Why can't two different genetically species reproduce to give a viable offspring?

Why can't two different genetically species reproduce to give a viable offspring?

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What is the reason that only organisms belong to same species can produce a viable and reproductively healthy offspring but organisms of two different species cannot?

The short answer is that "different species" are defined as "not being able to produce viable offspring". So the answer to your question is that it's a tautology; it's that way because that's how it's defined. You're asking a question about words, not about biology. If you were able to persuade people that a "species" was "anything living in the same area", then "different species" would be perfectly able to reproduce.

More specifically, there are many different definitions of "species", and you're asking a question about one particular definition, the "biological species" concept that was first presented by Ernst Mayr in 1942 in his book Systematics and the Origin of Species. Mayr's definition is a useful one, but it's far from the only one; there are literally dozens of other definitions of "species". See Wikipedia's The Species Problem for more information.

Still, it's worth asking why there biology does (to some extent) have "species" that don't interbreed; Mayr's definition has stuck because it is useful. The first thing to consider is how remarkable it is that any two individuals (of the same species) can interbreed at all. It's not the default option! There are thousands of highly specialized quirks that allow fertilization to happen at all. Males and females have to be incredibly fine-tuned to recognize each other, to have sperm and egg meet each other, to have the sperm identify the egg, to enter and mix genetic material… the cascade is tremendously unlikely and has to be rigorously maintained.

So when it's not maintained -- when there's no selection pressure on two populations -- inevitably there will be genetic drift that will randomly disrupt this fine-tuned system. If a population of, say, voles is isolated on an island, they will continue to have pressure to be able to interbreed with other voles on the island, but if they can't interbreed with those on the mainland there won't be any consequences, and so over long enough time they'll drift and lose that ability -- just as many apes, not suffering any consequences from not synthesizing vitamin C, gradually lost that ability from random drift.

There's another side to it. Two populations in the same location may be positively selected to not be able to interbreed. Think about two groups of finches, one with small fine beaks that eat tiny seeds deep inside pine cones, and one with heavy beaks that crush and eat thick-shelled nuts. They each do fine, but they can interbreed and produce offspring that have intermediate beaks -- too thick to reach the fine seeds that one parent eats, but too delicate to crush the nuts that the other parent eats. Those intermediate offspring will die off, and both parents will have wasted their resources raising them. Both parents would be better off not breeding with each other, but only breeding with their own kind to produce specialized and efficient offspring. There is now selection pressure on the birds to recognize their own kind (perhaps through songs or mating displays) and ultimately to be inter-sterile, so they never waste resources on the un-fit offspring. There's a gradation of separation over time, in which the different populations become more and more distinct. Eventually, at some arbitrary point, humans start calling them "species", but that's just us, not biology.

"Species" is an important concept, but it's not special in evolution; speciation is just one aspect of natural selection, there's nothing magical about it.

In sexually reproducing organisms, fusion of gametes starts of the process of embryonic development. If union of gametes between two genetically distinct species takes place, there may be two situations -

  1. Same number of chromosomes - Though the number of chromosomes are same, during the first mitosis, their centromeres may not be arranged in the same plane. As a result, cell division cycle arrest occurs, halting further development.
  2. Different number of chromosomes - The extra chromosomes which are not paired lead to differential tension on the microtubules which again disrupts the cell division cycle.

Description of cell division checkpoints.

Tree Genetics

Tree genetics is the study of tree genes – the units of transmission of hereditary characteristics within trees. Each gene is usually a segment of a DNA or RNA molecule within a chromosome that controls the production

of specific amino acids or proteins that, in turn, influence specific characteristics of development in a tree, such as leaf shape and function. Such characteristics are passed on from parent to offspring and, thus, are the fundamental controllers of what determines one species from another, as well as subspecies, or ecotypes.

Species are usually differentiated from one another because they have uniquely different genes and, therefore, identifiably unique shapes, sizes, functions, and, most importantly, reproductive characteristics. One definition of a unique species is that it cannot mate with another species and produce viable offspring. In trees, this may be because two different trees produce flowers and pollen at different times, pollen may be different shapes and sizes and not capable of entering and fertilizing other species’ flowers, or flowers may have characteristics that do not allow pollen of other species to fertilize them.

