200 years of data show evolution of smaller wings in non-migrating monarchs

Monarch butterfly (Daneus plexippus). Credit: USFWS

Monarch butterflies complete an epic and perilous migration across North America each year. Their route along a migratory corridor takes multiple generations to finish, and an individual butterfly may travel up to 3,000 kilometers in its lifetime!

While this migration is one of the defining characteristics of the species, lesser known is that monarchs have expanded beyond their ancestral range in the past 180 years. Monarchs are now found outside of North America throughout the Pacific and Atlantic. From these long-distance travels, there are several non-migratory populations found on remote islands; some time in the past, migratory monarchs reached these islands, and then stayed and stopped migrating.

This makes monarchs the perfect natural experiment to study evolution in action. Each time monarchs colonize a new island, it becomes a natural replicate of the same experiment again.

A new study used this natural experiment to determine whether the monarchs that founded new island populations had traits that favored long-distance travel, and whether these traits were lost over time once migration was no longer a strong selective force. Using over 6,000 historical museum specimens, dating as far back as 1856, the authors measured wing length and other features associated with migration.

They found evidence of evolution: while early migrants had long, larger wings (traits that help with long-distance travel), over time selection favored individuals with smaller wings, and the trait evolved. This pattern repeated in multiple populations, and happened in just the past 200 years.

Next, the authors paired their historical observations with an experiment using living monarchs. They collected specimens from migratory and non-migratory populations, and grew them in a common environment in the lab. By keeping the environment the same for all individuals, they could rule out that trait differences were due to plastic responses to life in different environments, and were indeed based on genetics. They found the pattern from their museum samples replicated in live monarchs, i.e. those from non-migratory populations had significantly smaller wings than those from migratory populations.

This study highlights the importance of long-term sampling and museum collections, and the exciting potential of pairing historical specimens with contemporary experiments.

Resources

Freedman, M.G., Dingle, H., Strauss, S.Y. and Ramírez, S.R. 2020. Two centuries of monarch butterfly collections reveal contrasting effects of range expansion and migration loss on wing traits. Proceedings of the National Academy of Sciences 117(46): 28887-28893.

Monarch butterfly deaths due to road collisions during fall migration (July 2019)

State	Year	Policy	Description
New Hampshire	2026	SB592	Enables regional, conservation, and energy resource planning for habitat strongholds and wildlife corridors
New Hampshire	2019	SB200	Requires the New Hampshire Department of Transportation to consider fish and wildlife crossings and mitigation measures in all future road and culvert projects
New Hampshire	2016	SB376	Requires the fish and game department to identify existing and needed wildlife corridors connecting wildlife habitats in the state and to make recommendations for legislative changes

State	Year	Policy	Description
Maine	2023	LD258	Appropriates $7.5 million to Department of Transportation to improve roadways with potential adaptations for wildlife
Maine	2023	LD1881	Establishes compensation fee program relating to fish and wildlife habitats and associated corridors, including compensation paid by solar energy development
Maine	2023	LD508	Requires Department of Environmental Protection to review its permitting laws concerning climate change, habitat connectivity, ecological resilience, and coastal environments

State	Year	Policy	Description
New Jersey	2025	S3618	Directs state agencies to develop a Wildlife Corridor Action Plan
New Jersey	2024	S3916/A5486	Expands eligibility for existing wildlife fencing programs by including farmers leasing farmland
New Jersey	2007	NJAC7:13	Requires the state to incorporate wildlife crossings or a natural bank for terrestrial threatened and endangered species and/or species of concern when a new bridge or culvert project would cause habitat fragmentation

State	Year	Policy	Description
New Mexico	2023	SB72	Appropriates money to new Wildlife Corridors Fund for implementation of wildlife crossing projects under New Mexico Wildlife Corridor Action Plan
New Mexico	2022	HB 2	Allocates funding for projects to reduce wildlife-vehicle collisions and allows the Department of Transportation and Department of Game to implement their Wildlife Corridors Action Plan
New Mexico	2019	SB 228	Requires the creation of a Wildlife Corridor Action Plan that includes comprehensive guidance to agencies for identifying, prioritizing and maintaining important areas for wildlife movement

State	Year	Policy	Description
Washington	2023	HB1216/SB5196	Requires Department of Ecology to prepare environmental impact statements for clean energy projects that consider impacts to habitat connectivity and wildlife migration corridors
Washington	2022	SB5689	Provides funding for US 97 Wildlife Crossing Improvements project
Washington	2019	SEO 1031.02	Recognizes the effect of roadways in creating barriers to wildlife movement, breeding, and migrations; the Washington State Department of Transportation will promote and support Practical Solutions and Planning and Environmental Linkages to integrate habitat connectivity and biodiversity plans; will make use of the highway prioritization map known as Habitat Connectivity Investment Priorities; and will corporate and coordinate with other agencies involved in wildlife habitat protection, specifically along highways

