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Types of Selection

Types of Selection

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...

Limits to Natural Selection

Limits to Natural Selection

Organisms that are well-adapted to their environment are more likely to survive and reproduce. However, natural selection does not lead to perfectly adapted organisms. Several factors constrain natural selection.

Frequency-dependent Selection

Frequency-dependent Selection

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.

Natural Selection and Mating Preferences

Natural Selection and Mating Preferences

The principle of natural selection posits that organisms better adapted to their environment are more likely to survive and reproduce. This principle is closely intertwined with mating preferences, a key aspect of sexual selection, which evolutionary psychologists believe is driven by instincts to propagate one's genes. Such instincts significantly influence mating behaviors and preferences between genders.
Females, due to their biological roles in conception, pregnancy, and nursing, inherently...

Evolutionary Relationships through Genome Comparisons

Evolutionary Relationships through Genome Comparisons

Genome comparison is one of the excellent ways to interpret the evolutionary relationships between organisms. The basic principle of genome comparison is that if two species share a common feature, it is likely encoded by the DNA sequence conserved between both species. The advent of genome sequencing technologies in the late 20th century enabled scientists to understand the concept of conservation of domains between species and helped them to deduce evolutionary relationships across diverse...

Gene Evolution - Fast or Slow?

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The genomes of eukaryotes are punctuated by long stretches of sequence which do not code for proteins or RNAs. Although some of these regions do contain crucial regulatory sequences, the vast majority of this DNA serves no known function. Typically, these regions of the genome are the ones in which the fastest change, in evolutionary terms, is observed, because there is typically little to no selection pressure acting on these regions to preserve their sequences.
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Related Experiment Video

Updated: May 26, 2026

Dissection and Flat-mounting of the Threespine Stickleback Branchial Skeleton

Dissection and Flat-mounting of the Threespine Stickleback Branchial Skeleton

Published on: May 7, 2016

Understanding selection for long necks in different taxa.

David M Wilkinson¹, Graeme D Ruxton

¹School of Natural Science and Psychology, Liverpool John Moores University, Byrom Street, Liverpool, L3 3AF, UK. D.M.Wilkinson@ljmu.ac.uk

Biological Reviews of the Cambridge Philosophical Society

|December 17, 2011

Summary

Evolutionary pressures for long necks in animals like giraffes and sauropods are primarily driven by foraging needs. While other factors exist, high browsing and accessing food sources are the most common explanations for elongated necks across diverse species.

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Area of Science:

Evolutionary Biology
Comparative Anatomy
Paleontology

Background:

The evolution of long necks in animals, such as giraffes and sauropod dinosaurs, has been a subject of ongoing scientific debate.
Understanding the selective pressures that lead to extreme neck elongation is crucial for comprehending evolutionary adaptations.

Purpose of the Study:

To synthesize current debates on the evolutionary pressures behind long necks in diverse taxa.
To investigate whether a common selective factor or separate explanations account for neck elongation across different species.

Main Methods:

Comparative analysis of neck evolution across a wide range of living and extinct taxa.
Review and synthesis of existing research on selective pressures including foraging, sexual selection, thermoregulation, and predation.

Main Results:

Foraging requirements, particularly high browsing, are identified as the dominant selective pressure for long necks in most cases (e.g., giraffes, tortoises, sauropods).
Alternative explanations like sexual selection, thermoregulation, and predation pressure are less supported.
Correlated selection for neck length with leg length for feeding/drinking (camels, wading birds) and predatory adaptations (fish-eating birds, plesiosaurs) are also discussed.

Conclusions:

Foraging is the primary driver for long neck evolution across most studied taxa.
Specific adaptations for accessing food (aquatic plants, carcasses) and prey capture are key.
Further empirical research is needed to fully understand the costs and benefits of neck elongation in various groups, with pterosaurs remaining an area of uncertainty.