In some cases, two closely related but different species may produce a viable offspring that has the characteristics of both species. This is called a hybrid and is often sterile, or unable to reproduce itself. In animals, a common example is the offspring between a horse and donkey, which produces a mule. In trees, the most common example are crosses between species in the poplar family – called hybrid poplars. As mentioned, hybrids are often unable to reproduce offspring of their own, and most hybrids do not develop into adults that have good survival characteristics. Occasionally, a hybrid may have specific traits that are highly desired, such as larger fruit, faster height growth, different colored leaves, which is why they may be artificially produced by humans for specific purposes.

In nature, the genetic variation that occurs and leads to different species is determined by five major processes:

Mutation The process where a mistake occurs when genes are duplicated or transferred to offspring. The vast majority of mutations are harmful and lead to the individual’s death, but occasionally a mutation results in a beneficial characteristic, such as greater sun or drought tolerance, elongated root growth, different leaf shape or color, and earlier flowering. If these characteristics give an individual a survival advantage, this mutation has a high probability of perpetuating itself within that individual’s offspring. Natural selection If the specific genes (such as those from mutations) of an individual organisms give it a survival advantage it has a greater likelihood of producing more offspring that will, in turn, have a higher survival advantage. This leads to the eventual dominance of this genetic trait in a local population. For example, trees in arid climates tend to have small, thick leaves to help them conserve water versus trees in wetter areas, where trees with large, thin leaves do not have a competitive disadvantage. Natural selection, therefore, allows for the development of populations within a species that may be more closely adapted to local characteristics, such as timing of winter and spring, frequent wildfires, water and nutrient availability, and pest and pathogen resistance. Such geographic variations in a species are called ecotypes, or subspecies. If they become isolated enough from the general population, they may develop characteristics that become unique enough to qualify them as a different species. Migration This is the general movement of genetic information across a larger population through the dispersal of pollen or seeds. Usually this movement does not occur over large distances and, thus, a large natural landscape covered by trees of the same species will consist of groups of interrelated trees called families and neighborhoods – each with specific and uniquely similar genetic traits, though not different enough to be considered ecotypes, or subspecies. Genetic drift Within a population, random differences or variations in genetic makeup can occur when a mutation interacts with natural selection to create a unique trait in a species. This often occurs in small, isolated populations in which there is little migration of outside pollen or seeds. The differences between Black Hills ponderosa pine (two needles per fascicle) and northern Rocky Mountain ponderosa pine (three needles per fascicle) are due to genetic drift as their populations are isolated from each other by hundreds of miles of dry prairie. Hybridization Breeding between two different species – often closely related – that result in an offspring is called hybridization. Most hybrids, much like mutations, do not produce viable or fit offspring and, thus, do not flourish. Occasionally, a hybrid will have superior competitive characteristics that allow it a reproductive advantage. Such hybrids eventually may become a subspecies or its own species.

Tree genetics and the processes that control them are the reasons we have many different species of trees and are vital in creating the biodiversity across the earth. Genetic adaptation allows species to colonize lands that may not have supported life and for adapting to more efficiently utilizing the resources available in any particular landscape.

Humans have used genetic breeding and, more recently, gene splicing (the artificial splicing of certain genes from one species into another species’ DNA) to create plants and animals that: produce greater quantities of food, such as many vegetable and cereal crops specific breeds of horses, such as Belgiums for pulling or Thoroughbreds for racing and breeds of trees that grow straighter and taller for wood production, such as loblolly pine, or that are more resistant to exotic introduced diseases, such as white pines resistant to European blister rust. Climate change may create conditions where trees that have specific genetic adaptations to the timing of frost and spring, moisture availability, and pest and pathogens, etc., are no longer well adapted to the new climate. Although genetic processes may allow for each species to adapt, this process may take multiple centuries because trees are slow to mature and produce offspring. The more rapid any climatic change that may occur, the greater the chances that a species or ecotype may go extinct. Human assistance in helping species migrate and adapt may moderate negative impacts if rapid climate change does occur.

