BSC2010 Unit 3 Lectures 9-10

Gonochoristic

Separate sexes; either male or female from birth to death

Protogynous hermaphrodites

Sex-changing; female first, then larger males

Simultaneous hermaphrodites

Produce male and female gametes at the same time

Species

Groups of organisms that mate with one another

Speciation

Divergence of biological lineages and emergence of reproductive isolation between lineages

Species concepts

Ways of approaching the question: “What are species?”

Morphological species concept

Groups organisms that look similar (Linnaeus)

Biological species concept

Groups organisms that have potential to mate with each other (Mayr)

Lineage or phylogenetic species concept

Groups organisms that share a branch on the tree of life (Simpson)

Ecological species concept

Groups related organisms that occupy specific ecological niches and become reproductively isolated due to behavior and habitat (Van Valen)

Limitations of morphological species concept

Regional variation, sexual dimorphism, cryptic species, phenotypic polymorphisms maintained by negative frequency-dependent selection

Reproductive isolation

Two groups of organisms can no longer exchange genes

Hybridization

Distinct species mate, but they do not produce fertile offspring.

Limitations of the biological species concept

Ring species, asexual species, hybridization, extinct species

Limitations of the lineage species concept

Interbreeding populations may be classified as separate species

Speciation requires ___.

interruption of gene flow

The Dobzhansky-Muller model

1. A population is subdivided and then the two groups evolve independently.

2. In each lineage, new alleles become fixed at different loci. The new alleles at the two loci are incompatible with one another.

3. Genetic incompatibility between the two isolated populations will develop over time.

Reproductive isolation may take ___ or ___.

millions of years; a few generations

Allopatric speciation

When populations are separated by a physical or geographic barrier

Adaptive radiation

Rapid diversification of a single ancestor into many specialized species

Sympatric speciation

Speciation without physical isolation

What is sympatric speciation correlated with?

Disruptive selection and assortative mating

Which is more common, allopatric or sympatric speciation?

Allopatric speciation

What can lead to sympatric speciation in a very short time span, particularly in plants?

Polyploidy

Incipient species

Populations that are in the process of diverging but still have the potential to interbreed

Postzygotic isolating mechanisms

Genetic differences in diverging lineages that reduce fitness of hybrid offspring

Postzygotic mechanisms result in selection for ___ and against ___.

prezygotic isolating mechanisms; hybridization

Reinforcement

Postzygotic mechanisms result in selection for prezygotic mechanisms and selection against hybridization.

Do sympatric or allopatric populations have stronger reinforcement?

Sympatric populations

Examples of prezygotic isolating mechanisms

Mechanical isolation, temporal isolation, behavioral isolation, habitat isolation, gametic isolation

Mechanical isolation

Differences in sizes and shapes of reproductive organs

Temporal isolation

Different species may breed at different times

Behavioral isolation

Individuals reject mating behaviors

Habitat isolation

Two closely related species evolve different habitat preferences.

Gametic isolation

Sperm and eggs don’t fuse.

Hybrid zones

Regions where reproductive isolation is incomplete