Connecting Science to Conservation

200 years of data show evolution of smaller wings in non-migrating monarchs

Resources

About the Author: Elizabeth Schultheis

State	Year	Policy	Description
Wyoming	2026	SF0052	Allocates funds for restoration of aquatic habitats including barrier removal for fish
Wyoming	2025	SF0088	Allocates funds for projects improving wildlife connectivity including fence removal, wildlife crossings improvements, and habitat restoration
Wyoming	2024	SF0060	Provides funds to help maintain primary migration corridors for ungulates in the Little Snake River Conservation District
Wyoming	2023	SF0106	Provides funds for large wildlife crossing projects across the state
Wyoming	2022	SF0048	Approves funding for specific wildlife crossings and aquatic connectivity initiatives
Wyoming	2021	SF066	Provides funds for large wildlife crossing projects across the state
Wyoming	2020	EO 1	Designates three mule deer migration corridors and establishes the process whereby future corridors can be designated
Wyoming	2020	HB 69	Establishes a system wherein funding from outdoor recreation can be appropriated for wildlife conservation efforts related to the state’s transportation system
Wyoming	2018	HB39	Proceeds from the newly established wildlife conservation license plates are allocated to wildlife connectivity improvements to the state transportation system

State	Year	Policy	Description
Vermont	2023	Act59	Vermont's 30x30 law that commits to conserving and maintaining an ecologically functional landscape that sustains biodiversity and maintains landscape connectivity
Vermont	2016	Act 171	Primarily focuses on timber harvesting regulations, but calls for a study of necessary revisions to planning statutes that protect contiguous areas of forestland from fragmentation and promote habitat connectivity between forestlands

State	Year	Policy	Description
Colorado	2026	SB141	Creates optional collision prevention fee to fund wildlife safe passage projects
Colorado	2025	HB1093	Restricts local limits on residential density unless offset elsewhere, exempts areas near wildlife crossings, and allows municipalities to seek judicial review of land use initiatives
Colorado	2023	SB 23-092	Encourages minimal impacts to wildlife migration corridors when constructing solar panel farms
Colorado	2023	SB 214	Appropriates funding to the Colorado Wildlife Safe Passages Fund for wildlife crossing projects
Colorado	2022	SB22-151	Creates a $5 million fund for measures that reduce wildlife-vehicle collisions and reconnect habitat fragmented by roads
Colorado	2022	HB22-1072	Expands the scope of Habitat Partnership Program to fund efforts to conserve wildlife migration
Colorado	2021	SJR 21-021	Aims to protect and enhance habitat connectivity in the state through the establishment of a working group to develop state policies on corridors, as well as the commission of a report on state corridors and voluntary policies to support willing landowners in improving connectivity
Colorado	2019	EO 11	Directs Colorado agencies to identify policy, regulatory, and legislative opportunities to ensure the ongoing conservation of seasonal big game habitat and migration corridors
Colorado	2018	SB 18-143	States explicit goals for Division of Parks and Wildlife to invest in habitat conservation and wildlife crossings

State	Year	Policy	Description
California	2026	SB1250	Requires the Department of Transportation (Caltrans) to include wildlife connectivity corridors and needs into its infrastructure project planning
California	2024	AB1889	Requires counties and municipalities to conserve wildlife corridors when planning new developments
California	2023	AB1760	Calls for a statewide analysis of wildlife corridors and connectivity to support conservation planning and climate change adaptation activities.
California	2023	SB145	Outlines provisions for transportation-related environmental mitigation, including funding for wildlife corridors, property and credit purchases for mitigation, reporting requirements, and mandates for wildlife crossings along Interstate 15.
California	2023	SB101	Provides funding for wildlife corridors and open spaces, including projects to improve connectivity and reduce barriers between habitat areas.
California	2022	AB2344	Establishes a state program to address roads with high wildlife-vehicle collisions, authorizes Caltrans to develop standalone wildlife connectivity projects, mandates an inventory and prioritization system for critical areas, and requires integration of wildlife passage features into relevant road designs.
California	2022	AB1384	Requires agencies to address vulnerabilities in Safeguarding California Plans, including protecting and enhancing wildlife corridors
California	2021	SB790	Approves compensatory mitigation credits for wildlife connectivity actions
California	2021	AB1183	Establishes the California Desert Conservation Program to protect biodiversity, including the protection and enhancement of habitat connectivity
California	2021	AB128	Appropriates funding for wildlife crossing projects, including the Liberty Canyon crossing for mountain lions
California	2015	AB 498	Amends the California Fish and Game Code to encourage, wherever feasible, voluntary steps to protect the functioning of wildlife corridors
California	2014	AB1471	Provides funds to the Sacramento- San Joaquin Delta Conservancy, in part to protect and restore aquatic, wetland, and migratory bird ecosystems including corridors.
California	2008	AB2785	Orders Dept. of Fish & Wildlife to investigate, study, and identify key wildlife corridors and linkages within the state, and to make those areas public