Related reading:
Harper John L. 1977. Population biology of plants. Academic Press, New York. 892 pgs.

Wenger Karl F. (ed) 1984. Forestry Handbook second edition. John Wiley and Sons, New York. 1335 pgs.

Evolution Connection

The Red Queen HypothesisGenetic variation is the outcome of sexual reproduction, but why are ongoing variations necessary, even under seemingly stable environmental conditions? Enter the Red Queen hypothesis, first proposed by Leigh Van Valen in 1973. Leigh Van Valen, “A New Evolutionary Law,” Evolutionary Theory 1 (1973): 1–30 The concept was named in reference to the Red Queen's race in Lewis Carroll's book, Through the Looking-Glass.

All species coevolve (evolve together) with other organisms. For example, predators evolve with their prey, and parasites evolve with their hosts. Each tiny advantage gained by favorable variation gives a species a reproductive edge over close competitors, predators, parasites, or even prey. However, survival of any given genotype or phenotype in a population is dependent on the reproductive fitness of other genotypes or phenotypes within a given species. The only method that will allow a coevolving species to maintain its own share of the resources is to also continually improve its fitness (the capacity of the members to produce more reproductively viable offspring relative to others within a species). As one species gains an advantage, this increases selection on the other species they must also develop an advantage or they will be outcompeted. No single species progresses too far ahead because genetic variation among the progeny of sexual reproduction provides all species with a mechanism to improve rapidly. Species that cannot keep up become extinct. The Red Queen’s catchphrase was, “It takes all the running you can do to stay in the same place.” This is an apt description of coevolution between competing species.


Last week, scientists announced that the human gene pool seems to include DNA from Neanderthals. That suggests that humans interbred with their primate cousins at some point before the Neanderthals went extinct about 30,000 years ago. Could we mate with other animals today?

Probably not. Ethical considerations preclude definitive research on the subject, but it’s safe to say that human DNA has become so different from that of other animals that interbreeding would likely be impossible. Groups of organisms tend to drift apart genetically when they get separated by geographical barriers—one might leave to find new food sources, or an earthquake could force them apart. When the two groups come back into contact with each other many, many years later, they may each have evolved to the point where they can no longer mate.

In general, two types of changes prevent animals from interbreeding. The first includes all those factors—called “pre-zygotic reproductive isolating mechanisms”—that would make fertilization impossible. After so many generations apart, a pair of animals might look so different from one another that they’re not inclined to have sex. (If we’re not even trying to mate with monkeys, we’ll never have half-human, half-monkey babies. *) If the animals do try to get it on despite changed appearances, incompatible genitalia or sperm motility could pose another problem: A human spermatozoon may not be equipped to navigate the reproductive tract of a chimpanzee, for example.

The second type of barrier includes “post-zygotic reproductive isolating mechanisms,” or those factors that would make it impossible for a hybrid animal fetus to grow into a reproductive adult. If a human were indeed inclined and able to impregnate a monkey, post-zygotic mechanisms might result in a miscarriage or sterile offspring. The further apart two animals are in genetic terms, the less likely they are to produce viable offspring. At this point, humans seem to have been separate from other animals for far too long to interbreed. We diverged from our closest extant relative, the chimpanzee, as many as 7 million years ago. (For comparison, our apparent tryst with the Neanderthals occurred less than 700,000 years after we split off from them.)

Researchers haven’t pinned down exactly which mechanisms prevent interbreeding under most circumstances. Some closely related species can mate even if they have different numbers of chromosomes. Przewalski’s horse, for example, has 33 pairs of chromosomes instead of the 32 most horses have, but it can interbreed with regular equines anyway—the offspring takes the average and ends up with 65 chromosomes.

Neanderthals weren’t our ancestors’ only dalliance with other primates. “Pre-humans” and “pre-chimpanzees” interbred and gave birth to hybrids millions of years ago. In the 1920s, Soviet dictator Joseph Stalin sent an animal-breeding expert to Africa in hopes of creating an army of half-man, half-monkey soldiers. Attempts both to inseminate women with monkey sperm and impregnate female chimpanzees with human sperm failed.

