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Evolution, Anatomy, Diversity & Life History of Solitary Bees

When biologists use the term diversity it can have several different meanings. First, diversity can refer to the total number of species in a group. Flowering plants are diverse because there are over 350,000…

When biologists use the term diversity it can have a number of different meanings.

First, diversity can refer to the total number of species in a group. Flowering plants are diverse because there are over 350,000 described species. Beetles are a diverse group because they include over 750,000 described species (roughly ¾ of all animals on earth). A more precise term for the number of species in a group would be species richness. But diversity can also refer to something else: the phenotypic variation among the members of a group. Flowering plants are extraordinarily diverse because they come in all sizes and shapes and they inhabit almost every possible terrestrial habitat. Likewise, beetles are an extraordinarily diverse group because they are enormously variable in size, shape, coloration, life history, and ecology.

Evolutionary History of Bees

How do bees compare in these measures of diversity, including species richness and phenotypic diversity? We know, from studies of the fossil record and phylogeny, that bees arose approximately 120 million years ago in the mid-Cretaceous. They evolved from hunting wasps, a group of solitary, mostly ground-nesting, non-social wasps that prey on other insects (including flies, crickets, caterpillars and many others). One evolutionary lineage of the hunting wasps began to combine pollen with insect prey and eventually evolved into bees feeding on a diet of pure pollen. Bees are essentially vegetarian wasps!

The Diversity of Bees and their Life Histories

Once the transition to pollen feeding had taken place, bees diversified rapidly into many habitats and they evolved diverse ways of exploiting flowering plants. Today, bees include over 20,000 described species divided into seven families, and over 500 genera . In North America alone there are over 4000 bee species and in New York we estimate that there are approximately 420 bee species. It is worth comparing bee species richness to other groups that you may be familiar with – vertebrates such as birds, mammals and reptiles. There are only 4000 mammal and 7000 bird species on earth, so bees are five times more species-rich than mammals and three times more species rich than birds.

Solitary – a single, adult female builds and occupies each nest. She constructs her own brood cells, provisions them with pollen and nectar, guards her own nest, and lays her own eggs. Approximately three-quarters of all bees are solitary (Figure 1). 

Social – Social bees exhibit three key features that distinguish them from solitary bees. In social bees there is:

(1) reproductive division of labor – meaning some females reproduce (the queen) and others remain as sterile workers.

(2) cooperative brood care – meaning females care for offspring that are not their own.

(3) overlap of generations – meaning multiple generations (mother and daughter) remain together over time.

When these conditions are permanent (meaning a female who is a worker remains a worker for her entire life) we refer to these societies as “eusocial”. When these conditions are temporary (meaning a female may serve as a worker for some part of her life but then later assume the role of queen) we refer to these societies as “cooperatively breeding ”.

Brood parasitic or “cleptoparasitic” – In brood parasitic bees, females do not build nests or collect pollen and nectar for larval nutrition. In fact, these bees lack the structures for gathering, manipulating and carrying pollen. Instead, they enter the nests of free-living, pollen-collecting bees and lay their eggs in either open or closed brood cells. The adult female or her first instar larva kills the host egg or larva, and the brood parasite then consumes the pollen provisions of the host bee. Female brood parasites are often heavily armored to defend themselves against the attack of the host female. Brood parasites are often, but not always, closely related to their hosts.

Socially parasitic – Social parasites enter the nests of social bees and kill or replace the host female as the primary egg layer. These bees only attack social hosts and are often closely related to their hosts (e.g., the subgenus Psithyrus in bumblebees).

1. Data Interpretation: Global Bee Diversity

In this activity, students will explore global bee diversity through several figures which examines the number of species in each of the seven bee families across the globe.

Part 1: Global Diversity Background

To highlight the impressive diversity of bee species, we will explore the variation in the number of species worldwide. The following figures, created using species richness data available from Discover Life, a free web tool that maintains a global encyclopedia of life ( In this activity, we will look at bee richness in five regions of interest: Australia, Brazil, North America (Canada, Mexico, and the United States), Southern Africa (South Africa, Botswana, Namibia, and Mozambique), and Spain.

To help you orient yourself, Figure 1 from Danforth et al. shows the family-level phylogeny of bees. For this activity, we will focus on the global distribution of these families and how species richness varies in each region.

Interpreting the Figures

Figure 2 shows the species richness (number of species) per area in millions of square Kilometers of reach region. Australia, Brazil, and North America have similar species richness per unit area even though they occupy very differentbioregions. Southern Africa shows moderately more species richness per unit area, possibly the result of the arid environment. Similarly, Spain's dry Mediterranean climate makes it a bee biodiversityhotspot. The relatively small size and lack of environmental diversity likely contribute to the high species richness seen in this area. 


Figure 1. The vast majority of the ~20,000 described bee species are solitary (figure adapted from Danforth, Minckley, and Neff, 2019).

Figure 2. Subfamily phylogeny of bees showing the relative abundance of each life history category (figure from Danforth, Minckley, and Neff, 2019).


Vocabulary PDF


Figure 1. Family-level phylogeny of bees Figure 2. Species richness by region Figure 3. Species richness per unite area (million sq Km) by region


Danforth, B. N., Cardinal, S., Praz, C., Almeida, E. A. B., & Michez, D. (2013). The impact of molecular data on our understanding of bee phylogeny and evolution. Annual Review of Entomology, 58, 57–78.

Koh, I., Lonsdorf, E. V., Williams, N. M., Brittain, C., Isaacs, R., Gibbs, J., & Ricketts, T. H. (2016). Modeling the status, trends, and impacts of wild bee abundance in the United States. Proceedings of the National Academy of Sciences of the United States of America, 113(1), 140–145.