Ch 24: Origin of Species & Macroevolution

How many species have scientists currently identified on Earth

About 2 million species have been identified and formally described by scientists.

What was an early estimate of known species in 1990

E. O. Wilson and colleagues estimated about 1.4 million known species in 1990.

What is the estimated total number of species on Earth

Scientists estimate that between 5 million and 50 million species may exist.

Why are many species still undiscovered

Many species are microorganisms or live in remote habitats that are difficult to study.

Why are bacteria and archaea hard to classify

Most cannot be grown in laboratory conditions, making identification difficult.

What is the estimated number of bacterial and archaeal species

Estimates range widely from about 10 million to 1 billion species.

What is a subspecies

A geographically restricted population of a species that differs slightly from other populations.

What is an ecotype

A genetically distinct population within a species adapted to a specific environment.

What is a species

A group of organisms that share key characteristics and a common evolutionary history.

What is the biological species concept

In sexually reproducing organisms, a species consists of individuals that can interbreed successfully.

Why are species boundaries often difficult to define

Differences between populations usually occur gradually rather than abruptly.

Why do biologists use different criteria to identify species

Different organisms require different methods of classification.

Why is bacterial species classification especially challenging

Horizontal gene transfer blurs genetic boundaries between species.

What is horizontal gene transfer

The transfer of genes between organisms that are not parent and offspring.

What traits are commonly used to identify species

Physical traits, reproductive ability, DNA, ecological role, and evolutionary relationships.

What are morphological traits

Physical and anatomical features of organisms.

What is the morphological species concept

Species are classified based on similarity in physical traits.

How is morphology used to classify microorganisms

Microorganisms are classified using cellular and structural features.

What is a limitation of morphological analysis

It is unclear how many traits should be compared.

Why are quantitative traits difficult to analyze

Traits like size vary continuously without clear cutoffs.

Can individuals of the same species look different

Yes, members of the same species can vary widely in appearance.

Can different species look similar

Yes, different species can sometimes appear nearly identical.

What are morphs

Different physical forms within the same species.

What is an example of morphs

Different color forms of the dyeing poison frog are the same species.

What is reproductive isolation

The inability of different species to successfully interbreed in nature.

Why can similar-looking organisms be different species

They may be reproductively isolated despite similar appearance.

What is an example of reproductive isolation

Northern and southern leopard frogs cannot interbreed in nature.

How do biologists use interbreeding to identify species

They test whether individuals can produce viable, fertile offspring.

What was Dobzhansky’s contribution to species concepts

He emphasized reproductive isolation as a defining feature of species.

How did Ernst Mayr define a species

A group that can interbreed in nature and produce viable, fertile offspring.

What is a key feature of sexually reproducing species

Members can interbreed with each other but not with other species.

What are reproductive isolating mechanisms

Biological barriers that prevent successful interbreeding.

When is reproductive isolation most useful

For identifying closely related, sexually reproducing species.

Why is reproductive isolation sometimes difficult to test

Populations may not overlap geographically.

Why does reproductive isolation not apply to all organisms

It cannot be used for asexual or extinct species.

What molecular features are used in species identification

DNA sequences and chromosome characteristics.

What molecular traits are commonly compared

DNA sequence, gene order, chromosome structure, and number.

What gene is commonly used to classify bacteria

The 16S rRNA gene.

What does very similar DNA suggest

Populations likely belong to the same species.

Why are molecular species boundaries debated

Scientists disagree on how much genetic difference defines a species.

How can ecology help identify species

Differences in habitat use can distinguish species.

What is the ecological species concept

Species are defined by how they use resources and their environment.

How are bacteria classified ecologically

By shared growth conditions and resource use.

What limits ecological classification

Different species may tolerate similar conditions.

How are evolutionary relationships used to identify species

By reconstructing ancestry using fossils and DNA.

What are evolutionary trees

Diagrams showing relationships among species over time.

Why is the fossil record important

Fossils help infer evolutionary relationships.

What is a species concept

A framework for defining what a species is.

Why are multiple species concepts needed

No single method works for all organisms.

What is the evolutionary lineage concept

A species is a unique evolutionary lineage.

What is the general lineage concept

All species are independently evolving lineages.

Why is the general lineage concept widely supported

It allows many types of evidence to be used.

What is prezygotic isolation

Isolation that occurs before fertilization.

What is postzygotic isolation

Isolation that occurs after fertilization.

Why is prezygotic isolation favored by natural selection

It prevents wasted reproductive effort.

What is behavioral isolation

Differences in mating behavior prevent interbreeding.

What is mechanical isolation

Incompatible body structures prevent mating.

What is gametic isolation

Sperm and egg cannot fuse successfully.

What is hybrid inviability

Hybrid embryos fail to develop properly.

What is hybrid sterility

Hybrids are viable but cannot reproduce.

Why are mules sterile

They have mismatched chromosomes that prevent meiosis.

What is hybrid breakdown

Hybrids are viable, but later generations are weak or infertile.

Which isolating mechanisms are most common

Prezygotic mechanisms are more common than postzygotic.

What is cladogenesis

The splitting of one species into two or more species.

What causes allopatric speciation

Geographic barriers block gene flow.

What is adaptive radiation

Rapid diversification into many species from one ancestor.

Why does adaptive radiation occur

Isolation and available ecological niches promote divergence.

What is a hybrid zone

An area where distinct populations meet and interbreed.

What are the three hybrid zone outcomes

Reinforcement, fusion, and stability.

What is reinforcement

Hybrid fitness is low, increasing reproductive isolation.

What is fusion

Hybrids are fit, causing populations to merge.

What is stability

Hybrids persist while species remain distinct.

What is sympatric speciation

Speciation without geographic isolation.

What mechanisms cause sympatric speciation

Polyploidy, hybridization, local adaptation, and sexual selection.

What is polyploidy

Having more than two chromosome sets.

Why does polyploidy cause instant speciation

New polyploids cannot reproduce with parents.

Why is polyploidy common in plants

Plants tolerate extra chromosome sets well.

What is hybrid speciation

A new species formed from interspecies hybrids.

What is gradualism

Evolution occurs slowly through small changes.

What is punctuated equilibrium

Long periods of little change interrupted by rapid evolution.

Which model is supported by the fossil record

Punctuated equilibrium.

What is evo-devo

The study of how changes in development cause evolution.

Why are developmental genes important

Small changes can create large morphological differences.

What is heterochrony

Evolution caused by changes in developmental timing.

Why is heterochrony evolutionarily powerful

It alters growth timing without creating new structures.