That doesn’t mean that tales of humans interbreeding with other animals haven’t endured. Rumored animal-human crosses from the past few hundred years have included a man-pig, a monkey-girl, and a porcupine man.

Got a question about today’s news? Ask the Explainer .

Explainer thanks Trenton Holliday of Tulane University.

Correction, Nov. 15, 2006: Due to an editing error, the original version of this piece suggested that interbreeding humans and apes might produce half-human, half-monkey babies. The offspring of such a union would be half-ape, not half-monkey. (Return to the corrected sentence.)

Biology 171

By the end of this section, you will be able to do the following:

  • Explain that meiosis and sexual reproduction are highly evolved traits
  • Identify variation among offspring as a potential evolutionary advantage of sexual reproduction
  • Describe the three different life-cycle types among sexually reproducing multicellular organisms.

Sexual reproduction was likely an early evolutionary innovation after the appearance of eukaryotic cells. It appears to have been very successful because most eukaryotes are able to reproduce sexually and, in many animals, it is the only mode of reproduction. And yet, scientists also recognize some real disadvantages to sexual reproduction. On the surface, creating offspring that are genetic clones of the parent appears to be a better system. If the parent organism is successfully occupying a habitat, offspring with the same traits should be similarly successful. There is also the obvious benefit to an organism that can produce offspring whenever circumstances are favorable by asexual budding, fragmentation, or by producing eggs asexually. These methods of reproduction do not require another organism of the opposite sex. Indeed, some organisms that lead a solitary lifestyle have retained the ability to reproduce asexually. In addition, in asexual populations, every individual is capable of reproduction. In sexual populations, the males are not producing the offspring themselves, so hypothetically an asexual population could grow twice as fast.

However, multicellular organisms that exclusively depend on asexual reproduction are exceedingly rare. Why are meiosis and sexual reproductive strategies so common? These are important (and as yet unanswered) questions in biology, even though they have been the focus of much research beginning in the latter half of the 20th century. There are several possible explanations, one of which is that the variation that sexual reproduction creates among offspring is very important to the survival and reproduction of the population. Thus, on average, a sexually reproducing population will leave more descendants than an otherwise similar asexually reproducing population. The only source of variation in asexual organisms is mutation. Mutations that take place during the formation of germ cell lines are also the ultimate source of variation in sexually reproducing organisms. However, in contrast to mutation during asexual reproduction, the mutations during sexual reproduction can be continually reshuffled from one generation to the next when different parents combine their unique genomes and the genes are mixed into different combinations by crossovers during prophase I and random assortment at metaphase I.

The Red Queen Hypothesis Genetic variation is the outcome of sexual reproduction, but why are ongoing variations necessary, even under seemingly stable environmental conditions? Enter the Red Queen hypothesis, first proposed by Leigh Van Valen in 1973. 1 The concept was named in reference to the Red Queen’s race in Lewis Carroll’s book, Through the Looking-Glass.

All species coevolve (evolve together) with other organisms. For example, predators evolve with their prey, and parasites evolve with their hosts. Each tiny advantage gained by favorable variation gives a species a reproductive edge over close competitors, predators, parasites, or even prey. However, survival of any given genotype or phenotype in a population is dependent on the reproductive fitness of other genotypes or phenotypes within a given species. The only method that will allow a coevolving species to maintain its own share of the resources is to also continually improve its fitness (the capacity of the members to produce more reproductively viable offspring relative to others within a species). As one species gains an advantage, this increases selection on the other species they must also develop an advantage or they will be outcompeted. No single species progresses too far ahead because genetic variation among the progeny of sexual reproduction provides all species with a mechanism to improve rapidly. Species that cannot keep up become extinct. The Red Queen’s catchphrase was, “It takes all the running you can do to stay in the same place.” This is an apt description of coevolution between competing species.

