Name: how species stay genetically separate
Uploaded: 2026-06-01T06:28:20.221Z
Duration: 1 min 17 s
Channel: JoVE

K12 Standards

north-carolina-k-12-science-standard-course-of-study

NC K-12 Science Standard Course of Study

9-12

Biology

Biological Evolution- Unity and Diversity

LS.Bio.9.1

Understanding Species and Reproductive Barriers

A species is a group of organisms that can interbreed and produce fertile offspring. They also tend to share similar characteristics because their DNA is relatively similar.On the other hand, some organisms may look similar even though they are different species, like the Monarch and Viceroy butterflies. Each is a butterfly with orange-and-black patterns, but they are from two different genera.Some individuals from different species can interbreed and produce offspring called hybrids, but these offspring are biologically incapable of further reproduction. For example, the resulting hybrid of a zebra and a donkey, called a zonkey, is usually infertile, preventing further gene flow between the parent species. This natural phenomenon is an example of a post-zygotic reproductive barrier, a mechanism that occurs after fertilization, preventing hybrid zygotes from developing into fertile adults, keeping species separate.The other reproductive barrier, called the pre-zygotic barrier, prevents mating between two species. For example, two frog species may live in the same rainforest habitat. But one is active only during the day and the other only at night, preventing contact between the sp

understanding species and reproductive barriers

A species is a group of organisms that interbreed and produce fertile offspring. Typically, individuals of the same species appear similar and share common characteristics due to their highly similar genomes. However, not all organisms that look alike are members of the same species. Various mechanisms keep most species discrete. While some mechanisms prevent reproductive behavior and fertilization (pre-zygotic isolation), others prevent the production of fertile offspring after mating has occurred (post-zygotic isolation).<h4>The Appearance of a Species</h4>Although individuals of a single species typically look very similar, variation in coloration or morphology is not uncommon. For example, the common wall lizard, Podarcis muralis has six distinct morphs which vary by the color of their throat and underbelly.Conversely, the monarch and viceroy butterflies are two genetically distinct species that appear similar, inhabit common habitats, and are both unpalatable to predators. This is an evolutionary strategy known as M&#xFC;llerian mimicry&#x2014;the species share warning signals, such as their bright orange and black appearance to educate common predators more effectively of their toxicity.<h4>Pre-zygotic Isolation</h4>Phenomena that prevent mating or fertilization from occurring are referred to as pre-zygotic isolation mechanisms. Geographic isolation and ecological isolation are two examples of environmental barriers. In the case of geographic isolation, two bird species might live in different forests on either side of an impassable mountain range; ecological isolation could be one insect species living solely in the treetops while another inhabits the soil.There may also be behavioral reasons a species remains isolated. Examples are differences in mating rituals or communication, like birds-of-paradise, where males of different species have novel dances to attract female conspecifics. In temporal isolation species are kept apart by different timing of daily routines or breeding seasons. For example, the red-legged frog Rana aurora has a breeding season from January to March, but its close relative the yellow-legged frog, Rana boylii, breeds from late March through May.The physical characteristics of different species may isolate them pre-zygotically. Insects can display an array of different genital morphologies that means they can only physically mate with members of their species&#x2014;referred to as mechanical isolation. Finally, prevention of gamete fusion is often the last pre-zygotic barrier which may be controlled by mechanisms like the prevention of pollen tube growth in plants, or an inability to physically fuse with or penetrate the outer layers of an egg cell.<h4>Post-Zygotic Isolation</h4>When individuals of different species overcome pre-zygotic isolation, post-zygotic isolation can prevent the resulting offspring from either surviving or reproducing. In some cases, a zygote may be formed but the embryo is not viable because of incompatible genetic material and thus dies. This is referred to as hybrid inviability.However, some hybrid embryos may survive to reach fertility. For instance, horses and donkeys can be crossed to produce hinnies or mules. But because horses and donkeys have different chromosome numbers (64 and 62, respectively), their hybrid offspring have an odd number of chromosomes (63) that cannot be sorted equally into gametes, rending the hybrids infertile. This post-zygotic barrier is also observed in crosses between zebras and horses or donkeys crosses.In cases where hybrids can survive to adulthood and copulate, non-genetic factors can prevent procreation. Hybrids of the sister species Drosophila pavani and D. gaucha produce viable gametes. However, the hybrids are unable to create offspring: the sperm of hybrid males cannot survive in the female semen receptors of either parent species or another hybrid, nor is the sperm of either parent line able to survive in a hybrid female.Other instances of post-zygotic factors may affect the viability of hybrids. Hybrid genomes contain material from two truly distinct species and can, therefore, harbor different genes and chromosomes that do not act harmoniously in the offspring, resulting in obvious fitness costs. Plants are an exception in some cases. Overall, pre- and post-zygotic isolation mechanisms cause most species to remain distinct.

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Education

JoVE Core

Evolution

Speciation and Diversity

KEY TERMS AND CONCEPTS

how species stay genetically separate

Species stay genetically separate through reproductive barriers that limit mating, fertilization, or the survival of offspring. A species is a group of organisms that can interbreed and produce fertile offspring. Members of the same species often look alike because they share very similar genomes. Even so, some organisms that look similar are not the same species.

Variation within a species is common. The common wall lizard, Podarcis muralis, has six morphs that differ in throat and underbelly color. Similar-looking species can also be different. Monarch and viceroy butterflies are genetically distinct, live in the same habitats, and are both unpalatable to predators. Their shared bright orange and black pattern is an example of Müllerian mimicry, where two species share warning signals to help predators learn their toxicity.

