Why reproduction is important to a species

The sperm refers to the male sex cell and the egg refers to the female sex cell. Tell students to look at organisms on their way home, to see if they can predict which ones reproduce by sexual or asexual means.

There are some reproductive strategies that are difficult to classify for students. Ferns, fungi, and some protozoa reproduce by spore production. Spores are formed by divisions of special mother cells and are released from the parent organism. A single spore develops into a new organism. A second, more serious argument is that sex generates variable offspring upon which natural selection can act.

This is one of the oldest explanations for sexual reproduction, tracing back to the work of German biologist August Weismann in the late s.

Although this explanation may very well account for why sexual reproduction is so commonplace, the explanation is far more subtle than many people realize for two reasons. First, sex does not always increase the variability among offspring. Second, producing more variable offspring is not necessarily favorable.

In the next two sections, we describe these flaws in Weismann's explanation for sex, so that we can better understand the processes that help and those that hinder the evolution of sex. To develop a better understanding of why sexual reproduction is so commonplace, it is helpful to start with an examination of some of the most common erroneous beliefs regarding the relationship between sex and natural selection , including those described in the following sections.

Many people assume that sexual reproduction is critical to evolution because it always results in the production of genetically varied offspring. In truth, however, sex does not always increase variation. Imagine, for instance, the simple case of a single gene that contributes to height in a diploid organism ; here, individuals with genotype aa are shortest, those with genotype Aa are of intermediate height, and those with genotype AA are tallest Figure 1. Now, for the sake of argument, imagine that the shortest individuals can hide safely, the tallest individuals are too big to be eaten by predators, and the intermediate-height individuals are heavily preyed upon.

Among those lucky few organisms who survive to reproduce, there will be a great deal of variation in height, with plenty of tall individuals and plenty of short individuals. What would sex accomplish in this case? Here, mating would bring the population back to Hardy-Weinberg proportions, producing fewer offspring at the extremes of height and more offspring in the middle. That is, sex would reduce variation in height, relative to a population that reproduces asexually.

Figure 1: Variability, built up by selection, is decreased by sex. Because the fitness surface exhibits positive curvature, the result of selection is a population with a great degree of variability in height middle panel.

Asexual reproduction in such a population preserves this variation bottom left , but sexual reproduction with random mating brings the population back into Hardy-Weinberg proportions and reduces variation bottom right. This example illustrates the fact that sex does not always increase variation. Figure Detail. This example is overly simplified, but it serves to illustrate a general point: Selection can build more variation than one would expect in a population in which genes are well mixed.

In such cases, sex reduces variation by mixing together genes from different parents. This problem arises in the case of a single gene whenever heterozygotes are less fit, on average, than homozygotes.

In this case, the heterozygote need not have the lowest fitness ; rather, its fitness must only be close to that of the least-fit homozygote. In general, mathematical models have confirmed that selection builds more variation than expected from randomly combined genes whenever fitness surfaces are positively curved, with intermediate genotypes having lower-than-expected fitness.

In such cases, sexual reproduction and recombination destroy the genetic associations that selection has built and therefore result in decreased rather than increased variation among offspring. The term " epistasis " is used to describe such gene interactions, and cases in which the intermediate genotypes are less fit than expected based on the fitness of the more extreme genotypes are said to exhibit "positive epistasis. Interestingly, even when sex does restore genetic variation , producing more variable offspring does not necessarily promote the evolution of sex.

Again, this reality refutes one of the arguments often raised in the attempt to explain the relationship between sex and evolution. To understand how this operates, consider another simple case involving a single gene, but this time, assume that heterozygotes rather than homozygotes are fittest. The gene responsible for sickle-cell anemia provides a great real-life example. Here, people who are heterozygous for the sickle-cell allele genotype Ss are less susceptible to malarial infection yet have a sufficient number of healthy red blood cells; on the other hand, SS homozygotes are more susceptible to malaria, while ss homozygotes are more susceptible to anemia.

Thus, in areas infested with the protozoans that cause malaria, adults who have survived to reproduce are more likely to have the Ss genotype than would be expected based on Hardy-Weinberg proportions. In such populations in which heterozygotes are in excess, sexual reproduction regenerates homozygotes from crosses among heterozygotes. Although this indeed results in greater genetic variation among offspring, the variation consists largely of homozygotes with low fitness.

Yet again, this simple example illustrates a more general point: Parents that have survived to reproduce tend to have genomes that are fairly well adapted to their environments. Mixing two genomes through sex and genetic recombination tends to produce offspring that are less fit, simply because a mixture of genes from both parents has no guarantee of functioning as well as the parents' original gene sets.

In fact, mathematical models have confirmed that when selection builds associations among genes, destroying these associations through sex and recombination tends to reduce offspring fitness. This reduction in fitness caused by sex and recombination is referred to as the "recombination load" or the " segregation load" when referring specifically to segregation at a single diploid gene.

The reason that the recombination load is a problem for the evolution of sex is better appreciated by looking at evolution at the level of the gene. Imagine a gene that promotes sexual reproduction, such as by making it more likely that a plant will reproduce via sexually produced seeds as opposed to some asexual process e.

Carriers of this gene will tend to produce less fit offspring because sexual reproduction and recombination break apart the genetic associations that have been built by past selection. The gene promoting sex will fail to spread if the offspring die at too high a high rate, even if the offspring are more variable.

Genetic Information. Species Interdependence. Nitrogen Cycle. Spotted a problem? BBC Bitesize. The notes and diagrams are licensed under a Creative Commons License.

What you need to know Reproduction Reproduction is vital to the success of a species. For a species to survive it must be able to produce more offspring than it loses though old age, disease, and predation.

Living things can reproduce in different ways — asexual reproduction and sexual reproduction. Asexual Reproduction A bacterium reproduces by asexual reproduction. When a bacterium reproduces it simply copies its DNA and then divides in half - we call this process cell division. This means in asexual reproduction there is only one parent and all the offspring are identical to that parent.

The diagram below shows asexual reproduction in bacteria. One bacterium divides to form two, which both divide to form four which each divide again to form eight bacteria. Both survival and reproduction are important fitness components, and thus critical to the viability of wildlife populations.

Preventing one death survival or contributing one newborn reproduction , has arguably the same effect on population dynamics-in each instance the population grows or is maintained by one additional member.