Life Cycles of Sexually Reproducing Organisms

Fertilization and meiosis alternate in sexual life cycles . What happens between these two events depends on the organism’s “reproductive strategy.” The process of meiosis reduces the chromosome number by half. Fertilization, the joining of two haploid gametes, restores the diploid condition. Some organisms have a multicellular diploid stage that is most obvious and only produce haploid reproductive cells. Animals, including humans, have this type of life cycle. Other organisms, such as fungi, have a multicellular haploid stage that is most obvious. Plants and some algae have alternation of generations, in which they have multicellular diploid and haploid life stages that are apparent to different degrees depending on the group.

Nearly all animals employ a diploid-dominant life-cycle strategy in which the only haploid cells produced by the organism are the gametes. Early in the development of the embryo, specialized diploid cells, called germ cells , are produced within the gonads (such as the testes and ovaries). Germ cells are capable of mitosis to perpetuate the germ cell line and meiosis to produce haploid gametes. Once the haploid gametes are formed, they lose the ability to divide again. There is no multicellular haploid life stage. Fertilization occurs with the fusion of two gametes, usually from different individuals, restoring the diploid state ((Figure)).

Most fungi and algae employ a life-cycle type in which the “body” of the organism—the ecologically important part of the life cycle—is haploid. The haploid cells that make up the tissues of the dominant multicellular stage are formed by mitosis. During sexual reproduction, specialized haploid cells from two individuals—designated the (+) and (−) mating types—join to form a diploid zygote. The zygote immediately undergoes meiosis to form four haploid cells called spores. Although these spores are haploid like the “parents,” they contain a new genetic combination from two parents. The spores can remain dormant for various time periods. Eventually, when conditions are favorable, the spores form multicellular haploid structures through many rounds of mitosis ((Figure)).

If a mutation occurs so that a fungus is no longer able to produce a minus mating type, will it still be able to reproduce?

The third life-cycle type, employed by some algae and all plants, is a blend of the haploid-dominant and diploid-dominant extremes. Species with alternation of generations have both haploid and diploid multicellular organisms as part of their life cycle. The haploid multicellular plants are called gametophytes , because they produce gametes from specialized cells. Meiosis is not directly involved in the production of gametes in this case, because the organism that produces the gametes is already haploid. Fertilization between the gametes forms a diploid zygote. The zygote will undergo many rounds of mitosis and give rise to a diploid multicellular plant called a sporophyte . Specialized cells of the sporophyte will undergo meiosis and produce haploid spores. The spores will subsequently develop into the gametophytes ((Figure)).

Although all plants utilize some version of the alternation of generations, the relative size of the sporophyte and the gametophyte and the relationship between them vary greatly. In plants such as moss, the gametophyte organism is the free-living plant and the sporophyte is physically dependent on the gametophyte. In other plants, such as ferns, both the gametophyte and sporophyte plants are free-living however, the sporophyte is much larger. In seed plants, such as magnolia trees and daisies, the gametophyte is composed of only a few cells and, in the case of the female gametophyte, is completely retained within the sporophyte.

Sexual reproduction takes many forms in multicellular organisms. The fact that nearly every multicellular organism on Earth employs sexual reproduction is strong evidence for the benefits of producing offspring with unique gene combinations, though there are other possible benefits as well.

Section Summary

Nearly all eukaryotes undergo sexual reproduction. The variation introduced into the reproductive cells by meiosis provides an important advantage that has made sexual reproduction evolutionarily successful. Meiosis and fertilization alternate in sexual life cycles. The process of meiosis produces unique reproductive cells called gametes, which have half the number of chromosomes as the parent cell. When two haploid gametes fuse, this restores the diploid condition in the new zygote. Thus, most sexually reproducing organisms alternate between haploid and diploid stages. However, the ways in which reproductive cells are produced and the timing between meiosis and fertilization vary greatly.

Art Connections

(Figure) If a mutation occurs so that a fungus is no longer able to produce a minus mating type, will it still be able to reproduce?

What would happen if two different species mated?

