This is a literature course on the book Evolution by Douglas J. Futuyma & Mark Kirkpatrick (Fourth Edition) during the first half of 2023. This write-up, authored by Louis Addo covers chapters 10 to 11.
All about Sex
Anisogamy, often known as sexual reproduction which involves large immobile eggs and small mobile sperm are present in plants, animals, and certain eukaryotes. Ansiogamy also describes the dimorphism in the sizes of gametes which defines the different sexes. When suitable partners are few, hermaphroditism is advantageous since the different sexes employ different methods to determine the sex of the embryos. Sexually dimorphic characteristics may not evolve due to genetic limitations, but they may nevertheless be produced by sexually antagonistic selection. Certain species, including deep-ocean anglerfish, have males that adhere to females and transform their bodies into practically complete testes to aid in fertilization.
Charles Darwin’s theory of sexual selection states that competition for mates among members of one sex results in the evolution of secondary sexual traits and mating habits. According to Bateman’s theory, the number of females a male mates with determines how successful his reproduction is, amplifying males’ secondary sexual traits in the long term. There are two major modes of sexual selection: selection by male-male competition and selection by female choice. In male male-male selection, males directly compete through interference or resource control. Males may acquire characteristics and behaviors such as horns and battles to get access to females. Another type of sexual selection is female choice, in which females can “choose” partners based on particular traits. For example, female salmon will choose the largest and strongest males among the options. In other mating systems called sex role reversal, females can actively seek males with males exhibiting parental behavior such as taking responsibility for incubation and rearing of the chicks. This is shown in Figure 1 where Two female red phalaropes (Phalaropus fulicarius) fight over the smaller, duller-plumaged male on their breeding ground. In contrast to most birds, female phalaropes court males, which care for the eggs and young in their nests.
Sexual selection is influenced by variables including resource availability and operational sex ratio. Flowering plants also engage in sexual selection, with the rivalry between pollen grains and beautiful flower displays. Direct selection on choice occurs when genes that affect preference also directly affect survival or fertility. Indirect selection, on the other hand, is when preference genes are connected to other genes that are spreading due to selection. Understanding the processes and results of sexual selection can provide insight into how secondary sexual traits and mating habits have evolved across a range of organisms.
Sex ratios, or the ratio of males to females, are typically equal at birth in most creatures with distinct sexes because meiosis in males typically transmits the X and Y chromosomes with equal likelihood. In some species, sex is determined not by chromosomes but by physical or social environment. This is known as environmental sex determination. In the insect order Hymenoptera, which includes ants, bees, and wasps, a female either lay an unfertilized egg that develops into a male or fertilizes an egg that develops into a female to decide their sex. There are several exceptions to the rule that natural selection favors equal sex ratios, such as fig wasps, where considerably more females than males are born. As fig wasps are often a single family of brothers and sisters, this tendency may be explained by considering how organisms within them are linked. A mutation that enables females to bear male offspring will be selected because only females leave the fig to start the next generation.
In asexual reproduction, mates are not required. The process is rapid and there is enormous production of organisms in a short time and a possibility for successive generations to inherit positive genes from parent organism. In addition, asexual reproduction can occur in various environments. Despite these advantages of asexual reproduction sexual reproduction predominates in the majority of life on Earth because of a number of perks. The benefits of sexual reproduction exceed its drawbacks because it permits genetic mixing, or the uniting of alleles from two parents. Parthenogenesis is a mystery in evolutionary biology since only a small proportion of plant and animal species reproduce asexually. The Red Queen hypothesis, which states that recombination increases the frequency of rare allele combinations that are helpful in fending off attacks from other species or parasites, has been studied in animals that exhibit both sexual and asexual reproduction, including water fleas and New Zealand mud snails. These studies suggest that sexual reproduction may be preferred in changing environments. Recombination, which may distinguish favorable mutations from detrimental ones and boost the efficiency of selection, reduces selective interference, where advantageous mutations fight for fixation in sexual reproduction.
Several hermaphrodite plants and animals employ self-fertilization as a method of reproduction to ensure reproductive success, however, it can cause inbreeding depression owing to harmful mutations. Animals with hermaphroditic sex often avoid self-fertilization. In order to prevent self-fertilization, plants have evolved defense systems (self-incompatibility) including the physical separation of anthers and stigmas. Several species of hermaphrodites still mate once in a while which helps avoid clonal interference and improve adaptability. Humans can experience inbreeding depression as well, which may help to explain why inter-family marriage is stigmatized in society.
How to Be Fit
Species have a broad range of life expectancies, from a few days to possibly indefinite. An organism’s fitness depends on its reproductive tactics, such as fertility, survival, and age of reproduction. Organisms allocate the energy and nutrients they get from their environment to self-maintenance, growth, and reproduction, and there are trade-offs between these functions due to correlated fitness benefits and costs. Reproductive effort is the allocation of nutrients and energy toward reproduction, while the cost of reproduction is the trade-off between reproduction and other functions. Numerous species have provided evidence for a cost of reproduction, since genotypes that devote more resources to reproduction may show reduced survival or development. Reproduction has a cost, according to genetic correlations that have revealed trade-offs between it and fecundity or survival. Fitness in iteroparous species is assessed by lifetime reproductive success, which is estimated by summing the number of births across all ages at which individuals reproduce. The example of an asexual lizard with a three-year lifetime can be used to demonstrate this. In sexually reproducing populations, we can use a life table to calculate the fitness of an allele by using the average values of survivorship and fecundity for individuals carrying that allele.
