Spotted skunks that mate at different times of the year

Habitat Isolation- prezygotic isolating mechanism

• populations are isolated because they breed in different habitats

• Ex. Garter snakes live in different habitats

• Ecology is important

Behavioral Isolation- prezyogtic isolation mechanism

• populations do not interbreed because their courtship display differs

• Ex. Male songbirds sing species-specific songs to attract females

Gametic Isolation- prezygotic isolating mechanism

• matings fail because eggs and sperm are incompatible

Ex. differences in the bindin protein determine whether sea urchin sperm will penetrate eggs

Mechanical Isolation- prezygotic isolating mechanism

• matings fail because male and female reproductive structures are incompatible

Hybrid (in)viability- Postzygotic isolating mechanism

• hybrid offspring do not develop normally and die as embryos

• Ex. When some species of

  salamanders mate their offspring

  have much lower viability

Hybrid Sterility- postzygotic isolating mechanisms

• hybrid offspring mature but are sterile as adults

• Ex. Horses and donkeys can mate and produce viable offspring, but their hybrid offspring (mules) are usually sterile

Haldane’s rule

• when in the F1 offspring of two different animal races one sex is absent, rare, or sterile, that sex is the heterozygous [heterogametic] sex

Morphospecies Concept

• individuals differ in size, shape, or other morphological features

• Distinguishing features most likely arise if populations are independent and isolated from gene flow

Disadvantages of Morphospecies Coencept

• One polymorphic species may be classified as more than one species

  • It cannot identify cryptic species that differ in non-morphological traits

  • The features used to distinguish species under this concept are subjective

The Phylogenetic Species Concept

• identifies species based on evolutionary history

• On phylogenetic trees, a monophyletic group consists of an ancestral population plus all of its descendants

  • Also called a clade or lineage

Monophyletic group

• an ancestral population and all descendants

• species are defined as smallest monophyletic group on tree of life

Synamorphy

trait unique to a monophyletic group (“unique forms”)

Advantages of Phylogenetic Species Concept

• It can be applied to any type of population (Ex. fossil, asexual, or sexual)

• It is logical because different species have different synapomorphies due to lack of gene flow and independent evolution of gene flow and independent evolution

Disadvantages of Phylogenetic Species Concept

• phylogenies are only available for only a tiny subset of populations on tree of life

• may lead to recognition of many more species than either of the other species concepts

Biological species concept Adv/Disadvantages

Advantages: Reproductive isolation=evolutionary independence

Disadvantages:  not applicable to asexual or fossil species; difficult to assess if population does not overlap geographically

Morphospecies concept Adv/Disadvantages

Advantages: widely applicable

Disadvantages: Subjective; misidentifies polymorphic species; misses cryptic species

Phylogenetic Species Concept Adv/Disadvantages

Advantages: widely applicable; based on testable criteria

Disadvantages: few well-estimated phylogenies available

Modes of Speciation

• Allopatric speciation

• Parapatric speciation

• Sympatric speciation

Allopatric Speciation

• genetic isolation occurs when populations become geographically separated

• populations that live in different areas are in allopatry

• MOST COMMON MODE

Speciation that begins with geographic isolation is called

Allopatric Speciation

Dispersal- of allopatric speciation

• a population moves to a new habitat, colonizes it, and finds a new population

• colonization events can lead to speciation

Vicariance- of allopatric speciation

the physical splitting of a habitat

Biogeography

• study of how species and populations are distributed geographically 

• can tell us how dispersal and vicariance events occur

Parapatric Speciation

• genetic isolation happens when populations change gradually along a cline

• Cline: a spatial gradient of character change along an environmental variable

• Ex. House mice increase in body size and build bigger nests in the north

Sympatry

• populations or species that live in same geographic area or close enough to interbreed

Sympatric Speciation can be initiated by two types of events:

(1) External events: Disruptive selection based on different ecological niches or mate preferences

(2) Internal events: Chromosomal mutations

Sympatric Speciation by Disruptive Selection

sympatric populations may be reproductively isolated by adapting to different habitats via disruptive selection

A niche

range of ecological resources that a species can use and the range of conditions it can tolerate

Disruptive Selection for Ecological Niche in Flies example

• apple maggot flies’ mate on apple fruits, and larvae use apple for food source

• hawthorn maggot flies feed and mate on hawthorn fruits

• apple maggot flies originated from hawthorn flies after apples was introduced in North America

What did the Fly and Apples experiment show?

• each species responds most strongly to its own fruit’s scent

• each species avoids the scent of the other fruit

• the ability to distinguish between scents has a genetic basis

Conclusion for Disruptive Selection in flies

• hybrid individuals do not orient to fruit scent as well as parents

• results in lower reproductive success

• thus, disruptive selection has led to sympatric speciation