The thing that allows us to to tell two species apart is also the thing that keeps them from mating successfully, their genetics. When two members of the same species mate, the haploid cells (egg and sperm) unite to create a full diploid cell with the full amount of chromosomes. In the case for humans a full set is 23 pairs or 46 individual chromosomes.

However, when two members of different species mate their genetic information is incompatible. The number of chromosomes, among other things, prohibits viable offspring in most cases. An example of successful cross-species breeding would be the mix between a Tiger and a Lion which is a viable creature and has been created before. However, it is genetically incapable of having children of its own which is why they don't propagate.

I know this seems counterintuitive because of all the species we have on Earth but just think of it in this way. When two individuals of the same species become different enough so they can no longer breed successfully, they're considered different species and their phylogenetic trees will most likely never merge again.

What is a Species?

A species is a group of individual organisms that interbreed and produce fertile, viable offspring. According to this definition, one species is distinguished from another when, in nature, it is not possible for matings between individuals from each species to produce fertile offspring.

Members of the same species share both external and internal characteristics, which develop from their DNA. The closer relationship two organisms share, the more DNA they have in common, just like people and their families. People’s DNA is likely to be more similar to their father or mother’s DNA than their cousin or grandparent’s DNA. Organisms of the same species have the highest level of DNA alignment and therefore share characteristics and behaviors that lead to successful reproduction.

Species’ appearance can be misleading in suggesting an ability or inability to mate. For example, even though domestic dogs (Canis lupus familiaris) display phenotypic differences, such as size, build, and coat, most dogs can interbreed and produce viable puppies that can mature and sexually reproduce (Figure 1).

Figure 1. The (a) poodle and (b) cocker spaniel can reproduce to produce a breed known as (c) the cockapoo. (credit a: modification of work by Sally Eller, Tom Reese credit b: modification of work by Jeremy McWilliams credit c: modification of work by Kathleen Conklin)

In other cases, individuals may appear similar although they are not members of the same species. For example, even though bald eagles (Haliaeetus leucocephalus) and African fish eagles (Haliaeetus vocifer) are both birds and eagles, each belongs to a separate species group (Figure 2). If humans were to artificially intervene and fertilize the egg of a bald eagle with the sperm of an African fish eagle and a chick did hatch, that offspring, called a hybrid (a cross between two species), would probably be infertile—unable to successfully reproduce after it reached maturity. Different species may have different genes that are active in development therefore, it may not be possible to develop a viable offspring with two different sets of directions. Thus, even though hybridization may take place, the two species still remain separate.

Figure 2. The (a) African fish eagle is similar in appearance to the (b) bald eagle, but the two birds are members of different species. (credit a: modification of work by Nigel Wedge credit b: modification of work by U.S. Fish and Wildlife Service)

Populations of species share a gene pool: a collection of all the variants of genes in the species. Again, the basis to any changes in a group or population of organisms must be genetic for this is the only way to share and pass on traits. When variations occur within a species, they can only be passed to the next generation along two main pathways: asexual reproduction or sexual reproduction. The change will be passed on asexually simply if the reproducing cell possesses the changed trait. For the changed trait to be passed on by sexual reproduction, a gamete, such as a sperm or egg cell, must possess the changed trait. In other words, sexually-reproducing organisms can experience several genetic changes in their body cells, but if these changes do not occur in a sperm or egg cell, the changed trait will never reach the next generation. Only heritable traits can evolve. Therefore, reproduction plays a paramount role for genetic change to take root in a population or species. In short, organisms must be able to reproduce with each other to pass new traits to offspring.

Wrapping Up the Differences between Autopolyploidy and Allopolyploidy

Polyploidy is an important phenomenon in the diversification of the angiosperm lineage. In some lineages, however, may have adverse consequences leading to deleterious genetic conditions. Allopolyploids and autopolyploids are crucial for the diversification of groups and present the opportunity to suppress lethal recessive properties. Allopolyploids seem to provide an ideal setting for asexual reproduction while costing the species less diversity loss than would diploids. Autopolyploids present a crucial platform for novel genes and novel gene expression in having spare duplicates per gene.