Natural selection does not act to prolong survival beyond the last age of reproduction, as increasing survival and fecundity at earlier ages has a larger effect on fitness than at later ages. Two major factors are responsible for the evolution of senescence and limited lifespan: mutation accumulation and antagonistic pleiotropy. Mutation accumulation causes mutations that compromise biological functions to reduce fitness less, the later in life they exert these effects, and selection against these mutations is weaker. Antagonistic pleiotropy suggests that many genes affect allocation to reproduction versus self-maintenance and incur a cost of reproduction. Alleles that increase allocation to reproduction early in life reduce function later in life, leading to a negative relationship between early reproduction and both longevity and later reproduction. While both factors can contribute to senescence, many biologists believe that antagonistic pleiotropy is often the more important factor.
The growth rate of a population is affected by ecological conditions, including resources, predation, and disease. The per capita growth rate of a population declines as the population size increases, eventually reaching a stable equilibrium number called the carrying capacity (K), which favors alleles that increase an individual’s ability to compete for limited resources. Species that are adapted to crowded conditions near K are called K-selected, while species that experience rapid, exponential growth and have higher fitness with higher fecundity are called r-selected.
There are different life histories and reproductive strategies in various species. Some species reproduce early and die young, while others delay reproduction and invest in growth and self-maintenance. Species with higher rates of adult survival tend to delay reproduction, while those with low adult mortality may benefit from iteroparity or repeated reproduction. Semelparity, or reproducing only once, may be favored if the probability of survival increases with body mass and if there is an exponential relationship between body mass and reproductive output. Conversely, iteroparity may be advantageous in fluctuating environments or in species with low adult mortality rates.
In species such as humans, albatrosses, and kiwis, there are trade-offs between the number of offspring and their size. Larger offspring require more care and resources from parents, and in some habitats or lifestyles, starting life at a large size greatly enhances the chance of survival. In these species, it may be advantageous to produce only a single, or a few, larger offspring rather than many smaller ones. Additionally, producing too many offspring can lead to decreased survival rates for both parents and offspring, as parents may be unable to adequately feed and care for a large brood. Therefore, the optimal clutch size for a bird, for example, maybe the number of eggs that yields the greatest number of surviving offspring, and this may be fewer than what could be produced given the parent’s resources. Similarly, among plants, larger seeds may be advantageous for species that germinate in the deep shade of closed forests, where the survival and growth of a seedling under adverse conditions are enhanced by the food stored in a large seed’s endosperm.
Sequential hermaphroditism, or the ability to change sex over the course of the life span, can be advantageous in species where reproductive success increases with size to a greater extent in one sex than the other. For example, in species that grow in size throughout reproductive life, a sex change can be beneficial if producing more seeds requires a larger body size, as in the case of squashes and other plants that produce male flowers when small and switch to producing female flowers when larger. In some species of slipper shells and fishes, sex changes occur when individuals reach a certain size or age, with almost all species of sex-changing animals changing sex when they have reached about 70 percent of their maximum size. These sex changes are a result of the costs of reproduction, which affect both males and females and the advantages of larger body size or reproductive output for each sex.
The evolution of life history variation is usually understood in terms of survival and reproduction components of fitness. However, a broader conception of an organism’s life history includes many other aspects of its life, such as dispersal and its use of habitats, food, and other resources. Specialization in ecological niches can be advantageous due to several reasons, including interactions with other species, the advantage of evolving a preference for a safe space, and trade-offs. The plasticity that often underlies broad tolerance can be disadvantageous because it has costs, including the costs of developing an altered phenotype and maintaining the ability to do so. Specialization may also be advantageous due to trade-offs, as a specialist is likely to become more effective or efficient than a generalist, in which performance of any one task is likely to be compromised by the characteristics needed to perform other tasks. Trade-offs in cognitive processing may account for host specialization in some herbivorous insects. Morphological trade-offs have been shown in many organisms, such as flower piercers, which have an unusual hooked bill with which they hold the flower and punch a hole in its base. When the hooked tip of the bill is clipped experimentally, the birds become less efficient at obtaining nectar but become more proficient at eating berries. However, specialization can also be disadvantageous in unpredictable environments, as specialists are more vulnerable to environmental change. Tropical species that already live near their upper thermal limit may be especially endangered by global warming, which could pose a threat to specialized species in general. Overall, understanding the advantages and disadvantages of ecological specialization can shed light on the evolution of life histories and the adaptation of organisms to their environments.
Futuyma, D. J., & Kirkpatrick, M. (2017). Evolutionary. Evolution (Fourth ed.). pp. 247-292. Sunderland, Massachusetts: Sinauer Associates, Inc.