Natural selection is not the only mechanism that causes evolution. In some taxa (flowering plants, for example) it may not even be the most important. This section discusses other phenomena that cause populations to evolve.

This competition takes one of two forms, either physical combat among males or competition for the attention of females. Large body size, strength and aggressiveness provide advantages in “male combat”, whereas male secondary sexual charactersTraits which are found in one gender only but are not directly connected to the act of reproduction, such as antlers, lion's manes, or a peacock's tail. (Figure 1) such as colorful male plumage and complex display behavior increase the attention of females (“female choice”)[#](Krebs & Davies 1993).

Darwin and Wallace proposed that males with bright plumage demonstrate their health and “good” genes, so that these males are more likely to attract a mate, and pass on the “bright plumage” genes. In fact, this idea has some empirical support[#](Møller & Alatalo 1999). For example, male blackbirds with the brightest bills have a stronger immune system than those without[#](Pennisi 2003), and peacocks whose tails are longer and have more 'eyes' have fewer parasites[#](Petrie, et al. 1991)(Ridley 1993).

Sexual selection illustrates why a focus on survival is misplaced. Many male traits that have arisen because of sexual selection actually decrease a male's chances of surviving. Ornate tail feathers like those of peacocks, birds of paradise, and widowbirds not only make it harder to fly, they make males more conspicuous - especially during display[#](Krebs & Davies 1993). Male sage grouse displays include inflating esophageal sacs, and females prefer males with larger ones.  But those males also a have higher mortality rate than other males[#](Young, et al. 1994).  Male fiddler crabs have one enlarged claw - subject to female choice - which makes it harder to feed and hide from predators[#](Croll & Klinger 1994).  Male Jamaican guppies with the brightest coloration suffer the highest mortality, but have highest reproductive success[#](Endler 1983). 

The traits persist because males who have them have more offspring than males without them, regardless of how long they live. 

Because choosy females only mate with males who possess certain traits, sexual selection has the potential to rapidly generate reproductive isolationThe inability of two or more populations to interbreed and exchange genes.. What’s more, sexual selection also has the potential for sympatric speciation - geographic isolation is unnecessary[#](Panhuis, et al. 2001). The diversification of cichlid fishes in Lakes Victoria, Malawi, and Tanganyika in East Africa suggests that sympatric speciation may have been a major driving force for this adaptive radiationThe rapid speciation from a single or a few species to many, because of the many open ecological niches, such as following colonization of new islands.[#](Kocher 2004)(Meyer 1993).

It should be noted, too, that males of many species have elaborate ornamentation that is not a result of female choice, but male-male competition. Rather than risk injury or death by combat, many species use ornamentation (such as antlers) as a signaling device amongst males to create a dominance hierarchyA social system by which individuals within a population or group control access to resources within the group. In some hierarchies, one individual dominates. In others, each individual has a rank in the hierarchy.[#](Krebs & Davies 1993). Dominant males assert their dominance not through physical combat but with displays and threats (although similarly-matched males sometimes do physically battle).

The costs and benefits of any trait - including altruistic behavior - are measured in terms of reproductive fitness (expected number of offspring)[#](Krebs & Davies 1993). So if altruism reduces fitness, this behavioral trait should be eliminated. And how can being sterile possibly be selectively advantageous?

In a classic paper WD Hamilton[#](Hamilton 1977) showed how. He showed that any genes that may be responsible for unselfish behavior can increase in frequency - if those benefitting from the altruism also carry genes for altruistic behavior.

If an altruist helps just anyone, the altruist sacrifices its own fitness, putting it at a selective disadvantage; the trait should disappear from the population. But suppose that altruists are discriminating in who they help. If they help only their relatives, this immediately changes things. Relatives are genetically similar, so when an organism carrying an “altruistic gene” shares its food, for example, there is a certain probability that the recipients of the food will also carry copies of that gene. This means that the altruistic gene can spread by natural selection: although the helper (or sterile worker) reduces its own fitness, it boosts the fitness of its relatives, who have a greater than average chance of carrying the gene themselves.

So the overall effect of the behavior may be to increase the number of copies of the altruistic gene found in the next generation, and thus the incidence of the altruistic behavior itself. This increase in “inclusive fitness” is known as kin selection[#](Futuyma 1998).