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Females are “choosy” males are not

“Females are choosy, while males are not” is a gross generalization, but biologically it tends to be true across most species that reproduce sexually, whether they reproduce with internal or external fertilization. Read on to explore how biological fitness and parental investment play into this major difference between male and females in many species.

Generally females invest more energy, care, and time in the offspring than a male does, and she has a limited number of eggs compared to practically limitless sperm produced by males of most species. Because of her higher investment, if a female mates with a male of poor genetic quality, and that choice results in no offspring survival, she has wasted energy and resources and ended up with nothing.

Alternatively, a male can mate with practically unlimited numbers of females with little loss of energy or resources, regardless of how successful those offspring may be. In other words, eggs are “expensive” and sperm are “cheap.” Thus, generally a female maximizes her reproductive success by mating with the “best” male she can, while generally a male maximizes his reproductive success by mating with as many females as possible.

Any situation where one gender selects specific individuals to mate with will result in a phenomenon called sexual selection. Sexual selection is a type of natural selection where one sex has a preference for certain characteristics in individuals of the other sex. As with any type of selection, this preference increases the reproductive success of individuals who have the preferred characteristic.

Because females of most sexually reproducing species are “choosy,” females are often the gender that sexually selects traits in males. As a result, males compete with each other for access to females or induce a specific female to mate with them. As a result of this competition, sexual selection often leads to sexual dimorphism, distinct differences in size or appearance between males and females. These differences in size or appearance, called secondary sexual characteristics, are exaggerated or showy traits that are associated with mating behaviors and reproductive success. Examples include breasts, showy tails and headpieces, and crazier traits like the length of the eye-stalks in stalk-eyed flies (see image below).

Stalk-eyed flies have eyes at the end of long stalks, and they compete for mates by measuring the distance between their eyes. Evidence shows that wider eye placement wins in these bouts of male competition. (Image credit: Jojo Cruzado – stalk eyed fly, CC BY 2.0,

Sexual dimorphism can lead to specific behaviors in males that increase their reproductive success. Significant energy is spent in the process of locating, attracting, and mating with the sex partner.

Competition among males occurs whether species mate via internal or external fertilization. In species with internal fertilization, only one male can mate with a female at a time, so males compete with each other for mating with a particular female. In species with external fertilization, the female controls how and when the eggs are released, and males compete for access to her eggs outside of her body.

HYBRIDS: If lions and tigers were intelligently designed, why are offspring of crosses between them infertile?

The original question was:
If the tiger and lion were intelligently designed, why are they able to interbreed, but create only sterile offspring? The same is true with horse and donkey, they produce sterile offspring. Why?

Answer by Diane Eager

We need to start by asking just why we think these animals should be able to mate and have fertile offspring anyway. The bottom line is that our classification system has put them in the same genus, so therefore we conclude they should be able to interbreed. The humbling factor is that whatever we label them does not necessarily make them part of the same original kind. Our classification system is simply a way of organising our knowledge of living things based on out observations of their structure, function and genetics today. We were not there to see the original kinds.

We do know that some creatures that are classified in the same genus, but labelled different species, can combine to produce fertile offspring, which is one indicator they were all part of and original kind. The best example of this is the Galapagos finches. See our report Finch Gene Flow here.

Furthermore, the world has degenerated since the original created kinds were made. It is possible some that some similar, but mismatched, species can mate and reproduce for one generation only because there has been loss of whatever mechanisms (genetic, cellular, behavioural, etc.) originally kept them separate.

Let’s look at the two examples given in the question.

Horses and Donkeys

The sterile offspring of the horse/donkey combination, i.e. a mule, can be explained due to a mismatch in the number of chromosomes. Horses have 64 chromosomes and donkeys have 62. A mule ends up with 63. To understand why this results in sterility we need to look at what happens to chromosomes when living things reproduce, and the importance of even numbers.

