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What is a Species?

What is a Species?

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Speciation describes the formation of one or more new species from one or sometimes multiple original species. The resulting species are discrete from the parent species, and barriers to reproduction will typically exist. There are two primary mechanisms, speciation with and without geographic isolation—allopatric and sympatric speciation, respectively.

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When the fitness of a trait is influenced by how common it is (i.e., its frequency) relative to different traits within a population, this is referred to as frequency-dependent selection. Frequency-dependent selection may occur between species or within a single species. This type of selection can either be positive—with more common phenotypes having higher fitness—or negative, with rarer phenotypes conferring increased fitness.

Types of Selection

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Natural selection influences the frequencies of particular alleles and phenotypes within populations in several different ways. Primarily, natural selection can be directional, stabilizing, or disruptive. Directional selection favors one extreme trait and shifts the population towards that phenotype while selecting against individuals displaying alternate traits. Stabilizing selection favors an intermediate trait with a narrow range of variation. Deviation from the optimal phenotype towards an...

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Related Experiment Video

Updated: Dec 20, 2025

Visually Sexing Loggerhead Shrike Lanius Ludovicianus Using Plumage Coloration and Pattern

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Published on: March 8, 2020

Species Differences in Phenology Shape Coexistence.

Christopher Blackford, Rachel M Germain, Benjamin Gilbert

The American Naturalist

|May 30, 2020

Summary

This summary is machine-generated.

Phenological differences in germination timing can limit species coexistence within a year. However, year-to-year variation in germination timing is sufficient to allow long-term coexistence of competing plant species.

Keywords:

Vulpia coexistence theory competition germination timing phenology priority effects

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Area of Science:

Ecology
Evolutionary Biology
Plant Science

Background:

Ecological theory presents conflicting views on whether phenological differences aid or hinder species coexistence.
Phenological separation can create temporal niche differences, promoting coexistence, but may also favor one species, increasing fitness differences and limiting coexistence.

Purpose of the Study:

To experimentally test how relative germination timing affects species coexistence in two annual grass species.
To investigate whether phenological separation increases or decreases niche and fitness differences.
To assess the role of year-to-year variation in germination timing on long-term coexistence.

Main Methods:

Experimental manipulation of relative germination timing for *Vulpia microstachys* and *V. octoflora*.
Parameterization of a competition model to estimate niche and fitness differences.
Analysis of within-season and year-to-year variation in coexistence dynamics.

Main Results:

Increased germination timing separation led to parallel shifts in niche and fitness differences, ultimately reducing within-year coexistence.
An earlier germination advantage benefited both species, with a 4-day head start enabling the weaker competitor to exclude the stronger one.
Phenological separation limited coexistence within a single year.

Conclusions:

Phenological differences significantly influence competitive interactions and species coexistence.
Year-to-year variation in germination timing is crucial for maintaining long-term coexistence, despite potential within-year limitations.
Quantifying temporal variation in phenological differences is essential for understanding species coexistence.