Pre-zygotic isolation happens before fertilization. Geographic isolation keeps species apart by physical barriers, such as a mountain range between two bird populations. Ecological isolation also blocks contact, such as one insect species living in treetops and another in soil. Behavioral isolation comes from different mating rituals or signals. Birds-of-paradise are an example, since males of different species perform unique dances to attract females of the same species.

Temporal isolation depends on timing. Species may breed in different seasons or carry out daily routines at different times. The red-legged frog, Rana aurora, breeds from January to March, while the yellow-legged frog, Rana boylii, breeds from late March through May. Mechanical isolation can also prevent mating when body structures do not match. In insects, differences in genital shape can keep individuals from mating with the wrong species. Another barrier stops gametes from fusing. In plants, this can involve blocking pollen tube growth. It can also happen when sperm or egg layers cannot physically fuse or be penetrated.

Post-zygotic isolation acts after fertilization. A zygote may form, but hybrid inviability can cause the embryo to die because the genetic material is incompatible. Some hybrids survive but are infertile. Horses and donkeys can produce mules or hinnies, but their different chromosome numbers, 64 and 62, give the hybrid 63 chromosomes that cannot divide evenly into gametes. This same barrier can appear in crosses between zebras and horses or donkeys.

Other hybrids survive and may even produce gametes, yet still cannot make offspring. Hybrids of Drosophila pavani and D. gaucha are one example. In these flies, sperm from hybrid males cannot survive in the female semen receptors of either parent species or another hybrid, and sperm from either parent line cannot survive in a hybrid female. Hybrid genomes can also contain genes and chromosomes that do not work well together, which lowers fitness. Plants are an exception in some cases. Together, pre-zygotic and post-zygotic barriers help most species remain distinct.

How Species Stay Genetically Separate

Species stay genetically separate through reproductive barriers that limit mating, fertilization, or the survival of offspring. A species is a group of organisms that can interbreed and produce fertile offspring. Members of the same species often look alike because they share very similar genomes. Even so, some organisms that look similar are not the same species. Variation within a species is common. The common wall lizard, Podarcis muralis, has six morphs that differ in throat and underbelly...

formation of species

How New Species Form

Speciation is the process that creates new species from one original species or from multiple original species. The new species become separate from the parent species, and barriers to reproduction usually develop. The main routes are allopatric speciation, which happens with geographic isolation, and sympatric speciation, which happens without it. In allopatric speciation, gene flow between two populations is blocked by a geographic barrier such as a mountain range or habitat fragmentation.

genetics of speciation

Reproductive Barriers in Speciation

Reproductive barriers drive speciation by stopping gene flow between populations. Speciation is the evolutionary process that forms new, distinct species, which are groups that cannot freely reproduce with one another. The genetics behind this process focuses on traits and isolating mechanisms that keep populations reproductively isolated. Some barriers act before a zygote forms, and these are called pre-zygotic mechanisms. They prevent fertilization and create the strongest block to gene flow...

speciation rates

Two Paths to New Species

Speciation can happen at different speeds, and evolutionary biologists describe those patterns with gradualism and punctuated equilibrium. Both models explain how new species arise. They also show that evolutionary change comes from heritable variation in a population, with natural selection often favoring traits that help survival and reproduction in a given environment. Gradualism is the slow, steady path to speciation. Small changes build up over many generations. In one example, birds on...

hybrid zones

How Hybrid Zones Shape Species Change

Hybrid zones are narrow regions where two closely related species meet, mate, and produce hybrids. Hybrids can show different traits or genes from either parent species. Those differences can affect survival and reproductive success. Because of this, hybridization can change species diversity and influence speciation within the hybrid zone. Gene flow and natural selection shape what happens in a hybrid zone. Gene flow moves genes between populations and can spread, blend, and preserve genetic...

key terms and concepts

Explore how pre-zygotic and post-zygotic barriers keep species separate, with examples from frogs, birds-of-paradise, butterflies, mules, and flies.

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biology

LS.Bio.10.1

Pennsylvania State Academic Standards

Life Science

interdependent-relationships-in-ecosystems

Interdependent Relationships in Ecosystems

3.1.9-12.V

Natural Selection and Evolution

3.1.9-12.X

Louisiana Student Standards for Science

High School

Life Sciences

Biological Evolution: Unity And Diversity

HS-LS4-5

Environmental Science

ecosystems-interactions-energy-and-dynamics

Ecosystems: Interactions, Energy And Dynamics

HS-LS2-7

Virginia  Standards of Learning

HIgh School

LS.11

LS.11.a

Minnesota Academic Standards in Science

4.1.1 9L.4.1.1.3

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New Jersey Student Learning Standards for Science

Biological Evolution: Unity and Diversity

Arizona Science Standards

HS.L4

HS.L4U1.27

Next Generation Science Standards

heredity-inheritance-and-variation-of-traits

Heredity: Inheritance and Variation of Traits

HS-LS4-3

Florida Department of Education

SC.912.L.15

SC.912.L.15.9

Texas Essential Knowledge and Skills

BIO.10

BIO.10.C

south-carolina-college-and-career-ready-science-standards

South Carolina College and Career Ready Science Standards

B-LS4-2.

B-LS4-5.

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New York State Science Learning Standards

HS-LS4-3.

HS-LS4-5.

Georgia Standards of Excellence

Ecology

SEC3.

SEC3.d

SB6.b

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LS.Bio.9.2

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LS.Bio.9.3

Construct an explanation to explain how natural selection leads to adaptations within populations.

LS.Bio.9.4

Extinction

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Use models (including dichotomous keys, scientific nomenclature, cladograms, phylogenetic trees) to identify organisms and exemplify relationships.

LS.Bio.10.2

Understand natural selection as a mechanism for biological evolution.

How Species Stay Genetically Separate

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