EVOLUTION: INSECT SPECIATION

ISOLATING MECHANISMS

Insects have numerous isolating mechanisms that enable sub-populations of species to avoid hybridization and take different evolutionary routes to formation of new species.  Darwin found the isolation of finches on different Galapagos islands may have been responsible for enabling them to evolve specializations leading to different species.  Island populations of insects may produce new species by a similar mechanism.

Geographic isolation and population size

These two factors interact.  In the island populations of finches in the Galapagos Islands.  The ancestral finches that first reached the islands after their volcanic origin presumably were few in number and consequently had fewer variations in the total "gene pool" than the larger continental populations from which they originated.  When new genes or combinations arose, that were better suited to survival on the various islands with varied conditions, selection might act more rapidly than in enormous potentially interbreeding continental populations.  Such selection can also operate on small semi-isolated populations of continental populations on the periphery of the species range.

Lock and key genitalia

Insect species have an effective way of preventing interbreeding between previously interbreeding populations.  Because of the need for, or value of, internal fertilization for reproduction of terrestrial adults there has been the evolution of specialized male organs for introducing sperm into the reproductive system of the females.  Selection has made the male and female genital openings of the semi-rigid exoskeleton match up in a "lock and key" arrangement of various shapes dependent on species.

The lock and key relationship of male and female genitalia presumably becomes better established over time so closely related species can no longer have cross-fertilization possible.  Other mechanisms may have helped the reproductive isolation occur.  Geographic isolation is not the only spatial mechanism to facilitate reproductive isolation.

Micro-habitat isolation

Some butterfly species in tropical forests isolate themselves from other similar species living in the same region, but at different elevations from the ground, some near the ground, other high in the canopy, and still others at an intermediate height.  Other insects may isolate themselves by their preference for a single species of plant species.  Many animals have parasitic species of insects found only on their species.  Various environmental conditions are often required by small organisms, sometimes in very small patches within what is a generally similar habitat by casual inspection; moisture, nutrients, soil particle size and texture, and chemical factors are features of micro-habitats that determine suitable environment for small organisms.

Imprinting

Lorenz discovered imprinting when he found that young geese responded to the first large moving animal they see after hatching as the object to follow.  They would follow him instead of the mother goose if they saw him first.  Imprinting of various types may occur at other times in the life cycle

A similar phenomenon occurs with the imprinting of the olfactory cues of a stream being imprinted on young salmon as guides to return to the same stream to breed.  It is thought that some insects preferentially lay eggs on the same plant type they fed upon as larval insects; a few times of use of a different plant variety could lead to separate evolutionary lines of the same species.

Temporal isolation

When the breeding season is extended over time it is possible for new species to evolve from populations separated by time of breeding.  I think this may have been a factor in evolution of species of isopods in Tasmania when the life cycle took three years for production of a new brood.  Cross-breeding would be less likely and three new species could evolve, especially when adults did not survive for a second breeding season.  Some intertidal populations of invertebrates have pairs of closely related species with separate reproductive seasons.  Many marine species have external fertilization and would benefit from a short breeding season giving specialized predators less time to prey upon them.

Pheromones

Chemical signals species give off include sex attractants given off by the females to facilitate their being found by the opposite sex.  Some tortricid moth species have been found to have sex attractants composed of two or three chemical components; ratios of the different compounds were different in each species and males only responded to the ratio characteristic of the species.

Pheromones of insects include other behavioral controls.  Formic acid is an alarm pheromone common to most species of ants; in fact, their family name, Formicidae, is based on that fact.

Beetle speciation

Beetles have more species than any other order of animals.  Their especially thick exoskeleton made the lock and key genitalia more effective in preserving the genetic isolation of new species once other adaptions became specialized.  The appearance of the new species may be almost identical to related species, something less common among related vertebrate species.  The hardened first pair of wings of beetles adapts them for survival without damage to their underlying membranous wings when crawling in forest floor debris.

Genetic factors in speciation

The important role of the genes in controlling development and function of organisms may be complicated by the complex life cycles of those with complete metamorphosis from larvae to pupae to adult.  It would seem that a lesser sequence of shifting controls would be found in insects with a gradual metamorphosis from wingless stages otherwise similar but preceding the adult stage.  The hormones regulating such changes have some parallels with vertebrate hormones.

Population size may affect the rate of evolutionary change although the loss of a better gene can occur by chance from mortality unrelated to a gene's value.  Local and/or broad scale catastrophes can ignore the fitness of a genome.  Barring loss of all with a better gene, it will probably become the most prevalent gene in a small population sooner than in a large population; if it is not lost, it will eventually be the norm in both.

The advantage of a complex life cycle

Most insects have a sequence of stages from egg, to larva, to pupa to adult.  The complexity might seem like a disadvantage exposing them to many different hazards during the course of their life cycle.  But consider the different ways they have developed to survive winter in temperate regions.  A species may overwinter in the egg stage, hidden away from predators and not a target food item for birds or other predators specializing on eating the adult or larval stage.

The sequence of stages through the year are less likely to enable excessive buildup of a predator population specialized to feed on one or two of the the stages.  When all stages are present at the same time, their different requirements may isolate them from competition with the other stages in feeding; it may also enable species survival by providing replacements if a particular stage has excessive predation.

An advantage of proper timing of stages of the insect species occurs when growth of the individual and its food organism, whether plant or animal, is at an optimal stage of growth.  Many insects feed on dead and/or decaying organisms that are present in accumulations soil or aquatic sediments.  Termites utilize wood effectively because they have symbiotic protozoans and bacteria in their gut that enable them to utilize cellulose, a plant material not digestible by large animals without such symbionts.

Speciation

Speciation is undoubtedly continuing among insect groups.  Most orders and families are quite ancient in their origin.  Fossils of insects much like those today have been found that are as ancient as the dinosaurs.

The previous post, http://evolutioninsights.blogspot.com/2015/10/evolution-insect-size.html , gives a probable reason insects have remained small.  Their small size has been a factor for their successful speciation into the largest number of species of any group of comparable sized organisms, with numbers of individuals far exceeding those of vertebrates and advanced invertebrates.

It is amazing that such diversity can be packed into the same adult body format- a head with compound eyes, antennae, and mouthparts; a thorax of three segments, three pair of legs and often two pair of wings; and an abdomen of about ten segments usually lacking appendages but bearing the genitalia.

There is great diversity in the specialization that insects have evolved for survival. It is a topic that could provide information for many interesting posts.

Joseph G. Engemann     Kalamazoo, Michigan     October 12, 2015