Chromosomes normally come in pairs – one member of the pair inherited from the mother and one from the father. Therefore, horses have 32 pairs, and donkeys 31 pairs. This is important when it comes to sexual reproduction, which involves the union of two cells, i.e. a sperm and an egg, to form a new individual. If the sperm or egg (sex cells) had the same number of chromosomes as any other cell in the body there would be a doubling of chromosome numbers with each generation. That wouldn’t work. Within a few generations there wouldn’t be room for them all. Therefore, during the formation of sex cells the number of chromosomes is halved by splitting up each of the pairs and giving one of each pair to the sex cells. Therefore, a sex cell from a horse will have 32 chromosomes and a donkey sex cell will have 31. When these are combined the total is 63 made of 31 pairs and 1 odd one. Because horse and donkey chromosomes are similar enough, the new combined cell is able to live and grow into a new individual, a mule. Growth involves making many more cells with the same number of chromosomes. To do this all the chromosomes are copied, and each new cell gets one whole set.

However, when the adult mule tries to makes sex cells, there is a problem. There pairs have to be split up. The normal process of making sex involves getting all the chromosomes to line up neatly in pairs in order to split off one of each pair to give to the sex cells. But for an animal with an odd number of chromosomes this can’t happen. There will always be an odd one out, so the process tends to stop there.

This problem will occur with all hybrids resulting from parents with mismatched numbers of chromosomes, not just horses and donkeys.

Lions and Tigers

Lions and tigers have the same number of chromosomes, 38, or 17 pairs. Therefore, an odd one out caused by unequal chromosome numbers is not the problem. In spite of this, hybrids resulting from a lion and tiger combination are usually sterile, so there must be a different reason.

Lions and tigers are both classified as felines, but even if they were part of an original Cat Kind, they have been separated for many generations of degeneration since their ancestors left the ark. Since then mutations have occurred in all animal populations, including lions and tigers, but not all mutations are the same in every population. This means that lions and tigers of today have accumulated different mutations. Not all mutations are lethal, they can simply result in variation in gene activity, or they can result in genes being moved out of their original place on the chromosome, which is how another problem can occur during the formation of sex cells. To see where this can happen we need to look at the process of copying the chromosomes during sex cell formation.

We will look at what happens to one pair of chromosomes, but it will happen with each pair. Remember each pair consists of one chromosome from the mother and one from the father. In the diagrams below they are coloured differently, but they are a matched pair.

The first step is to copy each chromosome of the pair. The copies remain attached to one another until the last step in the process, when they are separated and each new chromosome goes into the mix with the other new chromosomes to be incorporated into sex cells. The diagrams to the right and below show the process.

The double pairs then line up, and during this process, sections of chromosome are exchanged between the pairs, as shown in the diagram below. This process is known as “crossing over”, or recombination, and results in some come chromosomes containing a mix of genes from the two different parents. After recombination the attached copies, some with recombined sections, split up into separate chromosomes.

During this process it is important to get the chromosomes of each pair lined up very precisely, so that the pieces that are exchanged contain exactly the same genes, including the sections that regulate the activity of those genes, i.e. turn them on and off at appropriate times. This precise alignment may not be able to happen in the offspring of similar parents from two separate populations if mutations have resulted in displacement of genes or alteration in gene activity. The newly made composite chromosomes may be missing some genetic information, or it may have a combination of gene regulators that do not work well together. Genes do not act alone, they need to function in a coordinated way with other genes. Therefore, gene regulators, i.e. switches that turn genes on and off are just as important as genes that code for the structure and function of the body.

This recombining process happens for every chromosome pair. Therefore, it is almost inevitable to each newly formed sex cell will have at least one of the new re-combinations in its chromosome mix.

Lions, tigers, horses and donkeys were intelligently designed in the beginning, with no dysfunctional mutations. Since then all living things have been affected by the degeneration that came into the world as a result of man’s sin and God’s judgement. Therefore, it is possible that some kinds have lost some mechanisms that kept them separate to other kinds, and they can now reproduce, but give rise to dysfunctional offspring or some original kinds have been split into subpopulations that have been subject to different mutations that stop them from reproducing fertile offspring when they meet up again. Whatever the cause it is the result of degeneration in living things, not because there was anything wrong with the original designed Kinds.

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