The actual probability depends on how closely related they are. On average, full siblings share 50% of their genes, as do parents and offspring[#](Snustad & Simmons 2005). Grandparents and grandchildren share 1/4 of their genes, and full cousins share 1/8, and so-on (hence Haldane’s quote in the margin to the right). A gene for altruism can spread by natural selection as long as the fitness cost incurred by the altruist is offset by a sufficient amount of benefit to sufficiently closely-related relatives[#](Hamilton 1977).

This explains why, for example, it has been found in various bird species that ‘helper’ birds (Figure 2) are much more likely to help relatives raise their young, than they are to help unrelated breeding pairs. Similarly, studies of Japanese macaques have shown that altruistic actions, such as defending others from attack, tend to be preferentially directed towards close kin[#](Krebs & Davies 1993).

In most social insect species, a phenomenon called haplodiploidyA genetic system in which one of the sexes has haploid cells (usually male) and the other has diploid cells. The males develop from unfertilized eggs, females develop from fertilized eggs: the sperm provides a second set of chromosomes when it fertilizes the egg. means that females on average share more genes with their sisters than with their own offspring[#](Snustad & Simmons 2005). So by helping the queen reproduce, the genes responsible for a female's "helping" behaviors are more likely to be passed on if she helps raise her sisters, rather than by having offspring of her own.  This means that "helping" behaviors will not be replaced by selfish ones.

Kin selection does not require that animals have the ability to discriminate relatives from non-relatives[#](Krebs & Davies 1993). For example, in many species relatives live near each other or are part of same colony, pack or herd. If an individual simply behaves altruistically towards those in its immediate vicinity, the recipients of the altruism are likely to be relatives (who are likely to have and pass on the altruistic trait).

Polyploidy has long been recognized as a prominent force shaping the evolution of plants, especially flowering plants[#](Goldblatt 1980)(Soltis, et al. 2007)(Stebbins 1985). Various estimates suggests that as many as 30-70% of flowering plants originated via polyploidy. Many important crop plants, such as alfalfa, cotton, potato, and wheat, are polyploids.

Polyploidy can naturally arise in a number of different ways. In some cases a somatic mutation (in non-reproductive cells) can occur due to a disruption in mitosisThe process of nuclear division in eukaryotes, the first step in cell division, so each resulting daughter cell has a full complement of chromosomes., resulting in chromosome doubling in a cell that will subsequently produce a new (and polyploid) shoot, which may then produce polyploid seeds.

Polyploids can also result from the union of unreduced gametesReproductive cells which fuse to form a zygote. Gametes are haploid, differentiated into egg and sperm in animals. - an error in a plant's production of its pollen or ova which results in reproductive cells containing too many copies of each chromosome. When that pollen or egg unites with another polyploid pollen or ovum (from the same or different species), the offspring is a plant with extra chromosomes.

Speciation via polyploidy is instantaneous. It takes just one generation, and it doesn't require that the new species be geographically isolated. A 4n (tetraploid) individual cannot produce viable offspring with diploid (2n) individual[#](Snustad & Simmons 2005) - they are different species.

Genetic drift has negative effects on populations. It reduces genetic variation, and acts faster and has more drastic results in smaller populations - which means it can affect rare and endangered species. Reduced genetic variation means that the population may not be able to adapt to new selection pressures because the genetic variation that selection would act on may have already “drifted” out of the population.

Weaver, TD, CC Roseman, & CB Stringer. 2007. Were neandertal and modern human cranial differences produced by natural selection or genetic drift? Journal of Human Evolution 53:135-145

Doerr, ED & VA Doerr. 2007. Positive effects of helpers on reproductive success in the brown treecreeper and the general importance of future benefits. Journal of Animal Ecology 76:966–976

Gray, DA & WH Cade. 2000. Sexual selection and speciation in field crickets. PNAS 97:14449–14454

Soltis, DE, PS Soltis, DW Schemske, JF Hancock, JN Thompson, BC Husband, & WS Judd. 2007. Autopolyploidy in angiosperms: have we grossly underestimated the number of species? Taxon 56 (1):13-30